Direct measurement, or monitoring, can allow users to estimate source contributions and/or pollutant load reductions through sampling and observations. Direct measurement relies on actual measurements within the watershed of interest versus representative values obtained from literature. This methodology can provide an instantaneous value or snapshot of water quality conditions (e.g., flow, pollutant concentration), but to obtain an actual load or reduction over time, conversions or adjustments are often required (e.g., extrapolation over multiple rainfall events). Depending on the site and location of measurement, this methodology can provide values for specific sites or practices — or characterization of a larger geographic area. Direct measurements attained from sites representative of larger geographic areas (e.g., downstream MS4 point) may have broader applicability and capture loads and reductions resulting from collective efforts (e.g., household hazardous waste collection, irrigation reduction, enforcement actions). Special studies involve direct measure to obtain data to answer specific questions related to source contributions or pollutant load removal or reductions. Special studies may be designed to investigate pollutant sources or to evaluate pollutant load removal from site-specific BMPs or collective BMP efforts. Special studies may also be performed to identify conversion or adjustment values so that efforts from BMP activities, such as street sweeping, MS4 cleaning, or used oil collection, can be quantified in terms of pollutant removal. Conversions or adjustment values that can be obtained from special studies include, but are not limited to, material to constituent conversions, volume to weight conversions (for bulk waste), and assumptions of percent waste likely to reach the MS4. Example special studies to assist with the evaluation of source contributions and pollutant load removals include, but are not limited to:
- City of San Diego. 2010. Targeted Aggressive Street Sweeping Pilot Program Phase I and II Effectiveness Assessment Study. Download (Updated) Study (1MB)
- City of San Diego. 2010. Targeted Aggressive Street Sweeping Pilot Program Phase III Median Sweeping Study.
- City of San Diego. 2011. Targeted Aggressive Street Sweeping Pilot Program Phase IV Speed Efficiency Study. Download Study (9.9MB)
- City of San Diego. 2015. Targeted Aggressive Street Sweeping Pilot Program Phase V Limited-Hour Posted Route Study.
- City of San Diego. 2013. Volume: Weight Correlation Study Technical Memorandum. March 6, 2013. Download Memorandum (1.4MB)
- City of San Diego. 2013. Catch Basin Inlet Cleaning Pilot Study: Final Report. Download Study (4.4MB)
For successful implementation of direct measurement, development of a project plan or standard operating procedures (SOP) and a quality assurance project plan (QAPP) are recommended. Resources for considerations and procedures for this methodology include, but are not limited to, the following:
- U.S. EPA. 1997. Volunteer Stream Monitoring: A Methods Manual. EPA 841-B-97-003.
- U.S. EPA. 2002. Urban Stormwater BMP Performance Monitoring, A Guidance Manual for Meeting the National Stormwater BMP Database Requirements. EPA-821-B-02-001.
- California Department of Transportation (CalTrans). 2004. BMP Retrofit Pilot Program. Final Report. Report ID CTSW-RT-01-050. Download CalTrans Report
- Donald W. Meals, R. Peter Richards, and Steven A. Dressing. 2013. Pollutant load estimation for water quality monitoring projects. Tech Notes 8, April 2013. Developed for U.S. Environmental Protection Agency by Tetra Tech, Inc., Fairfax, VA, 21 p. Download Report (1.5MB)
- County of San Diego. 2001. BMP Effectiveness Assessment Strategies Model Program Guidance. November 2001. Download Guidance (2.1MB)
- USGS. 2006. National Field Manual for the Collection of Water Quality Data. Download Manual (5.5MB) Direct measurement is a methodology that may be used during the planning phase or implementation phase of a stormwater program.
Considerations: Methodology may be used during the planning phase or implementation phase of stormwater program.