Groundwater Characterization and Assessement of Contaminants in Marine Areas of Biscayne National Park [DRAFT]

Halley, Robert; Reich, Chris; Hickey, Todd; Swarzenski, Peter

Biscayne National Park is located next to the Miami-Dade urban complex and is adjacent to the Dade County South Dade Landfill Facility and the Miami-Dade Water and Sewer South District Plant. The base of the landfill is lined to separate it from the underlying Miami Limestone, the host rock for the Biscayne Aquifer. The sewer plant injects treated sewage into the lower Florida Aquifer that is overlain by an aquitard termed the Middle Confining Unit. The Biscayne Aquifer borders the western margin of the Park and the Floridan Aquifer underlies the entire park. There is concern about leakage of contaminated aquifer water into the National Park and its potential effects on water quality. Water samples from shallow near-shore and offshore wells in Biscayne National Park have been analyzed to determine the character of the groundwater beneath the Park and to assess the potential for contaminants entering the Park from subsurface flow. Samples were collected approximately quarterly from June 2002 until December 2003 from seven sites. The sites form a transect from the western shore of Biscayne Bay at Black Point southeastward across the shelf to Pacific Reef. Samples were analyzed for conductivity (salinity), dissolved oxygen, temperature, redox potential, nutrients, metals, strontium isotopes, radon, sulfate, and wastewater compounds. No significant evidence of freshwater flow from groundwater into Biscayne Bay was found during this study. Low-salinity water was identified from near-shore wells and may indicate some leakage from the Biscayne Aquifer and/or surface water intrusion into the rocks along western Biscayne Bay. Elevated nutrients indicate surface water exchange is more likely than groundwater flow. Lack of seasonal variation in groundwater salinity suggests sluggish exchange either with the surface water or with fresh groundwater. The groundwater beneath the shelf can be characterized as reduced seawater, modified by microbial respiration to remove oxygen, and interacting with sediments and minerals in the host limestone. Analyses of 109 water samples collected from wells across the Florida shelf beneath BNP between August 2002 and March 2004 show no consistent evidence of wastewater contaminants occurring in groundwater beneath BNP. No significant leakage from the Floridan Aquifer was detected in the groundwater beneath BNP. At Black Point, the western edge of Biscayne Bay is influenced by surface water and perhaps Biscayne Aquifer water, but the rest of the Florida Shelf, as examined for this study, is underlain by uncontaminated marine groundwater.