Evaluation of Groundwater Supply Alternatives Within Bernal Subbasin

Page 3 along with the typical 24-hour specific capacity value used in most well yield calculations. Available drawdown relates to regional groundwater elevations and varies seasonally and year to year, depending on how wet or dry the recent and preceding years have been. Well capacities were estimated for relatively high and low available drawdown estimates. Well efficiency can be highly variable and is heavily dependent on well engineering design considerations (e.g., screen slot size, filter pack quality and gradation, well drilling/development specifications) and drilling contractor performance (e.g., driller efficiency, driller adherence to drilling specifications). Each of these three key inputs is described in more detail below. Data from the constant rate aquifer tests were reviewed to calculate the 24-hour specific capacity by extending the trend of water level declines (on a semi-log time-drawdown plot) from the 8-hour test out for a 24-hour period. The 24-hour specific capacity was calculated by dividing the average pumping rate during the 8-hour constant rate test by the total estimated drawdown over a 24-hour period. In addition, specific capacity values were determined in a similar manner (extending the semi-log plot time-drawdown line from 8 hours to 30 daysand 90 days) for 30-day and 90-day continuous pumping scenarios to evaluate the difference in assumptions regarding specific capacity values. The specific capacity values for different time periods are summarized in Table 3. Table 3. Summary of Test Well Specific Capacity Values tĞůů/Pumping Rate (gpm) 24-Śƌ ;ĨƚͿ 24-Śƌ Q/s (gpm/ft) 30-ĂǇ ;ĨƚͿ 30-ĂǇ Q/s (gpm/ft) 90-ĂǇ ;ĨƚͿ 90-ĂǇ Q/s (gpm/ft) Tennis Park 549 9.6 57.2 10.3 53.3 10.6 51.8 Hansen Park 594 19.5 30.5 21.5 27.6 22.5 26.4 Del Prado Park 252 15.6 16.2 16.2 15.3 17.2 14.7 As summarized above Tennis Park specific capacity ranges from 57 to 52 gpm/ft, Hansen Park specific capacity ranges from 30.5 to about 26.5 gpm/ft, and Del Prado Park ranges from 16.2 to 14.7 gpm/ft for 24 hours to 90 days of pumping. Given the relatively high groundwater elevations at the time of the pumping tests, the measured groundwater level at the start of testing was used as the basis for the high-end well capacity estimate. The difference between the measured groundwater level and the top of the uppermost screen was used as the available drawdown for the well capacity estimate. As described above, historical groundwater elevation contour maps (for critical dry years) and municipal pumping were reviewed to determine an appropriate regional groundwater elevation to use at the test well sites as the basis for low-end well capacity estimates. The water years considered for the low-end estimate were 2021 and 2022. The difference between groundwater elevations in these dry years and the top of the well screen were used as available drawdown for low-end estimates. Well efficiency relates to the difference between the pumping water level inside a production well casing and the groundwater level in the aquifer formation just outside the well filter pack. In theory, a well would be 100% efficient if these water levels are the same; in practice, this rarely occurs for high-capacity