in karst systems is very rapid because it has these caves or conduits that essentially have infinite permeability. there is no resistance to water flow. dr. graham: and there's very little biological activity that would reduce nitrate levels, and that's one of the concerns. if you have a typical riparian zone or a sort of sandy superficial aquifer feeding a river, there's lots of biological activity along the riverbanks. but these springs are just coming out in sort of holes in the rock, and there's no biological remediation that happens. narrator: in a typical water basin, water moves slowly through soil layers containing a vast array of biological organisms that help to remediate excess nutrient loading. but because of the geology of this region, water and dissolved nutrients move quickly to the aquifer with little chance for remediation. and a second feature of this landscape poses an even greater risk. underground caves can collapse, creating sinkholes that are a direct connection between nutrient runoff and drinking water. dr. clark: one of the critical aspects about these direct connections