Quantitative microbial source apportionment as a tool in aiding the identification of microbial risk factors in shellfish harvesting waters: the Loch Etive case study

Sanitary surveys of shellfish harvesting waters are now a routine component of regulatory monitoring. These provide a qualitative appraisal of potential pollutant sources impacting on shellfish microbial quality. The information provided by this type of screening level appraisal is very useful, but does not afford quantitative assessment of the different pollution sources and their complex dynamic relationships which result in a highly episodic flux of microbial parameters into shellfish harvesting waters. The potential fluxes derive from treated sewage and industrial effluents, intermittent discharges from the sewerage system and diffuse sources of pollution, principally from livestock farming areas, but also from urban surface water drainage. None of these sources are routinely monitored for the faecal indicator parameters used as compliance measures by regulators worldwide and almost no high-flow information is available with which to construct any quantitative flux estimates to provide a credible evidence base for the design of remediation strategies where there is a need to improve water quality within a harvesting area. This study was conducted at Loch Etive, near Oban, Scotland, UK. It applies an approach to Quantitative Microbial Source Apportionment developed to inform management and remediation of water quality at bathing water locations. The results suggested that, in this case study location, diffuse sources of microbial indicator organisms derived from livestock farming activities in catchments draining to the loch were the dominant high-flow contribution of bacterial loadings. This finding was unexpected by local managers who had perceived 'environmental' water quality as 'high quality' in this traditionally pristine area of west Scotland. The findings led to a series of recommendations for future management of Scottish shellfish harvesting waters directed at appropriate data acquisition, through a detailed sampling programme design, to acquire microbial flux data from all sources, particularly during high-flow event conditions. It was recommended that such data acquisition was essential to the design of any remediation strategies that need a credible evidence base directing appropriate investment in interventions designed to attenuate microbial flux from either the sewerage infrastructure and/or adjacent farming activities. The utility of this study could be further enhanced through microbial tracer studies to establish connectivity between the key hydrological inputs (both those studied here and potential sources outside of the lower basin) and the shellfish beds.