CHARACTERIZING THE EFFECTS OF MACROPORES ON HYPORHEIC ZONE HYDRAULICS IN MEANDER BENDS

The hyporheic zone is the area beneath and adjacent to stream beds where the mixing of surface water and shallow groundwater occurs within sediment. Surface water tends to be high in dissolved oxygen with large diel temperature fluctuations, while groundwater tends to be low in dissolved oxygen and high in inorganic solutes with relatively constant diel temperatures. The mixing of these two water bodies can create unique conditions that attenuate pollutants, cycle nutrients, and buffer surface water temperatures. These functions of the hyporheic zone are important to stream ecosystems, aquatic organisms, and overall water quality. Evidence suggests that flow through connected voids (macropores) may play an important role in water and solute exchange and potentially affect hyporheic zone functions. However, no studies have been undertaken to address this, presumably because macropores are difficult to locate and characterize. Here we present data collected across a well-defined meander bend to quantify the effect of a 2cm-diameter constructed macropore on meander bend hydrology and solute transport. Solute tracer tests showed 28% faster travel times and 25% greater peak tracer levels down-gradient of the macropore compared to the partially plugged macropore. Electrical Resistivity Imaging (ERI) was utilized to track the tracer flowpaths through the meander bend over time. Data generated from ERI indicated the tracer was preferentially transported when the macropore was present and was more dispersed after we partially plugged the macropore. Falling-head tests across multiple depths and locations verified these results with hydraulic conductivities near the constructed macropore 18x higher than the average. These results provide strong evidence that macropores act as preferential flowpaths and may therefore have a profound impact on hyporheic zone function in meander bends and in turn, stream health and water quality.