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HomeMy WebLinkAbout2015_12_17 modeling_memo - FinalTechnical Groundwater Flow and Transport Memorandum Modeling December 17, 2015 Three-dimensional groundwater flow and constituent transport models were built and calibrated to data from each of the 14 Duke Energy coal ash basin sites. The goal of this effort was to produce the most reliable models for each site given the available field data. By reliable, it is meant that the models are capable of reproducing the observed current -state field conditions such as hydraulic heads and constituent of interest (COI) concentrations, and that the models are useful for the intended predictions. The models have three main components: a three dimensional (3-D) geologic model, a 3-D flow model, and a 3-D COI transport model. The 3-D geologic models were constrained by the numerous well logs and other known surface and geologic data for each site. The flow models were constrained by known hydrologic features such as lakes, rivers, streams, swamps, and ditches. The flow models specifically considered the potential effects of local water table mounding from the ash basins due to enhanced infiltration from the basins. The initial groundwater flow models constructed in this way represented the initial conceptual flow model for each site. The initial conceptual flow models were calibrated by adjusting the hydraulic conductivities of the hydrostratigraphic layers and zones to best match the observed hydraulic heads in monitoring wells at each site (-50 to 100 wells per site). This provided refined models where the normalized root mean square error on the heads was substantially less than 10 percent (the industry standard). A major part of the development process was evaluating alternative conceptual models that considered various heterogeneities including fracture zones, and both high and low hydraulic conductivity zones. These refinements to the flow models continued until the model accuracy could not be further improved at a site. The COI transport models used the calibrated flow models with COI sources placed in the footprint of the ash basins and site-specific experimental data for the Kd values. The transport models have fewer parameters that can be adjusted in the calibration process because they are already constrained by the calibrated flow models. The key input to the COI transport models was the COI concentration and distribution within the ash basins at each site during historical operations. These values were initially estimated from the currently observed COI concentrations in the ash pore water. This initial conceptual model was then refined by adjusting the COI concentrations and source locations to produce best matches with the currently observed concentrations in monitoring wells. For each COI, the transport model result was compared to the observed COI concentrations in the 50-100 observation wells (depending on the site). The most helpful constituent for evaluating flow and transport at the ash basin sites is typically boron. Boron occurs in high concentrations in the ash basin pore water at all of the sites, and it usually has a low background value. Boron is nonreactive, and has a low Kd value. These properties mean that boron can serve as a robust indicator of COI transport at the sites. While there are many other COIs at each site, these other COIs will follow the same flow paths as the boron, though usually at slower velocities (depending on the COI Kd). The models generally provided good matches with the field data for boron concentrations at the sites. This fact, combined with the calibrations that were achieved with the flow models give confidence in the accuracy of the groundwater flow fields predicted by the models. A primary concern at each of the ash basin sites is possible impacts to domestic and public wells from the ash basins. The calibrated groundwater flow and transport models were used to assess these possible impacts by considering pumping from all of the domestic and public wells within the model domain of a given site. Two modeling approaches were used here. The first approach was to include the pumping wells in the calibrated flow and transport models, and then assess the degree to which the wells were impacted by boron and other COIs that have originated at the ash basins. This approach was used in Part 1 of the Corrective action plans for the Sutton, Weatherspoon, Cape Fear, H.F. Lee, Asheville, Roxboro, and Mayo sites to evaluate the impacts to supply wells. COI concentrations close to or above 2L standards in supply wells were predicted to be occurring at the Sutton Site, and to a very limited degree at the H.F. Lee Site. No impact to supply wells above 2L standards was seen in the model simulations for the Asheville, Weatherspoon, Cape Fear, Roxboro, or Mayo Sites. The second assessment approach was a numerical capture zone analysis. This is similar to the first approach, but instead of using the COI transport model to predict concentrations, a computer program called MODPATH is used with the calibrated now model. MODPATH is a widely used USGS program that is designed to interface with the MODFLOW flow model that was used. MODPATH is a "particle tracking" model that traces the groundwater flow lines from any desired starting position. MODPATH can also do "reverse tracking" to identify where water originates in the flow model. MODPATH is a standard part of the GMS modeling platform that was used to construct the flow and transport models. MODPATH can be used with the reverse tracking feature to trace the groundwater flowlines around each well to see where the water that is pumped from the well originates. This well-known procedure is called a well capture zone analysis, because it identifies the zone from which all of the water entering the well is captured. The capture zone analysis has been performed for