HURRICANE PRECIPITATION ISOTOPE SIGNATURE AS A CALIBRATION TOOL AND RECHARGE FLUX MARKER FOR MODELING A TROPICAL KARSTIC AQUIFER

Groundwater flow and transport models such as MODFLOW although based on porous media hydraulics, are widely applied to fractured or karstic aquifer systems. Additional flux data to assess and constrain the application of such models is valuable. Tropical storm precipitation has a distinctive isotope composition and is deposited as a large, discrete slug of readily identifiable water over a wide area. Movement of this natural tracer through an aquifer can be used to calibrate groundwater models in the karstic aquifers common in certain hurricane-prone areas e.g. Yucatan, Florida. This study incorporates the signature from Hurricane Isidore into a MODFLOW-based model of the karstic aquifer of the Yucatan Peninsula, Mexico.

Isidore crossed northern Yucatan on September 23-24, 2002. It left a strong isotopic signal in six observation wells located along two transects perpendicular to the northern Yucatan coast, monitored between July 2003 and August 2004. These were used as a calibration tool and recharge marker in a Groundwater Vistas (ESI) model. A transient 2D section model has been produced, which also incorporates the SEAWAT module (Langevin et al. 2003;USGS Report 03-426) to simulate the saltwater interface, using isotope data from three of the wells. The model, initially calibrated using observed heads and salinities, has been refined using the hurricane signature as a separate data set to constrain recharge and calibrate hydraulic properties.

The isotope signature pulse, which entered a wide area of the aquifer from a specific time-linked event, is an independent recharge marker. Constraining recharge is especially important in calibrating permeability values, which are difficult to model in karst. The tracked isotope signature provides one of the few possible ways to calibrate porosity, a critical parameter in transport modeling. A hurricane's isotope signature is both time-linked and conservative and hence is a reliable velocity marker. Better recharge and porosity controls together will particularly help constrain vertical flow and transport aspects of the model. Use of the isotopic signature of hurricane precipitation as a tool for estimation of hard-to-measure parameters is promising. Best results will require detailed monitoring of the precipitation event.