Operation and maintenance of stormwater infiltration practices
- added information on sustainable service life of bioretention BMPs based on pollutant of concern;
- added information on maintenance of vegetation;
- minor modifications to the bioretention-construction inspection checklist.
The most frequently cited maintenance concern for infiltration practices is surface clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include
- standing water;
- clogged filter surface; and
- inlet, outlet or under-drains clogged.
Recommendations described below are aimed at preventing these common problems.
Contents
- 1 Design phase maintenance considerations
- 2 Construction phase maintenance
- 3 Post-construction operation and maintenance
- 4 Estimated Hours to Perform Maintenance Activities
- 5 Erosion Protection and Sediment Monitoring, Removal, and Disposal – Protecting Your Investment
- 6 Sustainable service life for infiltration and bioretention BMPs
- 7 References
Design phase maintenance considerations
Implicit in the design guidance is the fact that many design elements of infiltration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include
- limiting drainage area;
- providing easy site access (REQUIRED);
- providing pretreatment (REQUIRED); and
- utilizing native plantings (see Plants for Stormwater Design).
For more information on design information for individual infiltration practices, link here.
Construction phase maintenance
Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with construction of bioretention practices, the most important action for preventing operation and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line.
Inspections during construction are needed to ensure that the bioretention practice is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided here.
BIORETENTION - Construction inspection checklist
Post-construction operation and maintenance
Inspection and Maintenance Planning
A maintenance plan clarifying maintenance responsibilities is REQUIRED. Effective long-term operation of bioretention and infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value.
Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards:
- A site-specific O&M plan that includes the following considerations should be prepared by the designer prior to putting the stormwater practice into operation:
- Inspection checklists
- Routine maintenance checklists
- Operating instructions for outlet component
- Vegetation maintenance schedule
- A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority.
- Adequate access must be provided for inspection, maintenance and landscaping upkeep, including appropriate equipment and vehicles.
- Maintenance activities should be careful not to cause compaction. No vehicles will be allowed within the footprint of the filtration or infiltration area. Foot traffic and stockpiling should be kept to a minimum.
- The surface of the ponding area may become clogged with fine sediment over time. Core aeration or cultivating of non-vegetated areas may be required to ensure adequate filtration.
- BMP areas should not be used as dedicated snow storage areas:
- Areas designed for infiltration should be protected from excessive snow storage where sand and salt is applied.
- Specific snow storage areas should be assigned that will provide some filtration before the stormwater reaches the BMP areas.
- When used for snow storage, or if used to treat parking lot runoff, the BMP area should be planted with salt tolerant and non-woody plant species.
- Practices should always be inspected for sand build-up on the surface following the spring melt event.
- General maintenance activities and schedule are provided below.
Summary of Typical Maintenance Regime
The list below highlights the assumed maintenance regime for an infiltration or bioinfiltration basin, trench, tree trench, or dry swale with check dams.
- First year after planting
- Adequate water is crucial to plant survival and temporary irrigation will be
needed unless rainfall is adequate until plants mature
- As needed
- Prune and weed to maintain appearance
- Stabilize or replace mulch when erosion is evident
- Remove trash and debris
- Mow filter strip
- Renew mulch to replace that which has broken down into organic matter
- Replace vegetation whenever percent cover of acceptable vegetation falls below 90 percent or project specific performance requirements are not met. If vegetation suffers for no apparent reason, consult with horticulturist and/or test soil as needed.
- Semi-annually
- Inspect inflow and pretreatment systems for clogging (off-line systems) and remove any sediment
- Inspect filter strip/grass channel for erosion or gullying and sod as necessary
- Herbaceous vegetation, trees and shrubs should be inspected to evaluate their health and replanted as appropriate to meet project goals
- Remove any dead or severely diseased vegetation
- Annually in fall
- Inspect and remove any sediment and debris build-up in pre-treatment areas.
- Inspect inflow points and infiltration surface for buildup of road sand associated with spring melt period, remove as necessary and replant areas that have been impacted by sand/salt build up.
- Annually in spring
- Cut back and remove previous year’s plant material and remove accumulated leaves if needed (or controlled burn where appropriate).
Estimated Hours to Perform Maintenance Activities
All estimated hours listed below would be to perform maintenance on a commercially sized bioinfiltration or bioretention basin approximately 1,000 SF in size that has adequate pretreatment, has been planted with containerized plants, and mulched appropriately.
