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D R A F T January 4, 2012 applicable objectives, and the Central Valley Water Board has not made a determination on this issue. The Board notes that the mound exerts such a strong influence on the underlying groundwater that it makes a determination of"naturally occurring" background concentrations extremely difficult. The mound, which is composed of wastewater draining from the surface, induces flow down and away from the Facility. Because the mound interferes with groundwater flow across the site, it is difficult at this time to determine what upgradient, or background, conditions might be. In any event, the City bears the burden of providing sufficient evidence to demonstrate that its discharge complies with the Basin Plan, and, in particular, that the discharge meets Basin Plan objectives or naturally occurring concentrations, whichever values are higher. 4. Additional Contentions a. Disposal of Biosolids During the summer months, the City mixes a biosolids slurry with storage pond wastewater and industrial wastewater and applies this mixture by flood irrigation to the agricultural fields. The solids content of the slurry is between approximately 2 and 4 percent. CALSPA contends that land application of the biosolids wastewater mixture is not exempt from Title 27 because the bulk concentrations of waste constituents in the sludge in units of milligrams per kilogram (mg/kg) violate water quality objectives. In addition, CALSPA asserts that the wastewater mixture, when applied to the agricultural fields, will result in groundwater degradation, due to the very shallow groundwater depths at the site. The evidence in the record is insufficient to determine whether the field application of the biosolids wastewater mixture complies with the Basin Plan. As the Board concluded above, the monitoring that has been done to date at the Facility has been inadequate to demonstrate that the City's land disposal activities comply with the Basin Plan. In addition, it is infeasible to isolate and assess the water quality impacts of applying this wastewater mixture to land due to the masking effects of the nitrogen-rich and salt-rich groundwater mound underlying the facility unless other waste constituents are tested. The bulk content of waste constituents in the sludge is not relevant. The bulk concentrations do not indicate what the resulting concentrations will be once the slurry is diluted in the wastewater mixture and applied to the fields. The City's monitoring of the biosolids wastewater mixture applied to the fields for priority pollutants indicates that metals are not a concern. While the bulk concentrations of coliform and nitrogen are high, it is not clear what coliform values or nitrogen concentrations would be mobilized for these constituents once the biosolids are diluted with wastewater and applied to the fields. 17.