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HomeMy WebLinkAboutDWR Overview of HRL Chlorophyll a Site-Specific Standard Proposal - Version 2 1 Overview of High Rock Lake (HRL) Chlorophyll a Site-Specific Standard Proposal N.C. Division of Water Resources Version 2 Updated 7/1/2022 Introduction After a multiyear evaluation process conducted in accordance with North Carolina’s Nutrient Criteria Development Plan (NCDP), the N.C. Division of Water Resources (DWR) is proposing a site-specific chlorophyll a standard for High Rock Lake (HRL) for adoption by the Environmental Management Commission (EMC). Site-specific standards may be established where a state identifies conditions in a waterbody that that differ from national or statewide criteria. The NCDP was mutually agreed upon by North Carolina and the U.S. Environmental Protection Agency (EPA) in 2014 and was renewed in 2019 with minor revisions. The plan commits North Carolina to evaluate site-specific nutrient-related criteria for three pilot water bodies, each representing a distinct water body type. Those pilot water bodies include High Rock Lake (lake), Albemarle Sound (estuary), and the Middle Cape Fear river system (river and streams). Based upon lessons learned from these site- specific evaluations, North Carolina will be better positioned to reevaluate nutrient-related criteria statewide. The NCDP also established two advisory bodies to assist with criteria development. The Scientific Advisory Council (SAC) comprises experts in the fields of water quality, water quality engineering, nutrient biogeochemistry, nutrient response variables, nutrient management and point and non-point source nutrient abatement. The Criteria Implementation Committee (CIC) advises on the social and economic implications of implementing proposed nutrient criteria to inform and assist DWR with fiscal note preparation. The SAC reviewed several nutrient-related parameters to determine if changes were warranted or if criteria for new parameters should be adopted into site-specific standards. For High Rock Lake the parameters reviewed included dissolved oxygen, clarity, algal assemblages, pH, cyanotoxins, chlorophyll a, nitrogen, and phosphorus. No new criterion parameters were recommended, and the only standard recommended for amendment was chlorophyll a. The chlorophyll a site-specific standard proposal reflects a combination of the SAC’s recommendations to DWR, CIC input, and the expertise of DWR staff. High Rock Lake Chlorophyll a Criterion Proposal Overview The SAC began its work to evaluate site-specific criteria for High Rock Lake in 2015, ultimately concluding its recommendations in a report published in May 2020 (Appendix I). DWR staff reviewed the SAC’s recommendation, considered all components brought forward by the SAC, and has proposed a 2 scientifically-based site-specific chlorophyll a standard for High Rock Lake. The SAC’s report provides detailed justification for the necessary components of a water quality standard. DWR has carried forward the SAC’s chlorophyll a standard proposal with a magnitude of 35 ug/L, a frequency of not-to-exceed more than once in three years, and a seasonal duration, calculated as a geometric mean (or geomean). The DWR recommendation includes the spatial extent and depth to which the site-specific standard would apply within a waterbody and identifies the waterbodies to which the proposed site-specific standard would be applicable, as is required. The proposed language, to amend 15A NCAC 02B .0211(4), is as follows: (4) Chlorophyll a (corrected): except as specified in Sub-Item (a) of this Item, not greater than 40 ug/l for lakes, reservoirs, and other waters subject to growths of macroscopic or microscopic vegetation not designated as trout waters, and not greater than 15 ug/l for lakes, reservoirs, and other waters subject to growths of macroscopic or microscopic vegetation designated as trout waters (not applicable to lakes or reservoirs less than 10 acres in surface area). The Commission or its designee may prohibit or limit any discharge of waste into surface waters if the surface waters experience or the discharge would result in growths of microscopic or macroscopic vegetation such that the standards established pursuant to this Rule would be violated or the intended best usage of the waters would be impaired; (a)Site-specific High Rock Lake Reservoir [Index Numbers 12-(108.5), 12-(114), 12-117-(1), 12- 117-(3), 12-118.5, and the uppermost portion of 12-(124.5) to the dam of High Rock Lake] Chlorophyll a (corrected): not greater than one exceedance of a growing season geometric mean of 35 ug/L in the photic zone within a three-year period. For purposes of this Sub-Item: (i) The growing season is April 1 through October 31; (ii) Samples shall be collected in a minimum of five different months within each growing season with a minimum of two growing season geometric means collected in a three-year period; (iii) The photic zone shall be defined as the surface down to twice the Secchi depth. (iv) Samples shall be collected as a composite sample of the photic zone; and (v) Samples that do not satisfy the requirements in Sub-Item (iv) of this Item shall be excluded from the calculation of the geometric mean. Table 1 provides a comparison of SAC and DWR recommendations. 