Abstract
A methodology is proposed to define indices for quantifying risks under the threat of reducing in groundwater levels, the existence of saltwater intrusion (SWI), and an increasing nitrate contamination load in submarine groundwater discharge (SGD). The proposed methodology considers coastal regions under geological heterogeneity and it is tested on a groundwater system in Nassau County of Long Island, New York (USA). The numerical model is constructed with the SEAWAT code. The parameter uncertainty of this model is evaluated by coupling the Latin hypercube sampling method (as a sampling algorithm) and Monte Carlo simulation to consider the uncertainty in both hydraulic conductivity and recharge rate. The indices are presented in spatial maps that classify areas of risk to potential threats. The results show that two of the water districts have a high risk under conditions of decreasing groundwater level. Salinity occurs in the southern and southwestern parts of the Nassau County aquifer and a considerable area of high risk of SWI is identified. Furthermore, the average SGD rate with the associated fluxes of nitrate is estimated as 81.4 million m3/year (average 0.8 tons of nitrate through SGD per year), which can adversely affect the quality of life in the local coastal ecosystems. The framework developed in this study could help the water district managers to identify high-risk areas for short-term and long-term planning and is applicable to other coastal settings.
Highlights
Groundwater, as a critical source of freshwater, has a vital role in coastal regions and needs to be assessed against threats
The contribution of this paper is to consider the risk analysis concerning the decreasing groundwater levels, saltwater intrusion (SWI) based on the salinity concentration and the so-called filling ratio, and Submarine groundwater discharge (SGD), which can threaten the quality and quantity of coastal aquifers and ecosystems
After developing a conceptual model based on the collection of data and information, uncertainty analyses are applied to a real-case study, with a three-dimensional (3D) numerical model, simulating variable-density groundwater flow and coupled salt transport
Summary
Groundwater, as a critical source of freshwater, has a vital role in coastal regions and needs to be assessed against threats. The groundwater system is usually recharged by direct infiltration from precipitation and Hydrogeol J (2020) 28:2519–2541 surface-water resources. The regional flow of groundwater is commonly toward the ocean, but saltwater intrudes into the aquifers at many locations (Motallebian et al 2019). Submarine groundwater discharge (SGD) to the ocean includes water flow on continental margins from the seabed to the ocean (Burnett et al 2003; Moore 2010); SGD is associated with fluxes of nutrients that can damage coastal ecosystems (Slomp and Van Cappellen 2004; Moore 2010; Zhou et al 2019). Coastal aquifers are affected by decreasing groundwater levels which can cause subsidence and serious damage to infrastructures (e.g., Minderhoud et al 2017). A comprehensive review of factors that affect SWI and SGD is given by Moore (2010), Werner et al (2013), and Ketabchi et al (2016a)
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