A.J. Erickson, P.T. Weiss, J.S. Gulliver, R.M. Hozalski, B.C. Asleson
After visual inspection (level 1) and capacity testing (level 2) have been considered and either dismissed or performed, synthetic runoff testing should be considered if warranted by the goals of the assessment program.
Synthetic runoff testing can be used to evaluate the infiltration rate or the removal of pollutants by a stormwater treatment practice. Synthetic runoff testing uses a clean water source (e.g., a fire hydrant or water truck) that may contain targeted pollutants at predetermined concentrations or loads which is applied to the stormwater treatment practice under well-controlled conditions and while performance is measured. Adding targeted pollutants to synthetic stormwater may require authorization from local governments (municipal, watershed districts, or state). It is recommended that the reader investigate authorization requirements before performing synthetic runoff tests with pollutants. If the required discharge of water is outside the reasonable discharge of the water source, then synthetic runoff testing is not likely to be feasible.
Filtration, infiltration, and biologically enhanced practices
One application of synthetic runoff testing is to assess total drain time of stormwater treatment practices. The entire basin is filled with water and the change in water level in the basin is measured over time, which is a direct measure of the drain time. Another application of synthetic runoff testing is to measure pollutant removal efficiency. Pollutant removal efficiency can be evaluated by adding a well-characterized pollutant (e.g., suspended solids, phosphorus, etc.) to the influent water at the desired concentration and measuring the amount of pollutant retained by the stormwater treatment practice, the concentration exiting the stormwater treatment practice, or both. In both applications, the goal of synthetic runoff testing is not to mimic natural storm events, but to accurately measure the rate of infiltration or pollutant removal under specific conditions.
For filtration or infiltration rate assessment, the following three conditions must be met for synthetic runoff testing to be feasible:
- there must be a water supply that can provide the required discharge and total volume of runoff needed (see examples 8.2 and 8.3)
- outflow paths other than infiltration are either measurable or can be temporarily plugged
- the water surface elevation in the stormwater treatment practice can be measured continuously during the test.
When a stormwater treatment practice can be filled rapidly with synthetic stormwater, there is no need to measure the rate at which water is added because the infiltration rate is small in comparison and the measurement of change in water level with time occurs after the stormwater treatment practice is full. When the rate at which water is infiltrating is not negligible compared to the rate at which the stormwater treatment practice is filled, both the rate at which water is added to the stormwater treatment practice and the rate at which water is infiltrating into the stormwater treatment practice must be measured or estimated.
Synthetic runoff testing to assess drain time can be performed on the following stormwater treatment practices: bioretention practices (rain gardens), dry ponds, infiltration basins, sand and soil filters, underground sand filters, and underground wet vaults. The larger of these stormwater treatment practices, however, will likely not meet the first criterion above. Hydraulic conductivity in stormwater treatment practices may vary based on climatic season, soil conditions, etc., and therefore synthetic runoff testing for hydraulic conductivity should be performed at several different times throughout the year to get an overall estimation of hydraulic conductivity. An example schedule includes testing in the spring after the ground thaws, mid-summer, and late fall before the ground freezes.
The primary differences between measurement results from capacity testing (level 2) and synthetic runoff testing (level 3) for hydraulic conductivity relate to the size, vegetation, and subsurface characteristics of the stormwater treatment practice. Synthetic runoff testing is limited to stormwater treatment practices that are small enough to be filled with water, as outlined above. Synthetic runoff testing, however, accounts for the increased infiltration that occurs near and around the stems of vegetation that cannot be measured using capacity testing. Additionally, synthetic runoff testing will show when filtration is limited by the subsurface collection system and not by the surface or near-surface layers.
As with visual inspection (level 1) and capacity testing (level 2), the procedure for synthetic runoff testing varies for each stormwater treatment practice and assessment goal. Therefore, the reader should consider the recommendations located in each of the Filtration, Infiltration, Sedimentation, and Biologically Enhanced Practices sections of this manual. As with any field work, safety is an important concern and should be addressed when conducting synthetic runoff testing.
Synthetic runoff testing can also be used to measure the sediment retention by stormwater treatment practices. Research at the University of Minnesota has shown this technique to be repeatable and accurate for underground sediment retention structures (Wilson et al. 2008), but it has not been used on most other treatment practices. Manholes, grit chambers, and many proprietary devices can be classified as underground sediment retention structures. These structures are often suitable for synthetic runoff testing because of their relatively small design discharge. At a specific water discharge, a given quantity and size of sediment can be fed into the sediment retention structure. The sediment retained is then extracted from the structure and weighed. The difference in mass between the sediment fed and the sediment retained is presumed to have passed through the facility, and sediment retention efficiency can be computed for each sediment size and water discharge. Synthetic runoff testing with sediment is an effective means of determining how well a device will remove various sizes of sediment and to verify that a device is functioning as designed. These synthetic runoff tests can be conducted relatively accurately.
Sediment retention testing could be applied to other stormwater treatment practices. Some stormwater treatment practices (e.g., dry ponds) are constructed from soil, and in such cases, separating sediment added to synthetic runoff from the soil that makes up the bottom of the stormwater treatment practice can be difficult. An alternative solution for such stormwater treatment practices may be to use automatic samplers to capture synthetic stormwater samples at the outflow for comparison to sediment that was added to the influent synthetic runoff. Another alternative solution may be to paint sediments added during synthetic runoff testing so that they can be easily separated from sediments already in the stormwater treatment practice or that are part of the original treatment practice construction. With these alternatives available, it is anticipated that sediment retention testing can be applied to most stormwater treatment practices, including sand and soil filters, underground filters, hybrid filters, dry ponds, wet ponds, underground proprietary devices, wet vaults, rain gardens with a measurable outflow, constructed wetlands, filter strips, and swales.
Continue to Monitoring (level 4).