From the City of Pearland Facebook page:

Scam Alert: There have been reports of a company approaching residents stating that they represent the City asking residents to take water samples from their homes. This firm in not associated with the City, and the City’s contract laboratory staff is always accompanied by City staff. If someone comes to your door saying they work for the City of Pearland you should ask to see their identification card or credentials. In most cases City employees will also be in marked vehicles with the City of Pearland logo. City contractors receive magnets for the side of their vehicle to make it clear they are working on behalf of the City.