Madison water board sends out an annual drinking water report. The results shown in these reports are all average values. They switch wells and water routes depending on the season. Yes, they buy water from various sources depending on the price although the majority comes from Huntsville during the heavy grass watering period in summer. This shuffling affects the water quality considerably. My water has much higher nitrate and phosphate levels during the winter - my FW planted tanks love it!
If you are asking about the report because you want to use tap water for your salt water aquarium, I'd be really careful.
Although the drinking water report does not show any periodic spike in any parameters, I know it happens. My daphnia population used to drop drastically after water changes from time to time (I use tap water for them.) I suspect that copper level exceeded 0.5 ppm (1ppm is allowed by EPA standard.) When I started using either Amquel or Seachems Prime, both of which bind heavy metals, the daphnia population stabilized. I suspect that a low copper level can hurt salt water micro crustaceans such as copepod as well.
I recall that one of the curators of Tennessee Aquarium told us during our behind-the-scene tour that the minute amount of the copper in their tap water was affecting their cuttlefish display and that they had to switch the water to RODI water for them.
Tomoko
