WORST that could happen? Soil/water contamination risks (and remedies)

Wondering what the rough consensus is around genuine risks 'downstream' of improperly handled/treated solid waste in the ground from alternative toilet rigs, how long it could remain an issue, and what follow up steps could help reduce or eliminate it.

Consider the raw deposit case as a hypothetical - i.e. old school outhouse over a pit, that's been used to capacity and capped with a foot of soil on top. I expect the main issue is going to be how water moves in the ground through the area in question (several variables).

I'm assuming the top concern is pathogenic bacteria finding their way to unexpected human contact, through surface/soil skin contact, surface or ground water contamination, or food/crop tainting. Is there a ballpark sense of how far dangerous levels of bacteria can move from the source? What preventative measures might greatly reduce that (after the fact, with waste in ground as described)? How long would this continue to be a liability from a capped, static deposit? Anyone familiar with actual cases of illness/injury resulting from something like this, or even, say, a poorly utilized humanure compost heap out in the open?

Appreciate any insight or reference points, just trying to get a better understanding of real liabilities in this realm, beyond the usual squeamish paranoia

I would think that as long as the bottom of the pit is 2 feet above the high water table groundwater contamination shouldnt be too big of a concern....assuming the soil type has acceptable percolation rates. You know if this is just a single househould waste pit that has been abandoned I wouldnt be  concerned about transport...I wouldnt think enteric bacteria could persist too long in those conditions. Unless you have a well or pond nearby I wouldnt bother investigating it.

There are so many variables to consider when we look at this. Permeability, depth to groundwater, flora.
Use of a composting toilet and it wastes subsequent material require that it be buried under 12 inches of soil in OR. That's not raw sewage but material that has gone through a composting phase. https://www.oregon.gov/deq/FilterDocs/os-compostingtoilets.pdf
It is incredible the amount of pharmaceuticals and forever chemicals that are showing up in groundwater now days.

I know it's a little tough to pin down the threshold of "this is going to become a problem for somebody" but I'm interested in hearing some more specific instances or insight into how exactly it has been, or could (very likely) be one.

Obviously, guidelines err on the side of caution - I just don't have any sense of how far, and what the actual mechanisms or "failure modes" are. I'd like to be able to make more informed decisions around some possible edge cases or preemptive mitigation in the future.

One possible scenario for the sake of argument, what if the above described capped untreated waste pit was 50-100' uphill of a pond that people may swim in? Would that be a serious concern? And again, how long (1 year? 5, 10?) could that risk stay elevated?

One data point that comes to mind here are popular backcountry camping lakes that wind up with varying levels of fecal contamination, including one in Wyoming at almost 500X federal 'safety' guidelines...and yet, no evident reports of ill effects:

https://www.kunc.org/news/2025-07-19/wyomings-crowded-lonesome-lake-tops-epas-national-survey-for-fecal-contamination

I lived for three years in a place and time (rural Bangladesh in the 1980's) where humanure was commonly simply deposited wherever the urge overtook people on the roadsides, or in designated "open latrines" often near or even directly over bodies of temporary or permanent water. Diseases of various sorts were inevitable and widespread, even for those very careful of their own hygeine.  The stuff would dry out in the dry season and blow around in the wind so one could actually inhale the germs!   But by far, the majority of diseases were spread through three means....1. contaminated water 2. contaminated hands and 3. flies.  An ordinary pit latrine, enclosed to exclude flies or with each deposit covered with even a small amount of soil or mulch, would prevent the vast majority of cases.  Even moreso if water for handwashing were provided nearby.  One advantage is that most of the soil there is clayey, and so transport of bacteria through the soil, to the water table or elsewhere, is minimal.   Another danger common there, and to beware of elsewhere, is flooding, which would enter pit latrines and liberate their contents broadscale.  Humanure used in any way in a flood prone area complicates things hugely.

Historically humanure has been used for centuries as fertilizer.

Even here on the forum, there is an interest in humanure as a sustainable resource, using composting methods being used to safely create fertilizer from human waste.

