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D R A F T January 4, 2012 characterization was necessary to assess whether the City's discharge complies with the Basin Plan. Without this information, however, the Central Valley Water Board could not legally make the necessary finding that the City's land disposal activities meet the precondition for an exemption. Both the sewage and wastewater exemptions presuppose a monitoring program that is adequate to demonstrate compliance with the precondition. Both Title 27 and the Facility have been in place many years; it is reasonable to conclude that the City should, by now, have been able to provide sufficient information to the Central Valley Water Board demonstrating that it complies with the exemption criteria. As discussed above, the Basin Plan contains narrative and numeric groundwater objectives for waste constituents that apply to the City's activities, unless "naturally occurring background concentrations" exceed the objectives. The narrative and numeric objectives in the Basin Plan presumptively apply to groundwater unless the Central Valley Water Board has evidence in the record indicating that naturally occurring background concentrations exceed the objectives. A discharger who contends that naturally occurring background concentrations exceed the otherwise applicable objectives bears the burden of providing evidence to the Central Valley Water Board that supports this contention. Once the Central Valley Water Board is presented with this evidence, the board must then determine if it is sufficient to demonstrate that naturally occurring background concentrations exceed the objectives. If it is, then the naturally occurring background concentration of the constituent becomes the de facto objective. Absent evidence to the contrary, however, the numeric and narrative groundwater objectives in the Basin Plan apply. The City has presented preliminary evidence to the Central Valley Water Board indicating that electrical conductivity (EC) concentrations detected in groundwater at the site may be due to natural causes. Although the City has apparently been conducting groundwater monitoring since 1989, the City and the Central Valley Water Board nevertheless agree that naturally occurring background EC concentrations have not been adequately characterized. Hence, the Central Valley Water Board has not yet determined whether naturally occurring background EC concentrations are higher than the Basin Plan EC objectives. Likewise, prior monitoring has been inadequate to characterize the wastes discharged to the unlined ponds and agricultural fields at the Facility and to assess potential groundwater impacts. The sampling program for the ponds has focused primarily on nitrogen compounds and salts, such as EC or TDS, which are the constituents most difficult to distinguish from the historic legacy of other discharges in the area. There may be many other wastewater constituents of concern that are percolating through the bottom of the ponds to groundwater, 11.