THE ROLE OF A RIVER IN THE EVOLUTION OF A MIXED SILICICLASTIC-CARBONATE ENVIRONMENT

In Florida, surface water - karst aquifer interactions are complicated by the existence of a siliciclastic confining unit that covers a large portion of the peninsula. Rivers flowing on this confining unit have little interaction with the underlying carbonate Floridan Aquifer System (FAS). Where rivers flow off the confining unit, they interact with the unconfined FAS, introducing chemically aggressive water to the exposed limestone. These interactions can be studied in the Suwannee River, the largest river to flow from confined to unconfined FAS. In this study, we used a mass balance approach based on dissolved calcium load to assess the contributions of calcium to the river from confined to unconfined regions. The mass balance used daily river discharge values and results of quarterly sampling (collected by the US Geological Survey and Suwannee River Water Management District) to estimate calcium contributions over three years (2002-2004). Within the unconfined portion of the basin, a large portion (roughly 80%) of the calcium originates from groundwater and from tributary rivers. This ratio varies from >90% in low precipitation years to <70% during the highest precipitation year. The remaining calcium contribution (10 to 30%) is likely due to either dissolution of the river channel or hyporheic exchange around the river. As the river flows off the confining unit, undersaturated water with respect to calcite is potentially aggressive and likely to cause more dissolution. At this transition zone, roughly half of the calcium contributions cannot be attributed to incoming river sources or groundwater input. The analysis illustrates that spatial variability of chemical erosion in the river channel and basin and the differing erosion processes that occur down the river depend strongly on the source of water. More-detailed monitoring of the chemistry in and around the river should give us more insight into the role the river plays in contributing to the evolution of the Florida platform.