The role of the epikarst in karst and cave hydrogeology: a review
The epikarst (also known as the subcutaneous zone) comprises highly weathered carbonate bedrock immediately beneath the surface or beneath the soil (when present) or exposed at the surface. Porosity and permeability are higher near the surface than at depth, consequently after recharge percolating rainwater is detained near the base of the epikarst, the detention ponding producing an epikarstic aquifer. Such an aquifer is found only where the uppermost part of the vadose zone is very weathered compared to the bedrock at depth. Sometimes this contrast in porosity and permeability does not occur either because the epikarst has been scraped off by glacial scour or because high porosity exists throughout the bedrock. In some conditions porosity may even diminish near the surface because of case-hardening. The epikarst is best developed in pure, crystalline limestones or marble where it is typically about 10 m thick. It then contains a suspended aquifer that is under-drained and sustains the distal tributaries of cave streams and small perennial flows emerging on hillsides (epikarstic springs). Slow leakage paths from the epikarst maintain seepage to many stalactites throughout the year. A distinction should be recognized between the location (and form) of the epikarst and the function of the epikarst, because the near surface zone in carbonate rocks does not always contain a suspended aquifer.