Here is a podcast that might be of interest:

https://permies.com/t/132098/composting/older-civilizations-Humanure#1035598

This book might be of interest:

https://permies.com/wiki/131979/composting/Joe-Jenkins-Author-Humanure-Handbook

At 12,000 ft there is probably not a lot of flora for filtering waste at Lonesome Lake. In your scenario of a pit on a hill side above a pond, that could be mitigated by the riparian area prior to shore line. In my area and possibly yours if your location was affected by the Mt Mazama eruption ash fall. Pumice is an excellent filter for fecal waste but very porous allowing other easily diluted pollutants like nitrates to travel easily.
These would be factors I would consider. Quantity/volume of waste? Plants for uptake? Porosity of soil? Depth to groundwater? Is the waste vessel sealed from water infiltration? Depth of the bottom of the waste vessel/hole. Is the pond your only concern? Are there other bodies of water/streams nearby? Concerned about drinking water? The legal hell that comes from improper handling of human waste if that worries you might be a consideration.
Humanure and fertilizing food crops is a western no-no. In Europe the commercial collection of urine and using that for fertilizer is becoming more common.
A perc test and groundwater depth would be a good place to start in finding an answer.

Thanks for these inputs so far, some helpful perspective. I'm not trying to get to a concise answer for a particular scenario, just want to improve my intuitive sense about this topic because it can be consequential and I realize I don't have any hard data / case study type insight; I figured some people here are well versed and able to save me/others more extensive and technical research.

Haven't had any commentary yet on possible persistence of pathogens in or near buried waste - I know that humanure is typically 'binned' for a year or two to eliminate most or worst pathogen load before active composting, would that timeline look similar right in the ground? Could buried waste still pose a localized biohazard some 3,4,5+ years on?

'Historically humanure has been used for centuries as fertilizer.'

Historically, and in the present, intestinal parasites, hepatitis and other faecal-oral transmissible diseases have been, and are wide spread in areas using raw humanure (night soil).  Composted or vermicomposted humanure is much much safer.
Exposed populations generally build up some degree of resistance to at least some of the pathogens, hence traveller's diarrhea.

Worst-cases from leachate from an outhouse could include:  dysentery, scarlet fever (Enterobacter), haemorrhagic E. coli, Shigella, cholera, pin worms, tape worms, Giardia, Norovirus, hepatitis, skin infections... etc.

Best-case the outhouse users only deposit local, mild pathogens, the content vermi-composts in place, the leachate has low/no pathogens and the nutrient load feeds vegetation before reaching the water body.
I agree that access to handwashing, and avoiding direct faecal-oral transmission are big protections.

Pathogen survival and migration is highly conditional;  cool, dark , low oxygen conditions generally favour survival.
Rapid run off can over-whelm natural biofiltering; faecal coliform levels typically spike after heavy rainfall.
Earhworm action lowers loads; sunlight disinfects quite quickly.

Guidelines for swimming water levels of faecal coliforms are based upon epidemiological studies of risk of infection, roughly 1% risk of infection per season from swimming in the water body.
Remote, but contaminated, lakes would be used mostly by healthy people at relatively low risk, compared perhaps to a more accessible beach used by a wider population.

Aside:  Coliform, or E. coli, are used as convenient indicators of faecal contamination and thus wider pathogen risk; in and of themselves, they are minor part of the risk.

Residual pharmaceuticals are a parallel problem; not likely to cause acute illness, but possibly a bigger longer term issue.
(I researched & taught on these issues).

Ben, Excreta Disposal for Rural Areas and Small Communities section on Soil and Ground-water Pollution is a good reference for underground bacterial and chemical plumes, including distances.  The WHO Guidelines for the Safe Use of Wastewater, Excreta, and Greywater outlines the types of risks, quantifies the risk in terms of Disability Adjusted Life Years (DALY), and describes various prevention methods and their effectiveness (e.g. Volume 1 section 2).  The Humanure Handbook chapter on Worms and Disease provides a nice summary of persistence for pathogens.  I'm not sure about a good resource for summarizing various types of risk reduction steps that could be taken after the fact, but the use of fungi for remediation of certain types of issues was described in Mycelium Running: How Fungi Can Help Save the World.

Great stuff in these last two comments, thank you Burton and Douglas! Just the kind of info I was hoping for. Gives me a much better sense of what to anticipate with these situations, and where to dig deeper on case by case evaluations.

I'll highlight one key paragraph from Burton's first linked paper, which has some diagrams and elaboration starting on pg 28 (including details on groundwater transport which can carry much further, out to 50-100 feet):

"The study of methods of pollution of the soil and water by excreta also provides useful information concerning the design of disposal facilities, especially their location with respect to sources of drinking-water supplies. After excreta are deposited on the ground or in pits, the bacteria, unable to move much by themselves, may be transported horizontally and downward into the ground by leaching liquids or urine, or by rain water. The distance of travel of bacteria in this way varies with several factors, the most important of which is the porosity of the soil. Their horizontal travel through soil in this manner is usually less than 90 cm (3 ft) and the downward travel less than 3 m (10 ft) in pits open to heavy rains, and not more than 60 cm (2 ft) normally in porous soils."