POLLUTANT TRANSFORMATIONS AND RESIDENCE TIMES IN ENGINEERED HYPOHREIC ZONES OF URBAN WATERWAYS

Several new US programs provide financial incentives for stream restoration to improve degraded urban water quality. These efforts prioritize hyporheic zone (HZ) reconnection to enhance contaminant attenuation, but no current 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 utilize hydraulic conductivity modifications to drive hyporheic exchange and control residence times, combined with reactive geomedia to increase HZ reaction rates. Experiments were conducted in outdoor stream flumes (one all-sand control condition, the other featured BEST modules). Sodium chloride (conservative) and resazurin (surrogate for a reactive pollutant) injections were conducted, with observations analyzed by stream-groundwater transient storage exchange models. Results demonstrated that BEST increased the effective size of the hyporheic transient storage zone, and resazurin transformation, by ~50% compared to the control. Simulations of extended reach lengths up to 300m showed that BEST could achieve 1-log removal of resazurin in 125m, versus 190m in the control, and X and Y meters in two simulated urban streams. Additional experiments evaluated the ability of the BEST system to remove urban pollutants nitrogen and atrazine. The BEST was effective at removing atrazine and ammonium compared to the control, but improvements in nitrate removal over the control were minimal because geochemical conditions were not conducive to denitrification. These results emphasize the potential engineered hyporheic zones to improve urban stream water quality.