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Performance of a ‘Transitioned’ Stormwater Infiltration Basin for Managing Runoff
August 2013 (volume 8 - issue 8)
Contributed by Poornima Natarajan, Postdoctoral Research Associate, St. Anthony Falls Laboratory, University of Minnesota. Research Advisor: Professor Allen Davis, University of Maryland.
Urban stormwater runoff generated from impervious surfaces contribute large volumes of runoff and pollutant loads of suspended solids, nutrients, and heavy metals to nearby surface water bodies, resulting in ecological degradation. Infiltration basins have been widely implemented for stormwater quantity and quality control in urban areas. However, recent research studies (Dechesne et al. 2005; Emerson et al. 2010) and field inspections (Lindsey et al. 1992) have shown that infiltration basins show decreased infiltration performance and progressive signs of ‘failure’, based on evidence of inappropriate ponding of water, excessive sedimentation, and clogging. The functionality of such ‘failed’ infiltration basins in managing runoff, however, has not been investigated and was the focus of my Ph.D. research at the University of Maryland.
The research hypothesis was that a failed infiltration basin may naturally transition into a new practice, similar to a wetpond/wetland system, that is functional from a stormwater management perspective. The objectives of the research study were to determine how effective a ‘transitioned’ infiltration basin was in mitigating runoff flows and improving the water quality of runoff, and determine mechanisms controlling the performance. If found to be functional, such transitioned (or transitioning) infiltration basins can be permitted to remain as they are. This can save the resources involved in rehabilitating infiltration basins, classified as ‘failed’ facilities, to restore original conditions or reconstruction to a new stormwater control measure (SCM).
An infiltration basin, classified as ‘failed’ by the Maryland State Highway Administration, was the study site. The facility received runoff from a highway. Although originally constructed as an infiltration basin, the site showed clear signs of deviation from original design; it appeared to resemble a wetpond/wetland, but a specific name could not be used to describe this 'transitioned' system (Figure 1). Hydrophytes (some obligate wetland plants) were established along the edges of the infiltration basin.
Figure 1: Photograph showing the ‘transitioned’ infiltration basin site.
In a three-year monitoring and research study, the runoff hydrology and water quality performance of this transitioned infiltration basin were quantified. Runoff flows at the site were continuously measured, except at times of snow cover. Sampling of the runoff was performed during several natural rainfall events and input and output concentrations and loadings of pollutants including total suspended solids (TSS), nitrogen (nitrate, nitrite, TKN), phosphorus (total and dissolved phosphorus), and heavy metals (copper, lead, zinc) were monitored.
The hydrologic behavior and performance data from 188 rainfall events showed that the transitioned infiltration basin provided effective runoff control through detention and volume reduction. The sample hydrograph shown in Figure 2, recorded during a storm event in April 2010, shows the reduction in peak flow and total volume through the transitioned system.
Figure 2: Hydrograph recorded during a rainfall event at the transitioned infiltration basin site.
While all smaller rainfall events (<0.25” rainfall depth) were captured (100% runoff volume reduction), runoff flow volumes from moderate rainfall events (0.25 - 1”depth, any duration) and larger rainfall events (1 - 2.5” depth, any duration) were reduced, ranging between 4 and 100%. Minimal or no net hydrologic benefit was observed during colder periods, when the presence of ice-cover on the surface acted as a sheet for the runoff to flow through the facility, and during extreme events (Tropical Storm Lee and Hurricane Irene). The effects of storm characteristics, season, and weather, on the mitigation of runoff flows were visible throughout the study period.
The hydrologic behavior of the system had direct implications on the water quality performance during rainfall events. For runoff from rainfall events fully captured within the transitioned basin, the net pollutant mass removal was 100% for these events. For rainfall events producing outflow from the basin, overall reductions in event mean concentrations (EMCs) and pollutant mass loads were observed. The total suspended solids (TSS) and heavy metals (Cu, Zn, and Pb) removals were excellent; the outflow EMCs were lower than that of the inflow and outflow EMCs met their respective water quality criterion for all rainfall events. Sedimentation was determined to be the primary removal mechanism for TSS and metals, supported by the good correlation between the TSS and total metal concentrations.
Phosphorus and nitrogen removals occurred but varied throughout a seasonal year. For total phosphorus (TP), inflow EMCs were reduced through the basin for most rainfall events. The reductions, however, were not sufficient to meet the selected water quality criterion of 0.05 mg/L. With respect to nitrogen, good overall nitrate removal was observed and attributed to denitrification. TKN reductions were moderate (Figure 3). The exception to good water quality performance for nutrients was observed during select colder periods when export of TP and TKN masses occurred. Overall, total nitrogen (TN) was removed through the basin for most of the rainfall events.
Figure 3: Inflow and outflow event mean concentrations of total Kjeldahl nitrogen (TKN) measured during rainfall events at the transitioned infiltration basin. Open squares denote no outflow rainfall events.
In conclusion, the research study showed that the transitioned infiltration basin controlled the highway runoff by slowing and delaying flow rates and reducing the runoff volume. Improvements in the water quality of runoff through reductions in pollutant EMCs and mass loadings were also observed for all pollutants. The environmental conditions (hydroperiod, soil, vegetation) were, and must be, used as clues to attribute hydrologic and water quality benefits to such transitioning infiltration basin facilities. When feasible, this new practice could be part of a treatment train for good removal of solids, nitrogen, and metals, and moderate removal of phosphorus from stormwater runoff.
My Ph.D. research on transitioned infiltration basin provided me good understanding and experience on pollutant transformation dynamics and controlling processes, field monitoring, and laboratory analyses. The experience gained has been very helpful towards my next and current research project on ‘enhanced swales for prevention of pollution by stormwater runoff’ at the St. Anthony Falls Laboratory, University of Minnesota, for the Minnesota Local Road Research Board. In this research project, a filter prototype that can be installed in existing and new ditch blocks in swales sites is being developed. The prototype will utilize a sand filter enhanced with iron or steel to capture the phosphates and metals in runoff, a concept that has been successfully demonstrated (Erickson et al. 2007, 2012). Field performance testing of these enhanced ditch blocks will be conducted in the next few months.
- Dechesne, M., Barraud, S., and Bardin, J. P. (2005). “Experimental assessment of stormwater infiltration basin evolution.” J. Environ. Eng., 131 (7), 1090-1098.
- Emerson, C. H., Wadzuk B. M., and Traver, R. G. (2010). “Hydraulic evolution and total suspended solids capture of an infiltration.” Hydrol. Process., 24 (8), 1008-1014.
- Erickson, A. J., Gulliver, J. S., and Weiss, P. T. (2007). "Enhanced sand filtration for storm water phosphorus removal." J. Environ. Eng., 133(5), 485-497.
- Erickson, A. J., Gulliver, J. S., and Weiss, P. T. (2012). "Capturing Phosphates with Iron Enhanced Sand Filtration." Water Research, 46(9), 3032–3042.
- Lindsey, G., Roberts, L., and Page, W. (1992). “Inspection and maintenance of infiltration facilities.” J. Soil Water Conserv., 47 (6), 481-486.
We want to hear from you!!!
Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:
- If you have seen such ‘failed’ infiltration basins, what decision(s) and solution(s) were proposed? How successful or unsuccessful were these propositions?
- What and how do favorable conditions develop in a ‘transitioned’ infiltration basin that can make it functional?