In areas of the Florida Everglades receiving agricultural runoff, concentrations of dissolved nutrients are elevated in surface water and in sediments. This nutrient loading has been linked to changes in plant community distributions and to increases in peat accumulation rates. Elevated nutrient levels in the surface and subsurface are directly linked, and remediation efforts to decrease surface water nutrient concentrations will promote a release of nutrients that have built-up in ground water and sediments. In order to anticipate this release, we must increase our understanding of the exchange of water and chemicals between the surface and subsurface.

Exchange rates are slow in the Everglades and difficult to quantify with direct methods. We can overcome this limitation by employing naturally occurring isotopes of radium and radon. Radium isotopes are continually produced in the sediments from decay of their particle-bound thorium parents. The radium desorbs from the particles to surrounding pore water, and moves into surface waters via exchange processes (advection and diffusion). Determining the production rates of radium isotopes in the sediments and the radium concentrations in the surface water will allow us to quantify these exchange processes.

While the particle reactivity of radium is less than that of its thorium-parent, it is not a negligible effect. Radium is therefore transported through the subsurface more slowly than the water, due to its interaction with sediment particles. As an independent tracer of hydrologic exchange, we can use ²²²Rn, the short-lived daughter of ²²⁶Ra. Whereas radium occurs as a dissolved solid, radon occurs as a dissolved noble gas and has negligible particle reactivity. One disadvantage to using ²²²Rn is that it readily degasses from surface waters, and this effect must be estimated independently.

Plans call for a rigorous sampling effort across the nutrient gradient in Water Conservation Area 2A, collecting sediment, ground water and surface water samples for radium, radon, and dissolved nutrients. Using radium and radon as independent tracers, we will determine the hydrologic exchange between the surface and subsurface water, and will also ascertain the relative merits of each tracer.