The alkalinity of the Vamsadhara river downstream of the refinery has risen
An alumina refinery in east India run by a UK-based firm is causing pollution that threatens the health of local people, a human rights group says.
Amnesty International said those living near the Lanjigarh refinery in Orissa breathed polluted air and were afraid to drink from or bathe in local rivers.
It called on Vedanta Resources not to expand the refinery or mine for bauxite nearby before resolving the problems.
Vedanta has consistently rejected the allegations against it.
It points out that India's Supreme Court has approved its initial plans
The firm has previously argued it has support from the state authorities. But it did not respond immediately when asked for the comment by the BBC on the Amnesty report.
On Friday, the Church of England said it had sold its £2.5m stake in Vedanta.
The Church said it was not satisfied the firm had shown "the level of respect for human rights and local communities that we expect".
Vedanta said it was disappointed by the Church's decision and remained "fully committed to pursuing its investments in a responsible manner, respecting the environment and human rights".
In a report published on Tuesday, Amnesty International said the Orissa State Pollution Control Board (OSPCB) had documented widespread water and air pollution caused by the Lanjigarh refinery since it opened in 2006, but failed to share it with those affected.
The OSPCB found the alkalinity of the Vamsadhara river downstream of the refinery had increased, and believes the seepage, leakage or discharge of highly alkaline water from its waste storage facilities is responsible, it said.
Drinking slightly alkaline water is not associated with any health problems. But water with a high pH value can cause irritation of the skin, and exposure to water with a pH level of 12 or above can cause blistering or burns.
"We used to bathe in the river but now I am scared of taking my children there. Both my sons have had rashes and blisters," one woman told Amnesty.
Although it was required to ensure "zero discharge", the OSPCB reports indicated that the refinery had been operating without having put in place all the necessary systems to adequately manage water-borne waste and pollution, the report said.
Vedanta disputes that it is responsible for the river's alkalinity, saying it may be related to the increased use of fertiliser by famers.
Amnesty said the Lanjigarh refinery was also failing to limit the impact of air pollution on surrounding communities, caused mostly by dust from its bauxite, coal and lime handling areas and ash from its boilers.
Officials are considering a proposal for a six-fold expansion of the refinery
Local people told the group how the dust settled in their homes and on their clothes, left deposits on their trees, fruits, crops and water, and even on prepared food.
"I was finding it hard to see before and now this constant dust from the refinery is making it even harder for me to see. My throat is constantly sore as I inhale so much smoke and dust," said a woman in Bandaguda, who also described the continuous noise pollution as "mental torture".
Local health officials told Amnesty that the refinery's dust emissions were the most likely cause of the respiratory illnesses and discomfort experienced by those living nearby. The OSPCB also found in 2007 and 2008 that dust emissions were far higher than permitted.
However, Vedanta officials told Amnesty that there was no dust pollution, and a recent government environmental impact assessment stated that levels were within acceptable limits.
Despite the concerns of the villagers, the Indian government is considering a proposal for a six-fold expansion of the refinery.
"People have a right to water and to a healthy environment but Vedanta has failed to respect these rights in Orissa," said Amnesty's UK Director Kate Allen.
"Villagers were given scant and misleading information about the potential impact of the alumina refinery and mining project. Today they are living in the shadow of a massive refinery, breathing polluted air and afraid to drink from and bathe in a river that is one of the main sources of water in the region.
"The Indian government is responsible for protecting the rights of its people, but that doesn't absolve Vedanta from responsibility for the damage that it is causing people. Vedanta must ensure that its existing operations respect human rights before considering any expansion."
The Dhongria Kondh say their way of life is threatened by the planned mine
Amnesty also called on the Indian authorities to seek the consent of local people before allowing Vedanta to begin mining bauxite in the nearby Niyamgiri Hills.
It fears the proposed mine could destroy the area's ecosystem and threaten the entire way of life of the 8,000-strong indigenous Dongria Kondh community, who have lived in the hills for centuries and depend on them for their crops and water.
"The Dongria Kondh do not cut trees or cultivate the top of the hill as they believe that their deity Niyam Raja Penu lives there. Now they face the prospect of a 700-hectare open-cast mine in their holiest place," Amnesty said.
Vedanta did not respond immediately to Amnesty's report when asked for comment by the BBC, but last year the firm argued that it had the support of the Orissa state government and the Indian judiciary - and that before it went ahead with the project it had consulted exhaustively to assess its environmental and social impact.