A mechanistic study of nonlinear solute transport in a groundwater-surfacewater system under steady state and transient hydraulic conditions

Two laboratory experiments were conducted to investigate the effects of tid es and buoyancy on beach hydraulics in the presence of a seaward groundwate r 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 t he water of the regional aquifer was small, 2.4 g L-1, 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 similar to 32.0 g L-1, which creates a significant density gradient. This case corresponds to the prese nce of fresh groundwater in the subsurface of the coasts of the continental United States. The experiments were numerically simulated by the marine un saturated (MARUN) model, a numerical model for density-and-voscQsity-depend ent flows in two-dimensional variably saturated media. The long-term experi mental and numerical results showed that the seawater plume entered the bea ch 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 o f case 2, these results indicate an inverted salinity distribution in beach es subjected to tides with salt water from sea overtopping the freshwater l ens. 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-horizo ntal 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 app lying dissolved nutrients on the beach surface at low tide is superior to a pplying them in a trench landward of the beach. This is because the residen ce time of the nutrient plume in the bioremediation zone of the beach in th e prior situation is longer than that in the latter.