Relation of the spatial and temporal distribution of water quality parameters to hydrogeology at Indian River Bay, Delaware: field observations and modeling
Date
2012
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Publisher
University of Delaware
Abstract
Submarine groundwater discharge (SGD) has been recognized as a major contributor of inorganic nutrients and other dissolved constituents to coastal waters. The composition of SGD and the resulting nutrient flux is largely determined by the geochemical reactions that occur in the subsurface prior to discharge, especially in zones where redox gradients exist. These zones can be affected by heterogeneity and transient forcing which can potentially enhance mixing between groundwater of different chemistry, causing chemical reactions that may decrease nutrient concentrations. This field study aims to understand how tides, seasonal fluctuations of the water table and geologic heterogeneities affect the distribution of salinity, nutrients, and field parameters in the subsurface and their variation in time. The study site is located at Holts Landing State Park in Indian River Bay, Delaware; an estuary affected by nutrients derived from SGD. The geology of the study site is characterized by a paleovalley/interfluve system which has an important effect on groundwater flow and SGD. The paleovalley infill sequence, coarse sediments at the bottom overlain by low-permeability sediments, allows the flow of fresh groundwater offshore by impeding discharge nearshore. This has resulted in a freshwater plume which has been imaged with resistivity surveys and extends ~ 1 km offshore beneath the bay. Thirteen monitoring wells on land and eight multilevel wells (MLW) offshore within and away from this freshwater plume were sampled for salinity, pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) and nutrients. Selected MLWs were sampled over a tidal cycle on neap and spring tide. In addition, onshore monitoring wells and offshore MLWs were sampled every four months and monthly, respectively. Salinity measurements corroborate electrical resistivity data which mapped the geometry of the freshened groundwater plume offshore beneath the paleovalley cap. Salinity profiles from offshore MLWs show salinity gradients where shallow fresh groundwater (~2 ‰) underlies saline groundwater (~25‰) and show that transition zones do not move substantially in response to seasonal fluctuations in the water table elevation, tidal fluctuations or amplitude. Likewise, nutrient concentrations were stable over time. Fresh groundwater onshore had higher dissolved oxygen levels and ORP than groundwater offshore, independent of salinity. Ammonium was the dominant species in saline groundwater. Onshore shallow wells near the marsh and brackish wells also had high ammonium concentrations. The highest nitrate concentrations (755 μM) were observed in fresh groundwater onshore at a depth >12 m, while nitrate was nearly absent in groundwater offshore. The apparent non-conservative behavior of nitrate indicates that nutrients fluxes may be lower than expected when fresh flow paths are longer under paleovalley caps. Direct discharge of nitrate to the bay can occur if nitrate-bearing groundwater does not come into contact with reducing conditions. A numerical groundwater model was constructed with SEAWAT to simulate 3D, variable-density groundwater flow with site specific parameters of the field site. Simulation results support the influence of geologic heterogeneity on the subsurface salinity distribution. Models that included the paleochannel feature consisting of a high-permeability channel overlain by a low-permeability cap were able to reproduce patterns in observed salinities offshore. In contrast, models with the low-permeability cap without the high-permeability channel did not produce the offshore fresh plume. Modeling results indicate that the paleochannel feature strongly controls subsurface salinity and provides evidence that the sediments at the base of the channel are more permeable than the surrounding aquifer.
This investigation highlights the importance of geologic heterogeneity in controlling salinity distributions in the subsurface. The research demonstrated that transient forcing is not as important as the spatial variability in salinity and nutrients near shore at our field site. The non-conservative behavior of nitrate suggests that denitrification can occur along fresh groundwater flow paths beneath the paleovalley cap. It is possible that direct discharge of nitrate to the bay can occur if nitrate-bearing groundwater does not come into contact with reducing conditions. Saline groundwater offshore is a significant source ammonium and may be the dominant N-species in SGD in the vicinity of the paleochannel feature.