Flowing through the fractured, honeycombed rocks of karst regions, groundwater is highly susceptible to contamination. No thick layer of clayey till (a mixture of materials left by glaciers) or barely permeable bedrock, such as shale, slows down and filters water moving through the earth. During heavy rains, the water table rises rapidly and, within hours, the shallow aquifers are filled to overflowing.
Nothing protects these karst aquifers. Nothing seals them off from water that's just flushed through garbage, septic effluent, or recently fertilized fields.