Faecal indicator pollution from a dairy farm in Ayrshire, Scotland: source apportionment, risk assessment and potential of mitigation measures

The objectives of this study were (i) to apportion sources of faecal coliforms (FC) and faecal streptococci (FS) loads from a dairy farm to a stream in the Irvine catchment; (ii) to assess efficacy of pollution mitigation measures installed on this farm; (iii) to assess frequency with which observed FC loads contribute to high risk of failure of coastal bathing water to meet microbial standards. FC and FS loads in a primary stream running through the farm were estimated at up to four stations (above the farm, above the steading, below the farm and below a pre-existing 0.6 ha area of open water/wetland), in summer 2004 and 2005. During this period, steading and field mitigation measures were being installed. We estimated that farm FC loads likely to cause bathing water failure were 8.9 x 10(8) colony-forming units (cfu) ha(-1)d(-1) (guideline standards) and 1.7 x 10(10)cfu ha(-1)d(-1) (mandatory standards). In 2005, the guideline exceedance risk (42%) was associated about equally with field and steading sources, whereas the mandatory exceedance risk (11%) was mainly due to steading sources. The pond/wetland below the farm reduced these exceedance risks to 20% and <1%, respectively, but was a source of FS at high discharge. The exceedence curves below the farm were similar in 2004 and 2005, despite lower flows and installation of pollution mitigation measures. The results suggest that further mitigation efforts on this farm should be focused on improving management of the steading FC sources and that wetlands to receive steading runoff can be cost:effective mitigation tools for faecal indicator pollution.