The Avalon Green Alley project in south Los Angeles is designed to allow water to percolate into the soil and recharge the water table . Beneath the permeable pavement of the green alley, catch basins store storm water temporarily where it can be bio-remediated before flowing into surrounding soil. Scanlon et al. describe three categories of methods: physical techniques such as measuring flows , tracer techniques which include the use of chemical, isotopic, or historical tracers , and numerical modeling techniques . Challenges still exist such as those posed by spatial and temporal variability , but there are efforts to address them. For example, Tubau et al. developed a temporally explicit model for quantifying groundwater recharge. The value of groundwater recharge can be calculated using various methods; the most appropriate is dependent on the end-use of water. For example, in 2010, California withdrew 12,700 million gallons of groundwater per day of which approximately 22.2% was used for domestic purposes . The value of groundwater recharge can therefore be associated with the price of water to consumers. Replacement costs can also be used to assign a value to groundwater recharge. Artificial groundwater recharge is the spread of water on land to increase infiltration or the injection of water directly into the aquifer . These actions have associated costs and can be used to estimate a value for the same service performed by NTS.As storm water runoff flows over urban surfaces, it can acquire contaminants such as suspended solids, heavy metals, nutrients, and pathogens . Because southern California receives little precipitation, it allows for the build-up of contaminants and results in the “first flush” phenomenon: elevated levels of storm water pollution at the beginning of storm events . These contaminants can then enter larger bodies of water and degrade the local environment . Many highly urban areas,vertical hydroponics such as Los Angeles County, are coastal which can exacerbate this issue of water quality because storm water runoff can drain directly into the ocean with little opportunity for treatment.
There is a range of common storm water runoff pollutants. Industrial processes and traffic produce copper, lead, and zinc . Heavy metals can persist in the environment and accumulate in sediment, plants, and animals, leading to the degradation of environmental and human health . Major contributors to nutrient pollution are fertilizers, sewage, and erosion . The input of excess nutrients into streams, lakes, and the ocean can cause degradation of habitat and changes in community structure. Eutrophication can lead to harmful algal blooms, hypoxia, and anoxia, which can have cascading effects on the local ecosystem . Harmful organics from herbicides, pharmaceuticals, and industrial processes can be found in urban storm water runoff and be detrimental to biology . Pathogens also pose a risk to human health through exposure . Lim et al. found that captured storm water can be used for toilet-flushing with acceptable risk, but it does not meet required standards for showering and food-crop irrigation. Beach closures and advisories are often the result of bacteria levels exceeding water quality standards . In addition to the threat to human health, the resulting closures have associated economic costs, e.g. less use of parking lots, restaurants, and shopping . NTS remove contaminants through several pathways. Physical filtration removes debris and suspended solids . While this process may lead to clogging, and subsequent deterioration of bio-remediation functions, informed NTS design, such as plant selection, can help maintain infiltration capacity .A significant portion of bio-remediation in these systems is performed by soil microbial communities that can be stimulated by moisture . NTS can prevent contaminants from traveling further to pollute local bodies of water, but they can also concentrate pollutants in plants and filter media . These concentrated contaminants can leach into surrounding soils, e.g. due to lower oxygen levels that increase metal solubility. As a result, some maintenance is likely required in order to prevent build up and transport of contaminants into the environment. Contaminant removal may be the most well-studied service provided by NTS. Figure 5.3 shows examples of systems installed to remove debris and contaminants from runoff. The Grand Boulevard tree wells consists of seven water filtration systems that capture urban runoff from 6.8 acres of residential and commercial area before the water reaches the Santa Monica Bay .
The Filterra™ system utilized on Grand Boulevard has been shown to effectively remove suspended solids, heavy metals, and nutrients, at efficiency ratios ranging from 83-88%, 33-77%, and 9-70% respectively, by comparing pollutant concentrations between inflow and outflow . Removal rates of heavy metals by bio-retention systems can be quite high . Sediment and nutrient removal by NTS have also been found to be relatively high in several laboratory experiments . While these laboratory experiments provide a deep understanding of how NTS may function under specified conditions , in situ studies are also needed to put those laboratory experiments into context. How NTS operate over time is still an open question, both over long and short time scales, as well as how the timing of water quality measurements can affect results . There are many studies that estimate the value of improved water quality using a variety of methods. Contingent valuation, including discrete choice experiments, asks a sample of respondents about their willingness-to-pay for a spectrum of water quality . A production function approach can value improved water quality by comparing the cost of alternative methods for contaminant removal, such as a storm water treatment plant. The appropriate method is dependent on the fate of the storm water and the final ecosystem service it provides, e.g. clean water for drinking or clean water that hosts higher biodiversity.There are efforts to better quantify and value the water services discussed above , but NTS can also provide a range of non-targeted ecosystem services, linked to their utilization of natural structures and functions, and other co-benefits. Filter media and plant communities, which increase infiltration and remove contaminants, host biodiversity which contributes to ecological processes that can result in beneficial ecosystem services . These ecosystem services and other co-benefits are not generally considered during the design and assessment of NTS. One unique feature of NTS is that they are human-made, so they can be designed to provide specific benefits, which are discussed here to further expand design options that can enhance variety and value of services that can be provided by NTS.Vegetated NTS act as man-made ecosystems that contain a diversity of plants, animals, and microbes from which ecosystem services can be generated . They provide patches of habitat within an urban landscape, and potentially act as corridors through which organisms can move. This can be important for population connectivity and resilience in a changing environment due to removal of natural habitat, habitat fragmentation, and anthropogenic climate change .
