Streambed Hydraulic Conductivity Structures: Enhanced Hyporheic Exchange in Model and Constructed Stream

Hyporheic exchange is an increasingly prominent topic in stream and groundwater chemistry. The hyporheic zone (HZ) plays a unique role in improving water quality, mitigating temperature fluctuations, and shielding aquifers from contaminated surface water. Whereas streams generally act as transport conduits, the HZ is a dynamic mixing environment that can provide longer residence times necessary for contaminant biodegradation. This study presents the influence of a streambed hydraulic conductivity structure on hyporheic exchange in a constructed stream. The structure, termed a Biohydrochemical Enhancement for Streamwater Treatment (BEST), consisted of a relatively high hydraulic conductivity gravel box in an otherwise coarse sand HZ. Hyporheic exchange was measured during salt tracer additions using time-lapse electrical resistivity surveys. Observations are compared to a MODFLOW and MODPATH (particle tracking) model, and the results are placed in the context of biodegradation of contaminants in recycled water and stormwater.