A federal jury has convicted a Twin Falls dairy operator for illegally discharging wastewater into a canal, a violation of federal clean water laws.    The jury found 52-year-old Mike Vierstra guilty of one misdemeanor count for discharging without a federal permit last May.      Prosecutors say wastewater from an uncapped pipe on his feedlot was leaking into the Low Line Canal. Complaints were filed by neighbors and the canal operator, Twin Falls Canal Co.  At trial, Vierstra argued the pipe was uncapped by an unknown third party and that he later ordered an employee to fix the problem. Still, the jury found him guilty on one misdemeanor and acquitted him on two other counts. Sentencing is set for June 6 in Boise. He faces up to one year in prison and fines.

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