The Influence of Low-head Milldams on Upstream Riparian Groundwater in the Mid-Atlantic: Hydrologic Mixing, Redox, and Nitrogen Regimes

Numerous low-head milldams fragment streams throughout the Northeast USA, altering the hydrology and nutrient cycling of riparian corridors. Studies have shown that milldams elevate stream and groundwater (GW) levels upstream of the dam. However, few studies have investigated how higher water levels may affect nitrogen processing in riparian zones. We examined the effects of milldams on riparian GW flow paths, stream-riparian GW mixing, redox conditions, and nitrogen concentrations for two dammed sites along Chiques Creek (PA) and Christina River (DE) (3 m and 4 m tall, respectively), USA. Monitoring wells were drilled in three transects perpendicular to the streams above the dams and classified as near stream, mid-riparian, upland, or wetland. Pressure transducers and conductivity sensors were placed in wells, logging data every 30 minutes. Oxidation-reduction potential (ORP) and dissolved O2 (DO) readings as well as water samples were collected monthly. Water samples were analyzed for NH4-N, NO3-N, total nitrogen, and organic carbon concentrations. Data and results from the Chiques Creek site are presented. Using the GW and stream water level and conductivity data collected over 2 years, three hydrologic regimes were characterized: wet, dry, and storm (see Figure 1). During the wet regime GW flow was primarily from the riparian zone to the stream but the dry and storm regimes exhibited reversal (stream to riparian) of GW flow. Although GW and stream water levels generally fluctuate in tandem, conductivity data for near-stream wells indicated little to no mixing, suggesting stream pressure waves affecting GW levels. While the upland-edge well indicated an oxidizing regime characterized by high DO, positive ORP, and high nitrate-N concentrations, most of the riparian GW wells were strongly reducing with negative ORP, low DO, low nitrate-N (median ~ 0.1 mg/L) and high ammonium-N (median ~ 3 mg/L) concentrations. The low nitrate-N concentrations are likely due to denitrification while the elevated ammonium-N could be due to suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Understanding how milldams affect riparian N conditions is critical to assessing their pollution buffering potential and how dam removals could impact riparian N budgets and watershed management.