Comparison of various filter media mixtures' hydraulic properties from a laboratory testing program

This paper describes the laboratory investigation of hydraulic properties in filter media mixtures designed for use in bioslopes along roadways. The accumulation of pollutants on roadways can result in contaminated stormwater runoff that has a negative effect on receiving water quality, groundwater quality, and aquatic ecosystems. Additionally, roadways increase impervious surface area resulting in an increase in runoff volume and peak discharge intensity. Therefore, current requirements focus on retention of the first inch (2.54 cm) of highway stormwater runoff. Current Minnesota Department of Transportation (MnDOT) specifications were developed using mixtures of clean sand and organic compost. However, mixtures of locally available organic materials in rural areas have not been tested. While there are industry-accepted methods of measuring infiltration rates in situ (i.e., the double ring infiltrometer and the modified Phillip-Dunne infiltrometer (MPDI)), there is no current standard for testing new mixtures on a smaller scale prior to field implementation. This project compares filter media mixtures to material passing current MnDOT specifications using a laboratory testing program. Laboratory constant and falling head tests were used to determine steady-state infiltration rate on samples using different combinations of various locally available organic material with screened sand or taconite tailings. Data from new filter media mixtures are compared to filter material passing current MnDOT specifications. Alternative media mixtures of peat, muck and taconite tailings were found to match the hydraulic conductivity of compost-sand mixtures. These results indicate that alternative media mixtures are suitable for stormwater biofiltration applications, allowing the authors to select mixtures for larger-scale field testing. Applying this laboratory testing program to previously untested filter media mixtures will allow for better design of filter media mixtures.