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Home My WebLink AboutMNA Cap Guidance 171031 FINALMonitored Natural Attenuation for Inorganic Contaminants in Groundwater: Guidance for Developing Corrective Action Plans Pursuant to NCAC 15A .0106 (l). Companion guidance to the ‘15A NCAC 2L Implementation Guidance’, Division of Environmental Management, December, 1995. Department of Environmental Quality, Division of Water Resources, October 2017 _____________________________________________________________________________________________ MONITORED NATURAL ATTENUATION FOR INORGANIC CONTAMINANTS IN GROUNDWATER: GUIDANCE FOR DEVELOPING CORRECTIVE ACTION PLANS PURSUANT TO NCAC 15A .0106 (l) Companion guidance to the ‘15A NCAC 2L Implementation Guidance’, Division of Environmental Management, December 1995 October 2017 Division of Water Resources Department of Environmental Quality i Table of Contents 1.0 BACKGROUND, PURPOSE, AND SCOPE ............................................................................................. 1 2.0 CONDITIONS FOR MNA APPROVAL UNDER NCAC 15A 02L .0106 (l) ................................................ 2 3.0 DEFINING SOURCE AREAS ................................................................................................................. 4 4.0 COLLECTING DATA IN SUPPORT OF MNA CAP .................................................................................. 5 4.1 Sample Location and Data Needed to Evaluate Plume Characteristics and Natural Attenuation 5 4.2 Iterative Nature of Data Collection in Support of an MNA CAP ................................................... 7 4.3 Data Inventory .............................................................................................................................. 7 5.0 MINIMUM CONTENTS EXPECTED IN AN MNA CAP FOR INORGANICS ............................................. 8 5.1 Site Background and Regulatory Basis for Corrective Action. ...................................................... 8 5.2 Conceptual Model of Groundwater Flow and Contaminant Transport........................................ 8 5.3 Potentiometric Maps. ................................................................................................................... 9 5.3.1 Potentiometric contour map of shallow system ........................................................................ 9 5.3.2 Potentiometric contour map of bedrock system (as applicable) ............................................... 9 5.3.3 Vertical gradient map showing observed head differences ...................................................... 9 5.3.4 Potentiometric contour map of simulated shallow system ....................................................... 9 5.3.5 Potentiometric contour map of simulated bedrock system ...................................................... 9 5.3.6 Simulated vertical gradient map ................................................................................................ 9 5.4 Background Concentrations. ......................................................................................................... 9 5.5 Characterization of Source Areas................................................................................................ 10 5.5.1 Source history, volume, and characteristics. ........................................................................... 10 5.5.2 Horizontal and vertical extent of source material. .................................................................. 10 5.5.3 Horizontal and vertical extent of saturated source material. .................................................. 10 5.5.4 Source control and stability. .................................................................................................... 10 5.5.5 COI(s) associated with source area. ......................................................................................... 10 5.5.6 Base and iso-concentration maps showing horizontal and vertical extent of COI(s) .............. 10 5.5.7 Identification of receptors associated with the source area ................................................... 10 5.5.7.2 Supply wells ....................................................................................................................... 11 5.5.7.3 Future groundwater use areas .......................................................................................... 11 5.5.8 Evaluation of human and ecological risks ................................................................................ 11 5.5.9 Evaluation of alternative corrective actions ............................................................................ 11 5.6 Demonstration of Natural Attenuation. ..................................................................................... 11 5.6.1 Understanding plume characteristics in space and time .................................................... 11 ii 5.6.2 COI migration predictions ................................................................................................... 12 5.6.2.1 Predictions based on existing conditions. ......................................................................... 13 5.6.2.2 Predictions based on source control and corrective actions. ........................................... 13 5.6.3 Conceptual model of natural attenuation. ......................................................................... 14 5.6.3.1 Dilution and dispersion. .................................................................................................... 14 5.6.3.2 Sorption, if applicable. ...................................................................................................... 14 5.6.3.3 Precipitation, if applicable. ............................................................................................... 14 5.6.3.4 Phyto-attenuation ............................................................................................................. 14 5.6.3.5 Other mechanisms ............................................................................................................ 14 5.6.4 Demonstration that contaminated groundwater discharge to surface water will not result in violations of surface water standards contained in 02B .022. ........................................................ 14 5.6.4.1 Existing conditions............................................................................................................. 14 5.6.4.2 Future conditions .............................................................................................................. 15 5.7 MNA schedule, performance monitoring plans, and contingency plans .................................... 15 5.8 Access agreements, public and other notices, and adherence to other laws ............................ 15 5.9 Completed checklist for MNA CAP for Inorganics ...................................................................... 15 6.0 REFERENCES .................................................................................................................................... 17 7.0 APPENDICES .................................................................................................................................... 18 APPENDIX A – CHECKLIST FOR MNA CAP FOR INORGANICS....................................................................... 18 APPENDIX B. EPA FOUR TIER MNA EVALUATION. ..................................................................................... 20 APPENDIX C. CONTENTS EXPECTED IN BASE MAPS. .................................................................................. 24 APPENDIX D – 1995 15A NCAC 2L IMPLEMENTATION GUIDANCE ............................................................. 27 1 1.0 BACKGROUND, PURPOSE, AND SCOPE Monitored natural attenuation (MNA) has been proposed as a corrective action remedy for inorganics in groundwater at numerous facilities across the State, including coal ash sites. 15A NCAC 02L .0106 (l) allows for the use of MNA for the remediation of groundwater contamination and states that any person required to implement an approved corrective action plan “may request that the Director approve such a plan based on natural processes of degradation and attenuation of contaminants.” A contaminant is defined here as a constituent (or constituent of interest (COI) that exceeds the 02L .0202 standards or Interim Maximum Allowable Concentrations (IMACs), and is above background concentrations. The Division of Environmental Management produced a 15A NCAC 2L Implementation Guidance in December 1995 (1995 Guidance), that provided direction on the implementation of selected sections of 02L, including 02L .0106 (l) (MNA). However, the 1995 Guidance focused on organic contaminants that attenuate primarily through biodegradation to less toxic daughter constituents rather than inorganic contaminants that attenuate primarily through other mechanisms such as dispersion, sorption, and, in some cases precipitation or co-precipitation. Unlike organic attenuation that is permanent and irreversible, the attenuation of inorganics by sorption and (or) (co)precipitation may, in some cases, be reversible if subsurface conditions change. An inorganic contaminant that was otherwise immobilized within aquifer solids may re-mobilize and re-enter the groundwater system if geochemical conditions change. This possibility is assessed within an MNA Corrective Action Plan (CAP) for inorganics. It is recognized also that inorganics occur naturally in soils, bedrock, and groundwater which may affect the cleanup levels to which the MNA remedy must ultimately achieve. The purpose of this document - ‘MNA for Inorganic Contaminants in Groundwater: Guidance for Developing Corrective Action Plans’ – is to outline Division of Water Resource expectations for an approvable MNA CAP for inorganic contaminants. This document augments the 1995 Guidance and acknowledges that both documents are relevant to the implementation of MNA under 02L .0106 (l). It is the expectation that an approvable MNA CAP demonstrate adherence to the criteria set forth in 02L .0106 (l) for each source area and that the contaminants in question can be remediated to 02L/IMAC standards within an acceptable period-of-time. Hybrid remediation strategies (for example, MNA, coupled with a passive reactive barrier) may also be presented under a 02L .0106 (l) CAP. The decision to approve the use of MNA rests with the Division Director. This document describes the expectations for the use of MNA. While MNA shares some elements of risk based remediation, the two are separate and distinct, and this guidance is not intended to address risk based remediation1. 