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Abstract The tidal tributaries of the lower Chesapeake Bay experience seasonally recurring harmful algal blooms and the significance of submarine groundwater discharge (SGD) as a nutrient vector is largely unknown. Here, we determined seasonal SGD nutrient loads in two tributaries with contrasting hydrodynamic conditions, river‐fed (York River) vs. tidally dominated (Lafayette River). Radon surveys were performed in each river to quantify SGD at the embayment‐scale during spring and fall 2021. Total SGD was determined from a222Rn mass balance and Monte Carlo simulations. Submarine groundwater discharge rates differed by a factor of two during spring (Lafayette = 11 ± 17 cm d−1; York = 6 ± 10 cm d−1) and a factor of six during fall (Lafayette = 19 ± 27 cm d−1; York = 3 ± 7 cm d−1). Groundwater N concentrations and fluxes varied seasonally in the York (4–7 mmol N m−2d−1). In the Lafayette River, seasonal N fluxes (22–37 mmol N m−2d−1) were driven by seasonal water exchange rates, likely due to recurrent saltwater intrusion. Submarine groundwater discharge–derived nutrient fluxes were orders of magnitude greater than riverine inputs and runoff in each system. Additionally, sediment N removal by denitrification and anaerobic ammonium oxidation would only remove ~ 1–11% of dissolved inorganic nitrogen supplied through SGD. The continued recurrence of harmful algal blooms in the Bay's tidal tributaries may be indicative of an under‐accounting of submarine groundwater‐borne nutrient sources. This study highlights the importance of including SGD in water quality models used to advise restoration efforts in the Chesapeake Bay region and beyond.
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Septic Return Flow Pathlines, Endpoints, and Flows Based on the Urban Miami‐Dade Groundwater Model
Abstract Miami‐Dade County (MDC) has over 112,000 septic systems, some of which are at risk of compromise due to water table rise associated with sea level rise. MDC is surrounded by protected water bodies, including Biscayne Bay, with environmentally sensitive ecosystems and is underlain by highly transmissive karstic limestone. The main objective of the study is to provide first estimates of the locations and magnitudes of septic return flows to discharge endpoints. This is accomplished by leveraging MDC's county‐scale surface‐groundwater model using pathline analysis to estimate the transport and discharge fate of septic system flows under the complex time history of groundwater flow response to pumping, canal management, storms, and other environmental factors. The model covers an area of 4772 km2in Southeast Florida. Outputs from the model were used to create a 30‐year (2010 to 2040) simulation of the spatial–temporal pathlines from septic input locations to their termination points, allowing us to map flow paths and the spatial distribution of the septic flow discharge endpoints under the simulated conditions. Most septic return flows were discharged to surface water, primarily canals 52,830 m3/d and Biscayne Bay (5696 m3/d), and well fields (14,066 m3/d). Results allow us to identify “hotspots” to guide water quality sampling efforts and to provide recommendations for septic‐to‐sewer conversion areas that should provide most benefit by reducing nutrient loading to water bodies.
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- Award ID(s):
- 2149700
- PAR ID:
- 10598924
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Groundwater
- Volume:
- 62
- Issue:
- 6
- ISSN:
- 0017-467X
- Page Range / eLocation ID:
- 957 to 971
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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