MECHANISMS CONTROLLING THE INORGANIC AND ISOTOPIC GEOCHEMISTRY OF SPRINGS IN A CARBONATE TERRANE.

摘要

The purposes of this investigation were to (1) determine the processes that control temporal variations in the chemistry of springs in a karst terrane, (2) explain the apparent closed-system solution of carbonate minerals in a cavern environment, and (3) determine the sources of recharge to springs in the central Pennsylvania karst. Water samples were collected from a mountain-runoff stream, and from a conduit-fed and a diffuse-fed spring. The variables measured at the three sites were discharge, pH, specific conductance, the major ionic species, (delta)('18)O of the water, and (delta)('13)C of the dissolved carbonate.;The chemistry of the mountain-runoff stream is controlled by dilution, evapotranspiration, and leaching of leaf litter and the products of pyrite oxidation by precipitation. For the conduit-fed spring, variations in water chemistry are controlled by dilution, seasonal fluctuations in soil zone P(,CO(,2)), pollution, and contributions from mountain runoff. The processes that control the variation in the chemistry of the diffuse-fed spring are leaching of salts from the soil zone during storms, solution of carbonate minerals, and evapotranspiration. Discharge is the most important control on water chemistry at all three sites. This results from mixing of waters with different chemical compositions in proportions that depend on flow volume. Seasonal variations are primarily related to evapotranspiration, either directly, through concentration of salts, or indirectly, through a seasonal variation in the availability of water for discharge.;Carbon isotopic compositions of the conduit-fed spring are similar to values predicted by models of calcite solution in the absence of a coexisting gas phase. This indicates most limestone solution occurs in a diffuse flow system fed by percolation recharge. After emergence into the conduit, this diffuse component rapidly equilibrates with the P(,CO(,2) )in the conduit gas phase, but retains its original (delta)('13)C values. Sixty-seven percent of the variation in the calcium concentrations of the conduit-fed spring can be attributed to mixing. An additional 10% of the variation in spring-water calcium concentrations results from a seasonal variation in the solution of limestone. This seasonal cycle lags five weeks behind the cycle in spring-water P(,CO(,2)), and suggests a minimum transit time for percolation recharge of about five weeks.;The mountain-runoff stream is fed by a shallow aquifer in the colluvium on the mountain slope, and by deeper aquifers in the mountain-top hydrogeologic environment. The average residence time of water in the colluvium is 9 1/2 weeks, with 5 1/2% of the ground-water volume replaced each week. About 26% of the base flow of the stream is derived from the shallow colluvium. On the basis of a hydrologic budget for an 18 month period, 60% of the discharge of the conduit-fed spring is derived from sinkhole recharge. The remaining 40% is derived from percolation recharge on mountain-slope areas that have no integrated surface drainage system and from the carbonate rocks at the mountain foot. The attenuation in the amplitude of oxygen-18 annual cycles between sinkhole recharge and the conduit-fed spring indicates a 50/50 split between sinkhole and percolation recharge contributing to the base flow of the conduit-fed spring. Detailed study of a snowmelt-induced hydrograph peak in the diffuse-fed spring and a rainfall-induced peak in the hydrograph of the conduit-fed spring revealed that little precipitation runoff contributed to the peaks. The storm hydrograph peak in the conduit-fed spring was comprised almost entirely of water that was in the conduit before the storm began. During snowmelt, the discharge of the diffuse-fed spring increased 11% above base-flow levels, but the maximum determined contribution from melting snow was only 4.4%.