REDOX-SENSITIVE SOLUTE VARIABILITY ALONG AN URBAN STREAM HYPORHEIC RIFFLE-POOL UNIT

Stream hyporheic zones represent critical terrestrial-aquatic interfaces, serving as mixing zones of surface and groundwaters in contact with streambed sediment. Generally, pressure gradients force stream water to recharge bed sediment at the head of riffles, with subsequent upwelling of shallow groundwater in pools. This creates a redox gradient between the well-oxygenated stream water and deeper sediment in the riffle, and often results in pool pore-water anoxia. Thus, the geochemical environment of the sediment pore-water may result in dissolved solute concentrations varying by over an order of magnitude in less than 50 m of stream reach, potentially impairing stream water quality with increased phosphate and heavy metal concentrations. This may especially occur along urban streams, where inputs of these contaminants are much higher than natural environments. Here, a total of 48 mini-piezometers were installed along a riffle-pool sequence of a third-order stream in a watershed that was 85 % urbanized. Hydraulic measurements indicated sustained downwelling in the riffle and moderate upwelling in the pool. The magnitude of vertical hydraulic gradient throughout both the riffle and the pool, however, varied by a factor of two on most sampling days, indicating varying water flux through the sequence. This resulted in heterogeneous dissolved solute concentrations through the riffle and pool. In some pool locations, soluble reactive phosphate was an order of magnitude higher at 36 cm depth than in the stream; however, in other pool locations SRP levels were similar to stream levels, even at depth. The coefficient of variations in pore-water concentrations of the redox-sensitive Mn (69 % pool pore-water, 272 % riffle) and Fe (191 % pool, 73 % riffle) were generally higher than other metals (ranging from 175 % for Al to 25 % for Ni) along the 50 m stream reach. In contrast, homogeneous profiles were found for the conservative Li, (coefficient of variation of < 10 %). These results demonstrate the complex interaction of cm-scale hydrological flowpaths, redox chemistry, and mineral & biological reactivity, which make generalization and prediction of geochemcial function in stream hyporheic zones very difficult.