Biological treatment Many LID stormwater management practices are ecologically based, using plant materials and microorganisms to capture, degrade, and eliminate pollutants found in contaminated runoff, soil, and air. This biological treatment process, illustrated in the exhibit on the facing page, 61 is often referred to as phytoremediation. During the process of phytoremediation, stormwater runoff is typically conveyed over landscape surfaces (as opposed to in underground pipes) to distributed LID stormwater managment facilities. In these facilities, runoff is slowed and held for an extended period of time, allowing it to evaporate or infiltrate into the soil. In the soil, some contaminants are sequestered through absorption or storage around plant root zones (phytostabilization). Other contaminants are biochemically broken-down into a usable form through metabolic processes by microorganisms in the soil or on the surfaces of inundated plants and debris (phytodegradation). Still other contaminants are absorbed by plant roots (phytoextraction). 62
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Contaminants move from plant roots into stems and leaves through plant vascular systems in a process called “uptake.� Here, contaminant compounds, such as those containing nitrogen and phosphorus, are either processed and released into atmosphere as gases (phytovolatization) or are absorbed as nutrients in plant biomass. 63
These ecologically-based stormwater management facilities provide additional ecosystem benefits, such as creating wildlife habitat, air quality improvement, and ambient air temperature reduction. LID BMP facilities can also contribute to the site’s aesthetic, essentially participating as part of a designed landscape. This contrasts with conventional stormwater infrastructure that relies heavily, if not exclusively, on buried drains, pipes, and culverts as a means to concentrate and discharge contaminated stormwater runoff, as illustrated below.
conventional stormwater infrastructure (see p57) PA
N
P
Sediment Runoff
N
P
S
N
N
HC
HC
HM
HM
S
Hydrocarbon Runoff
Compacted Subgrade N
PA
P
Nutrient and Pathogen Runoff P
PA
HM
P
N
N
PA
S
S
P
PA
P
P
Compacted Subgrade
Contaminant Concentration P
N
HM
N
N
HM
HC
S
To Receiving Waters
HC
S
HM
S
PA
N
HC
P
PA
HM
Typical Storm Drain
P
N
Contaminant Index
HM Heavy Metals
HC Hydrocarbons
S Sediment
pa Pathogens
P Phosphorus Compounds
N Nitrogen Compounds