**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
t_{D}/r_{D}^{2} defined by Moench (132). “TYPE CURVE DIMENSIONLESS DRAWDOWN” is h_{wD }for
the pumped well and h_{D} for the observation well, and
“ADJUSTED TIME” is t/r^{2} (t is time since
pumping started, and r is distance to an observation well or the
effective well radius (r_{w}) 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/ft^{2},

Specific storage of the aquitards: 5E-4
(1/ft),

Dimensionless skin of pumped well: 1.0.

**Conclusion****s**

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.

**Addendum**

For questions or comments (especially
regarding errors and omissions), send an email to ddunn@dunnhydrogeo.com.

If you need a consulting hydrogeologist to apply DP_LAQ to your pumping test data or perform other hydrologic or geologic tasks, send an email to the address given above.