EP 1110-1-27
27 Jan 00
same rate, yielding the well efficiency at that rate. As a constant rate test approaches steady-state, the final
specific capacity at the discharge rate can be calculated. Neither the constant-rate or the slug test can
provide the means for predicting the well loss and the well efficiency that occurs over a range of
discharge rates. A step-drawdown test is needed.
(b) The constant rate test is conducted similarly to the step-drawdown test. As with the step-
drawdown test, accurate discharge, water level, and time measurements are essential. Again, Kruseman
and de Ridder (1994) provide an in-depth discussion of conducting and analyzing these tests. Computer
applications are available to aid in the analysis of constant rate tests. Boulding (1995) provides a useful
conceptual review of pumping test software which can be updated by research into current products.
(2) Slug tests. A slug test is also used to determine aquifer characteristics, not well performance,
and involves a different procedure and methods of analysis.
(a) Descriptions of procedures and methods of analysis are provided in Kruseman and de Ridder
(1994), Bouwer and Rice (1976), Hvorslev (1951), and ASTM D 4044, D 4050, and D 4104. The
computer applications available to aid in the analysis of constant rate tests, such as AQUITEST (Walton
1996), also provide analysis of slug tests. Because of the small volumes of water involved and the short
(or long) time span over which the test occurs, pressure transducers and digital data logging are generally
employed. Pressure tranducers are submerged in the well and register the pressure of the column of water
overlying them. Water-level changes are detected as changes in pressure as the height of the overlying
water column either increases or decreases. The data logger can be programmed to sample and record
data from the transducer at required time intervals. This feature of digital data logging is most useful
when conducting slug tests in high-permeability sediments where many water level measurements will be
required over a span of seconds as the water level rapidly recovers.
(b) As with constant rate pumping test data, calculations of aquifer characteristics based on slug
test data can be used for estimation of theoretical well mounding in injection wells.
2-3. Specific Capacity Data
a. Definitions. Specific capacity is a term used to express the productivity of a well, and is
defined as Q/s, where Q is the discharge rate and s is the drawdown in the well (Driscoll 1986). The
observed drawdown in the well is a function of aquifer and well loss; therefore, Q/s is a term
incorporating both aquifer and well performance. Step-drawdown tests described in Section 2-2 provide a
means of separating the aquifer and well loss components.
b. Use of Q/s calculations. Q/s calculations, using water-level change and well pumping data, are
used to assess pumping well performance and results of development and redevelopment (Helweg, Scott,
and Scalmanini 1983; Driscoll 1986; Borch, Smith, and Noble 1993). The data that need to be collected
(Q and s in pumping wells) are simple to obtain and the calculations simple to make. Specific capacity
and specific acceptance are relatively sensitive indicators of hydraulic performance change in wells.
Making valid calculations in turn depends on reliable data collection. Appropriate actions in response to
changing values depend on setting action levels that permit a response before performance is seriously
impaired.
c. Minimum data needed and standards for data gathering, reporting, and assessment. To
determine Q/s for a well, accurate static water-level, pumping water-level, and discharge rate data are
needed. Since the water table or potentiometric surface varies seasonally and with outside stresses, a
deeper pumping water level for a given discharge rate may not reflect a change in the well performance.
Therefore, some means will be required to determine the variation in the static water level, e.g., an
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