Isotopic and trace element constraints on the origin and evolution of saline groundwaters from central Missouri

Abstract

Na-Ca-Cl groundwaters with salinities of 1 to 30% discharge from natural springs and artesian wells in Mississippian carbonates and Ordovician sandstones and carbonates in central Missouri. Carbonate saturation and quartz supersaturation are maintained throughout the salinity range. Major and trace element and isotopic variations in the waters are used to place constraints on models for rock-water interaction and regional hydrology. The groundwaters have δD values that range from −108 to −45‰ and δ^(18)O values that range from −14.7 to −6.5‰ (SMOW). These data lie approximately along the meteoric water line, ranging from values similar to local recharge in Missouri, to significantly lower values that are similar to δ^(18)O and δD for meteoric recharge in high altitude and high latitude regions of North America. The more saline samples have the lowest ^(18)O and D contents, a trend that is opposite to that observed in most other studies of saline waters. The H-O isotopic correlation and the range in salinity and several major and trace element concentrations in central Missouri groundwaters are readily explained by a saline-dilute water mixing model. 87Sr/86Sr ratios for the groundwaters range from 0.7155 to 0.7161. These values are significantly higher than previously published values for: 1) estimates of ^(87)Sr/^(86)Sr in Phanerozoic seawater (0.7068–0.7092); and 2) carbonates of the Mississippian Burlington-Keokuk Fm. (0.7075–0.7105), through which some of the waters migrate as they pass to the surface. ϵNd(0) values in the groundwaters range from −10.9 to −8.1, and ^(147)Sm/^(144)Nd ratios range from 0.108 to 0.128. These values are similar to or lower than previously published analyses of carbonates and other authigenic phases from the Burlington-Keokuk Fm. (mean ϵNd(0) = −7.8 ± 0.8 (1σ); mean ^(147)Sm/^(144)Nd = 0.141). The H, O, Sr and Nd isotopic data and the results of model calculations preclude: 1) models involving the modification of ancient seawater for the origin of the saline waters, and 2) extensive interaction between the groundwaters and their host carbonates. The waters apparently acquired their δ^(18)O and δD values as meteoric recharge and their 87Sr/86Sr and rare earth element signatures from extraformational crustal sources (high ^(87)Sr/^(86)Sr; low ϵNd(0); low 147Sm/144Nd) and largely preserved these isotopic signatures during subsequent migration through the carbonates. Chemical exchange via rock-water interaction is required for isotopic exchange to occur. Therefore, the present state of chemical equilibrium between the groundwaters and carbonate aquifer minerals may serve to limit rock-water interaction, and hence preserve the extraformational isotopic signatures in the waters. The integration of geochemical, isotopic and hydrologic data on a local and regional scale suggests a history for the central Missouri groundwaters involving: 1) meteoric recharge in the Front Range of Colorado; 2) dissolution of Permian halite in the subsurface of Kansas; 3) interaction with predominantly silicate mineral assemblages in Paleozoic strata (and possibly Precambrian basement), with aquisition of crustal Sr and REE signatures; 4) dilution and migration to shallow aquifer levels in central Missouri; and 5) mixing with local meteoric recharge through Mississippian carbonates with no significant change of the isotopic signatures acquired in stage (3).