Opportunities and Challenges in Computing Fresh Groundwater Discharge to Continental Coastlines: A Multimodel Comparison for the United States Gulf and Atlantic Coasts

Abstract

The fresh component of submarine groundwater discharge (fresh SGD) transports nutrient and contaminant loads from land to sea. Fresh SGD fluxes are poorly known but depend on geology, topography, climate, and land use. Here we assess two general approaches for quantifying the magnitude of fresh SGD over regional to continental scales—water budgets (lumped parameter models) and groundwater flow models—focusing on the U.S. Atlantic and Gulf Coasts as a large‐scale case study. Estimates of fresh SGD fluxes from groundwater flow models are more variable than estimates from water budgets, and both are often less than field‐based estimates. Both approaches predict the volume of fresh SGD within approximately 50% of 12.3 km3/year, in line with previous estimates, and differences are largely driven by hydrographic data inputs. The simplified water budget approach, while computationally cheap, cannot resolve flow paths or continuous flux distributions and may neglect groundwater flow from deep aquifers or distal recharge zones. Conversely, three‐dimensional groundwater flow models can resolve flow paths and fresh SGD contributions from heterogeneous aquifer systems but are computationally expensive and require more parameterization. Ultimately, multiple models and integrated field data are needed to improve our estimates of fresh SGD fluxes over a wide range of scales.

Fresh groundwater discharge insignificant for the world’s oceans but important for coastal ecosystems

“…For the Gulf and Atlantic coasts of the USA our model estimates a coastal groundwater discharge of 6.6 (0.1-11.3) km 3 a −1 , which is at the lower end of recent estimates by Zhou et al 46 that range from 9.7 (7.2-12.0) to 27.1 (22.8-30.5) km 3 a −1 . These estimates are based on a series of regional groundwater models 47 , include more detailed hydrogeology and permeability structure and may therefore be more accurate than our model estimates, especially at local scales.…”

“…Our model results estimate a coastal groundwater discharge flux for the contiguous US of 8.5 (0.1-17) km 3 a −1 , which is in the same range as the 15 ± 4 km 3 a −1 estimated by Sawyer et al 11 . A recently published near-global estimate 12 yielded a fresh SGD flux of 489 ± 337 km 3 a −1 , which is likely an overestimate due to the relatively coarse watershed database that this estimate relies on 12,46 . This is higher than the CGD flux of 224 (1-500) km 3 a −1 and the fresh SGD flux of 78 (0.4-210) km 3 a −1 reported here.…”

Fresh Submarine Groundwater Discharge to the Near‐Global Coast

“…The combined error associated with our assumptions is difficult to quantify for the near‐global coast. However, in a comparison of 10 sites from the continental United States (Bokuniewicz, ; Bokuniewicz et al, ; Hays & Ullman, ; Mulligan & Charette, ; Reay et al, ; Russoniello et al, ; Santos et al, ; Simmons, ; Uddameri et al, ; Zimmermann et al, ), our water budget analysis yields similar estimates of fresh SGD as seepage meter studies (Sawyer et al, ), other water budget calculations, and validated three‐dimensional groundwater flow models (Befus et al, ; Zhou et al, ; supporting information Figure S1). Our estimates tend to be lower than field‐based estimates, likely because of our conservative approach for delineating coastal recharge areas, which would tend to exclude groundwater imports from upland basins.…”

“…If patterns of groundwater flow are similar to overland flow, the recharge zones that contribute groundwater to the coast are the same as the wedge‐shaped coastal catchments that lack streams and instead contribute runoff directly to the coast (Figure ). This assumption is best for unconfined, thick, homogeneous aquifers in wet climates (Haitjema & Mitchell‐Bruker, ) and may underestimate fresh SGD in areas with complex geology, especially in dry regions (Sawyer et al, ; Zhou et al, ). Assuming modest groundwater injection or withdrawal, the annual volume of fresh SGD ( Q SGD ) for each coastal catchment is then the linear average annual net recharge rate ( r ), or recharge adjusted for evapotranspiration losses, integrated across the recharge area ( A ; m 2 ): …”

Quantifying Regional Fresh Submarine Groundwater Discharge With the Lumped Modeling Approach CoCa‐RFSGD

“…Regional models are either water balance models with lumped parameters (Destouni et al, 2008;Sawyer et al, 2016) or discrete numerical groundwater models with high-resolution input data (Befus et al, 2017). Zhou et al (2018) compared these two different model approaches: A lumped water balance model based on the model of Sawyer et al (2016) and a distributed numerical groundwater flow model based on the model of Befus et al (2017). Both models are steady state models, which do not take seasonal changes into account.…”