Background - Thermal
Thermal pollution occurs when warm water enters a stream or a lake that is normally cold. Most people think of thermal pollution as a direct (or point) source such as a power plant that takes in cold water for cooling processes and then releases warm water back into the stream.>
Thermal stormwater runoff is a bit more complicated. There are a number of ways stormwater or snowmelt are warmed before entering a lake or a stream. >
As the state becomes more developed, there is a greater area of ground that is covered by pavement. Think about walking across a parking lot or road with bare feet on a hot summer day. It burns your feet. If a thunderstorm hits that afternoon, the rain landing on that pavement is going to cool the pavement but the water is heated up. Because pavement doesn't allow water to soak in, the heated water is going to runoff the lot, down a storm drain or through a culvert or ditch, into a stream.
Rip rap (large angular rocks used to stabilize banks) along a stream or lake also heats up in the sun. Rain falling on the heated rip rap at the waters edge can result in a rapid discharge of heated water to the lake or stream.
The heated stormwater runoff will heat up the water in the stream or lake. If the water is emptied into an important pool where brook trout are hanging out on a hot August day, it may have devastating effects on the fish population.
Trees and other plants are very important to water temperature. Vegetation naturally occurs along streams, rivers and lakes. Their presence is very important as they shade the water to help keep it cool. If the trees are not there, there is no shade. Tree roots also serve to hold soil in place. Without the trees, soil erosion adds its impact to thermal pollution. One of the side effects of soil entering streams, rivers and lakes is that it makes the water cloudy. Cloudy water absorbs more sunlight, which heats up the water. Another thermal impact of soil is that eroded soil fills in stream beds causing the channels to widen, become shallower and slow the current. Shallower channels are more easily heated up by the sun than deeper water.
Regardless of the source, thermal pollution can have severe impacts on aquatic ecosystems. First, warm water cannot hold as much dissolved oxygen as cold water can. Rapid loss of oxygen levels and increases in temperature can cause stress for many cold water fish.
Warmer water temperatures also encourage photosynthesis and growth in many aquatic plants, such as algae. Greater amounts of algae and other plants means more plant material to decompose with the help of bacteria in the fall. That process uses a lot of oxygen, which adds to the stress on fish and other organisms.
Go to: Bacteria, Nutrients, Soil, Toxics
