Long-term proficiency testing for the UK Acid Waters Monitoring Network

A series of proficiency test rounds to support the UK Acid Waters Monitoring Network has been conducted since the inception of the Network in 1988. These tests provide an illustration of the accuracy and comparability of analysis for laboratories that supply data to the Network. A summary of the tests is reported. Performance with respect to accuracy targets defined for the Network has been shown to be satisfactory for the majority of measurands of interest in studies of surface water acidification.