Trial court's denial of plaintiffs' petition for a writ of mandate, challenging the Central Valley Regional Water Quality Control Board's order (Order) regulating the waste from existing milk cow dairies, as violating the antidegradation policy because the Order does not require the best practicable method for regulating the discharge of waste, is reversed and remanded where: 1) the state's antidegradation policy applies to the Order because there is evidence in the record that at least some of the groundwater affected is high quality groundwater and the Order allows the discharge of waste to groundwater; and 2) because the monitoring plan upon which the Order relies to enforce its no degradation directive is inadequate, there is not substantial evidence to support the findings.