SPRINGFLOW HYDROGRAPHS: EOGENETIC VS. TELOGENETIC KARST

Traditional observations and interpretations of springflow hydrographs from karst areas are from telogenetic karst (e.g., mid-continent Paleozoic limestones), where the karst has developed after the limestone has been deeply buried and lithified. Limestones of telogenetic karst have very low porosities (less than 5%) and matrix permeabilities (on the order of 10^-15 – 10^-18 m²). Hydrographs of springs in telogenetic karst show a prompt response to precipitation events and a rapid return to base flow conditions. Hydrograph separation reveals components that represent storage within the conduits, a network of fractures, and the epikarst. Only a very small percentage of the full volume of the aquifer appears to participate in the precipitation event because the matrix permeability is so small.

Thirty-three first-magnitude springs, with mean flow in excess of 100 cfs, discharge from the eogenetic karst of Florida, where the Upper Floridan aquifer is unconfined. Interparticle porosity (20-40%) and matrix permeability (10^-12 – 10^-14 m²) of these Eocene and Oligocene limestones, which have not been deeply buried, are in striking contrast to those of telogenetic karst. So too are the spring hydrographs. With the exception of springs that are river-rise (flow-through) systems, springs of Florida’s eogenetic karst do not show single storm events, although they do vary according to seasonal or longer-period cycles. It appears that the huge interparticle pore volume of the aquifer mutes, and even eliminates, the spiky responses that have come to be featured in conceptualizations of karst springs.

Modeling efforts that seek to put simulated hydrographs in the context of the multi-continua that characterize karst aquifers will need to recognize the difference between eogenetic and telogenetic karst and pay special attention to the matrix of eogenetic karst.