Use of Geochemical Tracers to Identify Sources and Pathways of Water Flow in the Everglades

Attempts to manage water quality in the Everglades over the past 40 years generally have proceeded with little regard to the close hydrologic connection between surface water and ground water. Seepage of substantial quantities of Everglades surface water beneath levees was first quantified in the 1960s, and was shown to be an important factor depleting the water supply to Everglades National Park during dry periods. In contrast, the effect that seepage has on water quality has not been as widely considered. Although the variable sources of water inputs to the Everglades (for example, precipitation, agricultural runoff, Lake Okeechobee drainage) have been well documented, the chemical quality of those inflows is not always well known. In addition, the potential for ground water to function as a storage reservoir for surface-water contaminants, or as a site of biogeochemical reactions that affect contaminants, generally is not well understood. A thorough investigation will require study of the vast interior areas of the Everglades, in addition to the wetlands in close proximity to levee boundaries.

One reason that interactions between surface water and ground water are poorly understood in the Everglades is because the interactions are difficult to estimate or simulate. Measuring surface-subsurface interactions in wetlands is always a challenge, both because of the difficulties of deploying hydrological instrumentation in wetlands, and because the fluxes involved generally are small enough to challenge the precision of the readily available methods. For those reasons, hydraulic measurements alone may be inadequate to characterize interactions between surface water and ground water in the Everglades. The vast area of the Everglades, however, ensures that even small fluxes between wetland surface water and ground water potentially are of great importance to chemical budgets and water quality. Geochemical measurements, themselves, offer an alternative tool for researchers that potentially can supplement hydraulic measurements, revealing long-term patterns of interactions between surface water and ground water, and resulting effects on water quality.