The use of saline springs and the distribution of chloride as indicators of recharge to and groundwater movement within the Illinois Basin

The Illinois Basin, initially developed on Precambrian crust, occupies most of Illinois, and parts of Indiana, Kentucky and Tennessee, and is filled with Cambrian through Pennsylvanian-age sedimentary rocks. Paleozoic sandstones and carbonate rocks in northern Illinois are productive aquifers; however, groundwater in these rocks have a high salinity in central and southern Illinois. Seigel (1989) and Cartwright (1970) suggested that groundwater generally flows toward the center of the Illinois Basin and discharged as upward leakage across confining units. However, details of recharge to and groundwater flow within the Illinois Basin on a basin scale are poorly understood. Our investigation involves the use of saline springs and geochemical cross sections to explore this question. 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. Saline springs originate from Cambrian- to Pennsylvanian-age sedimentary rocks. Most springs were sampled for chemical and isotopic composition with emphasis on halide chemistry (Cl- , Br - , I - ). In addition, six cross sections across 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 saline springs and saline anomalies (plumes of saline groundwater discharging within freshwater aquifers). These springs and anomalies (also referred to as “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 on the frontier. Settlers converted some of these springs into lucrative commercial ventures as a source of salt. Later many became the focal points of health spas and/or state parks. Others were abandoned and forgotten, or destroyed. In general, Cl concentrations of these springs are typically around 500 mg/L, but are as high as 30,000 mg/L. Cl/Br mass ratios that range from 150 to 700 and were used to identify the source formations of the saline spring water. 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 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. The geochemical cross sections revealed active recharge to the basin from the north, west and east. Formations of low permeability (e.g., shales) tend to retain hypersaline brines, whereas formations with large permeability such as the St. Peter Sandstone tend to reflect focused recharge of fresh groundwater. A contour map of Cl concentrations of the St. Peter Sandstone was also prepared and revealed recharge areas (as above) and the effects of the LaSalle Anticlinorium on groundwater flow. The contour map suggests that the LaSalle Anticlinorium is providing a zone of greater permeability for groundwater movement from north to south. Structures are also important for downward movement of surface water and groundwater. Vertical pathways in the vicinity of the Ohio River in far southern Illinois where the river intersects the Cottage Grove Fault Zone appears to provide an avenue for recharge to near the base of Ordovician strata and groundwater movement south, into Kentucky.