Evaluation of infiltration testing methods for design of stormwater drywell systems

Abstract

The current infiltration testing and drywell design methods do not always provide accurate estimates of the capacity of production-scale dry wells. In numerous cases, the predicted capacities appear to be considerably lower than the actual drywell capacity, resulting in more drywells than required and significantly higher construction and long-term maintenance costs. In other cases, the uncertainty in the measurements results in the predicted capacities being higher than the actual drywell capacity, resulting in fewer drywells being required. In this study, we discuss the reasons for these inaccurate predictions of drywell performance using current methods, as well as the results of recent studies to develop improved methods for predicting the capacity of stormwater infiltration drywells. These studies, funded by the United States Environmental Protection Agency (EPA) and the Los Angeles County Safe Clean Water Program (SCW), include numerical simulations and field studies in California. These test methods rely on steady-state infiltration tests in small-diameter boreholes and produce an estimate of the saturated hydraulic conductivity, which can be used to reliably predict the performance of large-diameter drywells. The methods were validated by testing small- and large-diameter test wells in close proximity using three different drilling methods. The results of this study describe the design and conduct of a borehole infiltration test that accurately represents drywell performance. Furthermore, based on a combination of numerical simulations and field studies, reduction factors were developed to account for test uncertainty, test duration, analytical error, field variability, potential for groundwater mounding, and clogging.