Leaky Aquifer Testing (T)
Application of DP_LAQ to Leaky Aquifer Pumping Test Data
By Darrel Dunn Ph.D., PG (Unaffiliated hydrogeologist available for hydrologic and geologic consulting and contract work. Professional Synopsis)
This is a technical page on leaky aquifer testing. To see a non-technical page, press here.
The purpose of this web page is to describe the results of an application of DP_LAQ pumping test analysis software (132) to actual data. The pumping test selected for this study is described by Green and others (130). The test includes data from the pumped well and one observation well.
Description of the Aquifer Tested
The aquifer that was tested is composed of sand lenses interbedded with lenses of silt and clay. Its thickness is 320 feet. The aquifer is overlain by 320 feet of clay, siltstone, and very fine to fine grained sandstone, which is in turn overlain by a water table aquifer. This 320 foot thick unit is treated as a leaky aquitard in this pumping test analysis. The aquifer is underlain by 450 feet of sandy clay and siltstone which is in turn underlain by alternating layers of bentonitic clay and siltstone. This subjacent 450 foot thick unit is treated as a leaky aquitard.
Pumped Well Description
The pumped well was a 20-inch diameter bore-hole. The screened interval extended through the entire thickness of the aquifer, and consisted of alternating sections of 8-inch diameter casing and 8-inch diameter wire wrapped screen with 0.025-inch openings. The total length of screen was 180 feet. The annulus between the alternating screen and casing and the wall of the hole was packed with gravel.
Observation Well Description
One observation well was used. It was cased with 2-inch PVC pipe. The same intervals were screened as in the pumped well. The distance from the pumped well to the observation well was 100 feet.
Description of the Pumping Test and the Original Analysis
The original pumping test is described by Green and others (130). The well was pumped at a constant rate of 405 gpm for 98 hours with less than 3 percent discharge variation. The original analysis used a type curve method developed by Papadopulos and Cooper and presented in Reed (133). (The Papadopulos and Cooper method is a special case covered by DP_LAQ.) The analysis was applied to time-drawdown data from the the monitoring well. This method of analysis assumes a fully penetrating well of finite diameter in a non-leaky aquifer. It improves on the type curve method based on the Theis equation by considering the effect of depletion of water stored in the well resulting from drawdown during the test.
The result was as follows:
Transmissivity: 2,244 gpd/ft
Storativity: 3E-4 (dimensionless)
DP_LAQ Constant Rate Pumping Test Analysis
The present analysis of the data used DP_LAQ, which is described by Moench (132). DP_LAQ computes type curves for constant rate leaky aquifer tests for three cases:
- Aquitards above and below the aquifer have constant head boundaries on the top and bottom, respectively.
- Aquitards above and below the aquifer have have no-flow bondaries on the top and bottom, respectively.
- The aquitard above has a constant head boundary and the aquitard below has a no-flow boundary.
One can also specify whether leakage is from (1) both aquitards, (2) only the superjacent aquitard, (3) only the subjacent aquitard or (4) no leakage from either aquitard (non-leaky confined aquifer). The pumped well may be line-source or finite-diameter.
DP_LAQ generates type curves based on input of dimensionless variables that are derived from the following parameters:
- Thickness of the aquifer and aquitards,
- Specific storage of the aquifer and aquitards,
- Horizontal hydraulic conductivity of the aquifer,
- Vertical hydraulic conductivity of the aquitards,
- Distance to observation wells,
- Effective radius of the pumped well,
- Radius of the pumped well casing where the water level is declining,
- Pumped well “skin” constant related to well loss.
One can also enter dimensionless variables related to dual porosity fractured aquifers. However, the present study does not use the dual porosity capability.
Figure 1 shows the type-curve match with the observed drawdown data from the pumped well. In this figure, “TYPE CURVE DIMENSIONLESS TIME” is the expression tD/rD2 defined by Moench (132). “TYPE CURVE DIMENSIONLESS DRAWDOWN” is hwD for the pumped well and hD for the observation well, and “ADJUSTED TIME” is t/r2 (t is time since pumping started, and r is distance to an observation well or the effective well radius (rw) for the pumped well. Putting the observed data on the secondary axes and putting dimensionless time multiplied by S/T, and dimensionless drawdown multiplied by Q/(4πT) on the primary axes allows type curve matching by successive trial values of transmissivity (T) and storativity (S). This technique allows the matching to be performed on a spreadsheet rather than manually or electronically superimposing graphs and picking a match point.
Figure 1. Type curve match for pumped well and observation well with T=1448 gpd/ft and S=0.16.
Other parameters used to generate these type curves are:
Vertical hydraulic conductivity of the aquitards: 0.45 gpd/ft2,
Specific storage of the aquitards: 5E-4 (1/ft),
Dimensionless skin of pumped well: 1.0.
DP_LAQ worked well for analysis of this time-drawdown data from a leaky aquifer.
It is beneficial to have drawdown data from the pumped well and also from an observation well. During the development of the match shown in Figure 1, I found that data from the pumped well alone or data from the monitoring well alone could produce type curve matches that differed from the analysis matching data from both wells on the same graph.
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