Tracers of sewage sludge in the marine environment: a review.

A review of the available and potential tracers of the movement and distribution of sewage sludge ('sludge') in and on marine sediments is presented. A brief review of 'active' sludge tracers is followed by a more extensive review of 'passive' sludge tracers. These can be broken down into five categories: Natural organic components of sludge--organic carbon, faecal steroids, carbohydrate/TOC ratio, tomato seeds. Synthetic organic compounds--silicones, chlorinated hydrocarbons, non-ionic surfactants and their degradation products, linear alkylbenzenes. Stable isotopes of carbon, nitrogen, hydrogen and sulphur. Inorganic substances and their properties--minerals, paint pigments, trace elements, magnetic properties. Biological/Microbiological--faecal bacteria, Clostridium welchii spores, amoebae, viruses, nitrogen-fixing enterobacteriaceae. A wide variety of sludge tracers are available with the choice of suitable tracers in an individual situation governed by the available equipment and expertise, cost considerations, ease of sampling requirements and determination, and the nature of other pollutant inputs in the vicinity of the input of interest. Ideally, the sludge tracers chosen should be unique to the input of interest, but this will rarely be achievable in practice. In areas of gross sludge contamination, a marine geophysical survey technique can be used to delineate the area affected and this provides a rapid cost-effective method for large areas.