Distribution of Recharging and Vulnerability of the Tertiary Limestone Aquifer, South Carolina: Regional Gradients and Important Outliers

The distribution of ground-water radiocarbon ages from the Tertiary limestone aquifer (includes the Floridan aquifer) in South Carolina shows both a typical trend for a regional sedimentary aquifer plus a lesscommonly reported occurrence of disjunct outliers of recharging and thus high vulnerability to contamination located farther down the regional flow system. The main recharge area, and thus high vulnerability, is apparently in the updip Tertiary sand aquifers of the upper (inner) coastal plain that receive recharge directly and only later deliver this as ground water to the limestone formations by lateral coastward flow. In places, a considerable degree of isolation ("confinement") and protection is achieved by the time and location that this flow reaches the sand-to-limestone lateral transition near the outer (seaward) edge of the inner coastal plain. A substantial to high degree of isolation and protection is achieved or maintained in the limestone aquifer in a large part of the middle and lower coastal plain, basically where the Cooper marl and related confining layers occur. Notable exceptions exist though even within these downflow areas. Recharging and thus high vulnerability occurs in large or small-but-intense areas isolated within interior and coastal portions of the middle and lower coastal plain. THE TERTIARY LIMESTONE AQUIFER The major regional Tertiary limestone aquifer of southeastern United States extends into southern South Carolina where it is an important source of drinking water. Coastal plain aquifers have a generally seaward flow direction and thus both upflow and downflow boundaries of flowlines exist within the state. Upflow boundaries lie in recharge areas. Recharging is important not only in terms of ground-water replenishment but it is also the mechanism by which aquifers can most easily become contaminated by materials originating at the ground surface (e.g., accidentally spilled fuels or chemicals, disposed waste, leached fertilizer or pesticides). Recharge areas are the most vulnerable to such contamination and deserve special consideration in planning for protection of wells and in responding to existing contamination. The limestone aquifer is also widely confined and protected (effectively or partially) by younger sedimentary formations that overlie it and thus has areas of lower to low vulnerability. It is best protected where a thick sequence of low permeability (generally finer grained) materials overlie it. A “textbook” regional coastal-plain sedimentary aquifer is recharged in its inland, highest elevation portion (often the geologic formation’s outcrop area), becomes confined at some point downflow where younger formations with fine-grained strata bury and isolate it hydraulically, and remains highly confined until reaching some offshore location where water leaks out slowly or actively. Determining where the Tertiary limestone aquifer is highly vulnerable in this state is made more complex by several factors. The upflow boundaries of main regional flowlines lie not in limestone but in Tertiary sand formations and aquifers that connect with (interfinger or grade into) the limestone at its inland edge. A main question presents itself: is the limestone recharged principally by lateral flow into it across this boundary, or is vertical leakage from overlying sandy formations toward the limestone’s inland boundary also important, i.e., where is the seaward boundary of principal recharging? Secondly, the ground surface drops more steeply in a seaward direction than the seaward dip of the buried top of the limestone: the limestone becomes more shallowly buried rather than more deeply buried in a downflow direction when one crosses the Orangeburg Scarp (roughly parallel to the coast and inland at the city of Orangeburg). In the large limestone subcrop area just seaward of the scarp the aquifer is more shallowly buried and very possibly less confined than in areas just above the scarp farther upflow. Thirdly, hydraulic modeling suggests that a broad area nearer the southern tip of the state is “leaky” and this implies the possibility of widespread slow natural or more rapid pumping-induced recharging, which if rapid enough can raise vulnerability appreciably. Finally, small areas of active or rapid recharging were suspected or known from potentiometric data from near the coast, these being outliers of recharging far away from the major recharge area. These last could easily be overlooked in any vulnerability assessment envisioning a typical regional aquifer. In this study we sought geochemical evidence of local or nearby recharging from an array of well sites that span the geographic extent of the limestone aquifer and encompass the several main stratigraphic settings.