UNDERSTANDING PORE NETWORKS AND CHEMICAL TRANSPORT IN KARST AQUIFERS - SPATIAL AND TEMPORAL ANALYSES OF GROUND-WATER TRACERS

Some karst aquifers are characterized by an organized network of passages that strongly influence chemical transport. This network may include main conduits that are hydraulically connected to an annex system of smaller dissolution passages or fractures. Ground-water age can be estimated based on age-dating tracers such as chlorofluorocarbons (CFCs), SF₆, tritium, ¹⁴C, and ³⁶Cl. When several proximal sites are sampled, spatial anomalies of young ground water may indicate fast moving, focused flow and thus the likely presence of conduits. This occurs when samples near conduits have a larger fraction of modern water than do samples farther from conduits. Knowledge of tectonic features (e.g., faults, anticlines, synclines) also may help to interpret these spatial distributions because tectonics can influence the genesis of dissolution features.

For a particular well or spring, the distribution of ground-water age in samples may indicate how the karst pore network affects chemical transport. This age distribution can be estimated by convolution modeling of age-dating tracers, which are particularly useful if the age distribution spans decades. Convolution modeling is a useful method for understanding how the geochemical signal in recharge water is influenced as this water moves through the aquifer media, resulting in a particular geochemical response at a well or spring. Age distributions for multi-porosity karst aquifers may be multimodal and thus characterize two or more different porosity types, such as major conduits and associated annex systems. Temporal changes in an age distribution may reveal how fluctuating recharge rates affect the chemistry of water from a well or spring.

Modeling the signals of conservative tracers that fluctuate with higher frequencies than do age-dating tracers (e.g., δ¹⁸O or δ²H) may provide additional information concerning geologic controls that apply specifically to the modern water component. For example, transport in conduits might be distinguishable from transport in anastomosing passages based on these tracers. Also, a combination of spatial and temporal data analyses may reveal phenomena such as changes in conduit flow patterns resulting from changing climatic conditions.