We own an irrigation company. I am woefully ignorant myself in the area of mechanical operations of such systems, my husband being the expert in the design/installation area, but I seem to remember that he installs a backflow preventer in every new system, between the water meter and the main line of the system, and I don't remember it being that expensive. This backflow preventer inhibits, in the case of water lines/sprinkler heads installed in turf areas, the seepage of contaminants from that area back into the main water supply.
In most jurisdictions, Back-flow preventors have been required for a decade on new taps for water service. However, the thousands of existing homes, facilities, fire sprinkler systems and irrigation systems that pre-existed that requirement have the ability to allow contaminants into the localized portion of the water system to which they are connected.
If an arm or potion of a system becomes polluted or contaminated it can be flushed and, even, sterilized. However there can be a lot of time for the contaminant to move within the system prior to finding the locale and source of a problem. First the illness must incubate or manifest symptoms. Then they have to be diagnosed. Then, after diagnosis, the illness has to be vectored back to a particular water source such as home, office or short-visited establishment that was contaminated internally or from outside, nearby, utility contamination. Even then, samples must be gathered and tested.
Perhaps one of the biggest issues is that we are a society that is angered by inconvienience. Shutting down a potion of a town's water system at the first inkling of a problem will produce backlash as toilets and washing machines must stop as well. Boil Orders aren't carefully followed in instances of benign contamination during a flood.
Hi Goldfinch, ask your husband for more info on this. Thanks. ;9)