《9 The Chalk Aquifer of Dorset》.pdf

9 The Chalk Aquifer of Dorset, UK W. M. EDMUNDS AND P. SHAND The Dorset Chalk contains some of the high- 91 considered constituents in the groundwa- est quality chalk groundwater in the UK. ters of the area are presented. However, much of the area is given over to It is concluded that the properties of farming, the long-term impacts of which groundwater in the Dorset Chalk are over- may impact on the natural groundwater whelmingly determined by natural reactions quality. The main observed effect of agricul- between rainwater reacting with the bed- tural activity is the increase in nitrate over rock, especially in the topmost few metres. several decades from a baseline value of It is generally of high quality but is being around 1 mg L–1 in the early twentieth cen- progressively impacted by agrochemicals in tury to concentrations up to the maximum unconi ned areas, the effects of which are acceptable concentrations of 11.3 mg L–1 evident even in some of the deeper ground- NO –N. In addition, a rise in potassium from waters. Waters of pristine (pre-industrial) 3 agricultural sources above the natural back- quality are rare in the basin and restricted ground is demonstrated. Apart from these essentially to the coni ned aquifer. examples of diffuse pollution, there is no evidence for widespread contamination from urban and rural wastes or other point sources 9.1 Introduction of pollution. The natural baseline is expressed as a The Chalk aquifer is the most important range of values which are controlled by