Stormwater July/August 2012 : Page 32
Pollutant Removal Pathways HEY AND ASSOCIATES HAS PUT TOGETHER A LIST OF THE MOST COMMON NONPOINT SOURCE POLLUTION REMOVAL PATHWAYS USED TO TREAT URBAN STORMWATER RUNOFF AND PROTECT OR IMPROVE WATER QUALITY: S Sedimentation. Gravitational settling of suspended sediments, such as sand, o settling out in a catch basin or pond. This s process is considered very important as p many pollutants such as phosphorous are often carried by sediment. Filtration. The act of physically straining particulate matter such as sediment as it passes through a ﬁ lter media, such as silt fence at a construction site. Microbial action. Bacteria and other microorganisms break down pollutants into simpler or different compounds or elements that are less harmful to the environment. Examples: nitriﬁ cation and denitriﬁ cation. Volatilization. Allowing or providing opportunities for pollutants to evaporate from stormwater runoff; usually applies to hydrocarbons. Adsorption. Removal of pollutants, particularly dissolved pollutants, from stormwater runoff at the molecular level. Pollutants (reactants) from stormwater runoff adhere to the surfaces of solids (catalysts) via chemical interaction, resulting in a ﬁ lm on the surface of the catalyst. Example: sodium from road salt adhering to calcium in limestone. High cation exchange capacity and neutral to alkaline pH are typically necessary for adsorption to occur. Absorption. Stormwater runoff and associated pollutants soak into the soil. Example: inﬁ ltration. Pollutants may accumulate in the soil, be transported as water percolates through the soil, or dissipate via other processes such as microbial action, adsorption, and volatilization. Plant resistance and uptake. Vegetation slows runoff, creating opportunities for sedimentation and ﬁ ltration. Decaying plant material (detritus) increases adsorption and provides a suitable habitat for microbial action. Plants may also directly take up pollutants from the soil. If the plant material is harvested and removed, these pollutants are also removed (example: phytoremediation). However, direct uptake by plants is usually minimal and/or of secondary importance in treating nonpoint source pollution in urban runoff. of Well #1 in the west rain garden, water-level data were collected between November 29, 2007, and December 7, 2007, and between April 4, 2008, and October 23, 2008. Water-level data were collected from Well #1 in the west rain garden between November 29, 2007, and December 7, 2007, and between April 4, 2008, and September 18, 2008. Afterward, the meter was removed from the rain garden after it was hit by a vehicle. The water-level meters were conﬁ gured to record data on ﬁ ve-minute intervals, and the data were downloaded to Micro-soft Excel at least once every 30 days. The BMPs were surveyed to develop topographic contours over the surface of the bioswale and rain gardens and to ob-tain elevations of the monitoring wells. A Hobo Weather S-SMA Station Soil Moisture Smart Sensor was installed at a depth of approximately 4 inches in the engi-neered soil portion of each of the BMPs, conﬁ gured to record data on a ﬁ ve-minute interval. Soil moisture data were collected between November 29, 2007, and October 31, 2007. All soil moisture data were managed using Microsoft Excel. The data were used to determine the pre-rain soil satura-tion conditions of the BMPs and to evaluate the in situ po-rosity of the surface soils. The monitoring equipment was in-stalled in November 2007, but no storm events occurred after the date of installation. Following winter, monitoring resumed 32 July/August 2012 www.stormh2o.com on April 4, 2008, and concluded on October 23, 2008, for the bioswale and east rain garden. To determine the water-quantity performance of the rain gardens, the monitoring data were broken into discrete events. The initial screening of rainfall data looked at events that ex-ceeded 0.05 inch. Twenty-nine events were identiﬁ ed during the monitoring period for the bioswale and east rain garden, and 23 events were identiﬁ ed during the monitoring period for the west rain garden. For each of the BMPs, the volume of total runoff was computed by multiplying the rainfall by the tributary area to the BMP. The tributary area to the bioswale is the area of the ad-jacent parking lot that drains to the bioswale. The tributary area to each of the rain gardens is a portion of the rectory’s roof. The monitoring system was designed to account for the fate of the runoff through measurements made with the soil moisture meter and the two groundwater-monitoring wells. The soil moisture data were used to determine the porosity of the soil. The data acquired from the soil moisture meter indicate that the typical porosity of the soil in the bioswale and each of the rain gardens is 29%. A simple water budget was constructed for each mea-sured rain event. The cumulative results over many rain events allow each BMP’s performance to be characterized.
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