Well poisoning

Well-poisoning is the act of malicious manipulation of potable water resources in order to cause illness or death, or to deny an opponent access to fresh water resources.

Well poisoning has been historically documented as strategy during wartime since antiquity, and was used both offensively (as a terror tactic to disrupt and depopulate a target area) and defensively (as a scorched earth tactic to deny an invading army sources of clean water). Rotting corpses (both animal and human) thrown down wells were the most common implementation; in one of the earliest examples of biological warfare, corpses known to have died from common transmissible diseases of the Pre-Modern era such as bubonic plague or tuberculosis were especially favored for well-poisoning.

Additionally, well poisoning was one of the three gravest antisemitic accusations made against Jews during this period (the other two being host desecration and blood libel). Similar accusations were also made of Koreans living in Japan in the aftermath of the 1923 Great Kantō earthquake. In both cases the accusation was never substantiated, but did lead to widescale persecution and pogroms against the group so accused.

Well poisoning has been used as an important scorched earth tactic at least since ancient times. In 1462, for example, Prince Vlad III the Impaler of Wallachia utilized this method to delay his pursuing Ottoman Turk adversaries. Whilst retreating through Turkish-controlled Bulgaria, across the Danube River and back to the capital of Wallachia that same year, Vlad's army employed the poisoning of wells and other sources of water, as well as other scorched earth tactics en route to his country on both sides of the Danube, meaning that he deliberately polluted the water supplies of his fellow Romanians even at the cost of their lives if it slowed down his Muslim foes. Nearly 500 years later during the Winter War, the Finns rendered wells unusable by planting animal carcasses or feces in them in order to passively combat invading Soviet forces. During the 20th century, the practice of poisoning wells has lost most of its potency and practicality against an organized force as modern military logistics ensure secure and decontaminated supplies and resources. Nevertheless, German forces during First World War poisoned wells in France as part of Operation Alberich. A few religions have laws condemning such scorched earth tactics. Most notably Islam, in its scripture, dictates that water-bodies may not be poisoned even during a battle and enemies must be allowed access to water.

...
Wikipedia