A.J. Erickson, B.C. Asleson, J.S. Gulliver, R.M. Hozalski
Monitoring (level 4) is applicable to all four types of biologically enhanced practices but may be necessary only for constructed wetlands. Bioretention practices, filter strips, and swales are relatively small in size, and required monitoring equipment may be cost prohibitive. For example, bioretention practices that are located in residential areas on private property (i.e., resident's front yards) can range from 500 square feet (46.5 m2) down to 20 square feet (1.86 m2), and may occur on 50% or more of the properties on a given street. Monitoring each bioretention practice in this area would be costly in both materials and labor. Monitoring is appropriate, however, to investigate the impact of a number of bioretention practices in a sub watershed or drainage basin. Constructed wetlands and swales are large by comparison, and their design typically facilitates monitoring.
Bioretention practices (rain gardens)
Monitoring of bioretention practices requires that the practice have a subsurface pipe collection system to allow for effluent measurement and sampling. Monitoring bioretention practices without subsurface collection systems can be cost prohibitive and result in minimal conclusive data (Tornes 2005).
Constructed wetlands and swales
Numerous studies have been published in the literature concerning the assessment of constructed wetlands and swales with monitoring (Maehlum et al. 1995, Kadlec and Knight 1996, Oberts 1999, Barrett, et al. 1998, Carleton et al. 2000, Laber 2000, Yu, et al. 2001, Bulc and Slak 2003, Farahbakhshazad and Morrison 2003, Farrell 2003, Deletic and Fletcher 2006). Monitoring is the most comprehensive assessment technique for measuring the hydraulic and pollutant removal effectiveness of a constructed wetland. Runoff volume reduction (by evapotranspiration) can be estimated by comparing the total influent water volume to the total effluent water volume in the water budget for the constructed wetland. It is important to recognize that constructed wetlands typically do not infiltrate stormwater runoff and may receive substantial (> 5%) inflow from direct rainfall due to their large surface areas (see Water Budget Measurement for more information). Monitoring constructed wetlands for pollutant removal effectiveness requires that the volume and water quality of all stormwater inputs and outputs are measured. Refer to Water Budget Measurement for guidance on flow measurement, Sampling Methods for guidance on sampling techniques for gathering stormwater samples, and Analysis of Water and Soils for analysis techniques and recommendations. Data from monitoring should be analyzed according to methods described in Data Analysis.
Filter strips typically lack the inlet and outlet flow structures that would allow for discharge measurement or pollutant sampling. In this case, it is not recommended that filter strips be monitored for water quality or hydraulic performance.
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