EFFECTS OF PARTIAL CONFINEMENT OF MODERN CARBONATE PLATFORMS ON GROUNDWATER CIRCULATION AND GEOCHEMICAL EVOLUTION

Geochemical studies of groundwater circulation in carbonate platforms have emphasized platforms that are fully confined or fully unconfined. In both types of platforms, surface water is either absent (unconfined) or isolated from aquifers (confined) and flow paths and chemical evolution radiate from platform centers towards the coast. Less is known about groundwater geochemistry and circulation in partially confined platforms, such as the Santa Fe River Basin (SRB) in north-central Florida, that have complex interactions between surface water and groundwater. The SRB is an eogenetic karst watershed, 40% of which is confined by siliciclastics. During normal flow, allogenic runoff contributes some water to the lower Santa Fe River but groundwater is the only source of recharge downstream of the confining layer. During storms, allogenic runoff hydraulically dams groundwater input to the lower Santa Fe, causing the calcite saturation state of the river to switch from supersaturated to highly undersaturated. During droughts, all inflow to the lower Santa Fe is groundwater, the most chemically evolved water occurs at the edge of the confining layer (near the center of the platform) and solute concentrations decrease downstream. This evolution is the opposite of fully confined or unconfined platforms and is caused by the effects of rivers on groundwater circulation. Allogenic recharge drives river incision and creates topographic relief, orienting local and regional groundwater flow paths towards rivers instead of the coast. Water chemistry in the lower Santa Fe River can be explained by regional flowpaths upwelling at the edge of the confining layer with dilution downstream by groundwater from local flowpaths. Isolated groundwater basins formed by river incision may be one reason why the SRB lacks the extensive conduit networks found in fully unconfined platforms (e.g., the Yucatan Peninsula).