SITE INFORMATION AND CORRESPONDENCE_1992

Home Browse Search

3-D Model/RD Report Memorandum -6- 14 December 1992 DDRW, Sharpe Well Efficiency An efficiency of 65 percent was used in the Injection Scenario based on model calibrated estimates from the short-term injection tests completed in the Central Area (page 8-15) . The Modeling Report also states that pumping well efficiencies of 37, 54, 58, and 30 percent were estimated from head losses computed by the MODFLOW well package for the 4558, 437B, 453B and P4B aquifer tests, respectively, for the short-term transient calibrations (pages 5-9 to 5-11) . The Modeling Report correctly states that injection wells are generally less efficient than extraction wells (page 8-15) . Therefore, the 65 percent efficiency used for the Injection Scenario seems relatively high for gravity fed injection wells and in comparison to the estimated well efficiencies used in the transient state calibrations. The Modeling Report should have provided additional discussion justifying the appropriateness of these efficiencies. Sensitivity Analysis The sensitivity analysis indicated that the model was not highly sensitive to the following parameters: hydraulic conductivity and leakance, dispersivity and retardation factor. The removal efficiencies varied between 10 to 25 percent, 5 to 10 percent and 3 to 10 percent, respectively, when these parameters are decreased and increased in model simulations. The model appears to be less sensitive to agricultural well pumping. The model does not appear to be sensitive to rising regional water levels and continuous sources of TCE (pages 10-1 to 10-6) . As is common with many models, the 3-D model for the Sharpe site is most sensitive to changes in hydraulic conductivity and leakance. The lower sensitivity of the model to agricultural pumpage and rises in water levels appear to be directly affected by the assumptions used in development of the flow model (page 8-1) . The regional decline in ground water levels since 1945 and the ongoing drought were assumed not to continue into the future and the agricultural wells west of the site were assumed to be inoperative over the entire future simulation period. The model also does not account for contaminant transport from the unsaturated zone (source areas) . However, these model assumptions are acceptable only if the agricultural pumpage is discontinued and the source areas are remediated. In addition, the Modeling Report states that retardation factors used in the solute transport modeling were calculated from sediment sample results for Total Organic Carbon (TOC) . The TOC content was measured to range between 0.232 and 0.638 percent by weight for the samples. It is generally accepted that the organic content of soils in arid and semi-arid climates is relatively low. Therefore, these analytical results are surprisingly high. The affect of unusually high TOC content on the retardation factor and the resultant affects of the retardation factor on the sensitivity analysis were not sufficiently justified. These issues should be discussed more thoroughly in the Modeling Report. CKW:cw