Title of a movie in which infected water supply turns towns residents crazy?

Have been trying to find out what this movie i saw as a kid was called. I remember very little but from what I recall I believe it was a sci fi film possibly from the 70s or 80s (definately in colour) in which something gets into the water supply of a small desert town and strange happenings begin to occur. One scene that for some reason sticks in my mind involves a large slab of concrete with a crack in it and some kind of white liquid pouring out?. That is really about as much as i remember other than I found the movie quite chilling at the time. I also have a vague idea that the film's title could be the name of the town in which it is set but may be completely wrong on this. I would really love to know if this movie exists or wether i just dreamt it!