- Plants Establishment Period (First two years)
- Bi-Monthly Weeding – 4 Visits at 3 hours per visit
- Plant Replacement – 1 Replacement Planting in the Fall, 4 Hours (assuming 10% plant loss)
- Spring Clean-Up (Cut back of previous years vegetation) –2 Hours
- Erosion, Sediment, and Pretreatment Cleanout – 1 Hour (assuming vacuum truck cleanout of sump catch basin or sediment fore bay)
- Regular Maintenance (After first two years)
- Bi-Monthly Weeding – 4 Visits at 2 hours per visit
- Plant Replacement – 1 Replacement Planting in the Fall, 2 Hours (assuming 5% plant loss)
- Spring Clean-Up (Cut back of previous years vegetation) – 4 Hours
- Tree and Shrub Pruning – 2 hours (Every third year)
- Erosion, Sediment, and Pretreatment Cleanout – 4 Hours (assuming vacuum truck cleanout of sump catch basin or sediment forebay once per year and at least one bi-yearly sediment removal from the bottom of the basin)
Erosion Protection and Sediment Monitoring, Removal, and Disposal – Protecting Your Investment
Regular inspection of not only the BMP but also the immediate surrounding catchment area is necessary to ensure a long lifespan of the water quality improvement feature. Erosion should be identified as soon as possible to avoid the contribution of significant sediment to the BMP.
Pretreatment devices need to be maintained for long-term functionality of the entire feature. Accumulated sediment in forebays, filter strips, water quality sump catch basins or any pretreatment features will need to be inspected yearly. Timing of cleaning of these features is dependent on their design and sediment storage capabilities. In watersheds with erosion or high sediment loadings, the frequency of clean out will likely be increased. A vacuum truck is typically used for sediment removal. It is possible that any sediment removed from pretreatment devices or from the bottom of a basin may contain high levels of pollutants. All sediments, similar to those retrieved from a stormwater pond during dredging, may be subjected to the MPCA’s guidance for reuse and disposal.
If a grassed filter strip or swale is used as pre-treatment, they should be mowed as frequently as a typical lawn. Depending on the contributing watershed, grassed BMP’s may also need to be swept before mowing. All grassed BMP’s should also be swept annually with a stiff bristle broom or equal to remove thatch and winter sand. The University of Minnesota’s Sustainable Urban Landscape Series website [3] provides guidance for turf maintenance, including mowing heights.
Sediment loading can potentially lead to a drop in infiltration or filtration rates. It is recommended that infiltration performance evaluations follow the four level assessment systems in Stormwater Treatment: Assessment and Maintenance (Gulliver et al., 2010; also see [4]). Seeding, Planting, and Landscaping Maintenance – Keeping it Looking Good Plant selection during the design process is essential to limit the amount of maintenance required. It is also critical to identify who will be maintaining the BMP in perpetuity and to design the plantings or seedings with their capabilities in mind. The decision to install containerized plants or to seed will dictate the appearance of the BMP for years to come. If the BMP is designed to be seeded with an appropriate native plant based seed mix, it is essential the owner have staff or have the ability to hire specialized management professionals. Seedings can provide plant diversity and dense coverage that helps maintain drawdown rates but landscape management professionals that have not been trained to identify and appropriately manage weeds within the seeding may inadvertently allow the BMP to become infested and the designed plant diversity be lost. The following are minimum requirements for seed establishment and plant coverage.
- at least 50 percent of specified vegetation cover at end of the first growing season, not
including REQUIRED cover crop
- at least 90 percent of specified vegetation cover at end of the third growing season
- supplement plantings to meet project specifications if cover requirements are not met
- tailoring percent coverage requirements to project goals and vegetation. For example, percent cover required for turf after one growing season would likely be 100 percent, whereas it would likely be lower for other vegetation types.
For proper nutrient control, bioretention BMP’s must not be fertilized unless a soil test from a certified lab indicates nutrient deficiency. The one exception is a one-time fertilizer application during planting of the cell, which will help with plant establishment. Irrigation is also typically needed during establishment.
Weeding is especially important during the plant establishment period, when vegetation cover is not 100 percent yet, but some weeding will likely always be needed. It is also important to budget for some plant replacement (at least 5 to 10 percent of the original plantings or seedings) during the first few years in case some of the plants or seed that were originally installed don’t become vigorous. It is highly recommended that the install contractor be responsible for a plant warranty period. Typically, plant warranty periods can be 60 days or up to one year from preliminary acceptance through final inspections. If budget allows, installing larger plants (#1 Cont. vs 4” Pot) during construction can decrease replacement rates if properly cared for during the establishment period.
Weedings in years after initial establishment should be targeted and thorough. Total eradication of aggressive weeds at each maintenance visit will ultimately reduce the overall effort required to keep the BMP weed free. Mulch is highly effective at preventing weeds from establishing while helping retain moisture for plant health. Mulch renewal will be needed two or three times after establishment (first five years). After that, the plants are typically dense enough to require less mulching, and the breakdown of plant material will provide enough organic matter to the infiltration/filtration practice.
Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape. Trash removal is important for prevention of mosquitoes and for the overall appearance of the BMP.
Sustainable service life for infiltration and bioretention BMPs
The service life of bioretention practices depends upon the pollutant of concern.
Infiltration rate service life before clogging
Infiltration rate appears to drop immediately after installation and then level off at a sustainable level (Jenkins et al., 2010; Barrett et al., 2013). Planted bioretention columns even showed a slight increase in infiltration rate after the initial drop (Barrett et al., 2013). Plant roots are essential in macropore formation, which help to maintain the infiltration rate. If proper pre-treatment is present, service life for infiltration should be unlimited. However, if construction site runoff is not kept from entering the bioretention cell, clogging will occur, limiting or eliminating the infiltration function of the system and restorative maintenance or repair will be needed (Brown and Hunt]], 2012).
Nitrogen reduction
An important mechanism of nitrogen removal in bioretention systems is plant uptake since nitrogen is essential for plant growth. Soluble nitrogen is also removed through denitrification in internal water storage zones, a microbially-mediated process that only occurs under anoxic conditions. Denitrification requires organic matter as a carbon source, which is supplied by decaying root matter and mulch. Particulate bound nitrogen in stormwater runoff will typically be removed through sedimentation. All of these processes are self-sustaining, and the service life of a bioretention system designed for nitrogen reduction should be very long. In bioretention systems where denitrification is not an important process, leaching of nitrate is likely, particularly if the bioretention soil has a high organic matter content (Liging and Davis, 2014).
Phosphorus reduction
With design optimized for phosphorus reduction, phosphorus reduction service life can be more than three decades (Lucas and Greenway, 2011c). Sediment bound phosphorus is removed through sedimentation, while removal of soluble phosphorus in bioretention depends on the type of media used. If the media is already saturated with P (i.e. its P binding sites are full), it will not be able to retain additional dissolved P and the P in stormwater will tend to leach from the media as it passes through the biofilter (Hunt et al. 2006). It is highly recommended that the P-index of the media at installation be below 30, which equates to less than 36 milligrams per kilogram P, to ensure P removal capacity. Laboratory research has suggested an oxalate extractable P concentration of 20 to 40 milligrams per liter will provide consistent removal of P (O’Neill and Davis, 2012). After an effective loading of the equivalent of more than three decades of P into bioretention mecocosms optimized for P reduction, researchers in Australia showed that excellent P retention was still occurring. Keys to maximize P reduction in these systems included P sorptive soils or soil amendments (e.g. aluminum water treatment residuals [WTR] or Krasnozem soils [K40], a highly aggregated clay), use of coir peat (a source of organic matter low in phosphorus), and healthy vegetation. The systems with aluminum water treatment residuals still retained up to 99 percent of applied PO4-P in storm water after the equivalent of 32 years of treatment. After 110 weeks of effluent loading at typical stormwater concentrations, the equivalent to 48 years of bioretention loads, PO4-P retention from storm water by the K40 soils treatment was 85 percent. “Comparison with the K40 treatments over the loading and dosing regimes suggest that the WTR treatments will perform at least as well as the K40 treatment under similar exposure of 48 years” (Lucas and Greenway, 2011).
Heavy metals retention
Metals are typically retained in bioretention systems through sedimentation and adsorption processes. Since there are a finite amount of sorption sites for metals on a particular soil, there will be a finite service life for the removal of dissolved metals. Morgan et al. (2011) investigated cadmium, copper, and zinc removal and retention with batch and column experiments. Using synthetic stormwater at typical stormwater concentrations, they found that 6 inches of filter media composed of 30 percent compost and 70 percent sand will last 95 years until breakthrough (i.e. when the effluent concentration is 10 percent of the influent concentration). They also found that increasing compost from 0 percent to 10 percent more than doubles the expected lifespan for 10 percent breakthrough in 6 inches of filter media for retainage of cadmium and zinc. Using accelerated dosing laboratory experiments, Hatt et al. (2011) found that breakthrough of Zn was observed after 2000 pore volumes, but did not observe breakthrough for Cd, Cu, and Pb after 15 years of synthetic stormwater passed through the media. However, concentrations of Cd, Cu, and Pb on soil media particles exceeded human and/or ecological health levels, which could have an impact on disposal if the media needed replacement. Since the majority of metals retainage occurs in the upper 2 to 4 inches of the soil media (Li and Davis, 2008), long-term metals capture may only require rejuvenation of the upper portion of the media.
