Nitrate removal by watershed-scale hyporheic stream restoration: Modeling approach to estimate effects and patterns at the stream network scale

Abstract

Excess nutrient pollution and eutrophication are widespread, and stream restoration is increasingly implemented as a solution. Yet few studies evaluate the cumulative effects of multiple individual restoration projects on watershed-scale nutrient loading. We developed a new modeling approach linking the U. S. Army Corps of Engineers Hydrologic Engineering Center's River Analysis System (HEC-RAS) to an auxiliary R script that simulates hyporheic exchange. We used the modeling approach to simulate hyporheic enhancement by in-stream restoration features (e.g., structures, pool-riffles, gravel bars) implemented throughout a generic 4th-order gaining watershed in the eastern USA. We assumed groundwater was widely impacted by nitrate, thus the primary pollutant source was baseflow gaining. Model results indicated that hyporheic restoration throughout all streams of our 4th-order watershed would reduce nitrate loading to downstream waterbodies by ~83%. This percentage assumes removal of all nitrate that enters the hyporheic zone and is for a gravel/sand bed, so reductions would be smaller with finer sediments or incomplete removal. For example, when we reduced the hyporheic exchange rate by an order of magnitude, the maximum watershed nitrate load reduction decreased to ~25%. The relationship between the percent of watershed stream channels that have been restored and percent nitrate load reduction at the watershed outlet was nonlinear. This relationship was exponential in smaller streams (1st- and 2nd-order) due to efficient removal of all incoming nitrate, but became linear in larger streams (3rd- and 4th-order) due to “recycling” of channel flow through the hyporheic zone more than once. Yet restoration was more effective at overall nitrate load reduction in larger (e.g., 3rd-4th order) streams because the majority of nitrate enters the watershed through groundwater gaining in those larger streams. Thus, the location of restoration projects within a watershed is important in determining their effect on nitrate loads at the watershed outlet. Overall, our results indicate hyporheic restoration can significantly reduce watershed nitrate loading to downstream waterbodies, yet watersheds must be viewed as a whole to understand the potential impacts of any particular project under consideration.