Browsing by Author "Hripto, Johanna"
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Item Backed-Up, Saturated, and Stagnant: Effect of Milldams on Upstream Riparian Groundwater Hydrologic and Mixing Regimes(Water Resources Research, 2022-09-28) Sherman, Melissa; Hripto, Johanna; Peck, Erin K.; Gold, Arthur J.; Peipoch, Marc; Imhoff, Paul; Inamdar, ShreeramHow milldams alter riparian hydrologic and groundwater mixing regimes is not well understood. Understanding the effects of milldams and their legacies on riparian hydrology is key to assessing riparian pollution buffering potential and for making appropriate watershed management decisions. We examined the spatiotemporal effects of milldams on groundwater gradients, flow directions, and mixing regime for two dammed sites on Chiques Creek, Pennsylvania (2.4 m tall milldam), and Christina River, Delaware (4 m tall dam), USA. Riparian groundwater levels were recorded every 30 min for multiple wells and transects. Groundwater mixing regime was characterized using 30-min specific conductance data and selected chemical tracers measured monthly for about 2 years. Three distinct regimes were identified for riparian groundwaters—wet, dry, and storm. Riparian groundwater gradients above the dam were low but were typically from the riparian zone to the stream. These flow directions were reversed (stream to riparian) during dry periods due to riparian evapotranspiration losses and during peak stream flows. Longitudinal (parallel to the stream) riparian flow gradients and directions also varied across the hydrologic regimes. Groundwater mixing varied spatially and temporally between storms and seasons. Near-stream groundwater was poorly flushed or mixed during storms whereas that in the adjacent swales revealed greater mixing. This differential groundwater behavior was attributed to milldam legacies that include: berm and swale topography that influenced the routing of surface waters, varying riparian legacy sediment depths and hydraulic conductivities, evapotranspiration losses from riparian vegetation, and runoff input from adjoining roads. Key Points: - Milldams raise riparian groundwater levels, decrease hydraulic gradients, and cause reversals in groundwater flow - Milldam legacies contribute to reduced groundwater mixing in near-stream sediments - Altered groundwater regimes due to milldams could affect riparian water quality processes Plain Language Summary: Riparian zones can buffer streams from upland nitrogen pollution and are thus considered as important water quality management practices. How the presence of milldams affects groundwater flow paths and their buffering capacity is not known. This study showed that milldams back up stream water above dams, reduce the groundwater gradients from the upland to the stream, and also result in their reversal during summer dry conditions and floods. Milldams reduced the mixing of groundwaters for near-stream sediments. This response was attributed to the topographic and sediment conditions associated with the milldams.Item Nitrogen Sinks or Sources? Denitrification and Nitrogen Removal Potential in Riparian Legacy Sediment Terraces Affected by Milldams(Journal of Geophysical Research: Biogeosciences, 2022-09-19) Peck, Erin K.; Inamdar, Shreeram; Sherman, Melissa; Hripto, Johanna; Peipoch, Marc; Gold, Arthur J.; Addy, KellyRiparian zones are key ecotones that buffer aquatic ecosystems through removal of nitrogen (N) via processes such as denitrification. However, how dams alter riparian N cycling and buffering capacity is poorly understood. Here, we hypothesized that elevated groundwater and anoxia due to the backup of stream water above milldams may enhance denitrification. We assessed denitrification rates (using denitrification enzyme assays) and potential controlling factors in riparian sediments at various depths upstream and downstream of two relict U.S. mid-Atlantic milldams. Denitrification was not significantly different between upstream and downstream, although was greater per river km upstream considering deeper and wider geometries. Further, denitrification typically occurred in hydrologically variable shallow sediments where nitrate-N and organic matter were most concentrated. At depths below 1 m, both denitrification and nitrate-N decreased while ammonium-N concentrations substantially increased, indicating suppression of ammonium consumption or dissimilatory nitrate reduction to ammonium. These results suggest that denitrification occurs where dynamic groundwater levels result in higher rates of nitrification and mineralization, while another N process that produces ammonium-N competes with denitrification for limited nitrate-N at deeper, more stagnant/poorly mixed depths. Ultimately, while it is unclear whether relict milldams are sources of N, limited denitrification rates indicate that they are not always effective sinks; thus, milldam removal—especially accompanied by removal of ammonium-N rich legacy sediments—may improve riparian N buffering. Plain Language Summary: Floodplains adjacent to rivers are important ecosystems that provide valuable services including nutrient removal, especially nitrogen, from stream water. Because nitrogen is a major polluter of coastal waters, river floodplains are increasingly being restored as part of watershed best management practices. For example, millions of dollars are being spent annually in the Chesapeake Bay to install 900 miles of riparian buffers and on other watershed practices to mitigate nutrient pollution. However, the impact of small, colonial-era milldams on floodplain nitrogen mitigation is poorly understood, despite >14,000 such structures