Analysis of well hydrographs in a karst aquifer: Estimates of specific yields and continuum transmissivities

Abstract

Hydrograph analysis techniques have been well developed for hydrographs obtained from streams and springs, where data are cast in terms of total discharge. The data obtained from well hydrographs provide water level versus time; hence, a method of hydrograph analysis is required for situations in which only water level data are available. It is hypothesized here that three segments on a recession curve from wells in a karst aquifer represent drainage from three types of storage: conduit (C), fracture (F), and matrix (M). Hydrographs from several wells in a karst aquifer at the U.S. Department of Energy Oak Ridge Y-12 Plant are used to estimate the specific yields (S{sub y}) associated with each portion of the aquifer (C, F, M), as well as continuum transmissivities (T). Data from three short injection tests at one well indicate continuum T at this well bore is {approximately} 5m{sup 2}/d, and tests at numerous other wells in the aquifer yield results between 1 and 7 M{sup 2}/d. The T estimated with well hydrographs from two storm events indicates a T of 9.8 m{sup 2}2/d. Well developed conduit systems in which water levels in wells show a flashy response typically show S{sub y} values of 1{times}10{sup -4}, 1{times}10{sup -3}, and 3{times}10{sup -3}, for C, F, and M. Less well developed conduit areas show more nearly equal S{sub y} values (8.6{times}10{sup -4}, 1.3{times}10{sup -3}, 3{times}10{sup -3}). Areas with no evidence for the presence of conduits have only one, or in some cases two, slopes on the recession curve. In these cases, water level responses are slow. Recession curves with a single slope represent drainage from only the lower T matrix. Those with two slopes have an additional, more rapid response, segment on the recession curve, which represents drainage from the higher T, lower S{sub y}, fractures in the system.