Jeandeflorette

Hello everyone, This is my first post; I’m looking for some advice, please. We live in a rural hamlet of a dozen houses in Charente Maritime, with no mains sewage. Some of the neighbouring properties almost certainly have outdated fosse arrangements, meaning that water from washing machines and showers flows down the gutter in the street, past our garden gate. Occasionally ‘water’ from someone’s fosse also flows - with the accompanying odour. It all ends up in the stream which eventually ends up in the Charente. My question is this: who is the (water?) authority I could contact about this? I had thought the danger to the water system would be the way to go, but I may well be wrong. Grateful for any ideas. Thanks