Beaver dam analogues (BDAs) are a cost-effective stream restoration approach that leverages the recognized environmental benefits of natural beaver dams on channel stability and local hydrology. Although natural beaver dams are known to exert considerable influence on the hydrologic conditions of a stream system by mediating geomorphic processes, nutrient cycling, and groundwater-surface water interactions, the impacts of beaver-derived restoration methods on groundwater-surface water exchange are poorly characterized. To address this deficit, we monitored hyporheic exchange fluxes and streambed porewater biogeochemistry across a sequence of BDAs installed along a central Wyoming stream during the summer of 2019. Streambed fluxes were quantified by heat tracing methods and vertical hydraulic gradients. Biogeochemical activity was evaluated using major ion porewater chemistry and principal component analysis. Vertical fluxes of approximately 1.0 m/day were observed around the BDAs, as was the development of spatially heterogeneous zones of nitrate production, groundwater upwelling, and anaerobic reduction. Strong contrasts in hyporheic zone processes were observed across BDAs of differing sizes. This suggests that structures may function with size-dependent behaviour, only altering groundwater-surface water interactions after a threshold hydraulic step height is exceeded. Patterns of hyporheic exchange and biogeochemical cycling around the studied BDAs resemble those around natural beaver dams, suggesting that BDAs may provide comparable benefits to channel complexity and near-stream function over a 1-year period.

Residential development and urbanization have increased nutrient loads to streams and groundwater through increased use of fertilizers and discharge of wastewater effluent. Stream degradation in urbanizing areas has simultaneously reduced natural attenuation of nutrients. In this context, cemeteries are an often-overlooked land use that may contribute to nutrient loading in urbanizing watersheds. Although cemeteries provide ecosystem services, such as infiltration of stormwater, micrometeorology control, and greenspace, they also pose a unique threat to groundwater quality due to degradation and leaching of organic material. To assess the potential legacy impact of cemeteries on water quality, we explored the impact of a large cemetery that comprises 9% of the total area of a suburban watershed on groundwater nitrate concentrations and stream nitrate loads. We found nitrate concentrations were significantly higher in cemetery groundwater (median = 6.2 mg l−1) than in residential groundwater (median = 0.05 mg l−1). During summer months (June through September), the stream is consistently a gaining stream receiving groundwater discharge. During this time, stream nitrate concentrations increase by 1.4–1.9 mg l−1 between the upstream edge of the cemetery and the downstream edge (from 0.03–0.46 mg l−1 to 1.6 mg l−1–2.1 mg l−1, respectively). Stream nitrate loads observed at gauging stations located about 500 m upstream and downstream of the cemetery property show that the stream nitrate load is consistently 20–40 kg NO3 −/day higher downstream of the cemetery between June to September. Given that the cemetery handles about 350–500 burials per year, it is estimated that 25%–50% of the nitrate load between the gauging stations could be attributable to groundwater discharge of burial decay products. Our observations of nitrate concentrations in cemetery groundwater, coupled with the increases in nitrate loads in a stream traversing the cemetery property, suggest cemeteries may be an overlooked source of nutrient loading in developed watersheds.

Sodium chloride has long been used for winter deicing, although its legacy use has resulted in rising chloride concentrations in urban watersheds. Persistently high chloride levels impair drinking water resources and threaten the health of aquatic life and vegetation. In urban areas, chloride fate and transport is impacted by human modification of the environment, including increased impervious surface cover and disconnection of stream corridors from riparian groundwater. We couple continuous streamflow records with weekly chloride concentration data over two water years to create continuous chloride load estimates at three locations along a degraded, urban stream in upstate New York with contrasting channelized and intact reaches. Our results show that degraded reaches characterized by channelized, armored banks and minimal groundwater connection deliver chloride loads closer to chloride application rates in the surrounding watershed. In contrast, stream–groundwater interactions in intact reaches adjacent to riparian floodplains, including surface water losses to subsurface flow paths, result in stream chloride loads that are 50% less than those delivered from upstream channelized reaches. These findings show that longitudinal chloride load estimates along a stream channel can be valuable in identifying the timing and magnitude of chloride sources and sinks, which may be common but less apparent in urban environments.

Urbanization negatively impacts water quality in streams by reducing streamgroundwater interactions, which can reduce a stream's capacity to naturally attenuate nitrate. Meadowbrook Creek, a first order urban stream in Syracuse, New York, has an inverse urbanization gradient, with heavily urbanized headwaters that are disconnected from the floodplain and downstream reaches that have intact riparian floodplains and connection to riparian aquifers. This system allows assessment of how stream-groundwater interactions in urban streams impact the net sources and sinks of nitrate at the reach scale. We used continuous (15-min) streamflow measurements and weekly grab samples at three gauging stations positioned longitudinally along the creek to develop continuous nitrate load estimates at the inlet and outlet of two contrasting reaches. Nitrate load estimates were determined using a USGS linear regression model, RLOADEST, and differences between loads at the inlet and outlet of contrasting reaches were used to quantify nitrate sink and source behaviour year-round. We observed a nitrate load of 1.4 Â 10 4 kg NO 3 À per water year, on average, at the outlet of the urbanized reach while the nitrate load at the outlet of the downstream, connected reach was 1.0 Â 10 4 kg NO 3 À per water year, on average. We found the more heavily urbanized, hydrologically-disconnected reach was a net source of nitrate regardless of season. In contrast, stream-groundwater exchange caused the hydrologically connected reach to be both a source and sink for nitrate, depending on time of year. Both reaches alter nitrate source and sink behaviour at various spatiotemporal scales. Groundwater connection in the downstream, connected reach reduces annual nitrate loads and provides more opportunities for sources and sinks of nitrate year-round than the hydrologically disconnected stream reach. Mechanisms include groundwater discharge into the stream with variable nitrate concentrations, surface-water groundwater interactions that foster denitrification, and stream load loss to surrounding near-stream aquifers. This study emphasizes how loads are important in understanding how stream-groundwater interactions impact reach scale nitrate export in urban streams.

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