Performance of Engineered Streambeds for Inducing Hyporheic Transient Storage and Attenuation of Resazurin.

Several U.S. programs provide financial incentives for stream restoration to improve degraded water quality. These efforts prioritize hyporheic zone (HZ) restoration to enhance contaminant attenuation, but no stream restoration or stormwater best management practice (BMP) explicitly tailors hyporheic residence times to target specific contaminants of concern. Here we present the first physical demonstration of a new BMP called Biohydrochemical Enhancements for Streamwater Treatment (BEST). BEST are subsurface modules that use hydraulic conductivity modifications to drive hyporheic exchange and control residence times, combined with reactive geomedia to increase HZ reaction rates. Experiments were conducted in 15-m long outdoor flumes: one all-sand control, the other with BEST modules. Sodium chloride (conservative tracer) and resazurin (surrogate for a reactive pollutant) injections were conducted, with observations analyzed by stream transient storage models. Results demonstrated that BEST increased the effective HZ size and resazurin transformation both by ∼50% compared to the control. Numerical simulations of extended reach lengths showed that BEST could achieve 1-log removal of resazurin in 111 m, versus 172 m in the control, and 414 m and 683 m in two numerically simulated urban streams. These results emphasize the potential of BEST as a novel HZ BMP to improve streamwater quality.