Oil and water | An uneasy mix in farm fields of the Central Valley

Oil-produced water in a polishing pond operated by the Cawelo Water District near Bakersfield.   (Brian van der Brug / Los Angeles Times)

Water flows into a holding pond at a Kern County vineyard near Bakersfield. Water in the reservoir was tested last summer by Scott Smith, chief scientist at Water Defense.   (Brian van der Brug / Los Angeles Times)

Scott Smith, chief scientist at Water Defense, right, and assistant Skye Wallin retrieve foam sponges which absorbed canal water for testing near Bakersfield. The canal moves oil-produced water mixed with fresh water for use by Kern County agriculture.   (Brian van der Brug / Los Angeles Times)

A foam sponge to absorb test water from a canal operated by the Cawelo Water District near Bakersfield. The canal moves oil-produced water mixed with fresh water for use by Kern County farms.   (Brian van der Brug / Los Angeles Times)

Scott Smith, chief scientist at Water Defense, left, and assistant Skye Wallin retrieve foam sponges which were deployed to absorb test water from a canal in the Cawelo Water District near Bakersfield.   (Brian van der Brug / Los Angeles Times)