3 Table 1: SAC and DWR chlorophyll a criterion and assessment recommendations. Component SAC Recommendation SAC Notes on Selection DWR Recommendation DWR Notes on Selection Magnitude 35 ug/L Selected from a range of chlorophyll a concentrations deemed to be protective of HRL designated uses. Same Selection of 35 ug/L derived by SAC from a station by station analysis deemed to be protective of HRL designated uses. Period/ Duration Seasonal Geomean Calculated geomean based on all data from growing season. Same None Growing Season/ Duration April-October Include samples collected in at least five different growing season months for each year of data included in the analysis. Same None Frequency Maximum Exceedance Frequency of one- in-three years Compute the geometric mean for each year of individual data and apply a frequency component of not more than one exceedance out of three years of data. Maximum Exceedance Frequency of one- in-three years A minimum dataset of two seasonal geometric means within a three- year period is required. Incorporated the SAC’s recommended frequency component with the additional requirement to have data from at least two growing seasons. This acknowledges year-to-year variability in chlorophyll a concentrations and the need for more than one year of data before making decisions. Spatial Considerations Open Waters Photic zone composite based on twice the Secchi depth; shallow waters and isolated coves exempt from numeric criteria; all data within each assessment unit would be incorporated into the calculated geomean. All waters within the associated index numbers (12-(108.5), 12- (114), 12-117-(1), 12-117-(3), and 12-118.5). Same as SAC recommendation except no broad shallow water/isolated coves exemption. Data from shallow waters are only excluded when samples cannot be taken to twice the Secchi depth to allow for consistency in monitoring and assessment. Station by station assessment consistent with methodology used to develop the 35 ug/L geometric mean. 4 Narrative Criterion The SAC recommended the use of a narrative criterion for shallow waters and isolated coves but did not offer specific narrative language. DWR agrees that a narrative component is appropriate and proposes to rely on the existing narrative language for best usage in 15A NCAC 02B .0211(2) which reads: “The conditions of waters shall be such that waters are suitable for all best uses specified in this Rule. Sources of water pollution that preclude any of these uses on either a short-term or long-term basis shall be deemed to violate a water quality standard;”. This provides protection to any shallow waters and isolated coves, seasons, or instances not covered by the site-specific, seasonal geomean and is consistent with the SAC’s recommendation for shallow waters to be addressed by narrative criteria. Shallow Water Areas The SAC recommended the categorical exclusion of shallow waters and isolated coves from the proposed numeric criterion, with a parenthetical suggestion of all waters less than ten feet deep. This recommendation was included in the final SAC report but was not discussed as part of the 2018 meeting during which the SAC voted upon its criteria recommendation. DWR does not recommend incorporating this exclusion for several reasons. First, reducing coverage of numeric nutrient-related standards in state waters is not the NCDP’s goal. The NCDP’s purpose is to refine and expand the use of numeric standards to address nutrient issues, not to reduce Clean Water Act (CWA) protections. If the recommended site-specific criterion including the narrative component were not to apply to all waters under consideration, then the existing standard in 15A NCAC 02B .0211(4) would apply to any waters not subject to a site-specific standard. Shallow waters are often the very places in need of numeric standards, particularly for recreational and fishing uses. Second, the definition of shallow waters as being less than 10 feet deep (3.0 m) raises substantial pragmatic and operational issues and was never scientifically justified by the SAC. Water levels in High Rock Lake fluctuate 10 feet or more, making the application of this limitation uncertain in relation to many fixed sampling locations. Third, the Monte Carlo analysis used to derive this site-specific magnitude recommendation did not exclude data based