Research has been carried out over a 10 month period with the aim of identifying potential sources of Salmonella contamination in a Mediterranean lagoon (the Thau Lagoon). Two types of source have been sampled, permanent ones which are checked monthly, and incidental ones.

The highest occurrence of Salmonella is linked to episodic events brought on by rainfall. After summer low water levels, the rising of river water brings back into circulation water from pools previously formed by waste water from sewage treatment plants and sediments, thus creating a significant source of bacteria. The sewer system can also supply the Thau Lagoon with varying quantities of Salmonella when the system breaks down, and also when it is overloaded by an excess of rainwater following a storm. Monthly checks reveal that discharges of Salmonella occurred occasionally in permanent inputs, whereas the outflow from the Mèze pond provides a quasi-permanent, but negligible, source of contamination. The major risk of shellfish breeding contamination is thus associated with rainfall, and the occasional overflow of raw wastewater.The survival of Salmonella in the lagoon waters varies in function with the seasons, and the risk is greatest at those times of the year when autopurification can be affected.

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