Abstract:
It has been recognized that submarine groundwater discharge (SGD) may be one of the principal pathways for delivering nutrients to surface water bodies, resulting in eutrophication of many nearshore coastal areas throughout the world. A one-year study of the coastal aquifer system (A1, A2, A3-Aquifers) of Gulf Shores, Alabama was conducted to assess SGD fluxes, characterize contaminant and nutrient transport through the aquifer system, and determine the availability of future aquifer resources. A three-dimensional density-dependent groundwater flow and transport model (SEAWAT), based on the coupling of MOFLOW and MT3DMS, was used to simulate the transport of nitrate and sulfate through the groundwater system to the coast. The model was refined and calibrated using independently determined field-based radon (222Rn, t1/2=3.82 d) isotopic tracer time-series surveys across a portion of the model area to enhance estimates of nearshore SGD. Two SGD approaches, integrated with 222Rn-determined seepage rates, were developed to determine (1) localized; and (2) entire-shoreline SGD. Thirty-two groundwater wells within the study area were sampled to constrain the groundwater 222Rn end-member in the model and characterize the extent of nutrient contamination. The ArcGIS database was used to spatially plot and interpret nutrient and 222Rn data, and generate iso-concentration maps detailing groundwater contamination and aquifer piezometric surfaces across the study area. Radon concentrations measured in groundwater from the shallow A1 and deeper A2 Aquifers were statistically identical, an indication that there is direct connection between the two systems. Elevated nitrate and sulfate concentration (up to 30 mg/L and 724 mg/L, respectively) were observed through active monitoring with zones of principal discharge identified in the lower A2 Aquifer. A groundwater seepage rate of 18.3 cm/day, calculated through the radon mass-balance model at a model area lake, was used to calibrate the numerical model surficial aquifer zone. Final shoreline seepage fluxes of 6.41 and 8.62 cm/day were determined from the results of both the multi-cell and shoreface numerical model simulation SGD approaches, respectively. The results of the two numerical SGD methods demonstrate good agreement with the 222Rn-derived methods, and provided an effective approximation technique that can be inexpensively duplicated in other similar shoreline areas.
