Surface Runoff

Surface runoff is simply water flowing over land surfaces. Urban (city) runoff can come from many manmade sources, such as runoff from impervious surfaces, irrigation, washing vehicles, roofs, etc. Natural, direct rainfall can also be the cause of surface runoff. Heavy rainstorms often do not percolate through the ground fast enough, causing water to flow towards the lower ground and collect. Stormwater is an urban runoff to a receiving catchment (area) known as greywater infrastructure, especially stormwater infrastructure like sewer systems, and often then released directly into waterways without treatment. This causes many problems for us, the environment, and organisms.

Open channel greywater infrastructure. http://westfallstaticmixers.com/mixers/low-headloss-products/open-channel-low-headloss-static-mixer-5000/#toggle-id-2

Sources and discharge

In most developed countries and several developing countries, cities are responsible for a centralized method of stormwater management. The need for a centralized method is initiated where populations are increasing and flood control and routing of stormwater are required for public health and safety. Surface runoff is a result of precipitation not infiltrating the ground. If water could infiltrate at a faster rate than the falling rain, no runoff from ponding would occur.

Precipitation falls on all sorts of surfaces that may be natural or man-made. Man-made surfaces are usually impervious, meaning water cannot penetrate the surface. It flows off these surfaces and into catchment areas. The more cities grow, amount of stormwater generated increases due to fewer areas for infiltration. This greywater infrastructure used to control excess water often includes sewers, drainage basins, and retention ponds, storm drains, open channel infrastructure, treatment facilities. Some cities will direct water flow through green infrastructure before entering greywater infrastructure. Implementing Green Infrastructure is a new practice within the past decade or so. City planners are aware of the need for using more green space to reduce contaminants before discharge into local waterways. The final recipient of surface runoff is local waterways. Often these waterways, like rivers, are used as a source by water treatment facilities to provide a city's potable water. The concern with this is that some contaminants are not completely removed, such as organic and [xenobiotic]http://www.biologydiscussion.com/microbiology-2/bioremediation/xenobiotic-compounds-meaning-hazards-and-biodegradation/55625 pollutants, pathogens, and pharmaceuticals.

Differences in percolation between cities and green infrastructure or natural areas. Produced by Environmental Protection Agency.

Runoff quality is dependent on climate, sources, and surfaces of urban areas. Due to these highly variable factors, the resulting water quality in different catchments varies. A general trend is that as precipitation increases so does the amount of pollution in stormwater. Urban run off contains fecal wastes from animals and humans, which results in pathogens such as noroviruses and adenoviruses to collect in stormwater infrastructure, and routed to local waterways.

Health risks. There are health risks in recreational areas and surface waters due to these pathogens in urban runoff. Sealcoats used on driveways and parking lots are asphalt based or coal-tar based. Runoff from seal coats has a high concentration of polycyclic aromatic hydrocarbons (PAHs), which impairs receiving surface waters. PAHs are organic pollutants. Additionally, other organic contaminants such as polychlorinated biphenyls (PCBs), personal care products (PCPs), pharmaceuticals, and pesticides are found in surface runoff in urban areas.

Sediments. Roads have a higher amount of suspended sediments after precipitation than roofs, due to vehicle wear and sediment collection in road crevices. Atmospheric dry deposition from industry and vehicle emissions to roofs and roads contributes to sediments in urban runoff. There is a correlating linear relationship between total metal concentrations and dissolved concentrations on both roads and roofs. It is well known that roofs are a source and contributor of many pollutants, especially metals like copper and lead. Road runoff has high amounts of lead from wheel weights and old paint, zinc from sources like tires and brake pads, and copper from brake pads.

Temperatures. Temperatures in local waterways spike as initial precipitation runs off surfaces, such as transportation infrastructure, and continues to rise with increased precipitation. Increases in temperature are recorded to elevate up to 7°C at discharge points into waterways. Coldwater fish and macroinvertebrates are affected by the increased water temperature, resulting in high stress, lower growth rates, and even death.

Contaminated water flowing into a bioswale. http://northcurrybioswales.blogspot.com/

The contamination of gray water has devastating effects on the hydrology and chemical properties of waterways, and biota inhabiting them. Stormwater management that is sustainable is becoming increasingly necessary for areas that are urbanizing. Taking this into account, methods now exist for city planners and stakeholders to improve the water quality from runoff sources before the runoff enters storm drains and then waterways. Green infrastructure is one method that can have a significant positive impact in filtering water, reducing flow volume and rate, and reducing contaminant loads. Green infrastructure includes low impact development that promotes control of stormwater and reduces contaminant loads through creating vegetative areas, like bioswales or Treatment Wetlands. Overcoming objections to implementing green infrastructure is a reality faced by city stakeholders, as converted land for green infrastructure reduces the amount of land available for building businesses and making the city money.

Individuals can also reduce contaminated water entering local catchments and waterways. Driving less or carpooling will reduce the contaminants in the air, roads, and roofs. Washing vehicles on grass, properly disposing of chemicals, reducing fertilizer and pesticide use, and incorporating green areas will all help reduce the amount of water and contaminants within that water.

Sources

Charters, Frances J., Thomas A. Cochrane, and Aisling D. O'Sullivan. "Untreated Runoff Quality from Roof and Road Surfaces in a Low Intensity Rainfall Climate." Science of The Total Environment 550 (4/15/ 2016): 265-72.

Hathaway, J. M., R. J. Winston, R. A. Brown, W. F. Hunt, and D. T. McCarthy. "Temperature Dynamics of Stormwater Runoff in Australia and the USA." Science of The Total Environment 559 (7/15/ 2016): 141-50.

Lim, Keah-Ying, Andrew J. Hamilton, and Sunny C. Jiang. "Assessment of Public Health Risk Associated with Viral Contamination in Harvested Urban Stormwater for Domestic Applications." Science of The Total Environment 523 (8/1/ 2015): 95-108.

Lye, Dennis. "Rooftop Runoff as a Source of Contamination: A Review | Rainwater Resources." Science of the Total Environment 407 (2009): 5429-34.

Mahler, Barbara J., Peter C. Van Metre, and William T. Foreman. "Concentrations of Polycyclic Aromatic Hydrocarbons (Pahs) and Azaarenes in Runoff from Coal-Tar- and Asphalt-Sealcoated Pavement." Environmental Pollution 188 (5// 2014): 81-87.

Pennino, Michael, Rob Mcdonald, and Peter Jaffe. "Watershed-Scale Impacts of Stormwater Green Infrastructure on Hydrology, Nutrient Fluxes, and Combined Sewer Overflows in the Mid-Atlantic Region." Science of the Total Environment 565 (2016): 1044-53.

Taebi, Amir, and Ronald L. Droste. "Pollution Loads in Urban Runoff and Sanitary Wastewater." Science of The Total Environment 327, no. 1"3 (7/5/ 2004): 175-84.

Winston, Ryan J., William F. Hunt, Shawn G. Kennedy, Laura S. Merriman, Jacob Chandler, and David Brown. "Evaluation of Floating Treatment Wetlands as Retrofits to Existing Stormwater Retention Ponds." Ecological Engineering 54 (5// 2013): 254-65.

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