Stormwater program managers are challenged with meeting regulatory monitoring requirements to identify long-term trends and the effectiveness of the implementation of Best Management Practices (BMPs) and meeting multiple program requirements with finite resources. This paper provides stormwater managers with methods and several real-world scenarios using 10 years of data from the Los Angeles County Stormwater Monitoring Program to design long-term effectiveness monitoring programs. Potential tradeoffs in frequency reduction of sampling are assessed with increased focused monitoring of smaller drainage areas where BMPs have been implemented to optimize available resources.
The paper first presents the results of statistical analysis of up to 10 years of stormwater monitoring results collected by the Los Angeles County Department of Public Works for its Core Monitoring Program as specified in the Municipal Stormwater Permit. The data set represents runoff within a receiving channel from a highly urbanized watershed. The statistical analysis includes the evaluation of the inherent variability of urban runoff and the number of samples required to determine a significant difference for various confidence levels. The statistically required number of samples is determined for a selected metal (total copper) compared to suggested sample sizes in published guidelines to determine effectiveness of BMPs. The variability of the data set from the in-channel monitoring station at the base of the watersheds is also compared to the water quality variability of a sub-drainage area within the watershed. The findings of these evaluations are summarized to provide stormwater managers with the basis to plan and design BMP effectiveness assessment monitoring programs.
The paper also presents the results of the statistical evaluation using the Los Angeles County data of modifying sample frequency for determining long-term trends in stormwater quality. Several real-world scenarios are presented showing trends detected by different sample frequencies for different reductions in constituent concentrations through the implementation of BMPs. The results presented provide stormwater managers with methods and statistically based potential tradeoffs in monitoring programs to assess long-term effectiveness of stormwater management programs.