Iron-enhanced sand filters (IESFs) retain solids and associated pollutants by filtering and through adsorption of phosphate (soluble reactive phosphorus) from stormwater. A typical method for assessing the performance of BMPs with underdrains is therefore measuring and comparing pollutant concentrations at the influent to the filter and effluent from the underdrain outlet. Before developing an assessment program, it is important to have well-defined goals so that the effort required to develop and implement the program is focused to achieve the desired results. The need for assessment can be determined by permit, voluntary watershed management goals, TMDL allocation, or protection efforts, among others. Rather than proposing a series of rigid procedures, four assessment options are provided below that can be used in various combinations depending on information needs, budgetary constraints, time frames, and legal requirements. An online manual for assessing BMP treatment performance (Gulliver, et al. 2010) advises on a four-level process to assess the performance of a Best Management Practice.
Determination of Appropriate Monitoring Level(s) for a Stormwater Treatment Practice.
Link to this table
Level | When should I perform this assessment? | Advantages | Requirements/limitations | Recommended frequency | Can this be used to obtain a stormwater credit? |
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Visual inspection | Recommended for all practices | Quick and cost-effective | Available personnel. Does not necessarily identify causes of poor performance. | ≥ 1x / year, at start of rainy season | No |
Capacity testing | If there are suspected filtration rate problems with the practice, or to determine if the media has capacity for phosphorus removal | Applicable for practices of all sizes, quickly identify specific areas that require maintenance, less time and expense than monitoring | A Modified Philip-Dunne Infiltrometer is recommended for filtration rate testing. | Every few years | Consult with MPCA (Minnesota Pollution Control Agency) or regulatory agency to determine eligibility |
Synthetic runoff testing | If there are suspected problems with filtration or pollutant removal, or to establish a baseline condition or baseline performance level | Controlled method to accurately measure drawdown time and/or pollutant removal efficiency |
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Every few years | Consult with MPCA or regulatory agency to determine eligibility |
Monitoring | Goals include obtaining stormwater credits, assessing performance results and life of filter media, or complying with a permit or regulatory agency | Most comprehensive assessment technique and measures the response to natural rainfall events |
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Continuously from construction of the IESF | Yes |