A mechanistic study of nonlinear solute transport in a groundwater–surface water system Under steady state and transient hydraulic conditions

Abstract

Abstract. Two laboratory experiments were conducted to investigate the effects of tides and buoyancy on beach hydraulics in the presence of a seaward groundwater flow due to an elevated "regional" water table. In the first experiment, case 1, the difference in concentration between the salt water at sea and the water of the regional aquifer was small, 2.4 g L -•, such that it did not engender density gradients; the salt acts as a tracer in this case. In the second experiment, case 2, the difference was --•32.0 g L -•, which creates a significant density gradient. This case corresponds to the presence of fresh groundwater in the subsurface of the coasts of the continental United States. The experiments were numerically simulated by the marine unsaturated (MARUN) model, a numerical model for density-and-viscosity-dependent flows in two-dimensional variably saturated media. The long-term experimental and numerical results showed that the seawater plume entered the beach from the sea and occupied most of the intertidal zone. The maximum depth of the seawater plume was near the midsection of the intertidal zone, and it decreased near the low and high tide lines. When viewed in the context of case 2, these results indicate an inverted salinity distribution in beaches subjected to tides with salt water from sea overtopping the freshwater lens. For both cases, water from the regional aquifer moved seaward beneath the seawater in the intertidal zone and pinched out near the low tide mark. We also noted that beach hydraulics are highly two dimensional with water entering the beach at a near-vertical angle and leaving it at a near-horizontal angle, which casts doubts on analyses of beach hydraulics based on the Dupuit assumption. Findings from this work have direct implications within the practice of bioremediation of oil spills on beaches. We found that applying dissolved nutrients on the beach surface at low tide is superior to applying them in a trench landward of the beach. This is because the residence time of the nutrient plume in the bioremediation zone of the beach in the prior situation is longer than that in the latter.

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

“…Where the USP is present, FSGD travels below the USP and discharges near the low tide mark in a zone bounded by the USP and the saltwater wedge. Numerous field, numerical and 14 laboratory studies have observed this salinity distribution in tidally-influenced CUAs [e.g., Lebbe, 1981;Boufadel, 2000;Mango et al, 2004;Vandenbohede and Lebbe, 2005;Boufadel et al, 2006;Mao et al, 2006;Robinson et al, 2006;Robinson et al, 2007b;Yang et al, 2013;Buquet et al, 2016]. The USP, in comparison with the DZ of the saltwater wedge, is a more active zone of saltwater-freshwater mixing in a CUA and an important feature of the STE system [Robinson et al, 2007b;Thorn and Urish, 2013].…”

“…This time-averaged circulating flow can lead to the formation of an upper saline plume (USP, Figure 1) [Robinson et al, 1998;Boufadel, 2000;Mango et al, 2004;Vandenbohede and Lebbe, 2005;Robinson et al, 2007b]. Where the USP is present, FSGD travels below the USP and discharges near the low tide mark in a zone bounded by the USP and the saltwater wedge.…”

“…As such, numerical groundwater models used to quantify tidal effects are often two-dimensional (neglect alongshore variations) [e.g., Boufadel, 2000;Robinson et al, 2007b;Evans and Wilson, 2016], and field studies typically aim to obtain data along a single cross-shore monitoring transect [e.g., Michael et al, 2005;Robinson et al, 2006;Abarca et al, 2013]. Alongshore variations associated with, for instance tidal creeks, headlands and embayments and geological heterogeneities are common, but the three-dimensionality of the nearshore groundwater system is not well understood.…”

Influence of hysteresis on groundwater wave dynamics in an unconfined aquifer with a sloping boundary

“…Several authors have investigated periodic unsaturated groundwater flow via numerical solution of Richards' equation (e.g. Boufadel et al, 1998;AtaieAshtiani et al, 1999;Boufadel, 2000;Naba et al, 2002).…”

Simulation of rock salt dissolution and its impact on land subsidence

“…The solute mass conservation equation is written in terms of mass fraction as follows (Boufadel, 2000;Cardenas et al, 2007;Younes et al, 2009;Zidane et al, 2012):…”