Backed‐Up, Saturated, and Stagnant: Effect of Milldams on Upstream Riparian Groundwater Hydrologic and Mixing Regimes

Inamdar, Shreeram; Peck, Erin K.; Imhoff, Paul T.; Peipoch, Marc; Imhoff, Paul T.; Inamdar, Shreeram

doi:10.1029/2022wr033038

Cited by 6 publications

(28 citation statements)

References 58 publications

(102 reference statements)

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“…Hydraulic conductivity of the sediments across both sites was lowest for the near‐stream berm sediments (wells W1; berm position; 59–108 cm day −1 ) and greater upslope (wells W2 and W3; swale and hillslope positions; 85–468 cm day −1 ). Because of stream water backing up above the dam, riparian groundwater levels upstream of the dam were close to soil surface year‐round (Sherman et al., 2022). Groundwater flow gradients from the riparian zone to the stream were low for most of the year but reversed (stream to riparian) during the dry summer conditions (June through September) driven by high evapotranspiration (ET) loss (Sherman et al., 2022).…”

Section: Methodsmentioning

confidence: 99%

“…Because of stream water backing up above the dam, riparian groundwater levels upstream of the dam were close to soil surface year‐round (Sherman et al., 2022). Groundwater flow gradients from the riparian zone to the stream were low for most of the year but reversed (stream to riparian) during the dry summer conditions (June through September) driven by high evapotranspiration (ET) loss (Sherman et al., 2022). High‐frequency (30 min) specific conductivity data for the groundwater wells indicated that there was very little particle‐to‐particle groundwater mixing in the near‐stream wells (W1) during storms.…”

Section: Methodsmentioning

confidence: 99%

“…Manual grab water sampling was initiated in November 2019 and performed twice a week before COVID‐19 (February 2020) and monthly thereafter until December 2021 for all wells and upstream locations (Sherman et al., 2022). Sampling was interrupted during the months of March–May 2020 due to COVID‐19 travel bans.…”

Section: Methodsmentioning

confidence: 99%

“…In comparison, swale and upslope wells were more frequently flushed and mixed. The low event-scale mixing for near-stream sediments was attributed to the larger sediment depths and low hydraulic conductivity, whereas greater mixing of upslope groundwaters occurred because of smaller depths and the routing of upland and overbank stream flows along the riparian swales during storms (Sherman et al, 2022;Figure S3 in Supporting Information S1). This differential hydrologic regime and mixing for the riparian transect was likely instrumental in shaping the redox and biogeochemical conditions described in the results.…”

Section: Riparian Groundwater Hydrology and Mixing Regime Upstream Of...mentioning

confidence: 99%

“…We should note here that the Soil Survey classifications are likely for undammed riparian conditions and do not account the drainage effects of the milldams. Riparian zones upstream of the dams at both sites were defined by a berm and swale topography (Figures S1-S3 in Supporting Information S1) which had a strong influence on local hydrology (Sherman et al, 2022). The thickness of the riparian terrace sediments, upstream of the milldams varies between 1 and 4 m depending on the height of the dam.…”

Section: Site Descriptionmentioning

confidence: 99%

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Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

et al. 2022

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The 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.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Riparian Groundwater Hydrology and Mixing Regime Upstream Of...mentioning

confidence: 99%

Section: Site Descriptionmentioning

confidence: 99%

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Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

et al. 2022

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Influence of relict milldams on riparian sediment biogeochemistry

et al. 2023

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Dissimilatory Nitrate Reduction to Ammonium (DNRA) Can Undermine Nitrogen Removal Effectiveness of Persistently Reducing Riparian Sediments

Rahman,

Peipoch,

Kan

et al. 2024

ACS EST Water

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Denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) compete in reducing sediment conditions where DNF permanently removes nitrogen (N), while DNRA retains N with the conversion of nitrate (NO3 –) to ammonium (NH4 +). Thus, an increase in the level of DNRA can undermine permanent N removal. We investigated the relative magnitude and controls of these two processes at two milldam-affected riparian sites. DNRA (5.2–37.6 μg L–1 h–1) accounted for 10–79% of total NO3 – reduction and was highest in riparian sediments with higher iron (Fe) and sodium (Na+) in groundwater. DNF was the primary mechanism for NO3 – reduction when Fe and Na+ concentrations were low but when NO3 – was elevated. DNRA rates were higher for treatments with higher dissolved organic carbon (DOC):NO3 – and Fe:NO3 – ratios, indicating the stimulation of both heterotrophic and Fe2+ driven autotrophic DNRA. DNF and DNRA rates and their microbial functional genes decreased with increasing sediment depths. These findings imply that hydrologically stagnant and persistently reducing conditions associated with relict milldams and similar anthropogenic structures may enhance DNRA at the expense of DNF and undermine permanent N removal in riparian zones. Thus, the effects of such structures need to be accounted for in watershed N management strategies.

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Backed‐Up, Saturated, and Stagnant: Effect of Milldams on Upstream Riparian Groundwater Hydrologic and Mixing Regimes

Cited by 6 publications

References 58 publications

Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

Influence of relict milldams on riparian sediment biogeochemistry

Dissimilatory Nitrate Reduction to Ammonium (DNRA) Can Undermine Nitrogen Removal Effectiveness of Persistently Reducing Riparian Sediments

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