Potomac Highlands Watershed School

Understanding Pollution - A Primer


     Point Source pollution comes from an easily identifiable source - like a factory or a sewage treatment plant. The flow of pollutants from point sources is regulated by the State and Federal governments, is fairly constant and predictable, and control measures can be applied at the source.

     Non-point pollution sources are hard to control and study because they are everywhere - they include streets, parking lots, lawns, fields, barnyards and construction sites.  (Note that construction sites larger than one acre are regulated as point sources.)

     The flow of pollutants from non-point sources is very unpredictable, and mostly occurs when rain and snowmelt wash the surface of the land.




Fecal Coliform Bacteria in petri dish

     Concentration is how much of some substance is found in a certain volume of water - usually expressed as milligrams per liter (mg/L) or parts per million (ppm).  With bacteria (pictured above), you count how many are found in 100 milliliters (ml) of sample water.

     Water quality standards are designed to protect people and wildlife from damage, and are based on harmful concentrations of a pollutant. For example, the Nitrate standard for drinking water is 10 ppm because larger amounts than that have been linked to blue baby syndrome.

     The total amount of a substance that passes by some point in a certain amount of time - as in pounds per hour or tons per day. It is equal to the concentration times the total volume of water, a calculation that requires that an accurate measure of water volume be available.

     Water quality standards are not based on load, but on concentration. However, total load can be the most relevant way to determine the potential impacts of non-point source pollution, because, while major NPS pollution may occur rarely, (usually due to precipitation), the total amount delivered during these events may greatly exceed the sum of the loads delivered at all other times. For example, non-point sources of phosphorus usually "move" readily only with surface runoff. Because a few severe storms can create most of a watershed’s annual runoff, over 90% of the annual phosphorus load can be delivered during these few high water events - like the one pictured above at right.     

     The two pictures above show the same stream - at left a typical low flow day in early spring and, at right, near flood.  If the concentration (the amount in a certain volume of water) of some chemical, like phosphorus, was the same on both days, you can see that a lot more phosphorus would have been flowing downstream when the river was high because there was simply a great deal more water in the stream then.  

How do you measure the volume of water in a river?

     In order to calculate pollutant loads you need to know the volume of water flowing in a river over a period of time. To do this, you need to measure three things: width of wetted area, average depth from surface to river bottom, and the rate of flow (speed of the water) at a number of locations across the width of the stream. When you have these numbers, you multiply them together to obtain volume over some unit of time, also known as "stream flow" or "discharge" and is often expressed as cubic feet per second (cfs). This is a time consuming process and very difficult to do in large streams. The U.S. Geological Survey maintains flow stations throughout the United States, including many sites in the Potomac watershed. (See the following website for more information: http://water.usgs.gov/cgi-bin/waterwatch?state=us&map_type=real&web_type=map.




     How pollutants generated in a watershed affect the people and environment in that watershed.  For example, bacteria washed from the surface of a feed lot into the river can easily raise the bacteria levels in the river far in excess of the water quality standard, making the river potentially harmful for swimming.      How pollutants generated in a watershed affect the people and environment downstream of that watershed. For example, nutrient loads from sewage treatment plants in the Potomac drainage contribute to the excess nutrient problem in the Chesapeake Bay.


     Another tool for "assessing" pollution is known as the Total Maximum Daily Load (TMDL) process. This process is triggered when waters fail a State’s water quality standards: i.e. when the waters are impaired. Under the U.S. Clean Water Act Section 303(d), states are required to develop lists of impaired waters. The law requires that these jurisdictions establish priority rankings for waters on the 303(d) lists and develop TMDLs for these waters. A TMDL determines the pollutant loads that a water body can assimilate without violating water quality standards, and then allocates those loads to point source and non point source categories based on the best available science. Once established and approved through regulatory action, TMDLs are implemented through both regulatory and non-regulatory programs. Or, more simply, a TMDL provides a pollution budget for a watershed that allocates the amount each pollutant source is allowed to release while still attaining water quality standards.

Cacapon Institute, PO Box 68, High View, WV 26808 304-856-1385

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