The usage of HPC data in a drinking water context is apparently complicated and subjective. Some Countries use it for Regulatory/specific products, eg bottled drinking water, others use it more for assessing water treatment efficiency.
The 5 following links/attachments hopefully give a little detail on the US situation -
(1) Historical up to 2003 –
12.6.6 Regulations in the USA
Regulations for drinking-water quality from both private systems and public water utilities in the USA are provided by the US EPA. Drinking-water is under federal jurisdiction, so these regulations are enforceable across the country.
In the USA, acceptable HPC levels in municipal drinking-water have been set at less than 500 cfu/ml. Historically, as is the case in Canada, this level was recommended because higher colony counts interfered with the detection of total coliforms in lactose-based tests. During the development of the Surface Water Treatment Rule, it was decided that maintaining an HPC concentration below the allowable 500 cfu/ml limit could be used as a substitute for maintaining a detectable disinfection residual (US EPA 1989). More recently, the US EPA’s National Primary Drinking Water Standards (US EPA 2001) express HPC as a method of measuring the variety of bacteria present in a water sample but with no health significance. In this secondary standard, no maximum contaminant level goal is set, but the maximum contaminant level is still 500 cfu/ml. This is not an enforceable federal standard.
Other agencies, such as the American Water Works Association, have not recommended an operating level or goal for HPC bacteria in drinking-water. They do recommend minimizing HPC levels in water leaving the treatment plant and for water in the distribution system. It is suggested that each utility should establish baseline data for their water source based on at least two years of sampling of plant effluent, points of mean residency time in the distribution system and problem areas, such as dead-end reservoirs and sites downstream from pressure-reducing valves (AWWA 1990).
In the USA, bottled water is monitored by the Food and Drug Administration, and no HPC standards have been established (FDA 2001).
(Note1 - The AWWA viewpoint may now hv changed somewhat, eg this 2015 quote -
(Note 2- Regarding EPA, see (3) below for present scenario.[newer technology reduces the interference aspect]
(2) A more recent article (2016) presenting a quite readable "update" from a (unusual) POV –
HPC - health concerns,2016.pdf 501.07KB
(3) EPA’s general opinion on HPC in drinking water
(4) Missouri’s interpretation/implementation of EPA’s Guidelines
(5) Although not USA-specific, this detailed analysis on the Canadian Gov. website of a situation quite similar to that in OP is well presented IMO.
Further analysis will probably require data such as mentioned in Posts 6, 13.