Starting in 2023, large public water systems who serve 10,000 or more people will be required to sample. For the first time, medium-sized public water systems serving 3,300 to 10,000 people will be required to sample.Starting in 2023, large public water systems who serve 10,000 or more people will be required to sample. For the first time, medium-sized public water systems serving 3,300 to 10,000 people will be required to sample.