Combined Surface-Subsurface Stream Restoration Structures Can Optimize Hyporheic Attenuation of Stream Water Contaminants

There is a design-to-function knowledge gap regarding how engineered stream restoration structures can maximize hyporheic contaminant attenuation. Surface and subsurface structures have each been studied in isolation as techniques to restore hyporheic exchange, but surface-subsurface structures have not been investigated or optimized in an integrated manner. Here, we used a numerical model to systematically evaluate key design variables for combined surface (i.e., weir height and length) and subsurface (i.e., upstream and downstream baffle plate spacing) structures. We also compared performance metrics that place differing emphasis on hyporheic flux versus transit times. We found that surface structures tended to create higher flux, shorter transit time flowpaths, whereas subsurface structures promoted moderate-flux, longer transit time flowpaths. Optimal combined surface-subsurface structures could increase fluxes and transit times simultaneously, thus providing conditions for contaminant attenuation that were many times more effective than surface or subsurface structures alone. All performance metrics were improved by the presence of an upstream plate and the absence of a downstream plate. Increasing the weir length tended to improve all metrics, whereas the optimal weir height varied based on metrics. These findings may improve stream restoration by better aligning specific restoration goals with appropriate performance metrics and hyporheic structure designs.

Yearlong Impact of Buried Organic Carbon on Nitrate Retention in Stream Sediments

Carbon supply influences nitrogen transformation in ecosystems, but the longer-term effects of buried organic carbon on nitrogen processing in stream sediments have been rarely addressed. The effects of buried particulate organic carbon (red maple leaves) on net nitrogen retention, net dissolved organic carbon (DOC) production, and pore water dissolved oxygen concentration were assessed for 1 yr in a nitrogen-rich gaining stream (Emmons Creek, WI). Retention of nitrate and total dissolved nitrogen (TDN) and production of DOC were measured by comparing groundwater fluxes of nutrients at shallow and deeper depths in mesocosms inserted in the sediments. Buried leaves caused large increases in nitrate and TDN retention and DOC production relative to the control (combusted sand) that decreased in magnitude throughout the year. Nitrate and TDN retention occurred throughout the year in unmanipulated (ambient) sediments. Dissolved oxygen approached anoxia in most mesocosms containing buried leaves and ambient sediments, particularly early in the experiment. Collectively, these results indicate that buried leaves had persistent, diminishing effects on nitrate and TDN retention throughout a 1-yr period. The TDN retention in the ambient sediments throughout the year suggests that the deep sediments in Emmons Creek are a nitrogen sink.

Low transient storage and uptake efficiencies in seven agricultural streams: implications for nutrient demand

We used mass load budgets, transient storage modeling, and nutrient spiraling metrics to characterize nitrate (NO), ammonium (NH), and inorganic phosphorus (SRP) demand in seven agricultural streams across the United States and to identify in-stream services that may control these conditions. Retention of one or all nutrients was observed in all but one stream, but demand for all nutrients was low relative to the mass in transport. Transient storage metrics (/, , , and ) correlated with NO retention but not NH or SRP retention, suggesting in-stream services associated with transient storage and stream water residence time could influence reach-scale NO demand. However, because the fraction of median reach-scale travel time due to transient storage () was ≤1.2% across the sites, only a relatively small demand for NO could be generated by transient storage. In contrast, net uptake of nutrients from the water column calculated from nutrient spiraling metrics were not significant at any site because uptake lengths calculated from background nutrient concentrations were statistically insignificant and therefore much longer than the study reaches. These results suggest that low transient storage coupled with high surface water NO inputs have resulted in uptake efficiencies that are not sufficient to offset groundwater inputs of N. Nutrient retention has been linked to physical and hydrogeologic elements that drive flow through transient storage areas where residence time and biotic contact are maximized; however, our findings indicate that similar mechanisms are unable to generate a significant nutrient demand in these streams relative to the loads.