MULTI-WELL PUMPING TESTS OF A SMALL, 65-ACRE, OFF-STREAM GROUNDWATER BASIN IN THE LOWER CARMEL RIVER VALLEY IN CALIFORNIA DEMONSTRATING A LINEAR RATHER THAN A LOGARITHMIC TIME-DRAWDOWN RESPONSE

Three basic interpretations of pumping water level (PWL) responses to constant discharge pumping tests are given in the literature (Kruseman and de Ridder,1991) and are based on hydraulics defined by Darcy’s Law (1856), Dupuit (1863), Thiem (1906), and Theis (1935) with important observations, simplifications, and modifications by others (e.g., Bredehoeft, Boulton, Cooper, Hantush, Jacob, Moench, Neuman, Papadopulos, etc.). The three interpretations are: (1) constant drawdown slope corresponding to the response of an infinite laterally extensive aquifer; (2) recharge boundary where the cone of depression encounters an unlimited source of recharge such as surface water or a more permeable portion of the aquifer and the PWL stabilizes; and (3) discharge barrier boundary where the cone encounters an impermeable boundary such as the sides of an incised alluvial filled valley. These three basic responses can occur during a single test; e.g., delayed yield response will have three segments: an initial constant drawdown, transitional recharge boundary from delayed yield, and the third leg of the drawdown curve that resumes the initial drawdown slope. Complicating these interpretations are deviations from the ideal aquifer response: e.g., casing storage can mask initial PWL responses.

However, a fourth important interpretation, particularly in low-yield aquifers and isolated aquifers, involves the linear response of a pumping test rather than a logarithmic one. A linear drawdown response represents repeated barrier boundary encounters of the cone of depression indicating that the aquifer is of limited areal extent. Time-drawdown data plotted on semi-logarithmic graph paper results in ever-steepening curvilinear line representing an infinite number of straight lines. Re-plotting data on arithmetic paper yields a straight-line. The response is similar to conducting a pumping test in a large cylindrical tank: the PWL or drawdown is linear. Data from multiple pumping tests not only show consistent results but the pointlessness of additional pumping tests. Interpretations are supported with recovery data from the pumping test showing groundwater mining. This important observation and methodology can also be used to identify whether wells installed near mine workings tap country bedrock or mine workings.