11.1 Ion Exchange in The Lincolnshire Limestone Aquifer, Eastern England

Concentrations of F in groundwater increase progressively down the groundwater flow gradient in the Middle Jurassic Lincolnshire Limestone (Inferior Oolite Group) aquifer of eastern England (Figure 13). The limestone dips gently eastwards at an angle of less than one degree and is covered eastwards by low permeability marls, clays, shales, and limestones of Jurassic age. The aquifer is some 30 m thick at outcrop and reduces to around 20 m thick in the confined section (Edmunds, 1973). The aquifer has been well-studied over the years (Bishop and Lloyd, 1990; Edmunds, 1973; Lamont, 1959; Moncaster et al., 2000) and significant downgradient changes in water chemistry have been documented, arising through a combination of carbonate reactions, pollutant inputs, redox changes, cation exchange and mixing with brackish formation water.

Figure 13 – Downgradient variations in concentrations of major ions, fluoride, pH and saturation indices for calcite and fluorite in groundwater from the Jurassic Lincolnshire Limestone aquifer, eastern England (after Edmunds and Smedley, 2013) © UKRI, 2021.

The concentrations of F increase in response to cation exchange as groundwater composition changes from Ca-HCO₃ composition at outcrop and subcrop to Na-HCO₃ composition further into the confined aquifer, ahead of the zone where groundwater encounters old brackish Na-Cl groundwater in the deepest parts of the aquifer. Downgradient decreases in Ca concentration are coincident with increases in Na concentration, and in the southern part of the aquifer, occur some 12 km downgradient of the aquifer outcrop (Figure 13). The downgradient changes also coincide with the increase in pH up to 8.4 (Edmunds and Walton, 1983). Throughout most of the aquifer, groundwater is saturated with respect to calcite, but becomes undersaturated in the downgradient section in response to loss of Ca by ion exchange. Saturation indices for fluorite indicate undersaturation throughout (Figure 13). This trend suggests that a carbonate-fluorapatite mineral may be responsible for controlling the F concentrations.