Draining the Landscape: How Do Nitrogen Concentrations in Riparian Groundwater and Stream Water Change Following Milldam Removal?

Abstract

Dams are increasingly being removed across the United States (US) (Bellmore et al., 2017;Foley et al., 2017). Since 1912, more than 1,490 dams have been removed across the US and Pennsylvania leads the nation in the number of milldams and their removals (American Rivers, 2020). Most (>90%) of these milldams are classified as low-head dams (height < 7 m) and are typically a relic of colonial and post-colonial era milling activities (Merritts et al., 2011;Walter & Merritts, 2008). Dam removal numbers could be higher since not all dam removals are recorded. This could particularly be true for the Mid-Atlantic Piedmont region, where thousands of small mill dams existed since the late 1600s (Walter & Merritts, 2008).Low-head dam removals are primarily being driven by needs for public safety, reduction in financial liability, recreational access, aesthetics, and/or improvement in fish habitat (

Nitrogen Sinks or Sources? Denitrification and Nitrogen Removal Potential in Riparian Legacy Sediment Terraces Affected by Milldams

“…(2014) suggested, wet periods of elevated denitrification can occur following dry periods of elevated nitrification (Kolbjørn Jensen et al., 2017; Peter et al., 2011). Indeed, following the initial decline in DEA rates just after the Krady dam removal, potential denitrification rates showed evidence of seasonality (Lewis et al., 2021). These results are in line with our conceptual framework between hydrodynamic variability and coupled nitrification and denitrification rates (Figure 6) but in situ measurements of denitrification and nitrification rates across seasons are needed.…”

“…In certain riparian settings where groundwater intersects, root zone microsites serve as denitrification hotspots where small volumes of sediment disproportionally account for the majority of nitrate-N removal (e.g., Jacinthe et al, 1998;Parkin, 1987). Elevated denitrification rates for surficial sediments (vs. the subsurface) have also been reported for legacy sediment terraces upstream of breached/removed milldams, reflecting nutrient profiles that sharply decline with depth (Lewis et al, 2021;Weitzman et al, 2014). This was despite the fact that groundwater levels were observed at considerable depth from the sediment surface because of the removal/breach of the dam.…”

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

“…In contrast, where milldams have been breached or removed, streams are incised through legacy sediments, resulting in drained and oxic riparian terraces/soils that may be “perched” and hydrologically disconnected from the stream waters (Inamdar et al., 2021; Merritts et al., 2011). While considerable attention has been paid to how milldams have affected stream geomorphology and fluvial erosion and transport (Cashman et al., 2018; Donovan et al., 2015; Miller et al., 2019), little research has focused on how milldams and associated hydrologic and biogeochemical conditions have altered riparian zone N processes and functions (Inamdar et al., 2021; Lewis et al., 2021; Peck et al., 2022).…”

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

“…Human‐made dams fragment stream ecosystems (Fencl et al., 2015); are detrimental to riverine wildlife (Bellmore et al., 2019; Henley et al., 2000); and can contribute heavily to suspended sediment and nutrient loads in streams when breached (Cashman et al., 2018; Gellis et al., 2017; Jiang et al., 2020). However, few studies have addressed how milldams alter riparian hydrology and mixing regimes and how this could affect riparian zone water quality processes and functions (e.g., Lewis et al., 2021).…”

“…and mixing regimes and how this could affect riparian zone water quality processes and functions (e.g., Lewis et al, 2021).…”