1 Legislation (SL 2015-286) prohibits the use of risk-based remediation for coal ash sites. 2 2.0 CONDITIONS FOR MNA APPROVAL UNDER NCAC 15A 02L .0106 (l) Pursuant to 02L .0106 (l), the use of MNA may be a viable corrective action alternative when certain criteria are met. These criteria are discussed below. 02L .0106 (l)(1). All sources must be controlled or removed. The 1995 Guidance discusses source control on p. 12 (paragraph 1) and on p. 17. In some cases, the responsible party (RP) may satisfy this requirement by being in the active process of source control or source removal. If an RP can remediate to the 02L standards/IMACs, source material may, under certain conditions, remain in place. For purposes of this guidance, capping a source in place may, in some instances, constitute “control”. In such a case a de-watered source may be essentially stabilized and prevented from contributing additional mass to a plume2. For source areas that are capped in place and contain inorganic waste below the seasonal high water table, robust evidence must be provided, using a method pre-approved by the Division, that demonstrates adequate source control of the submerged waste. In some cases, it may not be possible to effectively control a source with only a cap in place closure. While 02L .0106 (l) does not preclude the use of MNA at sites where a plume is migrating or changing shape, it does require that the source has been controlled and that contaminant mass is not being added to the existing plume at quantities that would result in a contravention of the rule. In most cases this will mean that the source has been removed or capped and de-watered. 02L .0106 (l) (2). The contaminant must have the capacity to attenuate under site conditions. The 1995 Guidance discusses the attenuation of organic, but not inorganic contaminants. The use of MNA under 02L .0106 (l) (2) does not require that a contaminant plume be fixed in space, if the plume characteristics are well understood and documented. However, attenuation should be occurring, sufficient for 02L .0106 (l) compliance, and persistent over time. The dominant attenuation mechanisms for inorganics often include some combination of dilution, dispersion, sorption, precipitation/co-precipitation, and (or) phyto-attenuation. The case for natural attenuation is evaluated by the Division using, as a guideline, the EPA four tier framework (EPA, 2007) (Appendix C). In this framework, the emphasis is on characterizing what is causing attenuation to occur and determining that this cause (or causes) is (are) persistent over time. EPA Tier 1 states that active COI removal be shown3. EPA Tier 2 states that the dominant attenuation mechanism(s) and their rate(s) be determined. EPA Tier 3 states that the long-term stability of attenuation be shown. And Tier 4 describes performance monitoring to show that attenuation is occurring as expected and proposed. Demonstrating that natural attenuation is sufficient and appropriate is the responsibility of the RP. The degree of demonstration that is expected by the Division is based on site conditions 2 Plume, as used in this document, refers to either a mass of dissolved inorganic constituent spreading from a source (boron, for example), or to a distribution of a dissolved inorganic constituent whose presence and concentration is dependent on local geochemical conditions (iron, for example). 3 While EPA Tier 1 requires that “plume stability” be shown, 02L rules do not preclude plume migration in an MNA remedy. For example, 02L .0114 (a) provides notification requirements and states that, “under 2L .0106(k), (l), or (m), the RP shall notify the Health Director and Chief AO of the political jurisdictions in which the plume occurs and all property and owners and occupants within or contiguous to the area underlain by the contaminant plume, and under the areas where it is expected to migrate”. 3 such as the location and behavior of the plume and the attenuation capacity of the unit through which the plume flows. For example, a stable plume that is distant from the nearest receptor and whose leading edge is flowing through highly sorptive soils would typically require less attenuation demonstration than a moving or expanding plume whose leading edge is in close proximity to a receptor and whose leading edge is flowing through open, poorly sorptive bedrock fractures. The RP may use a combination of methods to demonstrate sufficient COI removal, attenuation rates, and persistence of the observed attenuation. Any modeling in support of these demonstrations should, in most cases, use conservative assumptions and include sensitivity analyses that evaluate end member conditions. Section 4.0 of this Guidance discusses expectations for data collection in support of the MNA CAP, and Section 5.6 discusses expectations for demonstration of natural attenuation. 02L .0106 (l) (3). The time and direction of contaminant travel must be predicted with reasonable certainty. The 1995 Guidance discusses contaminant modeling and travel predictions on p. 12 (paragraph 3) and on p. 23-24. Predictions may be carried out using an analytical or numerical model whose construction, assumptions, and performance are acceptable to the Division. It is noted that contaminants travel at different rates based on their sorptive characteristics and the subsurface geochemistry and sorptive hosts. The chosen model should be able to account for this or the modeler should assume worst case travel predictions (fastest, farthest) of the most highly mobile, lead edge contaminant (boron or sulfate, for example at coal ash sites) and apply this assumption to all contaminants. 02L .0106 (l) (4). Contaminant migration has not and will not result in any 02L standards/IMACs violations to a receptor or foreseeable receptor. The 1995 Guidance discusses migration to receptors on p. 11, 12 (paragraph 4) and on p. 25-26. As used in 02L .0106 (l) (4), “receptor” is defined as a supply well, and “foreseeable receptor” refers to a future groundwater use area defined in the 1995 Guidance (page 25) as “property where the groundwater resources have a potential use, public water is not available, and the permission of the area property owners allowing contamination to migrate onto their land has not been obtained, including locations for which formal plans exist to use groundwater for public or private use; locations for which property owner(s) has expressed an anticipated or possible future use of groundwater resources; rural locations for which public water supplies will most likely not be available for future residential, agricultural or industrial development and the owner(s) has expressed a future anticipated use; and locations where the land ownership cannot be determined at present”. A foreseeable receptor (i.e. future use area) shall include all potentially affected properties near the contamination site for which a public water supply is not available or legally promised by the RP. Surface water receptors are addressed by 02L .0106 (l) (6). An understanding of plume characteristics in space and time is necessary to properly assess contaminant migration and its potential impact on receptors or foreseeable receptors. 02L .0106 (l) (5). Contaminants have not and will not migrate onto adjacent properties unless (a) a public water supply sourced by surface water or unaffected groundwater is available, or (b) 4 owner approval has been granted in writing. The 1995 Guidance discusses migration onto adjacent properties on p. 12 (paragraph 5). An understanding of plume characteristics in space and time is necessary to properly assess contaminant migration onto adjacent properties. 02L .0106 (l) (6). Contaminated groundwater discharge from the source area is not currently causing and will not in the future cause violations to surface water standards. The 1995 Guidance discusses surface water standard violations on p. 13. Understanding the plume characteristics in space and time is needed to properly select the surface water sample locations for assessing potential current violations and to model or sample for potential future violations. 