Every community council in north Wales has been contacted by Plaid Cymru's regional MS Llyr Gruffydd to assess sewage contamination problems locally.
The move comes after complaints about sewage overflow in the village of Capel Curig last year.
Dwr Cymru - Welsh Water revealed that there were 3,000 communities in Wales facing similar problems and that Capel Curig wasn't even in the top 500 - despite raw human sewage flowing down the village's main road and into the nearby river at the height of summer.
Llyr Gruffydd said: "The problem in Capel Curig was bad enough but to be told, in a fairly off-hand way, that there were 500 communities more badly affected in Wales was quite shocking. I'm hoping that community councils, which have their ear to the ground, will be able to provide more detail in terms of sewage hotspots - despite requests Dwr Cymru have not provided a comprehensive list."
The letter sent to community councils: