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work. To the contrary, the record shows the Petitioner made no such offer,but merely stated in the last sentence of its closing argument, that the Petitioner"is prepared to support" and provide the funding for such a study. The Regional Board had just listened to numerous experts testifying on behalf of the Petitioner. To suggest that a bona fide offer existed, much less that the Regional Board somehow acted inappropriately in not interrupting the board meeting to commission a new study based on that one comment, is not consistent with the events as born out in the record. The Petitioner has not demonstrated how the Regional Board abused its discretion in that regard, and we find no such abuse of discretion here. In appropriate circumstances, however,when a regional board determines that such outside assistance would be beneficial, it would not be an abuse of discretion for the regional board to avail itself of such an offer, and the regional board is encouraged to do so. Contention: The Petitioner claims that the Regional Board acted arbitrarily and improperly in denying dilution credits and a mixing zone for ammonia and other constituents. Finding: We find no abuse of discretion in denying the dilution credits and mixing zone. Some of the relevant constituents are non-priority pollutants for which the Water Quality Control Plan for the Central Valley Region (Basin Plan) contains applicable provisions. It provides that"the Regional Water Board may designate mixing zones within which water quality objectives will not apply provided the discharger has demonstrated to the satisfaction of the Regional Water Board that the mixing zone will not adversely impact beneficial uses." (Basin Plan, p. IV-17.00.) Other relevant constituents are priority pollutants, which are governed by the Policy for Implementation of Toxics Standards for Inland Surface Waters, Enclosed Bays, and Estuaries of California(SIP). It also authorizes the regional boards to allow mixing zones and dilution credits in appropriate circumstances. (SIP, § 1.4.2.) While the Regional Board has broad discretion to determine whether a mixing zone is appropriate, that discretion must be properly exercised. The Regional Board considered numerous factors in its decision to reject the Petitioner's flow studies and deny dilution credits. Included among these are real-time flow data from the Ultrasonic Velocity Meter(UVM), which shows that dilution has been minimal during above-average wet years for all conditions, the available UVM data are from above-average wet years, tidal influences permit the receiving water to be dosed with effluent multiple times, the inadequacy of the existing models, the fact that the receiving water is impaired, and the presence 3.