Were Dalits prohibited from drinking water from wells in ancient times? Is there any evidence to support this claim?

They were exempted from the payment of all dues with the exception of Padaippanam and Kappalvagai panam.

In December 1937, the Bombay High Court ruled that untouchables have the right to use water from the tank. It took ten years for the Dalits to get justice.

Source : Travancore Manual, Volume I, page number 196.

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From ‘Castes and tribes of India’ :

The text itself is variously dated from 2nd century CE to 4th century CE, but the legends are believed to be older.

North or South, East or west - you can find Dalits facing the same issues when it comes to fetching water from common wells and ponds.

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They (Dalits) can ask any of us to fetch the water for them and we will oblige. But they cannot do it on their own. This practice has been there for ages and it cannot be changed,”

On March 20, 1927, Ambedkar and his followers marched to the Chavdar Lake, where he drank water from it, asserting their right of equality and equal access to public resources.

Even as recently as 18th century, the custom of keeping away Dalits from the common wells was in practice.

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I am not sure whether Sayakkars were classified as Avarnas, but they were harassed for taking water from tanks and wells. The issue was taken before the government and they were given retrieve.

Even if a Dalit didn’t engage in any said impure activity or job, he or she would still be denied access to common well, because by birth they are considered to be ritually impure.

A note on the Paraiyans, Sonnerat, writing in the eighteenth century, says that,

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They were freed from persecution at the hands of Brahmins, Pillaimars and others who were in the habit of obstructing their passage to take water from tanks and wells, by putting up fences of thorns etc, assaulting and exacting unreasonable dues from them and interfering with and interrupting them in their public religious performances. Their residence was also prescribed within certain limits.

Evidence of this practice comes from early Buddhist literature. The story of Ananda and the Chandali girl Prakrit is an example. The story is found in Divyavadana - a collection of early Buddhist legends.

Mahad Satyagraha -

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So no matter what job the Avarnas choose , their touch was/is considered impure by casteists who stick to the practice of ritual purity.

Shockingly, the practice of keeping away Dalits from common wells, continues to this day in some places.

The Brāhmans cannot behold them, and they are obliged to fly when they appear. Great care is taken not to eat anything dressed by a Paraiya, nor even to drink out of the vessel he has used; they dare not enter the house of an Indian of another caste; or, if they are employed in any work, a door is purposely made for them; but they must work with their eyes on the ground; for, if it is perceived they have glanced at the kitchen, all the utensils must be broken.

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In 1927, Ambedkar launched a satyagraha to allow untouchables to use water in a public tank in Mahad (currently in Raigad district), Maharashtra, India.

The girl replies, ‘I am a Chandali’

Avarnas were people who were perceived to be doing ritually unclean jobs - not necessarily actual unclean jobs. For example, Toddy tappers were considered impure. Because, toddy tapping was considered impure.

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Even today a lot of villages only have common wells and ponds. Not every rural house has a well.

When an Indian of any other caste permits a Paraiya to speak to him, this unfortunate being is obliged to hold his hand before his mouth, lest the Indian may be contaminated with his ​breath; and, if he Is met on the highway, he must turn on one side to let the other pass.

There were Paraicheri for Paraiyars, Kammalancheri for Kammalas, Ilai Cheri for toddy tappers so on. Some inscriptions mention Theendacheri (untouchable hanlet) - the term Cheri is still in use and is usually a reference to slums.

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Pre modern times :

The well the girl was using is meant for Avarnas. The touch of an avarna is considered impure and hence she hesitates giving water to Ananda.

When Ambedkar became a professor of political economy in the Sydenham College of Commerce and Economics in Mumbai, other professors objected to his sharing a drinking-water jug with them.

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One such dispute is recorded in a granite pillar in the Parasuraman Peruntheru, Idalakudi, Nagercoil, dated to 661 M.E (Malayalam Era), that is 1486 AD

If any Indian whatever, even a Choutre, by accident touches a Paraiya, he is obliged to purify himself in a bath.

Medieval inscription time to time record caste disputes.

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Where ever the concept of Varnashrama and ritual purity spread, this was the situation.

I read another answer which is nothing but apologetics for the evil system. The OP claims :

They were allowed to appear before the sovereign during the royal processions.

Inscription granted certain privileges to the professional people called Sayakars of Idalakkudi viz.,

One of the caste Hindus told ‘The Hindu’:

And that Dalits dealt with corpses that’s why others fetched water for them.

History stands as a testimony to the atrocities committed on the oppressed section.

The story goes like this : Parched in the hot sun, Ananda approached a girl fetching water and asks her for water.

And,

The medieval Chōla era the indigenous early settlers were relegated to the status of Avarnas and they were given space to live outside villages and towns. They had separate ponds, cremation grounds, so as to not ritually pollute others.

Ananda replies, ‘I didn’t ask for your caste’ ‘I asked for some water’

Every house had a well and no one stopped Dalits so on.

Hope these modern so called Shudra priests actually learn the history of discrimination in the subcontinent instead of doing apologetics.

"they are prohibited from drawing water from the wells of other castes; but have particular wells of their own near their inhabitations, round which they place the bones of animals, that they may be known and avoided.

Yes. Avarnas were generally prohibited from deinking or fetching water from common wells and ponds. They had their separate wells and ponds.