Investigations over several years have found that even the “purest” groundwater is not immune to contamination from a carcinogenic chemical long used by a common business operation found in towns and cities across the state: dry cleaners. Since the 1940s, perchloroethylene, or PCE for short, has been a popular chemical employed in dry cleaning shops across the country. … [I]n dry cleaners’ common practice — before better equipment and regulations were developed — the chemical was often dumped down drains or splashed on porous floors. As a result, over the past 50 years, PCE leaked into the soil and groundwater under the handful of former South Lake Tahoe dry cleaners — and likely thousands of others in California.