Given the Case Study 2 Well 31B Report in your Reference materials:
a. Calculate aquifer transmissivity using the Cooper-Jacob method.
b. Calculate well efficiency for each step in the step-drawdown test.
7.1 Aquifer Testing Methods After well development was complete, the water level in Well 32B was allowed to recover to the pre-pumping water level. Pumping tests consisted of a step-drawdown test and a 1,440 -minute constant-rate test. 7.2 Step-Drawdown Test A step-drawdown pumping test of Well 32B was performed on September 7, 2016. The measured pre-pumping water-level was 80.21ftbg. The test consisted of pumping for 60 minutes at each of the following rates: 900;1,000;1,100;1,183; and for 4 minutes at 1,225gpm (Fig. 5) at which time the test was stopped because the pumping water level was less than 12ft above the pump. Specific capacities at the end of each step ranged from 5.69gpm/ft at 900gpm, to 4.98gpm/ft at 1,183gpm. Specific capacity and well efficiency were not calculated for Step 5. Table 3 summarizes the results for the step-drawdown pumping test and well efficiency calculations. Total sand production was less than 0.04ppm by volume at the end of the test. Table 3. Summary of step-drawdown pumping test data, City of Las Cruces Replacement Well 32B (LRG-420-POD59) - gaunds yer munuse At U - aeet veivw grumiti aevea gpm/ft - gallon per minute per foot of drawdown 7.3 Well Efficiency The step-drawdown test is performed primarily to evaluate the completeness of well development and well efficiency. The yield of the well, in terms of specific capacity, is governed both by the properties of the aquifer and the efficiency of the well. Well efficiency is a measure of the effectiveness of the well in transmitting water from the aquifer to the pump. Any well is more efficient at a low pumping rate than it is at a higher one because a smaller proportion of the drawdown is attributable to turbulent-flow losses that are an exponential function of the pumping rate. Efficiency may be calculated as the ratio between the projected specific capacity at a pumping rate of zero (at which the characteristics of the well have no effect) and at an actual pumping rate (Bierschenk, 1964). Table 3 also gives the estimated efficiency, by this method, at each of the pumping rates represented by the step-drawdown test. Well efficiency calculations are based on the assumption that the aquifer is homogenous and isotropic. Unfortunately this is rarely the case. In the case of Well 32B, the aquifer adjacent to the screened portion of the well consists of interbedded layers of sand, gravel, clay, and silt of various thicknesses. Also, based on the geophysical logs and the drill cuttings, the sediments become more consolidated with depth. Therefore, the calculated well efficiency results in values that may appear lower than expected, and are biased downward due to interbedded nature of the aquifer and the increase in sediment consolidation. 7.4 Constant-Rate Pumping Test A constant-rate pumping test was performed September 13 through 14,2016 . The well was pumped for 1,440 minutes at an average rate of 1,016gpm. The depth to water in the well prior to starting the test was 77.77ft bgl. Data were collected at 1-minute intervals using a Rodgers-supplied pressure transducer and data logger. Drawdown and recovery data from the constant-rate pumping test were plotted on a semi-logarithmic plot (Fig. 6 and Table 4), and the data were analyzed using the Cooper-Jacob (1946) "straight-line" method. The calculated transmissivity of the portion of the aquifer open to the well was 12,780 gallons per day per foot (gpd/ft) (1,708ft2/ day) from the drawdown curve, and 13,410gpd/ft(1,793ft2/ day) from the recovery curve. The specific capacity was 4.69gpm/ft of drawdown from the lowest drawdown recorded during the pumping period. The late portion of the drawdown curve was marked by a slight stepwise decrease in slope, which may be indicative of a recharge boundary that is likely associated with leakage from the storm-water retention pond directly adjacent to the well where water from development and test pumping was discharged. Table 4. Summary from the 1,440-minute constant-rate pumping test, City of Las Cruces Replacement Well 32B (LRG-420-POD59) gpm - yaluns per munue II vgl - leel veiow givund sevel gpm ft - gallons per minute per foot of drawdown 7.5 Pump Capacity and Setting Depth Future pumping water levels were projected using the following assumptions: an aquifer transmissivity of 12,780gpd/ft(1,708ft2/ day) and short-term specific capacity as interpolated from the step-test data based on JSAI's interpretation of the pumping test results, in addition to a regional waterlevel decline of 1ft/yr. Short-term effect of Replacement Well 32B's own pumping will continue to be approximately the same as during the step-drawdown pumping test, provided the well efficiency remains the same as it is currently. Projected long-term pumping water levels when the well is pumped 65 and 100 percent per day are provided in Table 5. The pumping water levels assume an initial non-pumping water level of 78ftbgl. Table 5. Projected pumping water levels at various pumping rates including assumed regional water-level decline of 1ft/yr, City of Las Cruces Replacement Well 32B (LRG-420-POD59) bold - pumping water level within 25ft of top of the screen DD - drawdown gpm-gallons per minute ft bgl - feet below ground level If the well will be pumped about 65 percent of the year, JSAI recommends that the pump be sized to produce 1,200gpm with a pumping water level of about 410ft bgl. If the well will be pumped more continuously, JSAI recommends that the pump be sized to produce 1,100 gpm from a pumping water level of about 410ft bgl. In either case, the pump setting should be between 450 and 460ftbgl. Once Replacement Well 32B is equipped and used to provide water for municipal supplies, JSAI recommends that the Utility incorporate the well into its water-level monitoring program, and collect pumping and non-pumping water levels including the pumping rate at the time of measurement. This will facilitate in identifying and remedying problems with water production should they occur over time. 7.6 Water Quality Water samples were collected by JSAI near the end of the constant-rate pumping test on September 14, 2016, and submitted to Hall Environmental Analysis Laboratory (HEAL) in Albuquerque for analysis. Table 6 contains a summary of water quality analytical results, and Appendix 6 contains a copy of the laboratory report. No constituents tested exceeded primary EPA or NMED primary drinking water standards, including arsenic, metals, anions, total coliforms, and radionuclides. Water produced from the well had no detectable concentrations of volatile organic compounds as analyzed by EPA method 8260B. Based on the parameters analyzed by HEAL, water produced from the well is of good quality, but hardness was 180mg/L, which is considered very hard
