Groundwater quality in the Illinois Basin, like most intracratonic basins in the Midwestern US, transitions from extremely fresh near surface (Cl = 1 to 13 mg/) to concentrated brines at depth (Cl > 100,000 mg/L). Naturally-occurring saline anomalies have been identified throughout the Illinois Basin within the states of Illinois, Indiana, Kentucky, and Tennessee. In the course of this investigation, more than 40 locations of “mineral” and saline springs within and at the margins of the Illinois Basin were identified and investigated. Most springs were sampled for chemical and isotopic composition with emphasis on halide chemistry (Cl, Br, I) in order to determine the source formations of the salinity. In addition, six cross sections through the Illinois Basin were prepared and included all available Cl concentrations to help evaluate local and regional groundwater movement into, within, and discharge from the basin.
Of the springs identified and sampled, only 23 were naturally-occurring NaCl-enriched, brackish and saline springs and NaCl anomalies (plumes of saline groundwater discharging within freshwater aquifers). These springs and anomalies (so-called “mineral springs” and “salt licks”) were recognized first by Native Americans and later by early settlers as good hunting grounds and a source of much-needed salt. Some of these springs were converted into lucrative commercial ventures as a source of salt in the early frontier. After the bottom fell out of the salt-making business in the mid-1800s, many sites were converted to health spas that thrived until the early 1900s. After the health spa business waned, many were converted into city and state parks. Whereas many of their occurrences and locations are a matter of historic record and are of historic significance, some of the once well-known brackish and saline springs have been abandoned, forgotten or destroyed. For example, the Vermillion Salines of nearby Kickapoo Creek State Park were buried under tons of waste as a result of coal mining in the area.
Overall, the saline springs of the Illinois Basin were found to be coincident with geologic structures within the basin and with basin margins to the south. Two major areas of saline anomalies coincident with geological structures stand out: 1) the La Salle Anticlinorium, and 2) the DuQuoin-Louden Anticlinal Belt possibly. Discharge along basin margins occur primarily in the southern end of the basin in Kentucky. In general, chloride concentrations of these springs are typically around 500 mg/L, but can be as high as 30,000 mg/L. Springs with the greatest concentrations of Cl usually contain H2S which supports colonies of white, filamentous, chemolithotrophic sulfide-oxidizing bacteria. Cl/Br mass ratios that range from 150 to 700 and were used to identify the source formations of the saline spring water. Spring water salinities originated from Cambrian- to Pennsylvanian-age sedimentary rocks; these springs may be the result of recharge-driven hydraulic gradients into and out of the basin. The existence of the saline springs is an indicator of pathways associated with geologic structures and saline aquifers within and near the margins of the Illinois Basin.
About speaker: Samuel V. Panno is a Senior Geochemist and has been with the Illinois State Geological Survey since 1988. Prior to joining the ISGS, Sam worked as a Geochemist/Hydrogeologist with Rogers and Associates Engineering Corporation, as an Associate Geochemist with Brookhaven National Laboratory, and as a mine geologist with Atlas Minerals. Currently, Sam is leading research efforts on various aspects of karst geology, and the hydrogeochemistry of groundwater in aquifers of Illinois and within the Illinois Basin.