Iron-enhanced sand filters (IESFs) retain solids and associated pollutants by filtering and through adsorption of phosphate (soluble reactive phosphorus) from stormwater. IESFs employ an underdrain. A typical method for assessing the performance of best management practices (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 to achieve the desired results. The need for assessment can be determined by permit requirements, voluntary watershed management goals, TMDL ( total maximum daily load) allocation, or protection efforts, among others. Four assessment options are provided below that can be used to assess the performance of a bmp depending on information needs, budgetary constraints, time frames, and legal requirements (see this online manual for detailed descriptions of these assessment levels. (Gulliver, et al. 2010)).
For information on pollutant crediting see Calculating credits for iron enhanced sand filter or Overview of stormwater credits.
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?|
|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||
||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||
||Continuously from construction of the IESF||Yes|
The adjacent table summarizes the four levels of assessment.
Levels of assessment for stormwater best management practices (stormwater control measures)
Link to this table
|Level||Title||Objectives||Relative||Typical elapsed time||Advantages||Disadvantages|
|1||Visual Inspection||Determine if stormwater BMP is malfunctioning||1||1 day||Quick, inexpensive||Limited knowledge gained|
|2||Capacity testing||Determine infiltration or sedimentation capacity and rates||10||1 week||Less expensive, no equipment left in field||Limited to infiltration and sedimentation capacity/rates, uncertainties can be substantial|
|3||Simulated runoff testing||Determine infiltration rates, capacity, and pollutant removal performance||10-100||1 week to 1 month||Controlled experiments, more accurate with fewer tests required for statistical significance as compared to monitoring, no equipment left in field||Cannot be used without sufficient water supply, limited scope|
|4||Monitoring||Determine infiltration rates, capacity, and pollutant removal performance||400||14 months||Most comprehensive. Assess stormwater BMP within watershed without modeling||Uncertainty in results due to lack of control and number of variables, equipment left in field|
Assessing the engineered filter media consists of collection and analysis of filter media for iron, soluble reactive phosphorus and total phosphorus to estimate remaining service life of the media. Current research is designed to enable a relationship between filter media iron and phosphorus in samples and the remaining life of the IESF.
Water sampling: Total phosphorus at the outlet of the iron-sand filter that consistently exceeds 60 to 70 micrograms per liter (mg/L or ppm (parts per million))) may be used as an indicator that the phosphorus binding capacity of the iron-enhanced sand bed has been consumed or that short-circuiting or bypass of the IESF media is occurring. Capacity testing, synthetic runoff testing, or monitoring can be used to determine if short-circuiting is the cause of poor performance. If these concentrations consistently occur, then it is recommended that samples be taken from the iron-sand bed and analyzed for total phosphorus and total iron.
Media: Take two samples of the IESF media at five roughly evenly-spaced locations throughout the media; the first sample at mid-depth and the second towards the bottom of the media. Total phosphorus to total iron ratios that exceed 5 milligrams of phosphorus per gram of elemental iron (Erickson et al., 2007, 2012) indicate the phosphorus binding capacity of the iron-sand bed is exhausted and should be replaced.
Alternatively, IESF media samples can be used in benchtop batch studies (aka jar tests) to determine whether the media has capacity to capture phosphate. Media samples of a known mass (e.g., 10g) can be added to clean, washed jars of known water volume at known phosphate concentration (e.g., 100-300 µg/L), and then mixed for between 10 minutes and up to 24 hours. Then, water samples from the jars can be collected and analyzed for phosphate concentration. From this data, the phosphate capture capacity can be estimated (see Erickson et al., 2018).
Example soil sampling guidance is found here, here, and here.
Monitoring is the most comprehensive assessment technique and can be used to assess water volume reduction, peak flow reduction, and pollutant removal efficiency by measuring discharge and pollutant concentration during natural runoff events through flow measurements and water sample analysis. The two types of monitoring recommended for IESFs are as follows:
If water quality samples are being collected, it is highly recommended to follow rigorous quality assurance-quality control procedures. An example of acceptable sampling and analysis protocol is described here.
Use these links to obtain detailed information on the following topics related to BMP performance monitoring.
The iron in an IESF has a limited capacity to retain phosphorus. This capacity is affected by several factors, including the following.
There is limited monitoring data to develop specific criteria for estimating the life expectancy of an IESF. Gulliver and Erickson (2022) prepared a memo summarizing analysis of three IESFs (link below). Life expectancy of the three practices was 1200, 2050, and 12,500 feet. A value of 1200 feet is recommended as the lower end for the lifetime of an IESF, though the results suggest this value can be increased with considerations for design. An estimate of the number of years for a practice can be estimated by calculating the annual inflow volume and dividing by the surface area of the practice.
File:Gulliver and Erickson 2022 - Analysis of IESF Monitoring Reports and Data.pdf
Use these links to obtain detailed information on the following topics related to BMP performance monitoring:
Additional information on designing a monitoring network and performing field monitoring are found at this link.
This page was last edited on 27 December 2022, at 15:31.