Steev,

The short answer (if you had to pick one): E. coli.

The long answer depends on what your "risk" refers to. If you are concerned about immediate drinking water quality in terms of health, based on your original samples (and this is likely most important), E. coli is still the best indicator of this risk. It is "the" indicator of fecal contamination.

Alternatively, if you are concerned about the possibility of future fecal contamination total coliform bacteria (usually measured in colony forming units (CFU)) may be important. Total coliform bacteria can, and normally are, found in water with bacteria sources other than fecal waste. However, their presence in water, particularly drinking water, indicates susceptibility for future contamination since the water is in contact with other coliform sources (e.g., soil, vegetation matter, etc.). If you are talking about treated water the presence of total coliform bacteria may be more sensitive to treatment short-comings (in cases that possessed coliforms, but at the time of sampling, didn't happen to contain E. coli).

I hope this answer helps.

Regards,

Matt

Most of my coliform work was decades ago, and we used total and fecal coliform, based on forest conditions, and we did not have many cattle so we ruled out fecal strep.  Much of the info I had was from Millipore Filter Company and from the Standard Methods for Analysis of Water and Wastewater decades ago.  We also used fecal coliform for impacts within the Chattooga River including Stekoa Creek which flow through Clayton, GA with a treatment system that is periodically overwhelmed with stormwater (that report about 1998 in researchgate).  This has been several decades ago, so more recently there has been more attention to eColi from food contamination by humans.  I would tend to agree with Dr Bolton that Ecoli may be the best for some things, but he posed different questions that suggest it may depend on specifics of the situation.  I am glad you are interested.  Most people think water comes from a well or a tap, they have no idea of where their water really comes from.  The uploaded document suggests the many types of contaminants of drinking from relatively natural sources of forests and grasslands.  More recently I have learned some about dissolved organic carbon and how if it is too high, it can prevent proper disinfection of water.  As far as I know, boiling water will kill most things, but then as in Flint, there can can be distribution issues, storage issues to consider such as in the medical field.  If you already have a water lab, you might decide Ecoli is the primary one to test, but also periodically follow-up with fecal, total coliform, fecal strep, cryptosporidium, giardia, various metals, pesticides, etc.  Learn about where the water comes from and how it is treated and distributed, etc.  At my home, we are on a well and live in a low density development, we use water treatment including reverse osmosis for drinking. The piping is plastic, with plastic bag in pressure tank.  I am not sure what's in the hot water heater, but we don't drink that.  We have a water conditioner that removes most of the iron, which is high without treatment.  I am not going into other details, but we have had our water tested, and assume being on a well in more or less an urban surrounding with some potential for contamination, we may be at more risk periodically than we might like, such as periods with a lot of stormwater might receive less filtration. 

Matthew and William thank you for your suggestions.

Now, it is more clear. Water contamination could be associated to different sources.

Matthew, personally it seems that if you use Ecoli, as a fecal indicator, is more likely that water can harbor other human pathogens or (as you mentioned) the susceptibility for being infected with other pathogens.

William, WHO indicates that diarrhea is partially caused by water contaminated. We know that some rural areas, mainly in developing countries, have access to unsafe water in terms of pathogens. In addition, some researchers suggest that contamination could be intermitent through the time. So, do you think that the constant exposure could lead us to "tolerate" them?

Thank you all. Best.

Yes, it is fairly well assumed that people develop a tolerance to certain bacteria from frequent exposure, such as if travelling to other areas, it is best to drink bottled water or filter and boil it.  Even though they may develop a tolerance, it is probably uncertain what the long term effects are, we might assume they are negative, but who knows, maybe their systems develop strengths and resilience that serve them well. 

Sir Steev, being in a developing country like mine, I conducted research on drinking water quality, we use total and fecal coliform as an indicator of drinking water quality. The mere presence of fecal coliform is already a manifestation that the water is contaminated with feces either from human or animal source, according to WHO, the mere presence of fecal coliform is enough indicator to alert authorities for possible occurrence of waterborne diseases simply because it is assumed that pathogenic organisms are present in water thus immediate action is necessary especially if found in water service providers. If fecal coliform level is greater than 1.1 MPN (WHO standard) regardless if  E. coli (O157:H7-the harmful sub-type of E. coli) testing was conducted, such water is not fit for human consumption, by the way not all E. coli are harmful unless it is the one that I indicated above, this is according to WHO. If ever, water samples where taken from treated sources heterotrophic place count (HPC) is another parameter that can help determine re-contamination of treated water. However, in a country as hi-tech as yours, I have no idea what kind of test standards you are using.

Total coliform is naturally found in nature thus its presence in water could mean false positive for presence of fecal coliform as such fecal coliform is also tested for confirmation, both coliform tolerable level is

I agree with all the above explanations. Very effective and informative discussions.