Specific Capacity Automatic
Specific Capacity Automatic Analysis
View | Specific Capacity Automatic
See Also:
Recommended for:
- Confined Aquifer
- As few as 1 drawdown measurement
- Only a rough estimate of transmissivity is required
- Aquifer storativity can be estimated from well logs
Implementation:
The specific capacity method used here is a completely automatic analytical solution. This method calculates transmissivity for each time and drawdown value using the specific capacity equation, which is an approximation of the Theis solution.
The Infinite Extent Specific Capacity method uses the equation:
Where:
Q = well discharge
s = drawdown in the well at time t
T = transmissivity of the aquifer
r = radial distance to the point the drawdown is measured
S = storativity of the aquifer
Q/s is defined as the specific capacity of a well, or the volume of water available per unit of drawdown. In the above equation, it is not possible to solve explicitly for transmissivity. Infinite Extent solves the equation first for an estimated value, and then adjusts the transmissivity until the equation is satisfied.
To use this method, an estimate of storativity is required when the well data is entered.
Typical storativity values are:
0.0002 for confined aquifer
0.002 for leaky aquifer
0.2 for unconfined aquifer
Unlike the Infinite Extent Specific Capacity Calculator, the method does not account for partial penetration.
Accuracy:
The most important factor in the accuracy is an accurate estimate of the aquifer storativity. When storativity is known correctly, the specific capacity method is nearly as accurate as the Theis Type Curve method. Of course, storativity will rarely be known before the test. Also the test is usually limited to a few data points, and therefore the benefit of averaging across many measurements is lost.
The specific capacity method may fail in aquifers with transmissivity below 200 m2/day (2000 ft2/day).
Applicability:
Applicability for only a single pair of time and drawdown values is also the biggest strength for this method. It is the only method in Infinite Extent that will work with one single time/drawdown measurement. This situation may arise when evaluating driller's logs for water supply wells where time and drawdown data is usually limited. For such a situation, the Specific Capacity calculator may be more appropriate.
The Specific Capacity method is also useful to look for trends in changing transmissivity from the beginning to the end of a pump test. This data may be used to indicate the presence of hydraulic boundaries or indicate heterogeneities in the aquifer.
The Specific Capacity method may be the only method available for poorly planned tests or incomplete data. For well planned pump tests other methods are recommended to substantiate the results of this method. If results differ significantly, the lessor emphasis should be put on the Specific Capacity method.