|Bearcock, J M, Smedley, P L and Milne, C J. 2015. Baseline groundwater chemistry: the Corallian of the Vale of Pickering, Yorkshire. British Geological Survey Internal Report, OR/15/048.|
The Corallian aquifer in the Vale of Pickering comprises three formations, although the formation cannot be distinguished from the groundwater chemistry, as its natural character is relatively unvaried. The chemical compositions have been characterised by investigation of the statistical distributions, mapping the spatial variability, assessment of temporal changes and interpretation of the most likely geochemical processes. For many analytes the 95th percentile can be taken to be a first approximation of the upper limit of baseline concentrations, for others it is likely the entire range represents baseline concentrations, but for NO3-N the upper limit of the natural range is more uncertain.
The main properties of groundwater are determined by rainwater recharge reacting with aquifer minerals. Of these minerals calcite is the dominant influence on the water chemistry, resulting in waters of Ca-HCO3 type. There is little variation in the major ion proportion in the Corallian groundwaters of the Vale of Pickering, suggesting these data represent groundwater in equilibrium with the minerals which give it its character. Most of the groundwaters are oxic reflecting the dominance of samples from the unconfined aquifer. Mildly reducing conditions are apparent only on the periphery of the confined aquifer.
The most obvious anthropogenic influences on the groundwater chemistry are seen in the spatial and temporal variations in NO3-N, which is likely to be dominated by modern intensive farming practices. One fifth of the spatial data exceed the current drinking water limit of 11.3 mg/L, and half of the EA’s monitoring data show a trend of increasing concentrations, some to more than twice the drinking water limit. This is a reflection of the prevalence of farming in the region, and probably reflects the intensification of farming methods over the last few decades.
Of the elements tested there was no evidence for widespread contamination by potentially harmful trace elements, any outlier concentrations tend to be localised and probably represent natural anomalies, a small point source pollution source, or contamination derived from the borehole head workings.
The groundwaters in this region were generally of good inorganic quality, with the exception of the prevalence of high concentrations of nitrate.