on depth. Of four monitoring stations chosen for that analysis, station HRL051 was included to represent riverine waters despite being well below the recommended ten-foot depth threshold. The SAC noted that “waters at HRL051 reflect turbid river conditions, and the average chlorophyll a is lower than in downstream waters.”1 Thus, the analysis, at least in part, supports applying the derived site-specific criterion in shallower waters. While not incorporating the exclusion of shallow waters from the application of this criterion, DWR understands the underlying concern that samples be representative. To ensure decisions are made based on representative samples throughout the lake, the proposed standard includes a requirement that only data from samples that can be collected to twice the measured Secchi depth will be used when calculating a geometric mean. In addition, current monitoring, quality assurance and transparency protocols are also employed to ensure representative sampling in High Rock Lake. 1 N.C. Nutrient Criteria Scientific Advisory Council, page 65. A Chlorophyll a Criterion for High Rock Lake. May 26, 2020. 5 DWR recommends that any new monitoring efforts in High Rock Lake, whether by DWR or by third parties, comply with the following existing protocols: • Photic zone composite and boat- or bridge-based sampling, which provide natural access and depth limitations • DWR or third-party compliance with the DWR Ambient Lakes Monitoring Program Quality Assurance Project Plan, which provides: “Actual sampling points are generally located within the center or main-stem of the lake, or as determined by field staff as representative of the lake or specific areas of concern within the lake.” • Submission of third-party data for public review during biannual integrated reports and associated Quality Assurance Protocol Procedures requirements These safeguards ensure nonrepresentative shallow water samples are not used for assessment purposes in High Rock Lake or statewide. Frequency The SAC recommended a “not greater than 1 in 3” frequency to implement the recommendation of a seasonal geomean for chlorophyll a in High Rock Lake. This means that in order to be found to be meeting criteria, there cannot be more than one excursion of the seasonal geomean in a three-year period based on a minimum of two geomeans. DWR staff have concerns as to the justification and feasibility of this approach. The SAC justification for the frequency component primarily relied on the fact that other states have implemented this approach. DWR currently monitors HRL once every five years; this approach may necessitate additional resources to monitor at an increased frequency. However, as this is a site-specific standard applicable only to HRL, DWR is proposing to proceed with the SAC recommendation. DWR is not proposing a change to the 303(d)/305(b) Integrated Report assessment methodology as part of the site-specific standard adoption process as it is not part of the rule-making process. After the adoption of a site-specific standard for High Rock Lake, DWR will incorporate the complementary site- specific assessment methodology into the state’s comprehensive 303(d) listing and delisting assessment methodology for EMC review and approval. Reservoir Framework As stated previously, the NCDP commits North Carolina to evaluate site-specific nutrient-related criteria for three pilot water bodies, each representing a distinct water body type, and based upon lessons learned from these site-specific evaluations, North Carolina will be better positioned to reevaluate nutrient-related criteria statewide. With regards to the process to evaluate the potential need and determine appropriate standards for site-specific criteria for other reservoirs, the SAC proposal states: “The SAC has not yet developed a detailed framework for deriving reservoir-specific chl a criteria. However, many elements of the SAC’s approach for High Rock Lake would be transferable to other water bodies. At various times, the SAC also discussed potential elements of a more formal framework for site-specific criteria derivation. This section attempts to document some of those 6 concepts in case they are useful during the future, statewide effort. Any of the framework elements discussed herein are subject to additional discussion by the SAC and DWR.” The SAC recommendation went on to document some of the concepts that were discussed, however no framework was formally approved for use by the SAC and as stated above, require further discussion by the SAC and DWR. Any criteria or site-specific standard proposed for other reservoirs will need the scientific justification to support the proposed criteria or site-specific standard to demonstrate that it is protective of designated uses and complies with North Carolina’s antidegradation policy.