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Section 4 - Discussions and Conclusions
Results of the two pumping tests correspond to that reported by Ashworth (1983) and Hammond (1984). Table 5 summarizes results of the CSSA pumping tests with those values obtained from the literature review.
Comparison of Middle Trinity Aquifer Parameters at CSSA to Literature Review Values
| | Ashworth (1983) | Hammond (1984) | CSSA (2001) | ||
| Aquifer Parameter | (Middle Trinity Aquifer) | Local System (Lower Glen Rose) | Regional System (Lower Glen Rose) | CS-10 (Middle Trinity Aquifer) | CS-16 (Middle Trinity Aquifer) |
| Specific Capacity (gpm/ft) | N/A | N/A | N/A | 1.13 | 0.71 |
| Transmissivity (gpd/ft) | 1,700 | 5,740 to 16,110 | 240 to 3,220 | 2,400 | 1,600 |
| Hydraulic Conductivity (cm/sec) | N/A | 1.4 x 10-3 to 3.5 x 10-3 | 3.4 x 10-5 to 1.0 x 10-3 | 5.7 x 10-4 | 4.2 x 10-4 |
| Storativity | N/A | N/A | 0.00003 | 0.0005 | 0.00008 |
| N/A - Not Available from Literature Review | |||||
The hydraulic conductivities calculated for CS-10 (5.7 x 10-4 cm/sec) and CS-16 (4.2 x 10-4 cm/sec) were similar. Drawdowns for both pumping wells were also similar (63 and 71 feet); however, their specific capacities were not (1.13 gpm/ft and 0.71 gpm/ft). In light of the differences in pumping rates (80 gpm at CS-10 and 45 gpm at CS-16), and given that CSSA performed an acid treatment on CS-10 to improve well yield, it would be expected that CS-16 would have significantly less hydraulic conductivity than calculated at CS-10.
The difference may be explained by the fact that the hydrologic setting deviates from the basic underlying assumptions of the Theis equation. More specifically, the Middle Trinity aquifer does not behave a homogenous and isotropic entity. For example, the Theis curve analysis indicates that the aquifer is behaving under a confined condition given the low values of storativity calculated. However, some literature considers the Lower Glen Rose as a water table aquifer and the Cow Creek member as a confined aquifer. Since the pumping well boreholes are completed across both units, the practical approach is to treat the Middle Trinity aquifer as one hydrologic unit. This �averaging� of aquifer types may account for the low storativity values calculated in this report.
Additionally, as explained in Section 3, a water table CSM was used in calculating the saturated thickness. Although it is not entirely known at this time, it is suspected that the Lower Glen Rose and Cow Creek members do not equally transmit water throughout their entire thickness. In fact, it is probable that significant intervals of those units yield little to no water.
For a given value of transmissivity, the resulting hydraulic conductivity is inversely proportional to the thickness of saturated unit. Ultimately, the calculation of hydraulic conductivity is extremely sensitive to the saturated thickness assumed. Because of the inherent uncertainty of the saturated thickness at CSSA, calculated values of hydraulic conductivity should be considered qualitative.
The Middle Trinity aquifer at CSSA possesses characteristics of anisotropy and heterogeneity. First, based on transmissivity and storativity values estimated from the CS-16 test, the drawdown of 0.1 ft should be observed at a distance of 4,200 feet from CS-16.
s = [264Q/T] log (0.3Tt/[r2S])
Where s = drawdown, 0.1 ft
Q = pumping rate, 45 gpm
T = transmissivity, 1,600 gpd/ft
t = time since pump started, 3 days
r = distance from the pumping well (ft)
S = storativity, dimensionless, 0.00008
Substituting the numbers into the equation, the estimated distance at 0.1 ft drawdown would be somewhere around 4,100 feet from CS-16. In other words, all the wells, including the farthest wells CS-MW4-LGR and CS-MW9-LGR, monitored during the CS-16 pumping test should observe drawdown of 0.1 ft within 72 hours of constant pumping if the aquifer is homogeneous and isotropic. The only well responding to CS-16 pumping is Well D, about 455 feet away. This further confirms that the assumption of isotropy and homogeneity is not entirely applicable to this aquifer.
This discussion presents the principal conclusions derived from execution of the aquifer pumping test analyses conducted for the Middle Trinity aquifer in the area of CS-10 and CS-16 at the CSSA facility. The conclusions are presented for each of the two pumping test programs below.
4.2.1 - CS-10
The specific capacity of CS-10 was calculated as 1.13 gpm/ft of drawdown.
Response to pumping was identified within CS-9 and CS-10 during the pumping test. CS-11 did not respond to pumping. Parsons attributes the lack of response to pumping at CS-11 to an aquifer boundary condition located between CS-10 and CS-11 and between CS-9 and CS-11. The physical location of the boundary condition is apparently located closer to CS-10 than CS-9 with respect to CS-11.
Transmissivity was calculated to be 2,400 gpd/ft. Storativity was calculated as 0.0005. The Middle Trinity aquifer exhibits confined properties at CS-10.
The hydraulic conductivity for the Middle Trinity aquifer at CS-10 was calculated to be 5.7 x 10-4 cm/sec.
4.2.2 - CS-16
The specific capacity for CS-16 was calculated as 0.71 gpm/ft of drawdown.
Response to pumping was limited to CS-16 and CS-D during the pumping test.
Transmissivity was calculated to be 1,600 gpd/ft. Storativity was calculated as 0.00008. The Middle Trinity aquifer exhibits confined properties at CS-16.
The hydraulic conductivity for CS-16 was calculated to be 4.2 x 10-4 cm/sec.