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Selecting Numerical Limits - 8 - � 20 August 2002 limit from being set at the same level as the USEPA limit. What these examples show is that, while an algorithm may be useful to guide the selection process, other information and good judgment need to be used in selecting the final water quality limits. To maintain defensibility, arbitrary selection of limits must be avoided. Selection should be based on sound rationale and should consider the circumstances of each case. Sufficiently similar circumstances can be addressed in the same manner. To that end, I have generated a table of applicable or relevant and appropriate limits for commonly encountered chemicals, based on the above algorithms. The table may be found on the server at W:\General\ WQ_Goals\Limiting WQ Limits.xls The table does not include numerical water quality objectives from the Basin Plans, because these often vary from location to location within our Region. So, make sure to consult the Basin Plan and add numerical objectives applicable to your particular situation. The table will be updated on a regular basis. If you don't find the chemical of interest to you, ask and I will add it. Controllable Factors and Antidegradation Policies The selection of numerical limits, as discussed above, has only considered compliance with water quality objectives and promulgated water quality criteria(CTR/NTR). Additional factors govern the final selection of water quality limits. According to the Controllable Factors Policy in Chapter 4 of the Basin Plans, "Controllable water quality factors are not allowed to cause further degradation of water qualityin_ instances where other factors have already resulted in water quality objectives being exceeded. Controllable water quality factors are those actions, conditions, or circumstances resulting from human activities that may influence the quality of the waters of the State, that are subject to the authority of the State Water Board or Regional Water Board, and that may be reasonably controlled." Natural background water quality is an example of a water quality factor that is not controllable or is "uncontrollable." Where natural background water quality exceeds a water quality objective or the numerical limit chosen to translate the objective, the Basin Plan does not require improvement over the natural condition. However, the policy prohibits controllable factors from making the condition worse. In other words, if the natural concentration of a substance exceeds the limit derived from the above algorithms, then the natural concentration should be chosen as the applicable water quality limit for the water body. If there is a chance that local background water quality has been influenced by controllable factors (e.g., an upstream or upgradient discharge of waste), then the water quality objective or numerical limit chosen to translate the objective must not be exceeded. This latter situation is the default assumption for setting effluent limits in the NPDES program, as discussed in footnote 3 above. State Water Board Resolution No. 68-16, the State's Antidegradation Policy, requires that the quality of high quality waters be maintained "to the maximum extent possible." High quality means that the water is of better quality than water quality objectives for the constituent in question; and this is a constituent by constituent evaluation. The policy allows water quality to be lowered but only if the discharger demonstrates that any change will: 1) be consistent with the maximum benefit to the people of the State; 2) not unreasonably affect the water's present and anticipated beneficial uses; and