Hyporheic nitrate processing in two western New York gravel-bed streams
Anseeuw, Sierra K.
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The hyporheic zone has been identified as an important component of stream ecosystems for its role in groundwater-surface water exchange and nutrient transformation. Two third-order stream reaches in western New York were observed to characterize and quantify nitrate transformation within the hyporheic zone of gravel streams. From May - September 2013 specific conductance, dissolved oxygen, nitrate, and sulfate were monitored at up to five depths within the subsurface of both streams as well as in the surface water and groundwater at both sites. Additionally, stream stage and discharge were monitored to compare variability in stream chemistry to variability in stream flow. Results of this study highlight the complex nature of stream ecosystems and suggest the hyporheic zone may not be as important for nutrient processing in homogenous gravel streams as it is in streams with greater heterogeneity in the subsurface. Sampling locations at Elton Creek are separated into three categories; Group 1 sites exhibit constant hyporheic thickness over the field season, Group 2 sites exhibit hyporheic expansion over the field season, and Group 3 sites have hyporheic zones that extend beyond instrumentation. Elton Creek has decreasing nitrate concentrations over the field season in the surface water (r 2 =0.54, p=0.0) as well as in subsurficial samples at sites in each of the three categories. In surface samples as well as subsurface samples at Group 1 and 3 sites these declines in nitrate coincide with seasonally stable sulfate concentrations, suggesting the causal mechanism is more likely nitrate processing (i.e. denitrification or assimilation) than dilution. Decreasing nitrate concentrations observed at Group 2 sites occur at a similar rate as a decrease in sulfate concentration, suggesting dilution as the causal mechanism. All sampling locations within Elton Creek maintain dissolved oxygen concentrations greater than 20% until September, which likely would not support denitrification. Results at Elton Creek suggest any denitrification that may be occurring at Elton Creek is occurring within the groundwater rather than the hyporheic zone and that short-term expansion of the hyporheic zone may interfere with nitrate processing at this site. In contrast, Cattaraugus Creek exhibits constant hyporheic thickness at all sampling locations due to a clay layer 0.5 - 1 m thick within the shallow subsurface, which limits the extent of the hyporheic zone. Similar to Elton Creek, nitrate concentrations at Cattaraugus Creek decrease in surface water (r 2 =0.64, p=0.03) as well as in subsurface samples at multiple sites. It is unclear whether this decrease in nitrate concentrations is due to dilution by low-nitrate groundwater or denitrification. However, increasing sulfate concentrations within the subsurface suggests the oxidation of sulfur, potentially due to denitrification. Low nitrate concentrations in groundwater monitoring wells and low dissolved oxygen within the stream piezometers at the site suggest that nitrate processing may be occurring within the hyporheic zone. Results at Cattaraugus Creek provide support for the importance of the hyporheic zone for nutrient processing in stream ecosystems.