Correlations between fluviatile sandstone lithofacies and geochemical properties and their importance for groundwater contaminant transport

Reactive groundwater contaminant transport is dependent upon hydraulic and geochemical property distributions. The aim of this research was to determine, for an example fluvial sandstone: (i) the degree of correlation between geochemical property values and lithofacies (LFs); and (ii) the effect of the correlations on reactive solute transport. A 60 m core from the carbonate-cemented, red-bed Triassic Sherwood Sandstone Group was examined. Five fluvial LFs were identified, in fining upwards cycles from a channel lag sandstone (LF5) to a low energy mudstone (LF1). The main geochemical properties investigated were carbonate content, oxide content, sorption capacity, cation exchange capacity (CEC) and selectivity coefficient; Surface area and colour were also studied. There is a clear distinction in carbonate content, CEC, selectivity coefficient and Fe and Mn oxide content between LF1 and LF2, the ‘mud’ LFs, and the matrix of LF3, LF4 and LF5, the ‘sand’ LFs. In addition, the pellets change the bulk geochemical properties of the sand LF units. Modelling indicated that the pellets can affect solute mobility significantly. Selectivity coefficients show a possible depth zoning. Distribution parameter values obtained characterize the sequence for geochemical modelling and uncertainty estimation. Oxide coating thickness was estimated at about 100nm. CEC was found to be well-correlated with surface area. Colour/Fe/Mn content correlations suggest a possibility in future to use colour logging to indicate oxide content and possibly even surface area and CEC. As the sequence examined appears typical of many red-bed fluvial sequences, it is suggested that the findings may also be typical of other similar continental sandstones.