Assessing the effectiveness of regulatory controls on farm pollution usingchemical and biological indices of water quality and pollution statistics

Water quality was measured in 42 streams in the Colebrooke and Upper Bann c atchments in Northern Ireland over the period 1990-1998. Despite ongoing. p ollution control measures. biological water quality, as determined by the i nvertebrate average score per taxon (ASPT) index, did not improve and there was no appreciable decline in recorded farm pollution incidents. However, the lack of decline in pollution incidents could reflect changes in detecti on policy, as a greater proportion of incidents were recorded from less pol luting discharges such as farm-yard runoff. In contrast, there was an impro vement during 1997 and 1998 in annual chemical water quality classification based on exceedence values (90th percentiles) for dissolved oxygen, ammoni um and BOD c oncentrations. In 1998, 11.9% of streams were severely pollute d compared to 26.2% in 1990. while the proportion classed as of salmonid wa ter quality, increased from 40.5% in 1990 to 59.6% in 1998. Although water quality in 1996 did not improve relative to 1990 values, there was a notabl e increasing trend from 1990 in the numbers of samples taken during the sum mer which had good water quality with low ammonium ( < 0.6 mg N l (1)) and high dissolved oxygen ( > 70% sat). The trend for samples with low BOD ( <4 mg I (1)) was more erratic, but an improvement was apparent from 1994. The se improvements in chemical water quality suggest that point-source farm po llution declined after 1990. The fact that this was not reflected in stream biology may reflect the limited time scale for biological recovery. An imp ortant factor preventing biological recovery may be the high pollution capa city of manures and silage effluent, so that even reduced numbers of farm p ollution incidents can severely perturb stream ecosystems. The intractable nature of farm pollution suggests that there is a need to consider an inter active approach to problem resolution involving both farmers and regulators . (C) 2001 Elsevier Science Ltd. All rights reserved.