Importance of the Vadose Zone in Analyses of Unconfined Aquifer Tests

Abstract

Analytical models commonly used to interpret unconfined aquifer tests have been based on upper-boundary (water table) conditions that do not adequately address effects of time-varying drainage from the vadose zone. As a result, measured and simulated drawdown data may not agree and hydraulic parameters may be inaccurately estimated. A 72-hour aquifer test conducted in Cape Cod, Massachusetts, in a slightly heterogeneous, coarse-grained, glacial outwash deposit was found to be a good candidate for testing models with different upper-boundary conditions. In general, under the commonly invoked assumption of instantaneous drainage, measured and simulated drawdowns were found to agree with one another only at late time and early time. In the intermediate-time range, because of delayed drainage, measured drawdowns always exceeded simulated values, most noticeably in piezometers located near the water table. To reduce these discrepancies, an analytical model was developed that can fully account for time-varying drainage given that the aquifer is not strongly heterogeneous. The approach is flexible as the model, which makes use of empirical relations, does not constrain drainage to follow any particular functional relation. By this approach, measured and simulated drawdowns agree over the complete time range, and the estimated parameters are consistent with prior studies and with what is known about the aquifer geometry, stratigraphy, and composition. By properly accounting for vadose zone drainage, it was found that realistic estimates of all hydraulic parameters, including specific yield, could be obtained with or without the use of late-time data.