4.19. The appropriate frequency of monitoring can be determined on the basis of the rate at which concentration levels of contaminants are predicted to change as a consequence of phenomena such as groundwater flow and natural attenuation processes, the degree to which the causes of this variability are known, the types of evaluations to be performed, the locations of possible receptors, and the objectives for remediation of the site. In situations where the hydrological, geochemical and contaminant trends are stable and the conceptual model of the site can be verified by existing monitoring data, a reduction in monitoring frequency may be possible. In situations where the variability is high, an increase in monitoring frequency may be warranted. If the data trends appear to be stable over a period of several years, a reduction in monitoring frequency may be possible. On the other hand, for example, more frequent monitoring of groundwater elevations may be warranted, particularly during the establishment of baseline conditions, to improve the characterization of groundwater flow patterns. Other factors for determining the appropriate frequency of monitoring include the relevance of parameters and any redundancy in information. If monitoring of a specific parameter is not expected to influence the performance assessment of a site significantly, then the monitoring frequency for that parameter could be greatly reduced or monitoring of that parameter could even be eliminated.