When you read about certain conflicts, like Romans invading Persia, or the Cruzades, there are many mentions of wells being poisoned to deny water to their enemies, but no one explains in detail how this was done, what kinds of poison they were using, and if this process was reversible
I imagine there must be a way for wells to become safe again, otherwise this practice wouldn't be so common because all the wells in the world would already be poisoned, but I have no idea how long it takes. Do you just wait?, do you add other substances in the well to neutralize the poison somehow?
This is a very interesting question. I am hopeful that a comprehensive answer will be written for your question, but I can offer a brief one in the meantime.
Wells were typically poisoned with the corpse of a human or animal. There are also reports of human waste being used. Polon supposedly used a large quantity of hellebore (poisonous flowering plant) to poisons the city of Kirrha’s water supply.
Wells were also sealed or filled with rubble or debris, a more labor-intensive option than corpse-poisoning, but offering destruction which was also much more labor-intensive to undo.
The general practice of desecrating or poisoning wells and cisterns in this way is documented going back to the Roman Era or even the Neolithic period, and as recently as WWII and the Yugoslav wars. But well-documented recoveries of human remains from historic wells by archaeologists remains rare.
Kanz et al state that “Archaeozoological findings of animals from wells and cisterns are known for several sites of western Asia Minor.” Going on to say these remains belong to a “broad variety” of species.
The well Kanz et al excavated contained the complete skeletons of two human males, a horse, and a canine; as well as other incomplete animal remains. This well was at a fourteenth-century Hamam (bath complex), believed to be destroyed in a fifteenth-century uprising against Ottoman rule. The complete skeletons are believed to have been deposited in the well all around the same time, in the following order: one human male first, then the horse, then the second human male, then the canine. Then rubble, possibly from a former roof over the well, was piled on top. The arrangement “suggests that efforts have been made to seal the well, most probably with the intention of encumbering the recovery the cadavers and thus avoiding the well being cleaned and/or reused as a fresh water supply.”
Additionally, there are a number of small cat and tortoise bones from the period when the well was being actively used as a water source. The authors state it “seems certain these small animals were accidentally introduced into the well, with only minor consequences for the water quality.” The conclusion of the archaeologist was the human bodies were the bodies of those involved in the revolt, and the various corpses were placed in the well to poison it.
This is a fairly neat example of an archeological case which fairly closely matches the trope and the well in question was both partly filled in and poisoned, matching both the of widely employed means of destruction.
As the above example shows, sometimes the well/cistern is abandoned after poisoning or being infilled.
I should note that cisterns/wells could be somewhat sophisticated, already in 800 BC some cisterns in Jordan had sedimentation basins to keep mud and sand from entering the main basis. In both the classical and medieval periods, water systems at bathing complexes, hospitals, monasteries, and other sites could employ a number of technologies. These sites might employ aqueducts, roof tanks, cisterns, piping or conduits, rainwater/snowmelt collection, taps/faucets, or waterwheels. They usually had networks of drains, pipes, etc. for disposing of dirty water which had already been used for bathing, cooking, cleaning; or for removing rainwater from interior courtyards or similar spaces. They might employ both piped “live” water and “still” well or cistern water.
Wells and cisterns were also extremely numerous, with each one usually serving only a single household (though not every household had one, many used springs, rivers, or streams). They were concentrated in the homes or the wealthy, and were expensive to build and to maintain.
I include all this to illustrate that water management was in some ways sophisticated, and there were notions, of varying correctness, of what made water safe and unsafe to drink (though germ theory is of course a long way away.)
When wells are abandoned they typically fill some of the way with mud and sand, and are sometimes used as waste disposal sites. But a well can be re-excavated, repaired if it has retaining walls that are damaged, cleaned, and then used as a water source again.
Wells and cisterns need maintenance, they must be periodically emptied for cleaning and repairs, only very deep reservoirs maintain the necessary temperature “to guarantee no plant or insect life would sprout in the water” (Squatriti). It is only at depths of 30 feet that soil temperatures are a year round constant of 10-15 degrees Celsius (50-60 Fahrenheit), and soil temperatures do not start rising again until you get below depths of 100 feet (though this can be dependent on groundwater).
None of the sources I found are explicit on poisoned or filled-in wells, but it is reasonable to assume the procedure would be the same as with a normal re-excavation or cleaning. Infill or corpse would be removed, the water would be drained, plant or algae growth and other contaminants would be cleaned as much as possible from the reservoir, and then the well-would re-enter use. I would speculate that cisterns might be allowed to dry out entirely, but the sources don’t give details on how thorough the cleaning was, only that cistern/well owners often neglected and put-off necessary maintenance.
The length of time it takes to clean/empty a well depends on the depth, diameter of the well or cistern, how many laborers you have to clean it, how many can work in the well at a time if you need to excavate, repair, or scrape down the sides. But the answer is likely in days or weeks. How long it takes a well or cistern to fill again varies widely depending on climate, groundwater levels, soil conditions, and rainfall, but can vary from 24 hours to months or years.
As we learned from Kanz et al, some amount of contamination need not lead to the abandonment of a well, it seems small animals could die in wells and never be cleaned out while the well remained in use.
Sources:
Kanz et al cite a paper for more info on animals in wells, but I wasn’t able to access it: