Interaction of Groundwater, Surface Water, and Seawater in Wolf Bay, Weeks Bay, and Dauphin Island Coastal Watersheds, Alabama
Type of Degreethesis
Geology and Geography
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Freshwater residing in coastal plain aquifers and watersheds represents one of our nation’s most important natural resources. Globally, the distribution and fluxes of freshwater in many coastal settings remains poorly understood. As population, agricultural, and industrial centers have expanded along sea coasts, demands for freshwater resources have resulted in widespread water depletion and contamination in coastal regions. Integrative models rooted in science are needed to characterize surface water and groundwater quality and quantity in estuarine and coastal environments. This research used the Wolf Bay watershed, an EPA classified “Outstanding Alabama Water”, and Weeks Bay, a coastal watershed with high-risk of mercury methylation, as a natural laboratory to gain an understanding of the hydrologic variables that affect water supply and water quality. To understand the hydrochemical conditions in which mercury methylates, water quality measurements of temperature, pH, oxidation reduction potential (ORP), dissolved oxygen (DO), turbidity, and electrical conductivity were collected at more than 60 locations in Wolf Bay and Weeks Bay. A bay cruise was conducted in July 2008 to sample bay water and measure water quality parameters. Major ion and stable isotope (oxygen and hydrogen) concentrations were analyzed in the laboratory to investigate the mixing of seawater and freshwater. The results indicated elevated concentrations of chloride (Cl) and sodium (Na) are high in bay water. Oxygen and hydrogen isotope analysis provides additional information on the degree of evaporation and water mixing in bays. Wolf Bay water is enriched in 18O and 2H relative to Weeks Bay water, river water, and shallow groundwater, indicating that its has received less freshwater input, or undergone greater evaporation and mixing with isotopically heavier seawater. In Weeks Bay high salinity seawater invades below acidic, low salinity water in the bay to form a wedge interface. Low DO and ORP values observed in this mixing zone indicate high microbial activities that may initialize Hg methylation. In Wolf Bay, by contrast, less freshwater inflow produces high salinity water, which may prevent key microbial processes that initialize Hg methylation and bioaccumulation. The results imply that Hg biotransformation is strongly influenced by hydrochemical conditions in coastal watersheds. Regional scale groundwater flow models of southern Baldwin County were developed in a cross section extending from the northern recharge areas (near Bay Minette) to the Gulf Coast. The models predicted two flow regimes in major aquifer zones. Both local and regional flow regimes are present in Aquifer A2 due to local variations in topography and water table undulations. In the deeper Aquifer A3, a regional flow regime dominates in which flow directions are more consistent (i.e., from north to south) and controlled by the net topographic slope. Groundwater discharges southwards into the coastal estuaries (e.g., Wolf and Weeks bays) and Gulf of Mexico. Calculated groundwater flow velocities in major aquifers range from a few to tens of meters per year. The model calculated that groundwater residence time of major aquifers ranges from 0 near the recharge area to about 7000 years near the Gulf Coast along a 70 km flow path. The calculated groundwater residence time is consistent with 14C and 4He ages measured by Carey et al. (2004).