Geochemical methods for distinguishing surface water from groundwater in the Knox Aquifer System

Abstract

The Knox Group, a thick package of Cambro-Ordovician rocks, occurs over a wide geographic area in the southeastern US. Characteristics of the Knox Group include strong structural control on porosity and permeability, deep near-vertical solution features, great depth of water circulation, dolomite, as well as limestone, hosting the karstic features, and extreme anisotropy and heterogeneity. In this study, geochemical methods were used to distinguish ambient groundwater in the Knox aquifer from surface water, specifically, water leaking from the Logan Martin reservoir in east-central Alabama. Major cations and anions, as well as stable isotopes of hydrogen and oxygen, were used to distinguish lake water from groundwater, and to determine mixed waters. Lake water and groundwater components for mixed waters were calculated, and mapped in plan view. A relatively narrow zone of mixing occurs in the vicinity of Logan Martin dam in map view, which is consistent with the hydrogeological conceptual model of deep near-vertical solution-widened fractures (fissures), oriented east–northeast and to a lesser extent northwest, in a much less permeable dolomite matrix.