SUPPLEMENTARY INVESTIGATIONS INTO THE FAILURE OF SOME AYRSHIRE BATHING WATERS TO MEET EC STANDARDS

SR(99)04F

Dec 1999

SUMMARY

Compliance failures against EC Bathing Waters Directive 76/160/EEC water quality standards in Irvine Bay bathing waters led to the need to investigate the fate and proportion of faecal pollution discharged from point and diffuse sources entering Irvine Bay. Principal point sources of faecal pollution include discharges, via Long Sea Outfalls (LSO), from Stevenston Sewage Treatment Works (STW) and Meadowhead STW. Principal diffuse sources of faecal pollution, from agriculture and Combined Sewer Overflows, enter Irvine Bay, via rivers which drain the Irvine catchment, including River Irvine and River Garnock.

Statistical evidence collated by Centre for Research into Environment and Health (CREH) indicated an elevation of coliform concentrations entering coastal waters during significant rainfall events when river flow increased possibly having a significant contribution to the failure of bathing water standards (CREH, 1998). Further studies by CREH (1999) showed that over a period of 8 weeks, between 3.8.98 - 28.9.98, 99.5% of the indicator organism load entering the marine coastal waters was discharged from the two STWs during base (low) river flow conditions. In contrast to base flow conditions, during high river flow conditions, 74% of the Faecal Coliform load (FC) was discharged from the two STWs with 11.3% of FC entering Irvine Bay originating from the River Irvine and River Garnock. CREH (1999) High levels of faecal pollution recorded in the riverine input to the coastal waters during periods of high rainfall and river flow were not considered as significant in terms of overall loading to Irvine Bay as the STW discharges.

Supplementary investigations were required to more fully understand the coastal processes occurring in Irvine Bay and determine how these processes control the relative contributions of faecal pollution from the main point sources and collective diffuse sources under different flow conditions at ECBW sites in Irvine Bay.

Environmental Tracing Systems Ltd (ETS) were commissioned by Scotland & Northern Ireland Forum for Environmental Research (SNIFFER) to carry out the supplementary investigations with SEPA West Region. The study was conducted by releasing different coloured fluorescent tracer particles (FTP), designed to mimic the behaviour and fate of sewage indicator organisms, from each source. The study was designed to utilise FTP to assess the rate and direction of plume advection and identify the relative contribution of faecal pollution load between diffuse and point sources within Irvine Bay bathing waters.

Two surveys were carried out, one during base river flow conditions (15.9.98) and one during slightly higher river flow conditions (12.5.99). The survey comprised the collection of duplicate water samples from coastal waters, near shore waters and beaches throughout Irvine Bay including Saltcoats ECBW and Irvine Beach ECBW to determine the concentrations of each FTP released and faecal coliform. In addition to the collection of water samples, salinity distribution and current velocity data were collected. Towed fibre-optic fluorimeters were used to measure FTP concentrations in real-time over a wide area.

During base flow conditions, the measure FTP concentrations indicate that Stevenston STW and Meadowhead STW have the highest relative contributions at Saltcoats ECBW site and Irvine Beach ECBW sites respectively. The load from the Irvine Estuary is less than 1% for sampling sites both north and south of the estuary.

During high flow conditions, the measured FTP concentrations indicate that the source or faecal pollution at Irvine Beach ECBW site is due to a combination of Meadowhead STW (approximately 60%) and the Irvine Estuary (approximately 35%). At Saltcoats ECBW site, Stevenston STW has the highest relative contribution.

A relatively strong relationship was measured between FC and FTP from Meadowhead STW on the base flow survey, carried out on the 15.9.98. A positive correlation was measured between FC and FTP for the water samples collected by both drogues on the 12.5.99 survey. Generally no relationship was observed between the FTP released in Irvine Estuary and FC for both surveys.

The main transport mechanism for FTP advection appeared to be wind-driven circulation with a surface wind-driven current counterbalanced by a return flow lower in the water column. Tidal currents were relatively weak by comparison. Wind-driven circulation led to wide and rapid advection of FTP emerging from the LSO and estuary. The rate of advection of FTP, FC and drogues (12.5.99 survey) was higher than the measured current velocities and often in the opposite direction. The current meter was positioned 0.5-4m below the water surface for both surveys hence recording data lower in the water column than FTP in the surface water.

The study carried out suggests a wide spread advection of faecal pollution from point and diffuse sources with Irvine Bay to bathing waters and beaches within a matter of hours. Measured FTP concentrations indicate estimated FC concentrations to exceed EC Mandatory standard for bathing waters at a few beach sampling sites and to exceed EC Guideline standard at many more beach sampling sites due to point sources on the 15.9.98 survey and from a combination of diffuse and point sources on the 12.5.99 survey.

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