Evaluation of Groundwater Supply Alternatives Within Bernal Subbasin

Page 9 ƐƐĞƐƐŵĞŶƚŽĨdĞƐƚtĞůů^ŝƚĞWƌŽĚƵĐƚŝŽŶtĞůůĂƉĂĐŝƚǇǁŝƚŚŽŶƐŝĚĞƌĂƚŝŽŶŽĨ Mutual Well Interference and Longer-Term Specific Capacities An extreme (and unlikely) scenario is that the mutual well interference drawdown calculations summarized above would be additive to the groundwater elevations observed during the Fall of 2021/2022. It is not anticipated that all the existing wells would be operating at the same time as the new test well sites. Assuming that the groundwater elevations observed in Fall 2021 and Fall 2022 reflect operation of all the existing wells (as is generally indicated by review of historical annual municipal groundwater pumping since 1974), the mutual well interference drawdowns calculated above would not apply to the relatively low groundwater elevations observed in Fall 2021/2022. However, in the case that Fall 2021 and Fall 2022 groundwater elevations are representative of the future conditions without any new production wells at the test well sites operating, and that three new production wells at the test well sites are operating as additional pumping capacity, the following analysis may warrant consideration. Table 9 summarizes the assessment of how the original baseline minimum well capacities may be impacted by the mutual well interference with all three test well sites pumping simultaneously for 30 and 90 days continuously. The results are characterized both by reduced well capacities that would result, and also by what the change would have to be in the well efficiency assumptions to obtain the same original well pumping capacity estimates. The results for Tennis Park and Hansen generally indicate a reduction in well capacities by approximately 10 percent or the need to assume well efficiencies on the order of 55 to 60 percent instead of 50 percent. In the case of Del Prado Park, either well capacities are reduced from 1,300 gpm to between 1,000 and 1,050 gpm, or well efficiencies of 93 to 99 percent would need to be assumed. Table 9 also shows the combination of long-term specific capacity assumptions (for 30 and 90 days) and mutual well interference drawdowns (for three test well sites pumping). This maximum worst-case scenario indicates Tennis Park and Hansen Park have well capacities reduced from 3,400 gpm to between 2,600 and 2,900 gpm, or the need to assume well efficiencies ranging from 59 to 66 percent. For Del Prado Park, well capacities for this worst case drop from 1,300 gpm to between 880 and 975 gpm, or assumed well efficiencies of 100 percent or greater (not possible). Overall, this maximum worst-case scenario indicates that it is still reasonable to assume the original low- end well capacities at Tennis Park and Hansen Park can be maintained, because achieving well efficiencies up to 66 percent is reasonable. However, well efficiencies of 100 percent or greater are not reasonable to assume for Del Prado Park, and it is likely that a reduced well capacity would occur under these worst- case conditions. However, on additional consideration, it is noted that these low-end well capacity estimates are based on Fall groundwater level conditions, whereas drought-year pumping will occur throughout the year when water levels may be slightly higher. Therefore, even in the worst-casescenario, the decline in well capacity may be limited to the Fall season.