Aquifer Step Testing (NT)
AQUIFER STEP TESTING, NON-TECHNICAL
(For a technical web page on this subject, click here.)
By Darrel Dunn, Ph.D., PG, Consulting hydrogeologist (Professional Synopsis)
Aquifer testing procedures (aka pumping tests) are valuable hydrogeologic tools that may be applied to water well problems, water well maintenance and rehabilitation, groundwater monitoring well testing, injection well testing, and groundwater sustainability planning. There are many such procedures that are applicable to different problems involving different types of aquifers.
Step testing is one of the more valuable and cost effective aquifer testing procedures. It is commonly used for testing a new water well to determine aquifer characteristics (especially transmissivity which is related to permeability) and to evaluate the efficiency of the well, so that the proper sized pump can be selected. It can also be used to determine the inlet condition of an old well to see whether its efficiency has has deteriorated enough to warrant rehabilitation by chemical treatment and/or surging to reduce clogging of the inlets. Step testing is also useful in testing new monitoring wells to design sampling procedures properly, and in testing aquifer exploration test wells to determine aquifer characteristics at the well locations. When appropriate, the aquifer characteristics may be used to develop a conceptual model that is preliminary to constructing a numerical groundwater model. It has been recommended as the first procedure in testing Aquifer Storage Recovery (ASR) wells (141). Step testing may be used as a preliminary test to help plan a constant discharge rate test. The step testing method described on the technical page can also be used for testing injection wells if appropriate modifications are made to the input and output of the computer program used to analyze the test data.
Step testing usually involves starting to pump the well at a low rate and then increasing the pumping rate in successive steps in which the pumping rate is kept constant. The steps may be an hour or two long, but the actual conduct of the test depends on its purpose and on other factors such as how much pumped water can be disposed of without adverse effects. While the well is being pumped, the water level is measured in the well and any monitoring wells nearby. Step testing can also be used to estimate appropriate injection rates for new injection wells (for wastewater disposal et cetera) and to evaluate the degree of clogging in operating injection wells. In this case one might inject liquid at successive constant injection rates. The evaluation of the data is the same as for successive constant pumping rates. The technical page associated with this non-technical one describes a useful step test data analysis procedure. The procedure is valid for a type of groundwater aquifer that is overlain and underlain by low permeable beds (confined aquifer). It may also be used for shallow aquifers that contain the water table if they meet certain criteria described on the technical page (generally permeable surficial sand and gravel).
The measured water levels are recorded as drawdown, which is the distance the water level in a well declines during pumping from the initial non-pumping level. Two step test drawdown curves are shown in Figure 1. One is a curve for a hypothetical pumping well with no "well loss," and the other curve is for the same well with severe well loss. (Well loss is drawdown caused by restrictions to groundwater flow interfering with the movement of water into the well.)
Figure 1. Step drawdown for a hypothetical pumping well, with and without well loss.
The usefulness of step testing stems from the following considerations:
- In a new well, one need not know how hard the well can be pumped prior to the test. The test can be started at a low rate that will not stress the capacity of the well, and then the rate can be increased in steps until the drawdown is the maximum tolerable or the capacity of the pump is reached.
- Step test results can help one plan any additional testing that may be needed. Often step tests are followed by constant discharge tests. Constant discharge tests may provide more accurate results, especially for water table aquifers, leaky aquifers, and large diameter wells.
- In a test well, one can distinguish between drawdown due to well loss and drawdown in the aquifer that is unaffected by well loss. When the production well is constructed, the factors causing well loss can be reduced or eliminated. However, the remaining drawdown in the aquifer is not avoidable. Knowing the unavoidable drawdown in test wells and what it tells one about the quality of the aquifer near the wells can help in locating subsequent production wells in the best part of the aquifer.
- Periodic step testing in production wells after they have been put into service can help track the deterioration of the wells and indicate when rehabilitation might be cost effective.
- Step testing before and after rehabilitation of production wells can help gauge the effectiveness of the treatment.
- Step testing monitoring wells can help in planning sampling operations.
Step test data may be adjusted mathematically so that quantitative estimates of aquifer permeability (transmissivity) and well loss can be made. A graph of adjusted data is shown in Figure 2. It is the adjusted equivalent to the curve showing the effect of well loss in Figure 1. The aquifer transmissivity may be determined by the slope of the lines on the graph of adjusted data. A quantitative indication of well loss may be obtained from the vertical distance between the lines.
Figure 2. Example of adjusted step test data.
Calculating the adjusted time is too laborious to do by hand. I use a Fortran computer program for the repetitive calculations required. The output of the program can be transferred to a spreadsheet and/or other software for graphing and any additional analysis needed.
A simple method of analyzing data from a constant rate pumping test to obtain a value for transmissivity called the Jacob method is a special case of this step-rate method in which there is only one step. Adjusted drawdown becomes the inverse of specific capacity (pumping rate divided by drawdown of the water level in the well, gallons per minute per foot of drawdown), and adjusted time simply becomes the time since the pumping started.
Some methods currently used for analyzing step test data were practical when computers were not available to do rapid repetitive calculations. Now that computers are universally available, the computationally intensive method described in the technical page on this subject may be used to get more complete and accurate results. It is important to have a step test performed in accordance with specifications that will produce good data for a useful analysis and have step test data analyzed by a hydrogeologist that thoroughly understands pumping test theory and subsurface geology.
Posted March 21, 2013
Revised November 21, 2018