Submarine groundwater discharge and associated chemical input to a coastal sea

Abstract

Abstract. This paper presents a theoretical model of flow and chemical transport processes in subterranean estuaries (unconfined brackish groundwater aquifers at the ocean-land interface). The model shows that groundwater circulation and oscillating flow, caused by wave setup and tide, may constitute up to 96% of submarine groundwater discharge (SGWD) compared with 4% due to the net groundwater discharge. While these local flow processes do not change the total amount of land-derived chemical input to the ocean over a long period (e.g., yearly), they induce fluctuations of the chemical transfer rate as the aquifer undergoes saltwater intrusion. This may result in a substantial increase in chemical fluxes to the ocean over a short period (e.g., monthly and by a factor of 20 above the averaged level), imposing a possible threat to the marine environment. These results are essentially consistent with the experimental findings of Moore [1996] and have important implications for coastal resources management. Although these discharge components have been studied previously, the above conceptual model of SGWD is proposed for the first time.Dn can be estimated using aquifer recharge data. It is normally small compared with the discharge due to river flows into the ocean, Dr; the estimate of D n as a percentage of D r ranges from 0.1 to 10 [Younger, 1996]. Note that we are considering these discharges over a large coastal area (e.g., a few hundred kilometers in the alongshore direction) and a long period (e.g., seasonal). Although they contribute greatly to D sowr>, the magnitudes of D w and D t have not been described in previous studies nor measured independently in the field.

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…While wave periods typically range from a few to tens of seconds in the nearshore zone, wave height also varies over long time scales with intensified wave conditions occurring, for 19 instance, in response to offshore storms. Waves drive large volumes of water exchange across the CUA-ocean interface [Li et al, 1999]. It is challenging to quantify wave-driven recirculation from field data alone, but estimates have been provided through numerical studies [e.g., Xin et al, 2010;Bakhtyar et al, 2011;Geng et al, 2014;Robinson et al, 2014;Xin et al, 2014].…”

“…Field and numerical studies quantified the tidal effect on SGD and showed that tide-induced recirculation can represent a major portion of total SGD [e.g., Li et al, 1999;Taniguchi, 2002;Robinson et al, 2007c;Li et al, 2009]. Seawater infiltration into an aquifer occurs on the rising tide and exfiltration (saline SGD) on the ebbing tide [Robinson et al, 1998;Sholkovitz et al, 2003;Robinson et al, 2007c].…”

“…In early work, SGD was taken to be a linear sum of (independent) fluxes estimated based on the various inland and oceanic forces [Li et al, 1999;Taniguchi et al, 2002;Burnett et al, 2006]. Indeed, these forces and associated fluxes have been often studied numerically in isolation as discussed earlier.…”

Swash‐aquifer interaction in the vicinity of the water table exit point on a sandy beach

“…[3] Time-averaging of the beach groundwater response to wave forcing reveals a localized seaward directed circulation cell [Longuet-Higgins, 1983; which has implications for the availability and quality of coastal groundwater resources and for the fate of contaminants [e.g., Li et al, 1999;Moore, 1999]. The dynamics of the water table exit point on the beach face have also been linked to the distribution of interstitial macrofauna [e.g., McArdle and McLachlan, 1991] and to aeolian sediment transport on beaches [e.g., Jackson and Nordstrom, 1997].…”

Groundwater Flow to the Coastal Ocean

“…For example, seawater circulates through a coastal aquifer under the force of gravity (Li et al, 1999;Li and Jiao, 2003), from oceanic forces such as waves and tides (Taniguchi et al, 2002;Burnett et al, 2003), as a result of dispersive circulation along the freshwater-saltwater boundary within the aquifer (Kohout, 1960), and from changes in upland recharge (Michael et al, 2005).…”