TSS Reduction with Different Eng. Soil (USGS # BQY40)


The proposed study is to determine if the pollutant reduction in a bioretention system is related to the thickness of the engineered soil. Bioretention systems are being used more frequently as a stormwater control measure in Wisconsin. Wisconsin’s Department of Natural Resources (WDNR) technical standard for bioretention systems (technical standard 1004) requires a minimum engineered soil thickness of 3 feet. The total depth of the system is usually about 4.5 feet. The thickness of the sand, compost, and sandy loam engineered soil mixture was selected to provide a high level of pollutant removal and a sufficient depth of soil to support plant growth. Unfortunately, many places in Wisconsin with shallow bedrock and groundwater tables restrict the use of bioretention systems. There is, however, a possibility that the benefits of the engineered soil could be achieved with less than a 3-foot thickness. Evaluations of bioretention in the laboratory using synthetic stormwater and field studies using runoff from parking lots have demonstrated a high level of removal for many of the typical stormwater pollutants .These studies used engineered soil thickness of at least 30 inches and the content of the soil was usually different than recommended in WDNR technical standard 1004. Some of the results indicate the thickness of the engineered soil does play a role in pollutant reduction. For example, a high level of heavy metal control could be achieved by only a few inches of soil, but a high level of phosphorus control would require many feet of soil. None of the field studies focused on the 18 to 24 inch thicknesses of engineered soil. Also, the level of treatment was not expressed as a function of the particle size distribution in the runoff. This study will evaluate pollutant reductions for different thicknesses of engineered soil and the effectiveness of the bioretention systems on different particle-size distributions of solids in stormwater runoff.


The specific objectives are the following. 1. To determine the concentration and storm loads of typical stormwater pollutants at the inlet and outlet of three bioretention systems. 2. To develop relations between engineered soil thickness and pollutant reduction. 3. To determine how much of the runoff is treated by the bioretention systems versus how much runoff bypasses the bioretention systems. 4. To determine the relations between engineered soil thickness and the amount of evapotranspiration of the bioretention system. 5. To evaluate the effect of engineered soil thickness on removal efficiency of stormwater-delivered solids by particle size. 6. To verify the infiltration rate of engineered soils. 7. To evaluate the effect of engineered soil thickness on the system recovery time between runoff events. 8. To determine the decline in treatment levels over time for each soil thickness. 9. To calibrate and verify the engineered soil treatment calculations in WinSLAMM. 10. Document any relationship between engineered soil thickness and plant survivability.

Study Design

One bioretention system will be installed on each of three adjacent employee parking lots in Neenah, Wisconsin. The quality and quantity of the runoff is expected to be similar for these parking lots, in that they are about the same size and are designated for employee parking. The design of the system will follow the criteria in Wisconsin’s technical 9 standard 1004. Only the depth of the engineered soil will vary for two of the systems. Instead of the required three feet, one will be two feet and the other 1-1.5 ft of thickness. To insure that the mixture of the engineered soil will be the same, it will be pre-mixed by a local sand and gravel business. All the systems will be planted with the same types of plants. To isolate the effect of the engineered soils, the bioretention systems will be lined with heavy plastic. This will eliminate any water from being lost by infiltration. The water will have to leave the device thru the drain tile, overflow, or by evapotranspiration. Measurements of soil moisture within each cell will help quantify the importance of evapotranspiration in the operation of each system. Water quality samples will be collected at the inlet and outlet for each system. Automatic water quality samplers will be programmed to collect flow composite samples. Samples will be collected for a total of 25 events at each site. Events less than 0.2 inches in rainfall depth will probably not be included in the water quality analysis. The constituents list includes TSS, SSC, VSS, PSD, Dissolved Solids, Total P, Dissolved P, Total recoverable zinc, dissolved zinc, total recoverable copper, and PAHs. Flow will be measured with HS flumes fitted with pressure transducers to measure the depth of the flow in the flume. Any bypass volume will be determined with a stage recorder installed in the surface storage area above the engineered soil. A raingage will be located on site to measure rainfall depth and intensities. Campbell CR1000 data loggers will be programmed to control the sampling equipment and store the data. Data will be retrieved twice per day and migrated to the USGS real-time data website.

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