02L .0106 (l) (7). Performance monitoring well(s) will be no farther than 5 years travel time downgradient from the plume front, and no closer to the nearest supply well or future use area than 1-year travel time from the plume front. The 1995 Guidance discusses performance monitoring on p. 13 and p. 27. 02L .0106 (l) (8), (9), (10). All pertinent access agreements, public notice, and adherence to all other environmental laws must be demonstrated. The 1995 Guidance discusses access agreements and public notice on p. 13, p. 27-28, and p. 30. 02L .0106 (i). 02L .0106 (i) states that “In the evaluation of corrective action plans, the Director, or his designee shall consider the extent of any violations, the extent of any threat to human health or safety, the extent of damage or potential adverse impact to the environment, technology available to accomplish restoration, the potential for degradation of the contaminants in the environment, the time and costs estimated to achieve groundwater quality restoration, and the public and economic benefits to be derived from groundwater quality restoration.” It is the expectation that the RP provide information that is needed to evaluate these considerations as discussed further in Section 6 of this guidance. 3.0 DEFINING SOURCE AREAS The requirements of 02L .0106 (l) must be met for each COI4 and each source area at a facility. Some facilities may be represented by a single source area. Other facilities may contain several source areas. Large waste areas (e.g. coal ash basins) may need to be divided into separate smaller source areas if, for example, contaminant transport is toward different sets of receptors5. Where appropriate, some source areas may be strategically combined based on geographic proximity (for example, conjoining or overlapping source areas), common source characteristics and impacts, common receptors, and a shared proposed remedy. The Regional Office should be consulted when identifying source areas for 4 A COI is defined as a constituent that occurs above 02L standards/IMACs and background levels at or beyond a compliance boundary, or a constituent that occurs above 02L standards/IMACs and background levels in bedrock within a compliance boundary in an area with vulnerable downgradient receptors. 5 Cliffside (Rogers Energy Complex) active ash basin is an example of a site with a large waste area (an active, unlined ash basin) that was divided into two source areas. This basin has two dammed outfalls over a half mile apart (one into Suck Creek and the other into the Broad River). Contaminated groundwater discharge moves toward two different areas and sets of receptors. Dividing this large waste area into two source areas is appropriate. 5 purposes of CAP development. It is also the expectation that a separate CAP, as deemed appropriate by the Regional Office, be developed for each source area (or each combined source area). 4.0 COLLECTING DATA IN SUPPORT OF MNA CAP It is the Division’s responsibility to approve or disapprove CAPs submitted by the RP. The decision whether to approve or disapprove a CAP is dependent upon the technical merits of the submittal. The need for appropriate data used to assess and document natural attenuation cannot be overstated. The following sections discuss the data collection and analysis that are needed to properly evaluate and review the appropriateness of MNA. 4.1 Sample Location and Data Needed to Evaluate Plume Characteristics and Natural Attenuation To allow a proper review of natural attenuation and receptor risk, (a) the characteristics of a plume in time and space should be assessed, and (b) the mechanisms of attenuation should be understood. The amount of data needed to adequately assess the plume characteristics and attenuation is dependent on the complexity of site conditions and its potential risk to receptors. More extensive data collection is usually required at sites with complex subsurface conditions (e.g. fractured rock settings). An understanding of the horizontal and vertical movement of contaminants from source to receptor is expected. Data typically are needed, at a minimum, along the longitudinal plume centerline from a location at the source to a location beyond the downgradient plume boundary. In some cases, wells positioned along a lateral transect (perpendicular to groundwater flow) may be used to augment, but not replace, the longitudinal plume centerline wells. Such wells would be used, for example, for plume mass balance, lateral plume spread, or related computations. Poorly connected bedrock wells and wells located on the side perimeter of a plume are generally inappropriate for demonstrating contaminant attenuation. Instead, an emphasis on longitudinal plume centerline wells is expected, and those plume centerline wells should be screened across the vertical interval of the flow system that contains the maximum contaminant concentrations at that location. Useful references for analyzing plume attenuation include: • EPA, 2002, ‘Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies’, EPA/540/S-02/500; • EPA, 2007, Monitored Natural Attenuation of Inorganic Contaminants in Groundwater, Volume 1, Technical Basis for Assessment, EPA/600/R-07/139; • Farhat, Newell, and Nichols, 2006, Mass Flux Toolkit to Evaluate Groundwater Impacts, Attenuation, and Remedial Alternatives; and/or • ITRC, 2010, A Decision Framework for Applying MNA Processes to Metals and Radionuclides in Groundwater; and others. Understanding the dominant attenuation mechanisms - dilution, dispersion, sorption, precipitation/co-precipitation, and (or) phyto-attenuation - and their relative importance requires adequate sampling of subsurface reactants (e.g. iron and aluminum hydrous oxides), solid and water phase contaminant concentrations, and geochemical conditions, and these data should be collected at properly located and screened wells (discussed above). The following 6 table lists selected parameters, measures, and constituents that are often useful in understanding and documenting natural attenuation. This list is not intended to be comprehensive. Selected parameters, measures, and constituents that are often useful in understanding and documenting natural attenuation of inorganics Alkalinity pH Aluminum hydroxides Selenium species, as applicable Anion exchange capacity Soil-water pair chemistry Arsenic species, as applicable Specific conductance Cation exchange capacity Sulfate Chromium species, as applicable Sulfide Eh Total dissolved solids Iron hydroxides Total organic carbon Iron species Turbidity Manganese species Others as identified based on site conditions 7 4.2 Iterative Nature of Data Collection in Support of an MNA CAP Data collection in support of an MNA CAP is normally an iterative process. Results of one phase are used to determine where and how to collect data in a subsequent phase. In this way, data collection is progressively focused and leads to a refined understanding of site conditions, plume conditions, and attenuation. The RP should install and sample a sufficient number of monitoring points to define the extent of groundwater contamination both horizontally and vertically, to understand contaminant movement and attenuation with distance and depth (within hydrostratigraphic flow units), and to understand contaminant trends over time. Wells installed to assess horizontal and vertical extent may or may not be useful in assessing the capacity of the subsurface to attenuate, via dilution, dispersion, sorption, precipitation, or other mechanism, contaminants along a plume centerline. If required timeframes preclude an iterative approach (e.g. Coal Ash Management Act of 2014 (CAMA)), data collection should be particularly robust in the early phase of investigation to ensure that adequate and representative data will be available within the timeframes needed to demonstrate the appropriateness of MNA to the satisfaction of the Director. It is the responsibility of the RP to ensure that the necessary number of monitor wells are installed in the appropriate locations and at appropriately screened/open intervals to allow the development of an acceptable and approvable MNA CAP. The Division will support the RP in this effort through written and verbal input during assessment and corrective action planning based on the information and data available at the time of the input, but the responsibility for ensuring that monitor well placement and screened/open intervals are sufficient to develop a defensible MNA CAP rests with the RP. 4.3 Data Inventory Monitor wells installed to assess horizontal and vertical plume extent (which includes most CSA wells) and many compliance boundary wells may or may not be properly positioned or screened to assess plume characteristics, attenuation, and MNA viability that are needed for an MNA CAP. To ensure that adequate data are available for MNA CAP development, the RP should inventory the available monitor wells for each source area and provide the Regional Office responses to the following questions prior to assembling the evidence needed to demonstrate adherence to 02L .0106 (l). (a) have background concentrations been formally established for all COIs in soil and groundwater? (b) for each source area, how many wells within each flow system are located along the contaminant plume centerline? Along a transect that is perpendicular to the plume centerline? (c) how many wells in (b) above are screened across the most contaminated vertical interval of a given flow unit or are screened across the full thickness of the flow unit? (d) what is the length of record and how many valid sample events are available for wells listed in (b) and (c)? (e) does turbidity, well construction (for example, grout contamination, etc.), or well “break in” issues preclude the use of data in (b), (c), and (or) (d)? 