Abstract from the 2002 Spring AGU meeting.

Peat sediments are present as a nearly continuous surface layer across large areas of the Everglades. These sediments have relatively low hydraulic conductivity, impeding water exchange between surface water and the underlying surficial aquifer. Although vertical water transport does occur across the peat layer, rates are slow enough that they are difficult to quantify. Even modest rates of vertical transport, however, become significant to water budgets and nutrient cycles when they occur over a large area. In the Everglades, common methods for measuring exchange across the peat layer are prone to complications: small hydraulic gradients are difficult to measure; seepage meters tend to be imprecise at slow rates; radon profiles or emanation rates are complicated by methane bubble ebullition; chloride profiles often exhibit a strong gradient only at the surface of the peat. We are employing a method that takes advantage of the different production rates of short-lived radium isotopes (Ra-223 and Ra-224) in the peat sediments and in the underlying sand or carbonate aquifer. Pore water radium concentrations are balanced by radioactive decay and production in the surrounding sediments. As the pore water is carried across the sediment interface a temporary excess or deficit of dissolved radium exists until decay is again balanced by the new production rate. We have derived steady-state, one-dimensional models to determine flow rates on the basis of this disequilibrium. This method has the advantage of being most sensitive at the base of the peat, away from transient perturbations occurring at the surface. In addition, Ra-223 and Ra-224 are collected simultaneously, providing two independent measurements. At present we have tested the method in Water Conservation Area-1 and WCA-2A, quantifying recharge and discharge on opposite sides of Levee 39, and also in more central sites in WCA-2A. Additional work is underway in Taylor Slough and Shark Slough.

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