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Abstract Subterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p < 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were <8 mmoles N m−2 d−1. The total groundwater discharge rate was 38 ± 11 L m−2 d−1; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m−2 d−1for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to >100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for <10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics.
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Improved Prediction of Management‐Relevant Groundwater Discharge Characteristics Throughout River Networks
Abstract Groundwater discharge zones connect aquifers to surface water, generating baseflow and serving as ecosystem control points across aquatic ecosystems. The influence of groundwater discharge on surface flow connectivity, fate and transport of contaminants and nutrients, and thermal habitat depends strongly on hydrologic characteristics such as the spatial distribution, age, and depth of source groundwater flow paths. Groundwater models have the potential to predict spatial discharge characteristics within river networks, but models are often not evaluated against these critical characteristics and model equifinality with respect to discharge processes is a known challenge. We quantify discharge characteristics across a suite of groundwater models with commonly used frameworks and calibration data. We developed a base model (MODFLOW‐NWT) for a 1,570‐km2watershed in the northeastern United States and varied the calibration data, control of river‐aquifer exchange directionality, and resolution. Most models (n = 11 of 12) fit similarly to calibration metrics, but patterns in discharge location, flow path depth, and subsurface travel time varied substantially. We found (1) a 15% difference in the percent of discharge going to first‐order streams, (2) threefold variations in flow path depth, and (3) sevenfold variations in the subsurface travel times among the models. We recalibrated three models using a synthetic discharge location data set. Calibration with discharge location data reduced differences in simulated discharge characteristics, suggesting an approach to improved equifinality based on widespread field‐based mapping of discharge zones. Our work quantifying variation across common modeling approaches is an important step toward characterizing and improving predictions of groundwater discharge characteristics.
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- Award ID(s):
- 1824820
- PAR ID:
- 10448344
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 10
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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