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Abstract
Karst aquifers in central Northern Puerto Rico (NPR) are highly productive but are also highly heterogeneous. Due to existence of high permeability pathways, the NPR karst aquifers are highly vulnerable to contamination from Superfund and other hazardous waste sites within the area. Our first study applies an Equivalent Porous Media (EPM) approach to simulate groundwater hydraulics and contaminant transport in NPR karst aquifers. The water-table fluctuation results indicate that the model can practically reflect the steady-state groundwater hydraulics (normalized RMSE of 12.4%) and long-term variability (normalized RMSE of 3.0%) at regional and intermediate scales. Additionally, the EPM approach is capable to reproduce the spreading of a TCE plume at intermediate scales with sufficient accuracy (normalized RMSE of 8.45%).
While the field data sparsity often limits the ability to locate and characterize the conduit system, a method is introduced in our second study for modeling the karst conduits. Connecting sinkholes and springs, implemented drain cells improve the developed regional model by simulating the drainage effects of conduit networks on local groundwater table. The MODFLOW drainage feature is also able to roughly reproduce the spring discharge hydrographs in response to rainfall. Hydraulic conductivities at NPR are found to be scale dependent and significantly increase with higher test radius, indicating scale dependency of the EPM approach. The analysis of historical slug test data suggests that the central NPR aquifers are more karstified that others in northwestern, eastern, and southwestern Puerto Rico. Similar to other karst regions in the world, hydraulic gradients are steeper where the transmissivity values are lower approaching the coastline.
