A unified Perc Test-Well Permeameter methodology for absorption field investigations

Absorption fields are soil-based onsite installations designed to infiltrate, drain, filter or biochemically renovate water. A critical component in absorption field development is determination of the soil's ability to transmit water. Two of the most important in-situ methodologies for measuring water transmission in absorption fields are the Perc Test, which determines Perc Time (PT) in an open borehole or pit, and the Constant Head Well Permeameter (CHWP), which determines field-saturated hydraulic conductivity (Kfs) in an open borehole. Although both methods have important features, they also have deficiencies that can impair their use in absorption field investigations. Features of the Perc Test include simplicity, intuitive measurements (decline in water ponding depth, H, with time, t), and inclusion of PT criteria in most absorption field design specifications and regulatory codes. Critical deficiencies of the Perc Test include its empirical nature (absence of theoretical foundation), lack of standardization, and strong dependence of PT on borehole/pit dimensions, water ponding depth and antecedent soil properties. Features of the CHWP include a solid theoretical foundation, formal accounting for borehole dimensions, water ponding depth and soil properties, and recognition that Kfs controls long-term water acceptance rates. A critical deficiency of the CHWP is the absence of Kfs criteria from many absorption field specifications and codes. This study develops a unified Perc-CHWP analysis and application protocol that maintains the features of the Perc and CHWP tests, but circumvents the deficiencies. The unified analysis combined numerical optimization with piece-wise analytical integration of a falling-head CHWP equation; and it yielded accurate estimates of PT and Kfs using H vs. t data obtained from numerical simulations, laboratory experiments, and field-based Perc Tests. The unified Perc-CHWP methodology improves measurement of water transmission in absorption field investigations by providing: physically based PT values required by many design specifications and regulatory codes; Kfs values required for estimation of long-term water acceptance rates; and functionally linked PT-Kfs data pairs that are accurate and based on rigorous borehole infiltration theory.