The exact lateral and vertical extent of that critical volume is not possible to determine with the data currently available. As noted in the 'Robinson Ash Basin Groundwater Assessment for the MW-7 Area, H.B. Robinson Steam Electric Plant, Darlington County, South Carolina' (HDR, February 26, 2015), arsenic was not detected in the MW-7D, indicating the vertical extent of contamination in the vicinity of MW-7 is delineated and, laterally, arsenic was not detected above the laboratory reporting limit in MW-120S and MW-120D nor the surface water in the discharge canal or Lake Robinson. Further, it is noted that none of the deep wells (including those installed within the boundary of the basin itself) detected arsenic. Based on these data, further refinement of the fate and transport modeling would be prudent following collection of additional sampling data within the basin that could better define the extent or arsenic within the thalweg.