8 (f) for each source area and within each flow unit, how many spatial locations were sampled for solid phase chemistry and were these locations associated with “end member” (maximum and minimum) groundwater concentrations for each contaminant6? How many of these spatial locations are associated with (b) or (c) above? (g) given that iron hydroxide (HFO) content is a good indicator of retention capacity for most metal contaminants, how many locations in (f) was HFO measured? Results of this inventory will help the review team evaluate whether the existing dataset is adequate for assessing plume characteristics, attenuation, and attenuation mechanisms (e.g. the capacity for solid phases to retain contaminant mass over time) with reasonable certainty. If available at the time of the data inventory, the review team would also benefit from model results described in Section 7.5.2. If data for (f) and (g) from other (different) source areas on site are used to augment the existing dataset, an explanation is expected for why this is appropriate (for example, an RP may show using lithology data from both areas that the subsurface geology and sorptive properties are similar in both source areas). Approximately 14 days after the Division’s receipt of this information, the RP should meet with the Regional Office to review the amount and location of existing data and discuss its adequacy for MNA CAP development. If the existing dataset is determined to be adequate, then the RP should proceed with the development of the MNA CAP. If not, the RP should reach agreement with the Regional Office on any additional well and solid phase samples (locations and depths) needed to adequately assess plume characteristics and (or) attenuation capacity. 5.0 MINIMUM CONTENTS EXPECTED IN AN MNA CAP FOR INORGANICS An MNA CAP should provide, at a minimum, data and information required by 15A NCAC 02L .0106 (h) and (l) and, if applicable, CAMA and (or) other regulations. The following sections discuss the minimum contents expected in an MNA CAP. 5.1 Site Background and Regulatory Basis for Corrective Action. This section should describe the site, its history, ownership, operations, relevant permits (NPDES, e.g.) and a summary of their requirements, and a list of environmental reports prepared for the site to date. This section should include the relevant regulatory requirements incumbent upon the RP, as well as the specific portions of 2L .0106, CAMA, and (or) other regulations that are being addressed by the MNA CAP. 5.2 Conceptual Model of Groundwater Flow and Contaminant Transport. This section should succinctly describe the site setting, regional geology and its effect on groundwater flow and quality, areas of recharge and discharge, hydrologic boundaries, hydrostratigraphic units, and heterogeneities and their effect on groundwater flow, quality, and contaminant transport. Heterogeneities may include: flow units that pinch out; flow properties 6 Understanding the solid phase contaminant concentrations in locations of both low and high groundwater COI concentrations are important in understanding the sorptive capacity of the system. This is particularly true in the case of non-linear isotherm adsorption models that describe most metals. That is, a soil has a limited ability to sorb contaminant mass due, for example, to limited sorption sites, so a soil can become less efficient at removing mass at higher dissolved concentrations. 9 that vary with hydrostratigraphic unit; artificial areas (filled and (or) re-worked) of the site; different geologic formations across the site and their demonstrated effect on groundwater quality; faults, dikes, or other geologic anomalies and their effect on flow and transport; spatially varying subsurface sorption capacities (e.g. iron or aluminum hydrous oxides, partition coefficients (Kd)); and (or) others. If portions of the conceptual model rely on inferences or assumptions rather than field data, this should be clearly stated along with a technically defensible rationale for the inference or assumption. For example, if a hydrologic boundary is inferred by topography but was not measured, this should be stated and rationale provided. Rather than referencing content from an earlier report submittal, it is preferred that the content for this section be included in the MNA CAP. 5.3 Potentiometric Maps. The following potentiometric contour maps should be provided: 5.3.1 Potentiometric contour map of shallow system 5.3.2 Potentiometric contour map of bedrock system (as applicable) 5.3.3 Vertical gradient map showing observed head differences (shallow minus bedrock, based on a representative shallow layer and a representative bedrock layer) contoured at 2 foot intervals. For sites at which flow modeling has been conducted, the following additional maps are expected: 5.3.4 Potentiometric contour map of simulated shallow system once steady state conditions have been reached after basin or waste area closure (excavation, engineered cap, or other alterations to the subsurface arising from corrective actions), as applicable 5.3.5 Potentiometric contour map of simulated bedrock system once steady state conditions have been reached after basin closure (excavation, engineered cap, or other alterations to the subsurface arising from corrective actions), as applicable 5.3.6 Simulated vertical gradient map showing head differences (shallow minus bedrock, based on a representative shallow layer and a representative bedrock layer) once steady state conditions have been reached after basin or waste area closure (excavation, engineered cap, or other alterations to the subsurface arising from corrective actions), contoured at 2 foot intervals. Each of the above maps should be superimposed on an orhtophoto base map showing source areas and waste and compliance boundaries, 2-ft contours, and all monitor wells, identified receptor supply wells, and jurisdictional surface waters. If figure clarity is compromised by the amount of information being presented, please contact the Regional Office prior to the CAP submittal. 5.4 Background Concentrations. Inorganic constituents occur naturally in subsurface soils and groundwater, and for purposes of 02L implementation, only those concentrations that occur above background levels are relevant to remediation and attenuation. The MNA CAP 10 should include a table showing all COIs and their representative background value in soil and groundwater. 5.5 Characterization of Source Areas. This section discusses the information needed to understand the source, its impact on soil and groundwater, associated receptors, risks, and remedial alternatives. It is expected that this information be provided separately for each source area that is proposed to be addressed by an MNA remedy. 5.5.1 Source history, volume, and characteristics. History of waste placement, list of waste materials and volumes emplaced, characteristics (behavior, leachability, etc.) of waste materials, and chemistry of source area pore water and underlying groundwater. 5.5.2 Horizontal and vertical extent of source material. A plan view map (showing source area and all jurisdictional waters and supply wells within a ½ mile radius) and a cross section map (showing seasonal high water level and depth of material and any associated liners or caps), along with an estimated volume of source material. 5.5.3 Horizontal and vertical extent of saturated source material. A plan view map and a cross section map (showing seasonal high water level and depth of material and any associated liners or caps), along with an estimated volume of saturated source material. 5.5.4 Source control and stability. Describe how the source has been controlled (excavation, engineered cap, de-watering, and (or) other) and the permanence or impermanence of the control strategy. Describe the stability of the source area, including (i) any dams or other structures associated with the source area and their hazard rating, (ii) the type, condition, and designed lifespan of any engineered liner or cap, if applicable, (iii) proximity to 100-year flood zone, (iv) type, age, and health of tree cover, if applicable, and (v) any hazards to the public health, safety, or welfare resulting from the source area that is not covered above. 5.5.5 COI(s) associated with source area. List the COIs that have been are associated with the source area. For each COI listed, state whether it occurs as a continuous plume or as a constituent whose dissolved concentrations are controlled largely by geochemical conditions along a flow path. 5.5.6 Base and iso-concentration maps showing horizontal and vertical extent of COI(s) in groundwater and soil at and downgradient of source area. A separate plan view base map should be provided for each source area. The base map should be large scale (often 1 inch = 100 to 200 feet will be appropriate) and show the contents listed in Appendix C. A plan view map and a cross section map should be provided for each COI within the source area in question depicting the horizontal and vertical extent of the COI. Areas with sparse concentration data should be indicated (e.g. dashed iso- concentration lines). 