Baseline nitrate concentration in groundwater of the Chalk in south Dorset, UK

Determining the degree of nitrate pollution in Chalk groundwater is difficult without a clear understanding of concentrations naturally present. In the UK, a general shortage of long-term records of nitrate concentrations in Chalk groundwater prevents a full quantification of baseline concentrations. This paper presents late nineteenth and early twentieth century data on nitrate concentrations in Chalk groundwater in south Dorset, UK and compares them with corresponding data for the last 25 years. The nitrate record is instructive in (1) providing an insight into the baseline nitrate concentration of groundwater; (2) defining the level of nitrate pollution in potable supplies from the Chalk aquifer; and (3) identifying the long-term variations in local nitrate concentrations. Over a period exceeding 100 years mean nitrate concentrations increased from 1.04 mg NO3–N l−1 to 6.37 mg NO3–N l−1. Anthropogenic modification of the local groundwater composition is not evident in the early nitrate record. Nitrate data throughout this early period reflect natural background concentrations of approximately 1 mg NO3–N l−1. Intensified fertilser use and increased livestock numbers are suggested as the fundamental cause of the rise in nitrate concentrations. The implications of the nitrate record for regional hydrogeological processes are discussed.