Victims of heating oil theft in Leicestershire and Lincolnshire are being urged to report the crime to the Environment Agency as well as police.
The agency said storage tanks were often damaged by thieves and could leak oil into any nearby water.
Cleaning up after a contamination of surface water or groundwater can cost thousands of pounds.
Heating oil thefts from rural areas across the UK have have risen by about 70% in recent months, police said.
Increasing fuel prices and the isolated positions of many properties are thought to be factors.
Police have advised residents to install tank locks and alarms, but the Environment Agency said the possible impact on water sources is often overlooked.
Agency spokesman Shaun Rowson said: "Being able to act quickly is vital to stop oil pollution spreading and, potentially, reaching important underground water supplies.
"We would therefore urge people to contact us as quickly as possible if they think there is any risk of pollution as a result of oil theft. This will help to minimise the potential impact on the environment, the public and the homeowner."