CHARACTERIZING GROUNDWATER DISCHARGE TO A STREAM IN SUPPORT OF NUTRIENT LOADING ASSESSMENT: A COMPARISON OF METHODS
Various techniques are available to quantify GWSWI, each with its own limitations in precision, and scope. New, innovative tools for GWSW studies are needed to overcome the above limitations. This study utilizes a relatively new tool, the Streambed Point Velocity Probe (SBPVP), to conduct a detailed survey of groundwater flow into Alder Creek (Ontario, Canada). The SBPVP is deployed into the top ~10 cm of a stream or lakebed where a mini tracer test is conducted, from which seepage velocity values near the GWSW interface can be quantified. This kind of information is important as it provides a direct measurement of the exchanging flow, which, when coupled with concentrations of contaminants of both the GW and SW, permits distribution profile of pollutants transport at the interface.
Two small surveys were conducted: Site 1 featured a meander with 1-15 cm diameter cobbles covering most of the streambed; Site 2 was a straight channel with a less dense accumulation of cobbles. The SBPVP provided a direct way of measuring seepage velocities between the cobbles, distinguishing it from other methods. Mini piezometer measurements and temperature profiling were also conducted to determine water flux values. The piezometer data required prior estimates of hydraulic conductivity (K) to derive fluxes, while the temperature profiles yielded flux estimates directly, from which K values in the upward direction could be estimated. Comparison of the temperature flux velocities to the SBPVP fluxes and piezometer velocities yielded results with orders of magnitude differences. These differences are hypothesized to be explained by the density of cobbles on the streambed, the variation in recharge/discharge along the stream reach, seasonal flux differences, and horizontal hyporheic flow in the streambed and streambed anisotropy.