Specific Yield as Determined by Type‐Curve Analysis of Aquifer‐Test Data

Type-curve analysis of water-table aquifer pumping-test data has often resulted in values of specific yield that are unrealistically low when compared with values obtained by volume-balance calculations. It has been suggested that such values are the result of inadequate representation of drainage processes in the unsaturated zone. The commonly used analytical solution developed by Neuman for a homogeneous, water-table aquifer assumes that the drainage of pores in the zone above the water table due to lowering of the water table occurs instantaneously. Published pumping-test data from three relatively homogeneous, unconsolidated, granular aquifers with significantly different hydraulic conductivities are used to show that type-curve analysis based on the Neuman model will result in estimates of specific yield that agree with volume-balance calculations, provided that established procedures are followed. These involve the use of composite plots of drawdown observed at more than one location and inclusion of effects of partial penetration. Noninstantaneous drainage of pores in the unsaturated zone accounts for the finding that both type-curve analysis and volume-balance calculations yield values of specific yield that are slightly less than those obtained from laboratory column-drainage experiments. It may also account for a slight underestimation of vertical hydraulic conductivity as obtained by type-curve analysis.

[1]  R. Gillham,et al.  Unsaturated and Saturated Flow in Response to Pumping of an Unconfined Aquifer: Numerical Investigation of Delayed Drainage , 1992 .

[2]  G. Dagan,et al.  A Method of Determining the Permeability and Effective Porosity of Unconfined Anisotropie Aquifers , 1967 .

[3]  T. Narasimhan,et al.  Transient flow of water to a well in an unconfined aquifer: Applicability of some conceptual models , 1993 .

[4]  S. P. Neuman.  Analysis of Pumping Test Data From Anisotropic Unconfined Aquifers Considering Delayed Gravity Response , 1975 .

[5]  R. Stallman.  Aquifer-test design, observation and data analysis , 1971 .

[6]  S. P. Neuman,et al.  On Methods of Determining Specific Yield , 1987 .

[7]  Michael A. Celia,et al.  Large-scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 3. Hydraulic c , 1992 .

[8]  S. P. Neuman,et al.  Effect of partial penetration on flow in unconfined aquifers considering delayed gravity response , 1974 .

[9]  K. Howard,et al.  The Unreliability of Open Observation Boreholes in Unconfined Aquifer Pumping Tests , 1982 .

[10]  Robert W. Gillham,et al.  Unsaturated and Saturated Flow in Response to Pumping of an Unconfined Aquifer: Field Evidence of Delayed Drainage , 1992 .

[12]  Leland K. Wenzel,et al.  The thiem method for determining permeability of water-bearing materials and its application to the determination of specific yield; results of investigations in the Platte river valley, Nebraska , 1935 .

[13]  R. Gillham,et al.  A Comparative Study of Specific Yield Determinations for a Shallow Sand Aquifer , 1984 .

[14]  A. Moench,et al.  Computation of Type Curves for Flow to Partially Penetrating Wells in Water‐Table Aquifers , 1993 .

[15]  D. A. Morris,et al.  Specific yield - laboratory experiments showing the effect of time on column drainage , 1964 .

[16]  S. P. Neuman.  Theory of flow in unconfined aquifers considering delayed response of the water table , 1972 .

[17]  C. V. Theis.  The relation between the lowering of the Piezometric surface and the rate and duration of discharge of a well using ground‐water storage , 1935 .

[18]  T. A. Prickett.  Type‐Curve Solution to Aquifer Tests under Water‐Table Conditions , 1965 .

[19]  S. P. Neuman,et al.  Perspective on ‘Delayed yield’ , 1979 .

[20]  N S Boulton,et al.  ANALYSIS OF DATA FROM NON-EQUILIBRIUM PUMPING TESTS ALLOWING FOR DELAYED YIELD FROM STORAGE. , 1963 .