There are many Bioswales located within city boundaries, though they are not isolated to those. This area gives a good example of a bioswale that has been created between the new city center and the marshland just slightly north. While you can get all the information you need in this specific area I would encourage you to walk the small path for a more complete understanding of how this bioswale is designed.
Bioswales are designed to provide a concentrated safe place for water runoff where water can more easily recharge the local aquifers and pollutants within the storm water can more easily be removed. The swale itself generally has mildly sloped sides (6% of less) and is influenced by local variables such as regional climate, rainfall patterns, site size, budget, and suitable vegetation for the area. As water runoff flows through a bioswale the pollutants are captured and settled by the leaves and stems of the plants and subsequently enter the soil where they decompose or can be broken down by bacteria.
Runoff from roads and parking lots often contain high nutrient loads compared to other impervious surfaces. It is also a source of sediment, heavy metals, and organic compounds. When this concentrated flow enters a typical stream, it causes degradation in the natural habitat, flooding and other negative hydrological impacts. These factors can be mitigated by such items as meandering paths and water dams, among others. A bioswale's health relies on annual maintenance through soil testing, visual inspection, and mechanical testing.
Bioswales are also known as infiltration swales, biofilters, grassed swales or in-line bioretention. Like open ditches that convery larger stormwater volumes from a source to a discharge point. However, unlike ditches, they intentionally promote to slowing, cleansing and infiltration of the water along the way.
Types of Bioswales
Low Grass bioswales utilize low growing grases that can be landscape, similar to lawns. These types of bioswales tend to be less effective thatvegetated bioswales in treating stormwater runoff and sustaining an adequate collection time.
Vegetated bioswales are created with taller growing plants, ornamental vegetations, schrubs and even trees. These types can also be lined with rocks to slow down the velocity of stormwater runoff that is flowing through bioswales to increase collection time for decontamination. Vegetated bioswales can also include vegetation that is highly useful in removing certain chemicals in runoffs very efficiently.
Low water use bioswales are helpful in areas that tend to be drier with hotter climate. Xeriscape bioswales are populated with runoff generally only after rain and storms and stay dry otherwise.
Wet bioswales are similar to wetlands in which they retain water for much longer period of time that allows for infiltration of stormwater instead of simply emptying the water at the end of the bioswale into storm drain inlets.
Dry swales exist in a different non-vegetated class as they incorporate a deep constructed soil bed into the bottom of a channel. This soil bed is designed to meet specific permeability levels and typically encase a perforated pipe similar to a french drain. Stormwater treated by the soil bed flows into the underdrain, which conveys treated stormwater back to the storm drain system.
Common contaminents and sources of pollutants:
Sediment - Sources fo sediment in urban runoff include construction activities, erosion of unvegetated areas, and winter sand application. Sediment also accumulates on impervious surfaces whether from atmospheric deposition or wind erosion and deposition. Sediment that is routed to bioswale areas can build up at inlets, clog soil media, and smother plants.
Nutrients - Phosphorus and nitrogen are the primary nutrients in stormwater runoff, originating from sources such as lawn fetilizers, leaf litter, grass clippings, unfertilized soils, detergents, atmospheric deposition, and rainfall. Highways and other transportation corridors can increase atmospheric nutrient deposition from the by-products of vehicle exhaust.
Heavy Metals - No, not the band! The primary source of heavy metals in stormwater runoff is wear of mtor vehicle parts, such as brake pads, and tires. Gasoline, motor oil, brak linings, rubber and ashphalt all contribute heavy metals to roadway surfaces. Roadway stromwater runoff may contain trace metals such as copper, lead and zinc. Where runoff is allowed to infiltrate, these metals may acumulate in soil and potentially leach into the groundwater once the soil sorption capacity is reached.
Petroleum Hydrocabons and Organic Chemicals - Many sources of petrolem hypdrocarbons exist in urban catchments, including leaky storage tanks, parking lot and roadway runoff, automotive emissions, elicit dumping, spills and tire particles. Petroleum hydrocarbons in stormwater, particulalry oil, grease, and organic compounds (e.g., nezene, toluene), can be traced to transportation activitires such as fuel spills and engine oil leaks. Organic chemicals can also be found in runoff-high concentrations of polycyclic aromatic hydrocarbons (PAHs) have been found in runoff from roads and parking lots the use sealants.
Salt - In cold climates, salt is common to mitigate icy street, sidewalks, and paved areas during winter months. Salt (e.g., sodium chloride) can build up on paved surfaces between melt events but will eventually be washed off into nearby SCMs and downstream to receiving waters. Salt can also accumulate in soils and can be transferred to the shallow groundwater system over time. High salt concentrations in soils can hinder a plant’s access to water and cause declines or loss of vegetation in bioretention cells and bioswales. Early spring is the most critical period for managing salt concentrations when plant leaves are emerging and rains have not yet flushed the soil media of excess salts.
To earn credit for this Earthcache:
Observe the area of the Phoenix Bioswale, read and learn about bioswales, and answer the following quesitons. The interior of the bioswale is restricted, but entry is not required to complete this Earthcache.
- Please list the geocaching names for all individuals logging this Earthcache (a single message or email for everyone is ok).
- What type of maintenance is crucial for a healthy bioswale?
- What type of bioswale do you see? What is your reasoning?
- Look around. What type of pollutants would you expect to find in this bioswale? What would be there primary source?
- While facing the information sign what type of surface is 10 feet behind you. How would you describe it and why do you think it was designed this way?
Please log your find after you send an e-mail or message with the answers to the questions above. Photos are optional but are always appreciated. Naturally don't post spoiler pictures.
Resources and Further Information:
- https://en.wikipedia.org/wiki/Bioswale
- https://www.epa.gov/sites/production/files/2016-11/documents/final_gi_maintenance_508.pdf
- https://www.canr.msu.edu/news/bioswales_can_improve_water_quality_resources
- https://www.crd.bc.ca/education/stormwater-wastewater-septic/green-stormwater-infrastructure/bioswales