Coastal pollution has been of particular concern in south Florida because of the great increase in population and urban development. Reef decline during the past three decades has paralleled the growth of the Miami metropolitan and Florida Keys areas. This growth, and associated pollution and fishing pressure, have placed BNP among the top 10 endangered National Parks (National Parks Conservation Association, 2004). Pollutants can enter BNP through many pathways. BNP is connected to the surrounding urban area by roads, canals, waterways, water pipes, and electrical grids. Less apparent are the connections through wet and dry atmospheric deposition, surface seawater circulation, and groundwater flow. This study addresses the threat of pollutants entering BNP along the groundwater-flow path.

Figure 1. Study area and well-cluster sites indicated by yellow circles. Goulds Canal exits into the bay near Black Point. Landfills are marked with orange dots; active landfill is the South Dade Landfill Facility and the Miami-Dade Water and Sewer South District Plant. The Old South Dade Landfill (OSDL) is an inactive landfill. [larger image]

Groundwater in two south Florida aquifers, the shallow Biscayne and the deeper Floridan, is known to flow east and southeast from the mainland toward BNP (Fish and Stewart, 1991; McNeill, 2000). In addition, the Biscayne Aquifer is immediately overlain by both decommissioned (OSDL) and active landfills that lie on the western edge of BNP (Figure 1). Near the active landfill, a deep portion of the Floridan Aquifer is used for wastewater injection. Historically (prior to 1900), fresh groundwater from the Biscayne Aquifer discharged along the western shore of Biscayne Bay at greater volumes than those observed today. To simulate modern-day groundwater flow from the Biscayne Aquifer to the bay, a hydrogeologic model (SEAWAT) has been developed (Langevin, 2001). Connectivity between the Floridan Aquifer and surface water of BNP is unknown, although wells drilled to the Floridan are artesian and historically have had wellhead pressures of 10-20 psi at sea level (Bush and Johnson, 1988). The pressures have been decreasing over time with changing climatic conditions and aquifer withdraws.

Small brackish-water lenses occur beneath the larger islands of the Florida Keys such as Elliott Key, Key Largo, and Big Pine Key (Halley and others, 1997). Perhaps more importantly, the islands (keys) act as a barrier to tidal flow due to a large separation between tidal inlets (up to several kilometers). The presence of the keys causes a difference in tidal cycles and hence water levels, creating a hydraulic head gradient. The gradient constantly changes as the tide changes, setting up a phenomenon known as tidal pumping. Tidal pumping is the primary control or forcing factor for groundwater flow near the islands (e.g., Halley and others, 1997; Reich and others, 2002).

The objective of this study is to determine whether the shallow groundwater beneath Biscayne Bay and the outer-shelf reefs is being affected by activities on the mainland.

The scope of this study included installation, sampling, and analyses of water from sub-sea monitoring wells aligned along a transect from the western shore of Biscayne Bay southeastward to the reef tract. Surface water at each well site was also collected. The water samples were then analyzed for potential and known contaminants in the Biscayne and Floridan Aquifers.