SOIL MOISTURE CHARACTERISTICS AS DETERMINED FROM AQUIFER TEST DATA

Analysis of a 72-hour, constant-rate aquifer test conducted in a glacial outwash deposit on Cape Cod, Massachusetts reveals that drawdowns measured in piezometers located at various depths and distances from the pumped well are all significantly influenced by effects of drainage from the unsaturated zone, particularly in the first 24-hours of the test. This is true in spite of the fact that the deposit is coarse-grained and highly permeable. Because of the exceptional quality and quantity of data and relatively small aquifer heterogeneity, it was possible by inverse modeling to accurately estimate specific yield, specific storage, saturated thickness, vertical and horizontal hydraulic conductivity, and a set of three or more empirical parameters used to define the drainage process. The analytical model used in the analysis differs from previously published models in that it accounts for drainage from the unsaturated zone with a finite series of exponential terms each of which contains one of the estimated empirical parameters. The hydraulic properties of the saturated zone and the set of empirical parameters were estimated simultaneously with the help of a nonlinear least-squares parameter-estimation algorithm. Comparison of measured and model-simulated drawdowns shows excellent agreement for all piezometers and the pumped well over the duration of the test. Such agreement cannot be obtained under the common model assumption of instantaneous drainage from the material above the water table.

In this paper, the set of estimated empirical parameters was used to produce a theoretical, one-dimensional, drainage-versus-time curve that is presumed to be representative of the bulk material overlying the water table. With the help of a numerical model for variably saturated flow (VS2D) and a parameter-estimation algorithm (PEST), the drainage-versus-time curve was inverted to provide estimates of the Brooks-Corey and van Genuchten moisture characteristics. These and the hydraulic conductivity properties of the saturated zone, obtained by analytical modeling, seem to compare favorably with estimates of the corresponding parameters obtained by PEST and a fully three-dimensional, axisymmetric version of VS2D for the Cape Cod aquifer test.