The U.K. Acid Waters Monitoring Network: an assessment of chemistry data, 1988–93

Abstract

1. The current extent and distribution of acidified surface waters in the U.K. is assessed using data collected between 1988 and 1993 as part of the U.K. Acid Waters Monitoring Network (AWMN). The data have been 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 Lake District and south‐east England. In addition, sites with mean pH < 5.5 are found in central and north‐east Scotland, the Pennines and north Wales. These areas receive, and have historically received, high levels of S deposition and xSO4 is identified as the main contribution to non‐marine acid anion concentrations.3. Mean total oxidized nitrogen (TON) concentrations are > 10 μeq 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 anion concentration is > 10% at sixteen sites and > 20% at six sites. Most sites, except for those with TON < 10 μeq l–1 and mainly located in north‐west Scotland and Northern Ireland, demonstrate strong seasonality in TON concentrations and during autumn and winter instantaneous contributions of TON to total non‐marine acid anions is up to 60%. This confirms the importance of N with respect to surface water acidification and strengthens the case for implementation of controls on future N emissions.4. The data period is currently too short for rigorous trend analysis given the monthly/quarterly sampling frequency in streams/lakes, respectively. Nevertheless, increased SO4 and xSO4 can be observed in the time series at sites in south‐west Scotland and north Wales despite constant deposition to these areas. Similar trends are evident in longer‐term more frequent data at two sites and may reflect hydrometeorological influences, especially the duration and intensity of hot and dry weather in summer which promotes mineralization of organic S.