Biodiversity is here loosely defined to encompass plant, animal, and microbial species richness, abundance, and distribution. While NTS biodiversity can be measured using a range of methods , how biodiversity translates into ecosystem services is more complex and will need targeted studies. Plants act as ecosystem engineers in bio-retention systems, influencing both hydrological and ecological features. Vegetation captures precipitation , undergoes evapotranspiration , maintains media porosity with roots, and assimilates pollutants . Plants determine photosynthesis and respiration rates, organic matter in soil, and ultimately, carbon sequestration and storage in NTS . Additionally, plant communities influence microbial and in faunal communities which subsequently impact ecosystem function. Microbial communities are most often assessed in terms of harmful taxa present or as functional groups . Fauna, except in wetland settings, are usually not considered in NTS design,hydroponic vertical farming systems despite their role in facilitating targeted functions and services. Kazemi et al. and Mehring et al. identify common bio-filter taxa, such as Megadrilacea , Enchytraeidae , and Collembola . Earthworms are known to increase water infiltration via burrows , while spring tails can impact plant growth and nutrient cycling . As a result, these soil invertebrates can be considered ecosystem engineers that move and aerate soil, shaping the microbial, floral, and faunal communities from which more ecosystem services can stem. Because urban NTS can receive more water than native ecosystems, they can host elevated biodiversity of local soil invertebrates relative to natural habitats . Higher biodiversity can be beneficial if they enhance service provision, or detrimental if they involve invasive species that disrupt function. The Ballona Freshwater Marsh is reported to have provided breeding and foraging grounds for 217 bird species in 2012 . Wetland bird species are used internationally to designate areas for conservation and, in addition to contributing to wetland biodiversity, provide recreational services . Increases in biodiversity have also been shown to increase human well-being in urban settings . Studies exist on the economic value of biodiversity in urban and engineered settings . These often employ contingent valuation, travel cost analysis, and hedonic property pricing methods to estimate society’s WTP for biodiversity and its conservation. Travel cost analysis equates the value of an amenity with the amount of resources used to enjoy that amenity. Hedonic pricing considers goods as a bundle of attributes that can then be manipulated in order to determine how people value those attributes. In urban areas, Dupras et al. estimated that urban forests in the Greater Montreal area in Canada host biodiversity that creates a value of $2623 2013 CAD per hectare annually. Brander et al. conducted a meta-analysis of wetland biodiversity that resulted in an average value of $17,000 1995 USD per hectare of wetland annually. Biodiversity associated with other human-made ecosystems, such as agricultural land, may also be relevant to constructed NTS .
Carbon dioxide emissions are the largest contributor to anthropogenic climate change , and as a result, climate-regulating services related to carbon , have become increasingly important especially in urban areas that contribute disproportionately to global emissions . NTS plants have the potential to contribute to this effort by converting atmospheric carbon dioxide into biomass through photosynthesis. How long this carbon is subsequently stored is dependent on several factors. While some carbon is quickly rereleased during respiration, some is stored as plant biomass and soil detritic compounds. Turnover rates vary with types of biomass. For example, woody biomass has slower turnover rates than biomass that contains chlorophyll . Turnover rates can also vary with soil moisture , soil oxygenation, soil organic matter , and microbial communities. Some bioretention systems contain saturated or submerged zones, designed to create anaerobic conditions for denitrification , but may also help prevent microbial breakdown of organic matter. Quantifying carbon sequestration and storage would necessitate measurements that include net carbon fluxes , soil and plant carbon density , and biomass turnover rates. This has been done in urban green spaces but not specifically in NTS. Nowak et al. estimated annual carbon sequestration in U.S. urban forests to be 25.6 million tonnes which, at $36 2015 USD per tonne of carbon , has a value of over $900 million 2015 USD annually. On smaller scales, green roofs have also been shown to sequester 375 g per m2 annually, additionally decreasing carbon emissions due to lowered electricity usage for cooling . Researchers at the University of California, San Diego, are currently working on evaluating net greenhouse gas fluxes over different urban landscapes, including NTS, that can potentially be associated with an economic value. In addition to physically storing carbon, increased green space can reduce air and surface temperatures, reducing electricity use and emission of greenhouse gases . The urban heat island effect occurs due to increased air temperature in urban settings, relative to undeveloped areas, as a result of replacing vegetation with pavement . Pavements, such as asphalt and cement, have lower surface albedo than natural vegetation and therefore absorb more heat. Evapotranspiration also contributes to plant regulation of micro-climates by increasing the amount of water in the air for a cooling effect. Additionally, vegetation can provide shade. Even small green spaces can have a significant impact on micro-climate . Factors that affect microclimate regulating ecosystem services include UV intensity, wind, and size of the green space. The value of micro-climate regulation by urban green space can, in many instances, be calculated using avoided costs methods. For example, if a green space makes an area cooler, people may not run their air conditioning as long or intensively.