5.5.7 Identification of receptors associated with the source area. 5.5.7.1 Surface waters, to include all jurisdictional wetlands, streams, ponds, lakes, rivers, etc. 11 5.5.7.2 Supply wells within a ½ mile radius (or other distance as defined by the Division) of source area waste or, if applicable, compliance boundary. 5.5.7.3 Future groundwater use areas. 5.5.8 Evaluation of human and ecological risks associated with the source area. 5.5.9 Evaluation of alternative corrective actions (e.g. pump and treat, reactive barriers, in-situ stabilization, phytoremediation, MNA, etc.) with consideration of the following factors: degree of protection of human health and the environment; regulatory compliance; time required to accomplish restoration; cost required to accomplish restoration; reduction of toxicity, mobility, or volume of contamination; permanence of the remedy; implementability; stakeholder acceptance; and others, as appropriate. 5.6 Demonstration of Natural Attenuation. This section should provide documentation of plume characteristics and a demonstration that MNA will fulfill the requirements of 02L .0106 (l) (i.e. protect human health and the environment, etc.) and any other applicable regulations. Hybrid remediation strategies (for example, MNA, coupled with a passive reactive barrier) may also be presented under a 02L .0106 (l) CAP. Documentation of adequate attenuation should include maps, graphs, models, and (or) attenuation equations and slope factors, and the data used to construct these should be from technically defensible well locations/depths (see Section 4 of this Guidance). Contaminant attenuation must be demonstrated separately for each source area proposed for MNA and to the satisfaction of the Director. The following sections discuss the expectations for natural attenuation demonstration for each separate source area. 5.6.1 Understanding plume characteristics in space and time Understanding and documenting plume characteristics in space and time are necessary components of an MNA CAP. This documentation is used by the Division to help understand otherwise un-identified risks to receptors, the quality of any attenuation or predictive models, and the appropriateness of the proposed MNA remedy for the source area under consideration. As discussed in Sections 4.1 and 4.2, the number of spatial and temporal monitoring points that are needed for plume assessment is dependent on the degree of source and subsurface complexity and the degree of uncertainty about potential contaminant transport to receptors. Highly variable COI concentrations in space or time observed in an extremely heterogeneous subsurface (fractured bedrock, for example) flow system near receptors would necessitate a higher number of monitoring points than a site that is mostly isolated from receptors and is associated with relatively uniform subsurface and groundwater quality conditions. There should be enough monitoring points to define the extent of groundwater contamination both horizontally and vertically, to understand contaminant trends with distance toward receptors, and to understand contaminant trends over time. The monitor well network for most sites typically will include an upgradient well, at least one set of wells screened across the transverse axis of the plume, one set of wells 12 screened along the longitudinal axis of the plume, and sentinel wells. If more than one flow unit exists, the well network should be constructed to measure maximum concentrations in the flow units as contamination moves horizontally and vertically from source to receptors. Monitoring frequency should be designed to detect any variability in plume concentrations and extent due to seasonal water table fluctuations, tidal influence, source removal and (or) groundwater remediation efforts, or other effects. Where formal statistical methods are needed to demonstrate increasing or decreasing trends in a well or wells, eight to ten sample events may be needed. The documentation of plume characteristics should include some combination of the following qualitative and quantitative methods, including contaminant concentration contour maps, concentration versus time graphs for selected monitoring wells along a plume centerline (including well(s) closest to receptors), and concentration versus distance graphs showing concentrations along the plume centerline at a selected timepoint. • Graphical methods (qualitative evaluation) o Concentration vs. time plots, concentration vs. distance plots, and concentration isopleths maps • Quantitative methods o Statistical methods that analyze single-well trends (for example, Mann- Kendall, Theil-Sen, linear regression) o Plume-based methods (plume area, plume mass, plume center of mass, and mass flux analyses) Wells selected for use in a given analysis must be appropriate to that method and be representative of the conditions being analyzed. For example, a side gradient well may not be used to analyze plume centerline conditions. See the report titled ‘Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies’ (EPA/540/S-02/500, November 2002) as an example of proper plume analysis. 5.6.2 COI migration predictions Pursuant to 02L .0106 (l) (2), each COI must have the capacity to attenuate under current and future site conditions as demonstrated by adherence to criteria 02L .0106 (l) (4), (5), and (6). Criteria 02L .0106 (l) (4), (5), and (6)7 require predictive modeling of travel time, direction, and distance, and according to 02L .0106 (l) (3), there should be “reasonable certainty” in the model results. Various predictive modeling approaches are acceptable. It is recommended that the RP work with the Regional office to better assure that the selected approach is appropriate and sufficient for use in the MNA CAP. In some cases, the RP will choose to use a three- dimensional flow and transport numerical model. The model approach should be 7 02L .0106 (l) (4) requires a demonstration that contaminated groundwater will not result in any 02L standards/IMACs violations to a receptor or foreseeable receptor. 02L .0106 (l) (5) requires a demonstration that contaminated groundwater will not migrate onto adjacent properties unless (a) a public water supply sourced by surface water or unaffected groundwater is available, or (b) owner approval has been granted in writing. 02L .0106 requires that contaminated groundwater discharge not cause a surface water violation. 13 dependent upon the quality and amount of data available for its construction and calibration. To meet the expectation of “reasonable certainty”, it is recommended that basic elements of model construction be pre-approved by the Division. For 3-D models these will often include the selected grid spacing, boundary locations, boundary conditions, flow zone layering strategy, target well selection, particularly important input data (hydraulic conductivities, sorption coefficients, monitor well adequacy for modeled source areas, supply well pumping inclusion, and others), and strategy for sensitivity analyses and reporting. Failure to include the Division in model selection and development often will result in the need for multiple revisions requiring model re- development and re-calibration. It is recommended that the RP discuss this step with the Regional Office prior to model development or at the earliest opportunity if model development is already underway and regional approval has not yet been obtained. 5.6.2.1 Predictions based on existing conditions. The predictive modeling of time, direction, and distance of contaminant travel should be carried out based on existing conditions. The following figures are expected: (a) a concentration-time plot for each COI corresponding to the following locations: (i) nearest supply well, (ii) nearest future groundwater use area, and (iii) nearest surface water. The following information should also be provided: the time it takes for the COI to reach (i), (ii), and (iii), the time it takes for the COI to reach (i), (ii), and (iii) at its 2L/IMAC concentration, the time it takes for the COI to reach (i), (ii), and (iii) at its maximum concentration, and the time it takes for the COI to reach (i), (ii), and (iii) at a concentration that is back below the 2L/IMAC concentration. It is recommended that this information be included in the plot margin. (b) a map (superimposed on the requested base map) showing the maximum predicted migration distance, at any detectable concentration, of each COI. (c) a map (superimposed on the requested base map) showing the maximum predicted migration distance, at the 2L/IMAC standard concentration, of each COI. 5.6.2.2 Predictions based on source control and corrective actions. The predictive modeling of time, direction, and distance of contaminant travel should be carried out for each of three source control options (excavation, engineered cap, and (or) a hybrid of partial excavation and cap). The modeling for each option should also account for any proposed corrective action alterations to the subsurface (e.g. permeable reactive barrier, slurry wall, groundwater removal, etc). The figures listed in 5.6.2.1 (a), (b), and (c) are expected for each scenario. 14 5.6.3 Conceptual model of natural attenuation. The natural attenuation of each COI should be described in terms of the following mechanisms, the relative strength of which may vary by source area: 5.6.3.1 Dilution and dispersion. These mechanisms may be considered together and are typically evaluated using conservative, mostly non-reactive constituents (e.g. boron or chloride). 