Surface water acidification in the South Pennines I. Current status and spatial variability

The South Pennines, an area of acid-sensitive geology at the centre of a ma jor industrial region, have undergone perhaps the most severe historic expo sure to sulphur and nitrogen deposition in the UK. This study addresses a l ack of existing research on the region by presenting the findings of a surv ey of 62 surface waters sampled during a I-week period in April 1998. Resul ts indicate that acidification in the region is acute; 27 of the sampled su rface waters had a negative acid neutralising capacity (ANC) and 28 had a p H below 5.0. Minimum recorded pH values were below 4.0. Non-marine sulphate levels were extremely high (median 222 mu eq/l), and widespread high nitra te concentrations (median 41 mu eq/l) suggest that soils in the region as a whole may be at an advanced stage of nitrogen saturation. A consistent rel ationship was identified between site acidity and the balance between the m ajor weathering-derived cations, calcium and magnesium, and sulphate. This could in turn be linked to catchment soil type and land use, with the most acidic conditions occurring in peat-dominated catchments, where weathering is minimal and the influence of atmospheric deposition most pronounced. Per centage of peat in each catchment was the single best predictor of surface water acidity. Nitrate concentrations, although not a dominant control on a cidity, varied significantly according to land use. Elevated concentrations were observed in catchments containing forestry, due to enhanced depositio n inputs, and in catchments containing improved land, linked to fertiliser use. Ammonium concentrations, although low at most catchments, were a signi ficant component of the inorganic nitrogen total in a number of surface wat ers draining waterlogged peat catchments. (C) 2000 Elsevier Science Ltd. Al l rights reserved.