Understanding the Hydrology of Karst

Abstract

Determining the nature of water flow and contaminant dispersion in karst requires far more information than can be provided by simple dye traces. Tracing can delineate drainage divides, flow directions, and flow velocities at various stages, but from water management purposes it is also important to determine such variables as groundwater storage, retention times, patterns of convergence and divergence, and response to wet-dry cycles in the soil. These are most significant in the non-conduit portions of the karst aquifer, which supply most wells. Dye tracing can be augmented by hydrograph analysis at various stages, tracing with tagged solid particles or microbes, evaluation of dissolved solids and chemical equilibria, and isotopic analysis. This paper concentrates on some of the uses of chemical equilibria and isotopes. Stable isotopes (e.g. ¹⁸O and deuterium) and the various radium isotopes are among the most useful. Ratios among the four radium isotopes (²²⁸Ra and ²²⁴Ra, with half-lives in years; and ²²³Ra and ²²⁶Ra with half-lives in days) are well suited to karst studies. These techniques are time-consuming and costly, so a full analysis of a karst aquifer is rarely feasible. Instead, it is recommended that selective analyses be made of representative parts of the aquifer, and that they be applied as follows: (1) Develop conceptual models based on field observation, which allow one to anticipate a range of probable scenarios of contaminant transport and remediation. (2) If digital models are used, it is most effective to design simple generalized models in which the boundary conditions are clearly defined, and then to gain insight into real aquifers by noting the differences between the model and field observations. (3) Use field techniques to become familiar with the local hydrology and then apply hydraulic and chemical principles to anticipating contaminant behavior, rather than reacting only to emergencies. These approaches encourage the growth of interpretive skills based on the same scientific principles that govern the origin of caves and karst.