still present across streams of the eastern United States. We studied the impact of two small milldams (Roller mill on Chiques Creek, Lancaster, Pennsylvania, and Cooch mill on Christiana River, Newark, Delaware) on the ability of floodplains to remove or store nitrogen. We found that the stagnant water that accumulates behind milldams restricts floodplains from effectively removing nitrogen and may actually cause the accumulation of nitrogen. Whether accumulated nitrogen is released back into streams is unknown but concerning. Removal of dams would likely improve many ecosystem services of both streams and floodplains, with minimal consequences for the nitrogen mitigation abilities of these ecosystems. Key Points: Riparian denitrification rates are similar above and below milldams but deeper, wider upstream zones result in more nitrogen removal Denitrification rates peak in shallow sediments of riparian areas above milldams with higher hydrologic variability Stagnant hydrologic conditions upstream of milldams promote nitrogen processes that result in ammonium accumulation at deep sediment depthsItem Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones(Journal of Geophysical Research: Biogeosciences, 2022-12-09) Inamdar, Shreeram P.; Peck, Erin K.; Peipoch, Marc; Gold, Arthur J.; Sherman, Melissa; Hripto, Johanna; Groffman, Peter M.; Trammell, Tara L. E.; Merritts, Dorothy J.; Addy, Kelly; Lewis, Evan; Walter, Robert C.; Kan, JinjunThe compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial-scale legacies of milldams and decadal-scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid-Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt-rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium-N accumulation in near-stream sediments and groundwaters. Riparian groundwater ammonium-N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L−1 while soil-bound values were 0.11–456 mg kg−1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L−1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium-N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant. Plain Language Summary: Human activities can combine to exacerbate environmental pollution. We studied the effects of milldams and road salt runoff on nitrogen (N) pollution in streamside/riparian soil and groundwaters in Pennsylvania (Chiques Creek) and Delaware (Christina River). While nitrate-N concentrations in groundwaters and soils were low, ammonium-N concentrations for both sites were unexpectedly high. We attributed the high groundwater ammonium concentrations to processes of ammonification and/or dissimilatory nitrate reduction to ammonium that occurred under stagnant and persistently reducing riparian groundwater conditions. Road salt runoff inputs from an interstate highway above the Christina River site likely exacerbated the groundwater ammonium concentrations because of sodium displacement of ammonium-N from sediment surfaces into solution. We suggest that dam removals could enhance the natural variability in groundwater, induce nitrification-denitrification removal of N, and thus mitigate N pollution in riparian zones. Greater consideration needs to be given to environmental impacts of human legacies in watershed management. Key Points: - The coupled effects of anthropogenic legacies for nitrogen dynamics are not well understood - Ammonium-N may accumulate in riparian groundwater and sediments upstream of milldams due to stagnant, poorly mixed, and reducing conditions - Road salt salinization may further enhance the concentrations of ammonium in riparian groundwatersItem The effects of low-head milldams on stream nitrogen processes(University of Delaware, 2022) Hripto, JohannaStream fragmentation is prevalent throughout the US mid-Atlantic due to historic low-head milldams altering stream hydrology and biogeochemical processes. While numerous dams have breached or been purposefully removed, many remain creating slower flows and enhancing organic matter (OM) and sediment deposition above the dam. These factors promote anaerobic nutrient processing including denitrification which may alter overall nitrogen (N) removal across the watershed. We studied two streams with existing low-head milldams (Christina River in Newark, DE; drainage area of 50.7 km2 and Chiques Creek in Manheim, PA; drainage area of 127 km2, dam heights 4.0 & 2.4 m, respectively) over two years. We expected streambed sediment denitrification rates to be higher upstream of the dam compared to below and to vary seasonally, with highest rates in summer. Denitrification enzyme assays (DEA), net mineralization and nitrification, sediment particle size, and % OM, were determined from streambed sediments collected seasonally. Monthly stream grab samples were analyzed for total nitrogen (TN), nitrate (NO3-N), ammonium (NH4-N), and dissolved organic carbon. Contrary to expectations, DEA rates and nutrient concentrations did not differ above and below the dams, and highest denitrification rates occurred during autumn at Christina and winter at Chiques. Streambed sediment was dominated by sand at both sites, which has less surface area for denitrification to occur compared to smaller silt or clay particle sizes. A multilinear regression model showed OM and sediment NH4-N accounted for 33% of variability in denitrification rates. This study indicated the two milldams did not have a significant effect on streambed denitrification or stream water N concentrations, possibly due to full sediment capacity above the impoundments reducing denitrification potential. These findings will help watershed managers make informed decisions on dam removals and potential consequences for N exports.