The weathering of minerals at shallow depths provides much of the sink for acidic atmospheric deposition. The geology of the UK has been divided into four classes according to the extent to which the bedrock is likely to provide buffering and possibly give rise to acidic surface or groundwaters. Other modifying factors including soils, superficial deposits and land use are also considered. The likely sources and sinks of H+ in shallow groundwater systems are also reviewed.
Most available groundwater data are derived from major aquifers, often in carbonate-bearing lithologies which afford excellent buffering and which are unlikely ever to be adversely affected by acidic deposition. Relatively few data exist for shallow groundwaters widely used for domestic supplies in rural areas, especially in western and northern Britain where groundwaters might be most affected.
Acidic groundwaters and/or surface waters are defined on the basis of their alkalinity and calcite saturation index (SIc). Acidic groundwaters are most likely to occur where (1) acid soils are found and glacial drift is thin or absent, (2) the source of acidic inputs is high, (3) carbonate minerals are absent, (4) the residence time of water is short. Low alkalinity groundwater is found in many sandstone lithologies including the Sherwood Sandstone (a major UK aquifer), many Lower Palaeozoic strata and granites. However, existing data for susceptible areas are insufficient to show whether or not there have been significant long-term changes in groundwater acidity or alkalinity.