Backed-up and saturated: effect of milldams on upstream riparian groundwater hydrologic and mixing regimes
Date
2022
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Publisher
University of Delaware
Abstract
Numerous low-head milldams fragment streams throughout the Northeast USA, altering the hydrology of riparian corridors. Studies have shown that milldams elevate stream and groundwater (GW) levels upstream of the dam. However, few studies have investigated how milldams affect riparian hydrologic and groundwater mixing regimes. We examined the spatial and temporal effects of milldams on riparian GW flow paths, geochemical conditions, and stream-riparian GW mixing for two dammed sites along Chiques Creek (PA) and Christina River (DE) (2.4 m and 4 m tall, respectively), USA. Monitoring wells were augered in three transects perpendicular to the streams above the dams and classified as near stream, mid-riparian, or upland-riparian. Piezometer nests were installed at select wells to characterize vertical flow. Pressure transducers and conductivity sensors were placed in wells, logging data every 30 minutes. Manual depth-to-water measurements and conductivity readings as well as water samples were collected monthly. Water samples were analyzed for B, Ca, K, Mg, Na and organic carbon concentrations. Combining the GW and stream water level with conductivity data collected over 2 years, three hydrologic regimes were characterized: wet, dry, and storm. 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. An additional longitudinal gradient reversal was observed during dry periods. Although GW and stream water levels generally fluctuated in tandem, conductivity data for near-stream wells remained relatively invariant, likely due to low mixing associated with large storage in thick layers of fine-grained legacy sediment. Principal component analysis of conservative chemical tracers indicated mixing between the near-stream and stream, which corroborated hydraulic gradient data. This apparent contradiction may be explained by a pressure wave phenomenon. GW flow paths and seasonal and event-scale mixing have implications for biogeochemical cycling. Understanding how milldams affect riparian hydrologic and geochemical conditions is critical to assessing their pollution buffering potential and how dam removals could impact riparian N budgets and ultimately the success of watershed management strategies.
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Keywords
Groundwater levels, Hydrology, Milldams, Riparian corridors, Christina River, Delaware, Chiques Creek, Pennsylvania