Aquifer pumping tests

Well spacing is determined based on aquifer pumping tests, which provide data on aquifer permeability and the radius of influence per well at various pumping rates. Wells are placed to provide overlapping cones of influence within the con-... 

A limited number of aquifer pumping tests have been conducted within the perched zone. Two separate tests were conducted on a recovery well located in the southwestern portion of the refinery. Low transmissivity values of 100 and 150 gpd/ft were calculated. 

This material will give the first picture of the aquifer and will be the basis for complementary site investigations. These may include test drillings, geophysical logs and pumping tests with water chemistry in order to fully understand the aquifer conditions. 

From a pumping test the permeability is evaluated as Transmissivity (m2/s). This parameter express the added permeability s meter by meter. By dividing the transmissivity with the thickness of the aquifer an average permeability is given. 

Figure 37. A pumping test is used to evaluate the hydraulic properties of the aquifer...

Based on the results a conceptual model is created and the hydraulic properties of the aquifer and its surrounding layers are derived from the pumping test. 

Hantush, M. S., 1966, Analysis of Data from Pumping Tests in Anisotropic Aquifers Journal of Geophysical Research, pp. 421 126. 

A pumping test may be performed to evaluate hydrogeologic characteristics of the aquifer. Derived data can be applied to computer models or at least included in the conceptual site model. 

On-site aquifer testing has also been performed in several phases. Typically, pumping tests are performed on newly installed injection and recovery wells to collect data used to determine optimum pumping or maximum injection rates. Monitoring wells are also tested to determine the location or feasibility of injection and recovery wells. 

One of the approaches in evaluating the permeability of an aquifer system is to perform an aquifer test by pumping a well and observing the water-level decline with time in nearby observation wells. In systems of low permeability, sufficient water for a pumping test is generally prima facie evidence that it is unacceptable as a disposal site. Consequently, a new set of "parlor tricks" is needed to define the permeability of those types of media. 

As stated in section 2.10, the velocity by which groundwater flows is commonly calculated from the water table gradient and the coefficient of permeability (k, or the related parameter of transmissivity). The k value is determined by a pumping test. During such a test a studied well is intensively pumped and the water table is monitored in it as well as in available adjacent observation wells. The change in water table level as a function of the pumping rate serves to compute the aquifer permeability. 

The common interpretation of pumping test data is based on the assumption that only one aquifer is pumped and tested. However, the intensive pumping during the test causes a significant local pressure drop in the pumped aquifer that may cause water from an adjacent aquifer to breach in (Fig. 4.14). If the pumping test is done in a phreatic aquifer, water of a lower confined aquifer may flow in. Similarly, in pumping tests in confined aquifers, an overlying phreatic aquifer may be drawn in. 

The breaching in of water from a second aquifer has to be detected so it can be included in the pumping test interpretation. Otherwise, the conclusions based on the test will be erroneous apparent conductivities that are too large may be deduced, and exaggerated operation pumping rates may be suggested. 

To obtain the necessary information on the number of aquifers effectively included in a pumping test, continuous measurements of temperature, conductivity, and other parameters are recommended before, during, and after the test. Temperature is sensitive to aquifer depth (section 4.7) and is most useful in distinguishing different water systems (section 4.8). A constant temperature value, as shown in Fig. 4.15a, is a favorable indication that the pumping test remained restricted to a single aquifer. In contrast, in the example given in Fig. 4.15b, warmer water intruded into the pumped aquifer, and the latter part of the pumping test included water from two aquifers. 

Fig. 4.14 Possible effect of a pumping test. The water table of aquifer I is drawn down near the well, a feature called a depression cone. Occasionally water from a lower confined aquifer may breach in (arrows across the aquiclude).

Fig. 4.15 Temperature measurements during pumping tests, expressed as a function of time and cumulative discharge (a) temperature remained constant, indicating pumping remained constricted to a single aquifer (b) temperature suddenly increased, indicating water from a warmer (probably lower) aquifer...

Fig. 4.17 Tritium and 14C measurements during a pumping test conducted in a confined aquifer at the Aravaipa Valley, Arizona (Adar, 1984). Recent water of high tritium and 14C concentrations intruded from the overlying phreatic aquifer.

The importance of a thorough follow-up of pumping tests has been addressed in section 4.6, and we will get here a further look into the needed action. Pumping tests are an intensive intervention in the local water regime, as the intense pumping reduces water pressures locally. As a result, water from adjacent aquifers, or water compartments, may encroach. Thus every pumping test should be accompanied by continuous measurement of temperature and electrical conductivity, and samples should be collected along the test for laboratory analyses. Basically, two types of outcome are experienced ... 

Answer 4.9 Provided two aquifers (e.g., one below the other) have the same temperature, breaching during the pumping test would not be noticed on the temperature discharge curve. So what may be recommended ... 

Two monitoring wells are located 150 m apart in an unconfined sandy aquifer. The second well is directly downgradient of the first well. The hydraulic head in the first well is 20 m and in the second well is 19 m the hydraulic conductivity is estimated using a pump test as 10 3 cm/sec. What is the specific discharge in the aquifer Given that the porosity is 0.2, what is the rate at which nonsorbing chemicals dissolved in the groundwater will move between the wells ... 

S. M. Lang, Pumping Test Methods for Determining Aquifer Characteristics, Paper presented at the Sixty-Ninth Annual Meeting of the ASTM, Atlantic City. 

No. 18 Delete any methods not compatible with local conditions. No. 19 Pertinent field tests include drop tests, pumping tests and piezometer installations. Methods of performing such tests may be found in Theory of Aquifer Tests, U.S. Geological survey Water-Supply Paper 1536-E, 1962, and the U.S. Bureau of Reclamation s Earth Manual (Appendix pages 541 to 562) and other technical publications. If desired, specific test methods may be specified in this paragraph. 

Determination of the permeability in the field can be done by measuring the time it takes for a tracer to move between two test holes. As with pumping tests, this tracer technique is based on the assumption that the aquifer is homogeneous and that observations taken radially at the same distance from the well are comparable. This method of assessing permeability requires that injection and observation wells be close together (to avoid excessive travel time) and that the direction of flow be known so that observation holes are correctly sited. 

When dimensionless inertia coefficient P equals tolO the test result of the air pressure slug test corresponds really nicely with the result we get in the pumping test. Therefore, during the calculation process in the test, the value of P may not be 10", we should select the appropriate dimensionless inertia coefficient according to the status of fracture aquifer systems. 

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