5.6.3.2 Sorption, if applicable. Sorption should be documented. The subsurface reactants responsible for sorption should be identified along with their sorptive capacity. Evidence should be provided demonstrating that reactant mass is sufficient and conditions are conducive to continuous long term sorption. 5.6.3.3 Precipitation, if applicable. Precipitation or co-precipitation should be documented, along with the conditions and timeframes under which the precipitation reaction occurs. Evidence should be provided demonstrating that the precipitation or co-precipitation reaction is irreversible or will otherwise persist indefinitely. 5.6.3.4 Phyto-attenuation, if applicable. Phyto-attenuation should be documented. Tree or plant species responsible for phyto-attenuation should be identified along with the conditions under which phyto-attenuation occurs. Evidence should be provided demonstrating that phyto-attenuation is irreversible or will otherwise persist indefinitely. 5.6.3.5 Other mechanisms, if applicable. Other attenuation mechanisms should be documented, as applicable, along with the conditions under which they occur and evidence showing their long term persistence. 5.6.4 Demonstration that contaminated groundwater discharge to surface water will not result in violations of surface water standards contained in 02B .022. Pursuant to 02L .0106 (l) (6), contaminated groundwater intercepting surface waters may not, now or in the future, possess contaminant concentrations that would result in violations of standards for surface water contained in 15A NCAC 2B .0200. Adherence to this requirement should be demonstrated in the MNA CAP as follows: 5.6.4.1 Existing conditions Surface water samples should be collected at strategic near bank locations where maximum groundwater contaminant concentrations are expected to discharge to waters of the state. Results should document, for each MNA CAP source area, that discharge of contaminated groundwater to surface waters does not result in violations of 02B surface water standards (2017, DWR 2L-2B Sampling Guidance Memorandum). Generally, surface water samples should be collected in locations expected to intercept maximum plume concentrations (e.g. near bank samples collected during low flow conditions and in a location where the plume centerline is expected to discharge to the surface water of 15 concern). The Regional Office should be consulted to determine locations and methods of surface water sampling. 5.6.4.2 Future conditions The MNA CAP should propose locations and frequency of future surface water sampling that will document, over time, whether the discharge of contaminated groundwater to surface waters results in violations of 02B surface water standards. The locations and frequency of sampling will be dependent upon site conditions and documented plume behavior. 5.7 MNA schedule, performance monitoring plans, and contingency plans The CAP should include a schedule for implementation of the proposed MNA CAP. Requirements for performance monitoring and contingency corrective action plans are discussed in 02L .0106 (l) (7), in the 1995 Guidance (p. 13 and p. 27), and in Section 2.0 and Appendix C of this Guidance. Performance monitoring must include monitor wells and surface water sample locations, and the MNA CAP should include a map and table of proposed locations. It is recommended that performance monitoring sampling be conducted quarterly during the two years following CAP approval, followed by semi-annually for the next three years. Sample parameters should be discussed with the Regional Office. The sampling locations, frequency, and parameters should be reviewed periodically. It is the RP’s responsibility to document and request where reduced sampling is appropriate. Alternative corrective action plans should be provided for each source area as a contingency for a case in which performance monitoring shows that contamination is not being attenuated as expected. At the approval of the Division, the Contingency Plans may be provided under a separate cover within 120 days of the MNA CAP submittal; in such case, the Division will review and approve the Contingency Plans separately from the MNA CAP. 5.8 Access agreements, public and other notices, and adherence to other laws The MNA CAP should include documentation showing all necessary access agreements, public notice announcements, and (or) adherence to other relevant environmental laws. These requirements are discussed in 02L .0106 (l) (8), (9), and (10), and in the 1995 Implementation Guidance (p. 13, p. 27-28, and p. 30), and in NCAC 15A 02L .0106 (k)(6), (l)(9), and (m)(D), and NCAC 15A 02L .0114. 5.9 Completed checklist for MNA CAP for Inorganics The RP should refer to Appendix A for a checklist that should be completed and submitted as part of the MNA CAP for inorganic contaminants. 16 17 6.0 REFERENCES EPA, 2007, Monitored Natural Attenuation of Inorganic Contaminants in Groundwater, Volume 1, Technical Basis for Assessment, EPA/600/R-07/139. https://nepis.epa.gov/Exe/ZyNET.exe/60000N4K.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2006+Thru+201 0&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=& QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A\\zyfiles\\Index%20Data\\06 thru10\\Txt\\00000002\\60000N4K.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|- &MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeek Page=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&See kPage=x&ZyPURL EPA, 2002, Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies, EPA/540/S-02/500, November 2002. https://nepis.epa.gov/Exe/ZyNET.exe/10004674.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+200 5&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=& QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data %5C00thru05%5CTxt%5C00000005%5C10004674.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h %7C- &MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeek Page=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&See kPage=x&ZyPURL# Farhat, Newell, and Nichols, 2006, Mass Flux Toolkit to Evaluate Groundwater Impacts, Attenuation, and Remedial Alternatives, User’s Manual. https://clu-in.org/download/contaminantfocus/ER-0430-MassFluxToolkit.pdf ITRC (Interstate Technology & Regulatory Council), 2010. A Decision Framework for Applying Monitored Natural Attenuation Processes to Metals and Radionuclides in Groundwater. APMR-1. Washington, DC: Interstate Technology & Regulatory Council, Attenuation Processes for Metals and Radionuclides Team. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwi766CYtKfUAhVKeyYKHbk BDkwQFggmMAA&url=http%3A%2F%2Fwww.itrcweb.org%2FGuidanceDocuments%2FAPMR1.pdf&usg=AFQjCNGK vOjhv48c4DQYa0OAz-lLvOfNdw&cad=rja NCDWQ, 2012, Evaluating Metals in Groundwater at DWQ Permitted Facilities: A Technical Assistance Document for DWQ Staff http://digital.ncdcr.gov/cdm/ref/collection/p16062coll9/id/251181 Smith and Gardner, 2016, Final Draft Monitored Natural Attenuation and No Further Action Evaluation Study, Prepared for N.C. Coal Ash Management Commission. 18 7.0 APPENDICES APPENDIX A – CHECKLIST FOR MNA CAP FOR INORGANICS The following checklist should be completed for each source area and included as part of the MNA for Inorganics CAP. For each source area, Y/N Consideration 1.a. Has the source of contamination been removed or controlled in accordance with 02L .0106 (l)(1)? 1.b. Has this been properly documented? 2.a. Will submerged ash (or other waste) remain on site? 2b. If so, will this result in additional mass loading to the existing plume? 2.c. If additional mass loading does not occur, why not? 3. Have provisional background threshold values (PBTVs) been formally established for all COIs in soil and groundwater? 4.a. Has downgradient surface water been tested for adherence to 2B standards? 4.b. Were any surface water locations impacted by the discharge of a contaminated plume? 5.a. Has downgradient surface water been modeled for future (predicted) adherence to 2B standards? 5.b. Were any modeled surface water locations impacted at any point in the future? 5.c. Has downgradient surface water been modeled for future (predicted) adherence to 2B standards? 5.d. Have long-term surface water sample locations been proposed in the performance monitoring plan to address future adherence to 2B standards? 5.e. Were any modeled surface water locations impacted at any point in the future? 5.f. Was a predictive groundwater-surface water mixing model developed for the source area and, if so, what are the most significant limitations that affect the model’s reliability? 6. Are any of the surface water receptors used as a water supply and, if so, how far away are the Intakes? 7. Has a “data inventory” been conducted and shared with the Division (see ‘Data Inventory’ Section)? 8. Are contaminants in bedrock groundwater above 02L standards/IMACs at any location inside or outside the compliance boundary? If yes, what source area is implicated and is it predicted that the plume in fractured bedrock will reach a supply well in the future? What model was used to predict contaminant migration in bedrock? What are the most significant limitations affecting the reliability of the model? 9. Have all supply wells within ½ mile radius of the source area waste boundary or, if applicable, compliance boundary been abandoned? List any wells that have not been abandoned or for which this information has not been obtained. Have the owners of these supply wells all converted to an alternative permanent source of clean drinking water? List all properties that have not been converted to an alternative, permanent clean drinking source or for which this information has not been obtained. 