Dante's Peak

Dr. Harry Dalton (Pierce Brosnan), a volcanologist of the United States Geological Survey, and his partner Marianne (Walker Brandt), are studying volcanic activity in Colombia when the volcano erupts. Marianne is killed by a volcanic bomb, leaving Harry remorseful about her death, believing it could have been prevented by evacuating sooner.
Four years later, Harry is assigned by his boss Dr. Paul Dreyfus (Charles Hallahan) to investigate seismic activity at Dante's Peak, Washington, a small town situated near a dormant stratovolcano in the Cascades. Harry arrives as the town is celebrating its founding, and meets the town's mayor, Rachel Wando (Linda Hamilton), and her two children Graham (Jeremy Foley) and Lauren (Jamie RenÃ©e Smith). Rachel offers to take Harry up the mountain to take readings while also visiting her former mother in-law Ruth (Elizabeth Hoffman). While near a hot springs, Harry stops the children when he sees dead wildlife near it, and they discover two dead bathers in the springs, boiled alive. Believing the excessive heat is a result of volcanic activity, Harry asks Paul to bring a team to study the mountain further. The team arrives with additional equipment, but they do not corroborate Harry's fears and suggest there is little concern. Regardless, Harry tries to convince Rachel to prepare the town for something drastic, while falling into a budding relationship with her.
A week passes without any signs of volcanic activity and Paul orders the team to pack up. While having a final dinner with Rachel, Harry discovers her water to be contaminated with volcanic residue, and shortly discovers the town's water supply similarly contaminated. Paul and Harry agree this is a sign of a pending eruption, and they begin to evacuate the town. The volcano erupts during their preparation, sending the townspeople into a panic. As Paul and the rest of the team contact the National Guard for help and try to maintain an orderly evacuation, Harry goes to help Rachel get her children, finding they have left to get Ruth.
Harry and Rachel reach Ruth's cabin just as the lava flow reaches it, and they are forced to abandon their vehicles and cross a nearby lake via a motorboat. Close to the shore of the lake, the boat's engine fails, and they realize the lake's waters have become acidic as a result of sulphur-rich gases dissolving in the water, with the resultant sulphuric acid eating away at the boat. Ruth willingly gets out of the boat to push it to the dock, leaving her with severe chemical burns from which she later dies.
Meanwhile Paul and the National Guard manage to evacuate the town and start leaving but the glaciers on the volcano have melted and flows into the river picking up trees and boulders now becoming a lahar. The lahar soon hits the nearby dam where the weight of the mud and water is too much and the dam collapses. Soon the lahar reaches the town. The team manages to get across the bridge but Paul is washed away in the flood. Harry and the family find a nearby ranger station and take a pickup truck to drive down the mountain. They come across a lava field and attempt to drive across it but they get stuck halfway. Harry tries to back up but the tires are melting and, to their horror, fresh lava is starting to flow down the hill. They finally get free and rescue Ruth's dog Ruffy as fresh lava flows across the road.
Harry, Rachel, and the children re-enter the town, where Harry retrieves a distress radiobeacon. The volcano erupts again and produces a pyroclastic flow, which quickly begins destroying the town. The family manages to escape by driving into an abandoned mine. Harry activates the beacon after they become trapped when the mine collapses. Days later, after the eruption has ended, Harry's team notices the activated distress beacon and dispatch rescue crews. Harry, Rachel, and the children are freed and flown out by helicopter.

Question: What contaminated Rachel's water?
Answer: Volcanic residue