Missouri is home to several major aquifers, estimated to contain 500 trillion gallons of groundwater suitable for drinking. Some of the most critical aquifers are the Ozark Aquifer in south central Missouri, Southeastern Lowlands Aquifer in southeastern Missouri, buried glacial channels distributed throughout northern Missouri, and various alluvial aquifers surrounding Missouri’s major rivers. Each aquifer has its own geologic properties that affect the flow, quantity and quality of the stored groundwater.
Unlike surface water bodies (rivers and lakes), it is difficult to observe and measure water stored in aquifers, because they lie hundreds to thousands of feet below ground surface. Useful information is derived by examining rock cuttings (from well drilling) and by employing geophysical techniques, but these methods focus more on the host rocks rather than the water. To observe and model groundwater flow, engineers and geologists rely on pump tests.
Pump Tests
A pump test, also known as an aquifer test, is performed by purposely pumping water from a well at a significant rate for a specified amount of time, and the effects on the water table (aquifer) from pumping is measured. When pumping begins, water closest to the well bore is pulled in and the water table close to the well begins to fall. As time goes on, the water table continues to decline and the area of decline continues to grow. The resulting shape is an upside-down cone called “the cone of depression.” The goal of a pump test is to measure the cone’s growth over time by continually measuring the water level in the pumping well and any nearby nonpumping wells (observation wells). When pumping ends, the water table will flatten and rise close to its static water level (SWL).
By measuring the growth of the cone over a period of time, geologists can calculate aquifer properties. The most commonly used characteristic is transmissivity, which describes how easily water can flow through an aquifer. Due to Missouri’s geology, transmissivity values are only locally accurate, and cannot be applied to other locations. Transmissivity can be used to model how an aquifer will respond to different levels of pumping, which can help determine if any nearby wells will be impacted by heavy pumping.
Procedure
The two main types of pump tests, the constant rate and step drawdown tests, are outlined in the Department’s minimum design standards for public water supply systems (at the end of this fact sheet). The most common pump test is the constant rate test. A general procedure for a constant rate test follows:
1. Determine if a pump test is required – Certain public wells are exempt from needing a pump test, though performing one is encouraged. Wells requiring pump tests include:
- Community wells with a total depth greater than 300 feet and casing diameter of 8 inches or more.
- Any community well drilled into unconsolidated (nonbedrock) material less than 300 feet deep.
- All noncommunity wells, unless exempted by the Missouri Department of Natural Resources.
2. Select a pumping rate – The pumping rate should be at least the designed production rate. Pump tests performed at a rate of 1.5 times the design rate may end after 8 hours if the water level in the well has stabilized for 2 hours. The pumping rate must be held within ±5% of the initial rate for the entirety of the test. If the pumping rate goes outside this range at any point, the pump test must be stopped to allow the aquifer to recover.
An aquifer has fully recovered when the cone of depression no longer exists and the water level in the well has returned to the SWL, though natural fluctuations in the aquifer may cause the SWL to be slightly different. Recovery time should be approximately the same as the duration of the test that was stopped.
Once the aquifer has recovered, any appropriate adjustments should be made (e.g., lower the pump if it was in danger of being exposed) and the test should be restarted from the beginning.
3. Measure the static water level and pump depth before pumping – Recording the static water level depth is essential. It is used to calculate drawdown. Water level measurements can be made using whatever method is best for the driller, as long as it can be converted to an actual depth to water level. The SWL depth should be measured as accurately as possible to the nearest foot (or better, which is ideal).
4. Begin pumping and record measurements – Measurements should be made at very specific time intervals in order for the test to be analyzed properly. Measurements should be taken less frequently as the test proceeds, because the water level decline occurs more slowly at later times. The amount of time between each measurement is outlined in the following table. Measurements may be taken more frequently than specified.
| Time since pump was turned on/off | Time between measurements |
|---|---|
| 1-10 minutes | 1 minute |
| 10-50 minutes | 5 minutes |
| 50-90 minutes | 10 minutes |
| 90-180 minutes | 30 minutes |
| 180 minutes - end | 60 minutes |
Each measurement should include depth to the water level (ideally, to the nearest foot), actual pumping rate, and any comments or irregularities that occurred since the last measurement.
For pumping rate, record the actual pumping rate at each step; not the target rate. Recording the actual pumping rate and any irregularities helps geologists identify natural changes in the aquifer. For example, if the water level begins to rapidly decline without any change in pumping rate, the aquifer likely contains a less-productive geologic unit.
5. End of pumping – If pumping at the design rate, the test should proceed for at least 24 hours. If the pump test needs to be ended prematurely (e.g., if the water level is about to fall below the pump), the test must be restarted at a lower rate. The second pump test must not start until the aquifer has fully recovered.
6. Record recovery – Upon completion of the test and immediately after the pump is shut off, measure the water level recovery at the same frequencies as shown in the table in Step 4. Record until the water level has reached the initial SWL or for 6 hours, whichever occurs first. Recovery data is analyzed in addition to the drawdown, and are used to verify the results of the pumping test.
7. Submit the data – Submit the pump test within 14 days of completion to the Department’s Missouri Geological Survey. Information to submit includes:
- Well owner, or the name of public water supply and well number.
- Driller and/or company performing the test.
- Well location.
- Static water level.
- Pump set depth.
- Pumping rate.
- Measurements made during the pump test and recovery.
These detailed pump test procedures and other topics related to public well construction and drinking water distribution are available in Minimum Design Standards for Missouri Community Water Systems – PUB2489 and Standards for Non-community Public Water Supplies - PUB979.