the Allen, Belews Creek, Buck, Cliffside, Marshall, Roxboro, and Mayo Sites. Pumping rates for the individual household wells at these sites were generally not available, and were assumed to be equal to the average US household water use rate of 400 gallons per day (USEPA, 2015). It was assumed that all of the wells in the model domain were pumping continuously unless they were known to be inactive. In general, the shape of a given capture zone is a function of recharge, hydraulic conductivity, flow rate, and hydraulic gradient. For these sites, the primary influence on the shape of the capture zone is the hydraulic gradient. When the prevailing hydraulic gradient is small and relatively flat, the capture zone approaches a circular configuration. As the hydraulic gradient increases, the shape becomes elongated in a locally upgradient direction. The well capture zone approach for the Roxboro Site is shown in Figure 1. The supply well capture zones are represented in this figure by the yellow particle tracks and the ash basins are outlined in orange. The blue arrows show the overall directions of groundwater flow. The supply well at the Woodland School (at the southwestern corner of the figure) was assigned a pumping rate of 4,000 gpd, which is equivalent to 15 gallons per day per student and staff. The supply well at the drywall plant in the northern part of the plant was likewise assigned a pumping rate of 4,000 gpd. It is apparent from the figure that the well capture zones are limited to the immediate vicinity of the wellhead, and that they do not extend towards the ash basins. Figure 2 shows a similar well capture zone analysis for the Mayo Site. Again, the yellow particle tracks show the well capture zones, and the ash basin is outlined in orange. The well capture zones are far removed from the ash basin. The capture zone analysis for the Allen site is shown in Figure 3, with the capture zone particle tracks in yellow. The ash basin is outlined in orange, and the model boundary is shown by the black line. Model results indicate no effect of pumping on the general site groundwater direction and gradient which is away from off-site wells. Although some of the capture zones are close to the western and southeastern limits of the ash basin, the well capture zones do not overlap the ash basin boundary except for a few wells that are located immediately adjacent to the basin. This area of the model currently has limited resolution (which will be improved with additional field data which is pending and with model enhancements). Groundwater data collected to -date in the bedrock wells does not show ash basin impacts in this area. Figure 4 shows the analysis for the Belews Creek Site, using the same color scheme. The well capture zones in the model domain are located away from the ash basins. Well capture zones for the Buck Site are given in Figure 5. Model results indicate no effect of pumping on the general site groundwater direction and gradient which is away from off- site wells. Most of the well capture zones are isolated from the ash basins, although some of the wells are located near the southern and southeastern limits of the ash basin. This area of the model currently has limited resolution (which will be improved with additional field data which is pending and with model enhancements). The analysis for the Cliffside Site is shown in Figure 6. The wells are located upgradient or sidegradient to the ash basins, and the well capture zones are far removed from the ash basins. The Marshall Site capture zone analysis is shown in Figure 7. The well capture zones are located upgradient of the ash basin and the well capture zones are far removed from the ash basin. Reference US EPA, 2015, http://www.epa.gov/watersense/pubs/indoor.html accessed 8/26/15. Well Capture Zone Ash Basin Boundary Groundwater Flow Direction Figure 1. Supply well capture zones for the Roxboro Site. The yellow particle tracks are the well capture zones and the ash basins are outlined in orange. The large blue arrows show the overall groundwater directions of flow. Well Capture Zone Ash Basin Boundary lz* Groundwater Flow Direction Figure 2. Supply well capture zones at the Mayo Site. The yellow particle tracks are the well capture zones, and the ash basin is outlined in orange. The large blue arrows show the overall groundwater directions of flow. Off-site wells r e� Groundwater flow direction Well Capture Zone Ash Basin Boundary �i Groundwater Flow Direction �\ Model Boundary Figure 3. Supply well capture zones at the Allen Site. The yellow particle tracks are the well capture zones, and the ash basin is outlined in orange. The model boundary is shown by the black line. Off-site well Well Capture Zone Ash Basin Boundary cz* Groundwater Flow Direction �\ Model Boundary 0 Figure 4. Supply well capture zones at the Belews Creek Site. The yellow particle tracks are the well capture zones, and the ash basin is outlined in orange. The model boundary is shown by the black line. F. Well Capture Zone Ash Basin Boundary Groundwater Flow Direction �\ Model Boundary Figure 5. Supply well capture zones at the Buck Site. The yellow particle tracks are the well capture zones, and the ash basins are outlined in orange. The model boundary is shown by the black line. Well Capture Zone Ash Basin Boundary Groundwater Flow Direction �\ Model Boundary Figure 6. Supply well capture zones at the Cliffside Site. The yellow particle tracks are the well capture zones, and the ash basins are outlined in orange. The model boundary is shown by the black line. Off-site wells x ,, Groundwater flow direction Off-site well Well Capture Zone Ash Basin Boundary Groundwater Flow Direction �\ Model Boundary Figure 7. Supply well capture zones at the Marshall Site. The yellow particle tracks are the well capture zones, and the ash basin is outlined in orange. The model boundary is shown by the black line.