DOES RAPID EXCHANGE OF SURFACE AND GROUND WATER PLAY A KEY ROLE IN REDUCING NITRATE LOADING IN STREAMS?: DRAWING CONCLUSIONS FROM INTER-SITE COMPARISONS

The exchange of surface and ground water in the hyporheic zone strongly influences stream water chemistry by increasing solute residence times and exposing solutes to biogeochemically active sediments. Microbial processes and associated nutrient transformations in the hyporheic zone may be key to maintaining surface water quality. Theoretical models suggest we should find a positive relationship between rates of nutrient uptake and hyporheic exchange in streams. Paired nutrient and conservative tracer tests are commonly used to simultaneously measure hyporheic interactions and nutrient uptake potential in streams. We report results of conservative and nitrate addition tracer tests used to quantify the degree of hyporheic interaction and nitrate uptake along three reaches of Red Canyon Creek, a semi-arid stream in Wyoming. We also used results from 15 other nitrate-addition tracer tests, presented in the literature, to explore whether nitrate uptake rates are generally higher at sites with more hyporheic interaction.

Nitrate uptake along the most downstream reach of Red Canyon Creek is rapid compared to results from other studies, but other reaches within the watershed show little nitrate uptake potential. Nitrate uptake was not correlated to measures of hyporheic interaction at the Red Canyon Creek sites. Statistical analyses of 16 nitrate addition tracer tests show that almost half the variability in nitrate uptake between sites can be explained by large differences in specific discharge and hyporheic zone size. Although large differences in these variables explain inter-site differences in nitrate uptake, other unmeasured variables, such as available labile organic carbon and microbial community composition may be more important for predicting nitrate uptake at sites with similar specific discharge and hyporheic zone areas, such as the reaches along Red Canyon Creek.