… State records show that Stanislaus County has about 20,000 water wells, with nearly half used domestically. Those wells are more likely than not to contain unsafe drinking water, according to test results reviewed by The Bee. They tend to be shallow and therefore more prone to surface contaminants like pesticide residue, heavy metals and nitrate contamination from fertilizer, dairies or septic tanks. Domestic wells are a blind spot for water quality data since the state does not regulate private wells. It’s only through voluntary programs like the Valley Water Collaborative that data on these wells are gathered. … [E]ven if nitrate stopped percolating into the shallow aquifers in the county today, it may take 50 years to get back to normal.

Other water quality news:

• SJV Water (Bakersfield, Calif.): Kern water district gets $230 million to settle contamination lawsuit