Hydrogeological modeling of submarine groundwater discharge on the continental shelf of Louisiana

Abstract

[1] A regional scale hydrogeologic model has been developed to estimate the magnitude of submarine groundwater discharge to the coastal waters of southeastern Louisiana. The model domain incorporates both the onshore recharge area of terrestrially derived freshwater, and fluid circulation within the sediments on the continental shelf. The hydrogeologic properties of these sediments, which form part of the Coastal Lowlands Aquifer System, have been well-characterized in earlier studies. The low topographic relief of the coastal plain and an extensive zone of seawater intrusion are key features of the groundwater flow system. Model calculations suggest that no water containing a substantial component of terrestrial origin discharges on the continental shelf. Rather the near-shore coastal zone serves as a groundwater recharge area of saline water that then forms the seawater recirculation system beneath the coastal plain. The modeling results are consistent with interpretations of the rates of submarine groundwater discharge derived from geochemical tracers, presented in a companion paper by McCoy et al. (2007).

Review of submarine groundwater discharge (SGD) in coastal zones of the Southeast and Gulf Coast regions of the United States with management implications

“…Many SGD studies have utilized radioisotopes of the U-Th decay series, which include 222 Rn Cable et al, 1996a;Corbett et al, 2000a;McCoy et al, 2007a) and 223,224,226,228 Ra (Charette et al, 2001;Krest et al, 1999;Moore, 1996). 3 H, 4 He have also been utilized in recent SGD studies (Castro, 2004;McCoy et al, 2007b;Top et al, 2001). Quantification of SGD based on these geochemical tracers is effective because the activities of tracers are generally elevated in groundwater relative to surface water, and sources and sinks of each tracer can be calculated, estimated, or measured.…”

“…Hydrogeologic models have developed over the past 40 years and have become an invaluable tool for understanding subsurface flow in coastal aquifers (Li et al, 1999;Smith and Zawadzki, 2003;Thompson et al, 2007). The benefits of numerical hydrogeologic models are that they provide the opportunity to simplify key features in aquifer systems and enable analysis of groundwater and saltwater movement under varying conditions (pre-pumping, pumping, future) that are not possible to estimate by other Cable et al (1996a) Radon 2-10 cm d À1 180-730 m 3 s À1 Over 620 km 2 , equivalent to 20 first magnitude springs over study area Cable et al (1996b) Radon and CH 4 1.4-11.5 cm d À1 Equivalent to a first magnitude spring every 11-85 km of shoreline Cable et al (1997) Seepage meter 2-24 L m À1 min À1 Equivalent to 13% of FL coastline required to equal Apalachicola River discharge Rasmussen (1998) Seepage meter 0. methods.…”

“…Recent research has focused on characterizing, differentiating (terrestrial versus recirculated sources), and quantifying sources of SGD to the coastal ocean (Cable and Martin, 2008;Martin et al, 2006Martin et al, , 2007McCoy et al, 2007a;Thompson et al, 2007;Wilson, 2005; see also review by Burnett et al, 2006). Lautier (1998) estimated the freshwater-saltwater interface to be up to 12 km offshore near Wilmington, NC with significant seasonal variability, while Cable and Martin (2008) suggest the freshwater-saltwater interface is on the scale of meters offshore based on research conducted in Flamengo Bay, Brazil utilizing four artificial and natural tracers.…”

Submarine and intertidal groundwater discharge through a complex multi-level karst conduit aquifer

“…Instead, studies have applied direct and indirect methods for measuring the mass transfer of groundwater across the sea floor (Zektser et al 2007) or towards a surface water body, including (1) measuring the seepage flow rate using seepage meters (Carr and Winter 1980;Corbett et al 2003) or multilevel onshore piezometers (Freeze and Cherry 1979;Taniguchi and Fukuo 1996), (2) applying mass-balance approaches using natural geochemical tracers such as electrical conductivity (EC), short-lived radium and radon isotopes, i.e. 222 Rn, 223 Ra, 224 Ra, 226 Ra, or oxygen-18 (δ 18 O) and deuterium (δ 2 H) (Moore 2006;Peterson et al 2008;Santos et al 2008;Cave and Henry 2011;Lee et al 2012;Null et al 2014;Knee et al 2016), (3) applying water balance approaches based on the contributing catchment (Sekulic and Vertacnik 1996;Smith and Nield 2003), (4) using hydrograph separation techniques to quantify the groundwater contribution of surface-water streams and extrapolating this contribution to the coastal shore (Zektser et al 2007), (5) employing numerical modelling (Thompson et al 2007;McCormack et al 2014;Taniguchi et al 2015), and (6) using thermal imaging from remote sensing (Johnson et al 2008;Wilson and Rocha 2012;Tamborski et al 2015). While methods 1-5 can be considered quantitative, method 6 by itself will only yield a purely qualitative result.…”

Geomicrobiology of nitrogen in a coastal aquifer : isotopic and molecular methods to examine nitrification and denitrification in groundwater

“…As a second approach, discharge rates can be estimated using tracer techniques usually involving the tracking of radium or radon isotopes (Moore 1996;Moore et al 2008) or artificially injecting compounds such as sulfur hexafluoride (Cable and Martin 2008) or bromide (Hall et al 1991) into the aquifer and quantifying the flux rate of the tracer to the coastal ocean. Finally, discharge rates can be estimated using various modeling approaches that are usually based on the mass balance of water and the assumption of steady state have been used to estimate groundwater flux into coastal waters (Thompson et al 2007;. Estimates of SGD suggest that it can be a significant contribution of water and chemical constituents into the coastal ocean, with water fluxes (freshwater and saltwater recirculation) as high as 40% of riverine and runoff inputs (Moore 1996).…”