19 Y/N Consideration 10. If contaminated groundwater has migrated (or is predicted to migrate) off site onto surrounding properties, are all those properties served by a potable surface water or non-impacted groundwater supply or have those property owners allowed in writing the trespass of the contaminated groundwater? 11. For each COI within the source area, 11.a. Does the constituent behave as a plume (for example, boron, sulfate, TDS, etc.)? If so, i. has the plume been defined in space (horizontally and vertically)? ii. has the downgradient edge of the plume been measured or is this not possible/practical due to the location of a major surface water or other access issues? iii. is a well located and screened to measure the maximum (“hot spot”) plume concentrations at or near the source? Is a well located and screened to measure the maximum contamination along the plume longitudinal centerline at a point on the downgradient plume edge? If not, why not? iv. are concentrations within the plume boundary increasing? decreasing? stable? iv. is the plume expanding? shrinking? v. Is the plume migrating? vi. has the plume migrated to any supply wells? to any future groundwater use areas? vii. is the plume expected to migrate to any supply wells? to any future groundwater use areas? viii. have plume characteristics been properly assessed and documented (see #5 of the Section titled ‘Minimum Contents Expected in MNA CAP for Inorganics’)? ix. has attenuation been properly assessed along plume centerline and documented (see #8 of the Section titled ‘Minimum Contents Expected in MNA CAP for Inorganics’)? x. is attenuation expected to be persistent over the long term and has this been demonstrated in the MNA CAP? 11.b. What is the predicted maximum distance the plume will travel? Has this been shown on a base map that contains all surface water receptors, supply well receptors, property boundaries, and future groundwater use areas? What concentration-distance model was used for the prediction and what are the most significant limitations that affect the model’s reliability? 11.c. What is the predicted length of time it will take to reduce concentrations to below 02L standards/IMACs at the nearest surface water receptor and at the compliance boundary? What concentration-time model was used for the prediction and what are the most significant limitations that affect the model’s reliability? 11.d. Does the COI occur not as a contiguous, identifiable “plume”, but as discontiguous areas of concentrations whose values are controlled by local geochemical conditions (for example, iron, manganese, and others)? If so, i. are the geochemical controls on the COI concentration understood? ii. have the geochemical controls been measured in wells associated with checklist #11 (a) (I, ii, and iii) above? 20 Y/N Consideration iii. does the COI occur or is the COI predicted to occur in the future above 02L standards/IMACs in any supply wells? future groundwater use areas? if so, has rationale been prepared that explains whether or not the source area is responsible for the COI occurrence? iv. has the COI been properly mapped, assessed, and documented (see #5 of the Section titled ‘Minimum Contents Expected in MNA CAP for Inorganics’)? v. has attenuation been assessed and documented for areas of maximum concentrations (see #7 of the Section titled ‘Minimum Contents Expected in MNA CAP for Inorganics’)? vi. is attenuation expected to be persistent over the long term and has this been demonstrated in the MNA CAP? 11.e. How does cap-in-place compare to excavation (or, if proposed, partial excavation), for purposes of checklist items #11 (a), (b), (c) and (d) above? 11.f. If presented, are soil-water pairs collected in horizontal and vertical locations representative of high and low COI concentrations? 11.g. If Kd lab tests were conducted using site soils, how much variation was observed across the source area? across flow units? across the site? across lab methods (for example, batch versus column tests)? Were lab-derived Kd’s used as the calibrated values in any transport modeling, and if not, why not? 12. Are performance monitoring wells able to be positioned no farther than 5 years travel time downgradient from the plume front, and no closer to the nearest supply well or future use area than 1-year travel time from the plume front. 13. Have all pertinent access agreements been obtained, public notices provided, and adherence to all other environmental laws been demonstrated? 14. Has a description of the proposed MNA remedy been provided, along with the reasons for its selection and a proposed schedule? 15. Has all relevant information needed to assess the MNA CAP been provided, including information on risk associated with the source area, the cost and benefits of cleanup, and the technology available to accomplish cleanup? 16. Has the CAP and any other associated reports and (or) appendices been sealed by a professional engineer and (or) geologist? APPENDIX B. EPA FOUR TIER MNA EVALUATION. This appendix summarizes the four tiers used in the EPA MNA evaluation process, along with data/analyses that, in some cases, are used to support such an evaluation. TIER 1. Demonstrate COI removal. COI removal is separate from and in addition to any “source” removal or control that is proposed. For purposes of this Guidance, active COI removal may be via dispersion8, sorption on/in a solid phase, precipitation/co-precipitation, phyto-attenuation, or other 8 Dispersion as used here is intended to mean both dispersion (from mechanical mixing due to longitudinal and transverse variations in the 3-dimensional velocity field) and dilution (from added recharge along a flow path). Degradation or phase change is another removal mechanism that typically is not relevant for the MNA of metals. 21 mechanisms. While dispersion does not actually “remove” COI mass from the groundwater system, it does displace and thereby dilute the mass by transferring it to areas of the groundwater system where the concentrations are below the groundwater standards. In most cases, some combination of dispersion and (or) sorption will be the dominant mechanisms removing inorganic contaminants from the groundwater system. Often, precipitation involves (mainly) iron or manganese which in turn could serve as sorption hosts for other COIs. Precipitation should thus be considered as a potential mechanism for selected constituents such as iron, manganese, sulfate, and, in some cases, other highly concentrated COIs. Various methods may be used for Tier 1 demonstration and may include the following: Water-solid pairs. For contaminants susceptible to sorption, Tier 1 may be demonstrated by comparing COI concentrations of water-solid pairs at representative locations within the area of the plume. If used, pair data generally should comprise at least three locations and be representative of the flow zone(s) (shallow, deep, or bedrock) through which that COI occurs above the 02L standards/IMACs. The locations of the pair data should be mapped (with water-solid pair location labels for identification) to show the proximity of the pair data to the source area and to areas of low and high groundwater concentrations. The pair data should show an increasing trend in sorbed mass from locations with low dissolved concentrations to locations with high dissolved concentrations. If these criteria are not met, the evidence would be considered inconclusive, and additional line(s) of COI removal evidence would be expected. Partition coefficient (Kd) lab tests. For contaminants susceptible to sorption, COI removal by sorption may be shown with Kd lab test results. Kd is defined as the ratio of the contaminant concentration associated with the solid to the contaminant concentration in the surrounding aqueous solution when the system is at equilibrium. Depending on the number and location of test samples, this may not produce a quantitative estimate of mass removal from the system nor is it an indication of the long term sorptive capacity, but it is strong evidence that COI mass is being removed from groundwater. Concentration-time and concentration-distance analyses. See Sections 4.1, 4.2, and 5.6.1 and EPA, 2002, ‘Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies’. TIER 2. Identify the dominant attenuation mechanism(s) and estimated rate(s) of attenuation. It is necessary to understand the dominant attenuation mechanism(s) in order to evaluate the long-term persistence of that (those) mechanism(s). An estimate of attenuation rates (that is, the rate at which contaminant mass is reduced (dispersion) and (or) transferred to solids via sorption or (co)precipitation) helps assess the length of time it will take to reach cleanup goals. Data needed for Tier 2 typically include stratigraphy, seepage velocities, contaminant concentrations in groundwater and solids, groundwater geochemistry (pH, Eh, etc), and a certain amount of subsurface mineralogy and contaminant speciation in groundwater and solids. Understanding and documenting heterogeneities at the scale of concern is also expected. To determine the dominant attenuation mechanism, a comparison should be made between the mass that is decreased due to dispersion versus the mass