Hyporheic zone chemistry and stream-subsurface exchange in two groundwater-fed streams

Data from two headwater streams in southern Ontario provide support for conceptual models that suggest that hyporheic zone influence on stream nutrient retention is determined by the extent of surface-groundwater exchange and subsurface chemical transformation rates. The hyporheic zone (>10% stream water) was delineated by a chemical mixing equation using differences in background stream and groundwater chloride concentrations and by injections of chloride to stream flow. Good agreement between the two methods confirmed that the extent of stream-groundwater exchanges can be successfully estimated using background conservative ions as a tracer technique. During low stream flows in May-October the depth of the hyporheic zone was 2-15 cm in a 12-m sand-bottom pool, debris dam, pool reach of Glen Major stream and 15-20 cm in a 16-m gravel riffle reach of Duffin Creek. Differences between observed NO3 concentrations and concentrations predicted from background chloride indicated depletion of NO3 in the hyporheic zone at a few locations in Glen Major and at 5-10 cm depth throughout the Duffin Creek reach. NO3 and NH4 injected into stream water were reactive at only a few hyporheic sites in the streams. Upstream-downstream comparisons during injections indicated that stream retention was minor.