Line 77: | Line 77: | ||
For proper nutrient control, bioretention cells 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. | For proper nutrient control, bioretention cells 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. | ||
− | Pre-treatment devices need to be maintained for long-term functionality. Accumulated sediment in the forebay will need to be cleaned out at a minimum when it is half-full, which should be approximately every 10 to 20 years. In an especially dirty watershed, the frequency may be increased to every 2 to 3 years. Sediment should also be cleaned out of rip rap and sumps. A vacuum truck is typically used for sediment removal. If a [[Vegetated filter strips|grassed filter strip]] or [[Filtration|swale]] is used as [[Pre-treatment|pre-treatment]], they should be mowed as frequently as a typical lawn. Depending on the contributing watershed, grassed BMPs 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 [ | + | Pre-treatment devices need to be maintained for long-term functionality. Accumulated sediment in the forebay will need to be cleaned out at a minimum when it is half-full, which should be approximately every 10 to 20 years. In an especially dirty watershed, the frequency may be increased to every 2 to 3 years. Sediment should also be cleaned out of rip rap and sumps. A vacuum truck is typically used for sediment removal. If a [[Vegetated filter strips|grassed filter strip]] or [[Filtration|swale]] is used as [[Pre-treatment|pre-treatment]], they should be mowed as frequently as a typical lawn. Depending on the contributing watershed, grassed BMPs 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 [https://conservancy.umn.edu/handle/11299/194835 Sustainable Urban Landscape Series website] provides guidance for turf maintenance, including mowing heights. |
Maintenance of vegetation after establishment is similar to adjacent gardens (except for application of fertilizer). 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) during the first few years after planting, in case some of the plants that were originally planted die. Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape, since the hydraulic loading ratio is high. Trash removal is important for prevention of mosquitoes. Mulch renewal will be needed two or three times after establishment (first five years). After that, the plants are typically dense enough to make it difficult to mulch, and the breakdown of plant material will provide enough organic matter to the infiltration/filtration device. It is recommended that bioretention performance evaluations follow the four level assessment system in ''Stormwater Treatment: Assessment and Maintenance'' (Gulliver et al., 2010; also see [http://stormwater.safl.umn.edu/sites/stormwater.safl.umn.edu/files/021611gulliver.pdf]). More detailed information about maintenance procedures, a maintenance schedule, and estimated maintenance costs are also available in Gulliver et al. 2010. | Maintenance of vegetation after establishment is similar to adjacent gardens (except for application of fertilizer). 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) during the first few years after planting, in case some of the plants that were originally planted die. Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape, since the hydraulic loading ratio is high. Trash removal is important for prevention of mosquitoes. Mulch renewal will be needed two or three times after establishment (first five years). After that, the plants are typically dense enough to make it difficult to mulch, and the breakdown of plant material will provide enough organic matter to the infiltration/filtration device. It is recommended that bioretention performance evaluations follow the four level assessment system in ''Stormwater Treatment: Assessment and Maintenance'' (Gulliver et al., 2010; also see [http://stormwater.safl.umn.edu/sites/stormwater.safl.umn.edu/files/021611gulliver.pdf]). More detailed information about maintenance procedures, a maintenance schedule, and estimated maintenance costs are also available in Gulliver et al. 2010. |
The most frequently cited maintenance concern for bioretention is surface and underdrain clogging caused by organic matter, fine silts, hydrocarbons, and algal matter. Common operational problems include
Recommendations described below are aimed at preventing these common problems.
Implicit in the design guidance for bioretention is the fact that many design elements of bioretention systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include
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.
Infiltration practices construction inspection checklist.
Link to this table
To access an Excel version of form (for field use), click here.
Project: | ||
Location: | ||
Site Status: | ||
Date: | ||
Time: | ||
Inspector: | ||
Construction Sequence | Satisfactory / Unsatisfactory | Comments |
---|---|---|
1. Pre-Construction | ||
Pre-construction meeting | ||
Runoff diverted (Note type of bypass) | ||
Facility area cleared | ||
Soil tested for permeability | ||
Soil tested for phosphorus content (include test method) | ||
Verify site was not overdug | ||
Project benchmark near site | ||
Facility location staked out | ||
Temporary erosion and sediment protection properly installed | ||
2. Excavation | ||
Lateral slopes completely level | ||
Soils not compacted during excavation | ||
Longitudinal slopes within design range | ||
Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence | ||
Verify stockpile is not causing compaction and that it is not eroding | ||
Was underlying soil ripped or loosened | ||
3. Structural Components | ||
Stone diaphragm installed per plans | ||
Outlets installed pre plans | ||
Underdrain installed to grade | ||
Pretreatment devices installed per plans | ||
Soil bed composition and texture conforms to specifications | ||
4. Vegetation | ||
Complies with planting specs | ||
Topsoil complies with specs in composition and placement | ||
Soil properly stabilized for permanent erosion control | ||
5. Final Inspection | ||
Dimensions per plans | ||
Pre-treatment operational | ||
Inlet/outlet operational | ||
Soil/ filter bed permeability verified | ||
Effective stand of vegetation stabilized | ||
Construction generated sediments removed | ||
Contributing watershed stabilized before flow is diverted to the practice | ||
Comments: | ||
Actions to be taken: |
Effective long-term operation of bioretention practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected life span of the facility, but will also improve aesthetics and property value. Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards.
For proper nutrient control, bioretention cells 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.
Pre-treatment devices need to be maintained for long-term functionality. Accumulated sediment in the forebay will need to be cleaned out at a minimum when it is half-full, which should be approximately every 10 to 20 years. In an especially dirty watershed, the frequency may be increased to every 2 to 3 years. Sediment should also be cleaned out of rip rap and sumps. A vacuum truck is typically used for sediment removal. 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 BMPs 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 provides guidance for turf maintenance, including mowing heights.
Maintenance of vegetation after establishment is similar to adjacent gardens (except for application of fertilizer). 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) during the first few years after planting, in case some of the plants that were originally planted die. Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape, since the hydraulic loading ratio is high. Trash removal is important for prevention of mosquitoes. Mulch renewal will be needed two or three times after establishment (first five years). After that, the plants are typically dense enough to make it difficult to mulch, and the breakdown of plant material will provide enough organic matter to the infiltration/filtration device. It is recommended that bioretention performance evaluations follow the four level assessment system in Stormwater Treatment: Assessment and Maintenance (Gulliver et al., 2010; also see [1]). More detailed information about maintenance procedures, a maintenance schedule, and estimated maintenance costs are also available in Gulliver et al. 2010.
The following are minimum requirements for plant coverage.
Owner’s Representatives may wish to consider deducts and liquidated damages for bad construction practices. Regulating authorities may wish to consider fines for bad construction practices.
The service life of bioretention practices depends upon the pollutant of concern.
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).
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).
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.
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).
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).