Subsurface drainage flow fluctuations impact denitrifying bioreactor hydraulic conductivity

Anecdotal evidence suggests the saturated hydraulic conductivity (Ksat) of woodchip media used in denitrifying bioreactors may change over time due to growth of microbial biofilms, woodchip decomposition, siltation, or other factors. The objective of this study was to analyze the in-situ Ksat of two full-size bioreactors in Illinois to investigate the influence fluctuating flow regimes have on this important physical property of the bioreactor matrix. It was hypothesized that biofilms are washed out from major pore spaces during high flow events, resulting in an increase in Ksat. Daily average- and higher frequency 15-min flow data and inlet and outlet water level data for two United States Department of Agriculture-Natural Resources Conservation Service-designed bioreactors were used to determine specific discharge and hydraulic gradient. Saturated hydraulic conductivity was calculated from those parameters and the bioreactor dimensions using Darcy's Law through porous media. Time series analysis of in-situ Ksat and flow showed Ksat was not constant throughout the monitoring period. Several high flow events during typically low summer flows appeared to result in a related increase in Ksat (e.g., from 1.0 cm/s to 10 cm/s). These early data may be evidence toward confirming the hypothesis that flushing of biofilms occurs during high flow events but deeper analysis is recommended.