The water supply of more than 2 million Californians has been exposed to harmful levels of nitrates over the past 15 years, a period marked by lax regulatory efforts to contain the colorless and odorless contaminant, a California Watch investigation has found.

Nitrates are the most common groundwater contaminant in California and across the nation. A byproduct of nitrogen-based farm fertilizer, animal manure, wastewater treatment plants and leaky septic tanks, nitrates seep into the ground and can be expensive to extract.

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