Hydrochemistry of the Mahomet Bedrock Valley Aquifer, East-Central Illinois: Indicators of recharge and ground-water flow

A conceptual model of the ground-water flow and recharge to the Mahomet Bedrock Valley Aquifer (MVA), east-central Illinois, was developed using major ion chemistry and isotope geochemistry. The MVA is a “basal” fill in the east-west trending buried bedrock valley composed of clean, permeable sand and gravel to thicknesses of up to 61 m. It is covered by a thick sequence of glacial till containing thinner bodies of interbedded sand and gravel. Ground water from the MVA was found to be characterized by clearly defined geochemical regions with three distinct ground-water types. A fourth ground-water type was found at the confluence of the MVA and the Mackinaw Bedrock Valley Aquifer (MAK) to the west.

Ground water in the Onarga Valley, a northeastern tributary of the MVA, is of two types, a mixed cation-SO₄^2- type and a mixed cation-HCO₃ type. The ground water is enriched in Na⁺ Ca²⁺, Mg²⁺, and SO₄^2- which appears to be the result of an upward hydraulic gradient and interaction of deeper ground water with oxidized pyritic coals and shale. We suggest that recharge to the Onarga Valley and overlying aquifers is 100% from bedrock (leakage) and lateral flow from the MVA to the south.

The central MVA (south of the Onarga Valley) is composed of relatively dilute ground water of a mixed cation-HC0₃ type, with low total dissolved solids, and very low concentrations of Cl^- and SO₄^2-. Stratigraphie relationships of overlying aquifers and ground-water chemistry of these and the MVA suggest recharge to this region of the MVA (predominantly in Champaign County) is relatively rapid and primarily from the surface.

Midway along the westerly flow path of the MVA (western MVA), ground water is a mixed cation-HCO₃^- type with relatively high Cl^-, where Cl^- increases abruptly by one to two orders of magnitude. Data suggest that the increase in Cl^- is the result of leakage of saline ground water from bedrock into the MVA. Mass-balance calculations indicate that approximately 9.5% of recharge in this area is from bedrock. Concentrations of Na⁺, HCO₃^-, As, and TDS also increase in the western MVA.

Ground water in the MAK is of a Ca²⁺-HCO₃^2- type. Mass-balance calculations, using Cl^- as a natural, conservative tracer, indicate that approximately 17% of the ground water flowing from the confluence area is derived from the MVA.