Water Well Journal
Issue link: http://read.dmtmag.com/i/498253
that must be satisfied in order for the model predictions to be valid. The less assumptions met, the less accurate the model's prediction. In conjunction with observation wells, the constant rate or steady state test can be used to estimate transmissiv- ity, hydraulic conductivity, specific yield, well efficiency, and storativity. Through the use of various modeling methods, numerous other aquifer char- acteristics can be defined. Characteris- tics like heterogeneity and homogeneity, no flow and recharge boundaries, frac- ture flow, radial flow, linear flow, spher- ical flow, anisotropy, isotropy, wellbore storage, aquifer leakage, and long-term production capabilities to name just a few of the numerous methods and mod- els available to characterize aquifer properties (Kelleher et al. 2013). Constant rate tests are pumped for extended durations in order to stress the aquifer. The more stress applied for a longer period of time to the aquifer, the more data and information that can be obtained about the overall performance and characteristic of that aquifer. Darrell S. Peckham, PG, of Water Quest Inc. in Odessa, Texas, designs pumping tests for his needs in the mu- nicipal and industrial water fields. The type of tests he uses is based on avail- able infrastructure. He says he prefers to have at least one monitor well for distance drawn. When asked what parameters he is trying to solve with the test, he explains: Since most are a single well test, it is the T (transmissivity) and K (hy- draulic conductivity) values. With dis- tance drawdown, Sy (specific yield) and S (storativity), as well as well efficiency. The duration of aquifer tests are al- ways a subject of debate, but Peckham explains how he determines the duration of time for his pumping test. Typically, a 24-36 hour constant rate pumping test followed by 12-hour re- covery test for a typical sand aquifer public drinking water supply well. The scale of the project, anticipated operational pumping times, and potential for boundary conditions typically dictate the testing duration. Some projects require over 30 days of constant rate pumping. There is no substitute for experience in determin- ing test duration. Allan R. Standen, a 20-year veteran hydrogeologist and consultant with Allan R. Standen LLC in Austin, Texas, conducts multi- and single-well tests. He points out there are important con- siderations when designing a test. Here are a few but not all of the possible considerations: Aquifer type (clastic vs. carbonate and unconfined vs. confined), selection of pump test wells and monitor wells, will pump test production rate mimic anticipated production rates, number and location (distance from proposed pumping well) of monitor wells, knowledge of pumping and monitor wells construc- tion, and potential of interference from other pumping activities during pump test. Standen further indicates surface recharge can occur from the discharge of water from the pumping test if the test is near unconfined carbonate sys- tems. He explains temporary storage may be necessary for the discharge water and the availability of infrastruc- ture, roads, and electricity as well as potential discharge water quality should be taken into consideration. When asked about the duration of the pumping test, Standen says he believes: Depending on the purpose of the production well and aquifer type, a minimum of 24 to 48 hours. If munic- ipal, at least 72 hours, and preferably longer to determine if there are any boundary conditions. The recovery test measures the change or rise in water level after the cessation of pumping. The measure- ments are taken on the same prescribed intervals as the constant rate test. The recovery test begins immediately after pumping stops and is continued until full recovery is reached or a predeter- mined percentage of full recovery has been achieved. The test is administered at the end of a constant rate or step drawdown test and is generally used to estimate trans- missivity and to provide an alternative and comparison to the data and estima- tions from the pumping test (Kasenow 2006). Recovery period duration varies depending on aquifer types. Willis D. Weight in his Manual of Applied Field Hydrology explains that in his general experience: Many unconsolidated aquifers take 1½ times the pumping rate time FIELD NOTES from page 41 Example of a pumping well and a curve response from a single well step drawdown test with recovery. Photo by Raymond L Straub Jr., PG 42 May 2015 WWJ waterwelljournal.com