DEFINING SPATIAL AND TEMPORAL VARIATIONS IN THE HYPORHEIC ZONE OF A LOW GRADIENT, MEANDERING AGRICULTURAL STREAM USING CONSERVATIVE TRACERS

The objective of this research is to define the extent and variability of the hyporheic zone along a 25 m reach of a low gradient, meandering stream using conservative geochemical parameters. This research is a part of a long term effort to understand nutrient cycling in agricultural streams, and complements a thermal modeling study underway at the same site. The site is instrumented with a 3D array of eight multilevel samplers, with each sampler having ports at depths of 0.3, 0.6, 0.9, and 1.5 m below the stream bed. Six samplers were installed 5 m apart along the stream thalweg, and two samplers were installed perpendicular to flow 1.5 m on either side of the third thalweg sampler. A groundwater well was also installed 1 m from the stream bank at the center of the array. Water from the hyporheic samplers, well, and surface water were periodically sampled and analyzed for major anions. The conservative anion chloride was used to calculate the thickness of the hyporheic zone using a two end member mixing model. One end member was the stream where the chloride concentration is usually >50 mg/L because of upstream road salting and the other end member was groundwater which has chloride concentrations <15 mg/L. For each sampling round, the measured stream and well chloride concentrations were used to calculate the percent surface water in each sampler. However, the model often produced negative percent surface water values, indicating the system is more complex than can be represented by a simple two end member model. We are currently installing additional wells and are preparing to conduct a bromide tracer test to better understand the temporal and spatial variations in groundwater chloride concentrations as well as subsurface flow paths and residence times.