" QUANTITATIVE GROUNDWATER TRACING: A POWERFUL TOOL FOR AQUIFER CHARACTERIZATION AND GROUNDWATER MODEL DEVELOPMENT

Quantitative groundwater tracing using injected tracers is a powerful tool for modeling flow and transport in karst aquifers. We have conducted nine groundwater tracing experiments in the Woodville Karst Plain of north Florida and have incorporated the results into a regional numerical groundwater flow model that calibrates to heads, spring flows, and measured velocities in conduits. Analysis of breakthrough curves allows calculation of important hydraulic parameters (velocity, and dispersion) that provide the critical constraints necessary for the development of representative numerical groundwater flow models and the accurate characterization of groundwater flow and contaminant transport in these settings. These data, in conjunction with simultaneously measured groundwater levels, rainfall, and gauged spring discharges provide significantly improved insights into the mechanisms responsible for spatial and temporal variations in groundwater flow and transport.

Our results clearly show that: 1) groundwater velocities often exceed 1.5 km/day; 2) the largest springs in the basin discharge water from sinking streams within days to months; and 3) the distribution of flow in the aquifer is dependent on the distribution and magnitude of recharge in and between primary and subsidiary conduits, and between the conduits and the surrounding aquifer. These results have greatly improved the modeling effort by providing an accurate hydraulic description of the conduits, which represent a small component of the aquifer volume but contain most of the flow, and the relationship of those conduits to the aquifer matrix, which provides most of the storage.

Quantitative tracing has thus proven to be our most valuable tool for aquifer characterization because the results define the most critical hydraulic variables and relationships that control the direction, rate, and timing of groundwater flow through the basin. Those results have also clearly illustrated the inherent danger of equating isotopic ages to effective groundwater velocities and springshed boundaries because they only reflect snapshot estimates of an average age of the composite spring discharge and provide no information on the aquifer hydraulics responsible for the discharge from which the ages were measured.