that is removed (transferred) from the aqueous phase to the solid phase due to sorption or due to precipitation. To estimate the rate of dispersion, contaminants shown to be relatively conservative (non-sorbing) may be used to estimate concentrations along a groundwater flow path. If it is assumed that the contaminant will be conserved as it moves downgradient, then any reduction in concentration is due to dispersion. The dispersion attenuation rate is valid and defensible only if the same flow path is being measured. Otherwise, the dispersion estimate 22 would be meaningless, as in the case of an upgradient concentration that is measured in a shallow flow unit within the heart of a contaminant plume and a downgradient concentration that is measured in a side gradient well screened across a disconnected lower flow unit. To estimate the rate of mass transfer via sorption, results from Kd lab tests or geochemical modeling may be used. Lab results that produce extreme Kd variability, for a given COI, between methods (batch versus column), replicated tests, locations, or depths would carry a fair amount of uncertainty, and additional line(s) of evidence usually would be expected in the MNA CAP to document the rate of attenuation by sorption. The greater the degree of sorption variability either between tests or between locations, the greater the degree of uncertainty in contaminant migration predictions obtained from flow and transport modeling that uses a single sorption coefficient per COI. To estimate precipitation, solubility diagrams may be used qualitatively and geochemical modeling may be used quantitatively. Assuming properly positioned and screened wells exist and have been sampled during a sufficient number of events, concentration-time and concentration-distance methods may be used to estimate the attenuation rate that results from any or all mechanisms (dispersion, sorption, precipitation). A useful reference for evaluating plume characteristics, demonstrating contaminant removal, and estimating attenuation rates is EPA, 2002, ‘Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies’. Another robust and quantitative line of evidence (mass balance) is the demonstration of mass attenuation as measured across a planar transect perpendicular to the plume migration centerline. This method accounts for all COI mass that passes a 2-D plume face and its concentration flux over time. This method assumes that the plume centerline is known and that monitor wells are located along a transect perpendicular to the centerline flow and that these wells are sufficient to estimate representative flow rates and COI concentrations. This method further assumes relatively isotropic conditions with little vertical variation in the plume’s flow field. A decreasing trend through this planar transect would be strong evidence of COI removal and would allow an attenuation rate to be estimated. TIER 3. Demonstrate that the attenuation capacity is stable over the long term. Once the dominant attenuation mechanism(s) is (are) understood, the long-term stability of that (those) mechanism(s) should be evaluated. Tier 3 should answer the question, “will current attenuation rates continue indefinitely?” Factors that could affect the attenuation capacity and stability include (a) changes in groundwater geochemistry (pH, Eh, etc), (b) insufficient mass flux of groundwater constituents that participate in the attenuation reaction, and (c) insufficient mass of solids that participate in the attenuation reaction. Long-term stability may be demonstrated by assessing differences between “end member” conditions (pH, Eh, ion loading, etc) using lab testing of site solids, geochemical modeling, and, in some cases, flow and transport modeling under a range of sorption coefficient values. Other methods may be considered and should be discussed with the Regional Office prior to their use. TIER 4. Conduct performance monitoring. Performance monitoring should be used to demonstrate, over time, that attenuation is occurring as originally demonstrated and proposed in the MNA CAP. Performance monitoring under Tier 4 is, for purposes of this document, the same as the performance monitoring required under 02L .0106 (l) (7). 02L .0106 (l) (7) states that performance monitoring wells should be installed (a) no farther than 5 years travel time downgradient from the plume front, and (b) no closer to the nearest supply well or future use area than 1-year travel time from the plume front. The Regional Office should be consulted if the performance monitoring wells cannot be located 23 according to these criteria either due to property access issues, plume discharge to a major surface water feature, or supply well locations. Performance monitoring wells should be screened and (or) open to the flow unit with the greatest likelihood for contamination and should be sampled for the same list of constituents and geochemical parameters (pH, Eh, etc.) that were monitored during the contaminant assessment phase. 24 APPENDIX C. CONTENTS EXPECTED IN BASE MAPS. Characterization maps (i.e. base maps and COI iso-concentration maps) generally should be orthophoto renderings (semi-transparent) that include the following information. Generally, one characterization map is expected for each source area proposed for an MNA remedy. (1) property boundary, waste boundary and, if applicable, compliance boundary (2) 2-foot topographic contours (semi-transparent). In some cases, 5- or 10-foot contours may be acceptable based on consultation with the Regional Office. (3) all jurisdictional surface waters, permitted (or proposed permitted) outfalls, and seeps. Indicate either “permitted” or “proposed for permitting”; perennial streams should be shown with a blue line that extends unbroken from its headwaters to its point of discharge; a dotted line should be shown for portions of a stream that are piped underground; all wetland areas should be clearly indicated. (4) all supply wells (referred to as receptors in 2L .0106 (l)(4)) and their associated property boundaries within the predicted maximum plume extent or half mile radius of the compliance boundary, whichever is a longer distance as defined by the most mobile COI. (5) all shallow monitor wells (different color for each flow unit). (6) all deep monitor wells (different color for each flow unit). (7) all bedrock monitor wells (different color for each flow unit). (8) all abandoned wells (use a gray color). (9) all wells used only as a water level piezometer due to water production issues (use a different symbol). (10) all surface water/seep/outfall sample locations. (11) transect lines associated with any geologic cross sections, attenuation computations, or vertical COI concentrations. (12) for each sample location in (4) through (10) above, use a line of small font to show the following data in the following order: Monitor well: Well identifier (include stratigraphic unit abbreviation in parentheses, followed by screened or open interval in feet below land surface). Ex: MW-3 (BR) 40-75. most recent leading edge indicator constituent concentration (e.g. boron for coal ash basins), in ug/L, with “up”, “down”, “stable”, or “?” for increasing, decreasing, stable, or uncertain boron trend with time most recent pH/Eh value list of abbreviated constituents above 2L/IMAC, along with most recent concentration (ug/L) 25 depth to bottom of ash/waste depth to top of transition zone (TZ) depth to top of bedrock solid phase HFO content; if unknown, state “NM” for not measured. geomean Kd (show both batch and column geomeans computed for that location) of a sorptive constituent of interest (e.g. arsenic)(e.g. Kd COI (batch geomean)/(column geomean) geomean Kd (show both batch and column geomeans computed for that location) of a mostly conservative constituent of interest (e.g. boron for coal ash sites) (e.g. Kd COI (batch geomean)/(column geomean) indication of whether solid phase was analyzed for the list of constituents (if yes, indicate with “COIs*”; if no, indicate with “—“) depth to seasonal mean high water level (WL)….in parentheses elevation of seasonal mean high WL (ft above mean sea level). Show WL = “dry” as applicable. geomean of slug test hydraulic conductivity (k) values (ft/day) Note: For all parameters, use “NM” for not measured. EXAMPLE: GW-2 (BR, 24-29) B=3200 up, pH=6/Eh=105…B=3200, Co=6.9, Fe=2900, Mn=280…..ash=41, TZ=45, BR=50…HFO=2200, Kd As=3/2900, Kd B=0.1/1, COIs*..…WL=25 (2428)….k=3.2 SW, seep, outfall: Location identifier (in parentheses include type of SW location….seep, outfall, permitted, etc) most recent leading edge indicator constituent concentration (e.g. boron for coal ash basins), in ug/L, with “up”, “down”, “stable”, or “?” for increasing, decreasing, stable, or uncertain boron trend with time list of constituents above 2L/IMAC (seep), 2B (SW), or permit limit (outfall) indication of whether sediment phase was analyzed for constituents (if yes, indicate with “COIs*”; if no, indicate with “—“). EXAMPLE: SW-14 (SW) B=200 down, Al, -- Supply well: well identifier total depth 26 casing depth most recent leading edge indicator constituent concentration (e.g. boron for coal ash basins), in ug/L, with “up”, “down”, “stable”, or “?” for increasing, decreasing, stable, or uncertain boron trend with time most recent pH/Eh value list of abbreviated constituents above 2L/IMAC, along with most recent concentration (ug/L) 27 APPENDIX D – 1995 15A NCAC 2L IMPLEMENTATION GUIDANCE