Polycyclic Aromatic Hydrocarbons (PAHs) reduction
Accumulation of polycyclic aromatic hydrocarbons (PAHs) in sediments has been found to be so high in some stormwater retention ponds that disposal costs for the dredging spoils were prohibitively high. Research has shown that rain gardens, on the other hand, are “a viable solution for sustainable petroleum hydrocarbon removal from stormwater, and that vegetation can enhance overall performance and stimulate biodegradation.” (Lefevre, 2012b).
References
- Aprill, W. and Sims, Ronald C. 1990. Evaluation of the Use of Prairie Grasses for Stimulating Polycyclic Aromatic Hydrocarbon Treatment in Soil. Biological Engineering Faculty Publications. Paper 41.
- Brown, R.A. and Hunt, W.F. 2010. Impacts of construction activity on bioretention performance. Journal of Hydrologic Engineering. 15(6), 386-394.
- Gulliver, J.S., A.J. Erickson, and P.T. Weiss (editors). 2010. Stormwater Treatment: Assessment and Maintenance. University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN.
- Hatt, B.E., Steinel, A., Deletic, A., and Fletcher, T.D. 2011. Retention of heavy metals by stormwater filtration systems: Breakthrough analysis. Water, Science, and Technology. 64(9), 1913-1919.
- Henderson, C.F.K. 2009. The Chemical and Biological Mechanisms of Nutrient Removal from Stormwater in Bioretention Systems. Thesis. Griffith School of Engineering, Griffith University.
- Hunt, W.F., Jarrett, A.R., Smith, J.T., and Sharkey, L.J. 2006. Evaluating bioretention hydrology and nutrient removal at three field sites in North Carolina. Journal of Irrigation and Drainage Engineering. 132(6), 600-608.
- Jenkins, G, J.K., Wadzuk, B.M., and Welker, A.L. 2010. Fines accumulation and distribution in a storm-water rain garden nine year postconstruction. Journal of Irrigation and Drainage Engineering. 136(12), 862-869.
- LeFevre, G.H., M. Raymond, P. Hozalski, J. Novak. 2012a. The role of biodegradation in limiting the accumulation of petroleum hydrocarbons in raingarden soils. Water Research 46: 6753-6762.
- Lefevre, G.H., P.J. Novak, R.M. Hozalski. 2012b. Fate of naphthalene in laboratory-scale bioretention cells: implications for sustainable stormwater management. Environmental Science and Technology 46(2):995-1002.
- Li, H. and Davis, A.P. 2008. Heavy metal capture and accumulation in bioretention media. Environmental Science & Technology. 42, 5247-5253.
- Liging, Li, and A.P. Davis. 2014. Urban stormwater runoff nitrogen composition and fate in bioretention systems. Accepted for publication in ES&T.
- Lucas, W.C. 2005. Green Technology: The Delaware Urban Runoff Management Approach. Prepared For Delaware Department of Natural Resources And Environmental Control Division of Soil And Water Conservation.
- Lucas, , W. C. and M. Greenway. 2007a. A Comparative Study of Nutrient Retention Performance In Vegetated and Non-Vegetated Bioretention Mecocosms. Novatech 2007 Session 5.2.
- Lucas, W. C. and M. Greenway. 2007b. Phosphorus Retention Performance in Vegetated and Non-Vegetated Bioretention Mecocosms Using Recycled Effluent. Conference Proceedings: Rainwater and Urban Design Conference 2007.
- Lucas, W. C. and M. Greenway. 2008. Nutrient Retention in Vegetated and Non-vegetated Bioretention Mesocosms. Journal of Irrigation and Drainage Engineering. 134 (5): 613-623.
- Lucas, W. C. and M. Greenway. 2011a. Hydraulic Response and Nitrogen Retention in Bioretention Mesocosms with Regulated Outlets: Part I—Hydraulic Response. Water Environment Research 83(8): 692-702.
- Lucas, W. C. and M. Greenway. 2011b. Hydraulic response and nitrogen retention in bioretention mesocosms with regulated outlets: part II--nitrogen retention. Water Environment Research 83(8): 703-13.
- Lucas, W. C. and M. Greenway. 2011c. Phosphorus Retention by Bioretention Mesocosms Using Media Formulated for Phosphorus Sorption: Response to Accelerated Loads. Journal of Irrigation and Drainage Engineering. 137(3): 144–153.
- Morgan, J.G., K.A. Paus, R.M. Hozalski and J.S. Gulliver. (2011). Sorption and Release of Dissolved Pollutants Via Bioretention Media. SAFL Project Report No. 559, September 2011.
- O’Neill, S.W. and Davis, A.P. (2012). Water treatment residual as a bioretention amendment for phosphorus. I: Evaluation studies. Journal of Environmental Engineering. 138(3), 318-327.