GROUNDWATER AND SEDIMENT ANALYSES OF A MANGROVE SWAMP NEAR BOVONI LANDFILL, ST. THOMAS, USVI

Mangroves are a unique and important coastal ecosystem, but in St. Thomas, United States Virgin Islands, very little is known about the hydrogeologic conditions of these systems. Groundwater hydraulic heads, sediment cores, and sediment surface temperatures were used in an integrated approach to better understand the physical processes governing mangroves in this system. Located adjacent to an ecologically important and protected wetland and marine protected area, the St. Thomas East End Reserve, is Bovoni landfill, an unlined landfill under Consent Decree for violating previously-issued federal orders regulating its operation. Groundwater elevations of 19 wells were used to determine flow direction while core stratigraphy was used to interpret past conditions and subsurface connectivity throughout the study area. Core samples were subjected to loss on ignition to calculate percent water content, organic content, and bulk density, while water and sediment samples were tested for trace metals. A one-dimensional finite-difference heat transport model was calibrated to temperature data collected at depths of 0, 7, 14, and 21 cm depths. Vertical groundwater velocity and porosity were adjusted in simulations to achieve a best fit with these temperature data. Water chemistry tests found concentrations of Cr ranging from 23 to 75 μg/L, Ni from 82 to 130 μg/L, Sn 105μg/L, Zn 68 μg/L, and TDN (total dissolved nitrogen) from 4 to 120 mg/L. Use of a SEM/EDX revealed the presence of 4-20 μm sized sediment particles enriched in Bi, Cr, Sn, Ti, and Zn. Hydraulic head data indicate shallow groundwater flow toward the center of the mangrove swamp, where open water occurs. Preliminary interpretations of the data suggest that groundwater discharge is likely driven by groundwater mounding beneath Bovoni Landfill and other uplands adjacent to the mangroves and that at least one site in close proximity to the landfill receives runoff with elevated concentrations of heavy metals and nutrients. High concentrations of heavy metals and total dissolved nitrogen were not measured in deeper groundwater wells, suggesting that surface discharge may be the more important pathway for contaminants entering the mangrove system. Future work should include additional water chemistry sampling and sampling during extreme precipitation events.