Two cancer-causing agents have been found in Sebastopol Water Wells. Traces of arsenic has been found in three aging wells while PCE was found in one of those same three. However, the amounts of those agents remain under thresholds deemed dangerous by state and federal regulators. Sebastopol gets its water from the Wilson Grove Formation Highlands water basin rather than Lake Sonoma, Lake Mendocino and the Russian River. The arsenic in the wells is due to naturally occurring volcanic layers within the Wilson Grove. The PCE, though, comes from an old dry cleaning business.