Most of the wells tapping into a vast underground sea in the north San Joaquin Valley provide water that meets human health standards for heavy metals or chemicals, according to a new federal analysis.

But the region still has hot spots where groundwater quality is a concern.

Arsenic, in particular, is prevalent in south San Joaquin County. The naturally-occurring metal was found at “high” levels – greater than health-based benchmarks – across 9 percent of the study area, which includes San Joaquin County and portions of Alameda, Amador, Calaveras, Contra Costa and Stanislaus counties.

Most of that 9 percent falls within an area stretching from south Stockton to Manteca.

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