THE UK ACID WATERS MONITORING NETWORK - AN ASSESSMENT OF CHEMISTRY DATA, 1988-93

1. The current extent and distribution of acidified surface waters in the U.K. is assessed using data collected between 1988 and 1993 as par t of the U.K. Acid Waters Monitoring Network (AWMN). The data have bee n collected against a background of constant S deposition across the U .K. throughout the monitoring period. 2. Surface waters with mean pH < 5.0 occur in south-west Scotland, south-east Northern Ireland, the La ke District and south-east England. In addition, sites with mean pH < 5.5 are found in central and north-east Scotland, the Pennines and nor th Wales. These areas receive, and have historically received, high le vels of S deposition and xSO(4) is identified as the main contribution to non-marine acid anion concentrations. 3. Mean total oxidized nitro gen (TON) concentrations are > 10 mu eq l(-1) at ten of the twenty-two sites which are found in areas broadly corresponding to areas of high N deposition. The mean contribution of TON to total non-marine acid a nion concentration is > 10% at sixteen sites and > 20% at six sites. M ost sites, except for those with TON < 10 mu eq l(-1) and mainly locat ed in north-west Scotland and Northern Ireland, demonstrate strong sea sonality in TON concentrations and during autumn and winter instantane ous contributions of TON to total non-marine acid anions is up to 60%. This confirms the importance of N with respect to surface water acidi fication and strengthens the case for implementation of controls on fu ture N emissions. 4. The data period is currently too short for rigoro us trend analysis given the monthly/quarterly sampling frequency in st reams/lakes, respectively. Nevertheless, increased SO4 and xSO(4) can be observed in the time series at sites in south-west Scotland and nor th Wales despite constant deposition to these areas. Similar trends ar e evident in longer-term more frequent data at two sites and may refle ct hydrometeorological influences, especially the duration and intensi ty of hot and dry weather in summer which promotes mineralization of o rganic S.