A 24-hr, constant-rate aquifer pumping test with MW1 as the producing well was conducted on February 17–18, 2004, to determine the aquifer’s response to pumping along the plume migration pathway. Water level monitoring continued through the subsequent 24-hr recovery period after pumping ended.
The hydrograph for well MW1 during the aquifer pumping test and measured flow rates are shown in Figure 4.13. The hydrograph has been corrected for barometric pressure effects as described in Appendix G. At most, approximately 10 ft of drawdown was observed in MW1, at an average pumping rate of approximately 1.1 gpm. Water levels in the well were extremely sensitive to small variations (0.1–0.2 gpm) in the instantaneous flow rate during the test, which were difficult to avoid at the overall low rate of pumping that was possible.
To estimate the transmissivity (T), hydraulic conductivity (Kh), and storativity (S) of the
Everest aquifer unit, the drawdown data from the pumping phase of the MW1 test were first corrected for barometric pressure effects as described in Appendix G. The corrected data were interpreted by using standard analysis methods that assume either confined (Theis 1935; Cooper and Jacob 1946) or leaky confined (Hantush and Jacob 1954, 1955; Hantush 1960) conditions, as implemented in the aquifer test analysis software package AqteSolv for Windows. The analysis methods were selected because of the relationships of static water levels in the aquifer unit and the responses of water levels to recharge events observed in the zone of influence of the pumping test.
4.3.2.1 Analysis with Methods for Confined Aquifers
The Theis and the Cooper and Jacob methods for confined aquifers can overestimate transmissivity or hydraulic conductivity if the aquifer being tested is not completely confined, but the overestimation is small when the aquifer is significantly more permeable than the overlying materials (Neuman and Witherspoon 1969). At the Everest site, the lithology indicates that this is the case. For all reported Cooper and Jacob analyses, only time-drawdown data satisfying the experimental criteria u < 0.1 were employed for the interpretive straight-line fits (Kruseman and deRidder 1991).
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Interpretive curve fits to the (corrected) drawdown data for MW1 from the Theis (1935) and the Cooper and Jacob (1946) analyses are shown in Figure 4.14. In each diagram, an initial offset of approximately 0.9 ft in the drawdown level affected the first minute of pumping. This is an artifact of a problem at the beginning of the test with a metering valve used to restrict and control the pump’s flow rate. Under the (February) conditions of the test, this valve froze before the pump was turned on and the test began, allowing a small volume of groundwater (equivalent to 0.9 ft of head in the casing) to flow from the well and into the riser pipe. As Figure 4.14 shows, the water level in well MW1 started to recover slightly after this initial withdrawal in the first minute of recording, and then the valve freed itself and drawdown resumed. In light of the planned duration of the test (24 hr), pumping was continued.
Figure 4.14 indicates that the subsequent data for MW1 fit the Theis and the Cooper and Jacob theoretical curves for confined aquifers reasonably well. The analyses yielded estimated transmissivities of 14.8 ft2/day and 13.6 ft2/day for the aquifer unit, corresponding to estimated
hydraulic conductivities of 1.48 ft/day and 1.36 ft/day, respectively. These values are similar to, but slightly lower than, the hydraulic conductivity estimates from slug testing of piezometers SB34 and SB01 (Figure 4.12) near MW1 and the former CCC/USDA facility. The Theis and the Cooper and Jacob methods do not support estimation of storativity from pumping well data.
Water level recovery data for well MW1 after pumping ended are shown in Figure 4.15. The observations yielded a very poor fit to the expected straight-line recovery trend (Theis 1935). Nevertheless, the statistical fitting of the straight line shown in Figure 4.15 yielded a transmissivity estimate (13.6 ft2/day) consistent with the values obtained from the MW1 pumping data. The relatively high value of interpretive parameter S′ (6.2) determined from the recovery analysis is qualitatively consistent with the observation (Appendix G, Section G.2.2) that melting snow might have recharged the aquifer during the pumping and recovery periods. (The dimensionless parameter S′ is the ratio of storativity during pumping to storativity during recovery. The ratio can be determined directly from the test data, but the individual storativity values cannot.)
Drawdown data for piezometers SB01, SB09, and SB34, corrected for barometric efficiency (Appendix G), are shown in Figure 4.16. The clear drawdown responses recorded at SB34 (141 ft east of the pumping well) and SB01 (304 ft southeast of MW1) demonstrate that the pumping of MW1 exerted a small hydraulic influence on water levels in the aquifer unit near the former CCC/USDA facility. A very small drawdown response (< 0.06 ft) is suggested during the early portion of the pumping period at SB09 (525 ft northwest and downgradient of MW1);
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however, this apparent trend was reversed later, as discussed in Appendix G. Because the absolute head changes occurring at the observation points during the pumping period were small, the measured drawdowns at these locations were particularly sensitive to the influence of several competing factors (other than the pumping of MW1) that also affected the water levels at these locations during the testing period, as described in Appendix G. The corrected drawdown levels shown for all three observation points (and particularly SB09) are uncertain in that the corrections for barometric efficiency required at these locations (Appendix G) represent a significant portion of the measured drawdowns (roughly 20–30% for SB34 and SB01; > 100% at SB09). Also, as noted in Appendix G, Section G.2.2, water levels later in the pumping period could have been influenced by local recharge due to melting snow, the effects of which could not be quantified. Both the observed barometric pressure changes during the pumping period and the inferred local recharge (for which no data correction could be applied) would have tended to decrease measured drawdown levels at the observation points, thus increasing calculated hydraulic conductivity estimates over values expected in the absence of these influences.
With these limitations in mind, the Theis (1935) and the Cooper and Jacob (1946) analysis methods for confined aquifers were used to fit interpretive curves to the drawdown data for piezometers SB34 and SB01 (not SB09), corrected for barometric efficiency per Appendix G. The results are in Figures 4.17 and 4.18, respectively. In each case, the theoretical trends were a reasonable approximation to the observations. The estimated transmissivities (358–360 ft2/day
for SB34; 416–440 ft2/day for SB01) and hence the hydraulic conductivities (35.8–36.0 ft/day
for SB34; 32.0–33.8 ft/day for SB01) calculated for the aquifer unit from these analyses were greater by an order of magnitude than those identified at MW1 and 5–10 times the hydraulic conductivity estimates from the slug tests at these locations. The values of storativity for the aquifer unit determined from the SB34 and SB01 data were similar, at 0.0003–0.0004.
Water level recovery data for SB01, SB09, and SB34 were recorded, but no attempt was made to interpret these results quantitatively because of the influence of a rising ambient water level trend at the testing site throughout the 24-hr recovery period (Appendix G, Section G.2.2).
4.3.2.2 Analysis with Methods for Leaky Confined Aquifers
An attempt was made to analyze the drawdown data for MW1, SB01, and SB34 by using both the Hantush (1956, 1960) and Hantush and Jacob (1954, 1955) methods for leaky confined aquifers. The analyses yielded no clear improvement in the fit of the theoretical interpretive
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curves to the test data, and the estimated transmissivity, hydraulic conductivity, and storativity values were comparable to those of the corresponding Theis and Cooper and Jacob analyses for confined aquifers.