Mixing of Rapid Recharge and Rapid Flowing Ground Water: Implications to Protecting Municipal Wells in the Woodville Karst Plain, North Florida

The Woodville Karst Plain (WKP) of northwest Florida has formed in a coastal carbonate terrane that includes the longest (~75 km explored and mapped) flooded cave system in the USA. Ground water tracing between swallets, sinkholes and springs has been done showing that velocities in conduits are near the global average of 1.7 km/day and discharge to large springs such as Wakulla spring occurs in days to weeks along continuous conduit pathways, some nearly 20 km long.

It has been shown that spring chemograph variability is strongly correlated with the percentage of concentrated recharge via swallets. WKP recharge is both concentrated via swallets that transmit large volumes of tannic waters and more distributed (but still rapid) direct recharge through thin sandy soils. Wakulla spring shows a quick response to storms even though the closest swallets, that tracing shows provide up to 20% of its discharge, are several kilometers away. Tracing data show that Wakulla only discharges a portion of waters in the basin, the remainder must discharge into the Gulf of Mexico via coastal springs.

Karst ground waters typically have strong meteoric signatures. The average electrical conductivity in wells of the WKP even including some north of the Cody Scarp cap rock is only about 270 uS/cm. Rapid recharge and circulation keeps the hardness low. Rapid circulation means there must be mixing and this is also shown by other isotopic and geochemical data. However, when other investigators determine isotopic ages the assumption is made that there is no mixing. Difficult questions need to be asked of that because a no mixing assumption is obviously incorrect, and the specious ages obtained are being used to calculate modeling parameters used to delineate municipal well-head protection zones.