OR/15/048 Baseline groundwater chemistry

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.

Regional baseline characteristics

The baseline chemistry of an aquifer is determined by a wide range of physical and chemical processes. The major source of most groundwaters is rainfall. The Corallian aquifer of the Vale of Pickering is recharged by rainfall on the slopes of the North York Moors, and by the river Derwent (Natural England, 2015). The aquifer discharges water to springs and the River Derwent and its tributaries (Environment Agency, 2013). A series of chemical reactions takes place during recharge; the most important of which are mineral dissolution and precipitation along with mixing, redox reactions, ion exchange and sorption/ desorption reactions.

The waters in this study are all Ca-HCO₃ type. This is a reflection of the dominance of calcite in mineral reactions. The waters are at equilibrium, or supersaturated with respect to calcite. Concentrations of solutes in groundwaters can span many orders of magnitude. However all the major elements (with the exception of NO₃-N), have concentration ranges which span less than one order of magnitude. These are all indicative of a fresh, shallow, oxidised aquifer.

For most determinands the 95th percentile can be taken to indicate the upper limit of the baseline concentrations. This removes obvious outliers for most elements. In the case of the Vale of Pickering groundwaters, there are elements without obvious outliers, which can mean the entire range is likely to be representative of baseline concentrations. A notable exception to this is NO₃-N, for which the 95th percentile is 22.6 mg L^-1, and 20% of the analyses are above the drinking water limit of 11.3 mg L^-1. This is highly unlikely to be naturally derived, as supported by the fact that the concentrations at a number of sites are increasing. The most likely source is from farming, which is extensive across the Vale of Pickering, and the predominance of this widespread diffuse source, coupled with the mainly oxidised nature of the Corallian aquifer means that NO₃-N remains the major concern to groundwater quality in the Vale of Pickering. Additional potential sources of NO₃-N include domestic pollution.

Trace elements in groundwaters can be derived naturally from minor minerals within aquifer strata. Their distribution is dependent on natural abundance as well as speciation and local aquifer conditions. The baseline can be elevated by anthropogenic influences, which are usually evident as outliers in the data set. The cumulative probability diagrams show that such outliers are limited in the Vale of Pickering groundwater data, which is a reflection of the predominantly rural setting.

Extent of anthropogenic influences

Superimposed on natural baseline concentration ranges are anthropogenic influences. Such influences can directly affect groundwater chemistry via point source or diffuse pollution.

As discussed above, the major anthropogenic influence is via farming practices. Nitrate is a useful indicator of human impacts from farming. Nitrate pollution is one of the largest problems facing unconfined groundwaters in the UK (Rivett et al., 2007). The main cause of these high concentrations is the development of intensive agriculture, which uses NO₃-rich fertilisers. Nitrate is extremely mobile and there are a number of sites within the study with high, and increasing, NO₃-N concentrations which present a real concern to the groundwater quality in the region. In addition to the NO₃-N, there is an outlier of K and high Na and Cl concentrations which can also be attributed to farming practices.

There is no evidence for widespread contamination by trace elements; there are only a few isolated exceedances of drinking water limits. The lack of urbanisation in the region means there are more limited sources of such solutes.