Turbidity Information Sheet

Background

Turbidity measures the cloudiness of water: the higher the turbidity, the cloudier the water. The numerical measurement is given in Jackson Turbidity Units (JTUs). Turbidity is caused by suspended organic (living or once living) and inorganic (non-living) particulate matter - the things you can see in the water. Erosion provides the clay (very small) and silt (a little bigger) sized particles that are suspended in water flow. Phytoplankton (algae, some protists, and cyanobacteria), fine particulate organic matter (FPOM, small pieces of dead plant and animal parts), and animal waste can also contribute to turbidity levels.

Turbidity caused by phytoplankton is usually highest in late summer - prime growing season. Turbidity caused by mineral matter is highest when discharge of the river is high; strong flows scour the waterway channel. Water containing sediment acts like sandpaper against the edges of a stream channel, scraping away more sediment. In addition, heavy rainstorms can increase soil and sediment erosion into the stream. These particles will settle out of the water column after flow conditions slow down, and they will become deposited on the stream bottom. This process is called sedimentation.

Cloudy water can complicate the lives of aquatic organisms. Turbidity directly affects the amount of light transmitted through water, possibly blocking out sun needed for algae and plant growth. Photosynthesis may decline, reducing the amount of oxygen provided to streams. High turbidity levels reduces visibility, making it difficult for predators (such as fish) to find their food (the insect on the stream bottom or the fisherman's bait). Suspended sediments further harm fish by clogging their gills, causing breathing difficulties. Clear water is needed by filter feeding organisms that siphon water through body parts to obtain food. Sediments in turbid water that eventually settle on the stream bottom can fill in the spaces between rocks where insects are supposed to live. It can also smother insect and fish eggs. Abundant bottom feeding fish like carp can kick up these settled sediments as they search for food, increasing the cloudiness of the water again.

Cloudy water also does something else to disturb aquatic life - it absorbs heat. The increased suspended solids absorb heat energy from the sun, raising the temperature of the stream. With a rise in temperature comes a decrease in dissolved oxygen. Aquatic life may not be able to tolerate the change in temperature and dissolved oxygen levels and may become inactive, unhealthy, or even die. Review the Temperature and Dissolved Oxygen Information Sheets for more information about how those parameters affect aquatic life.

Cloudy water can frustrate another animal - humans. When it comes to drinking water, we prefer sparkling clear water. Water authorities have an easier job of disinfecting and providing us with clear water if the source of water was clear to begin with. If the source water is turbid, water engineers must begin treatment with removing the fine solids. Then they can disinfect it and send it to our faucets. Humans also find clear bodies of water more aesthetically pleasing for recreation, vacationing, and wildlife viewing.

Some turbidity is from natural soil erosion, a main source of suspended inorganic matter and some organic matter. Geology of a watershed greatly influences turbidity. Regions with steep slopes composed of fine-grained sediments are most prone to erosion and contributing particles to the stream. In Western Pennsylvania, shale is a rock type that easily erodes compared to sandstone and limestone. Human impact on the geology and land in a watershed also attributes to higher turbidity.

Human Impact

In watersheds disturbed by people, improper land use practices can increase soil erosion into a stream. Removing streamside (riparian) vegetation for farming, construction, and timbering can increase erosion because plant roots are no longer holding the soil in place. Riparian zones also slow the flow of storm runoff before it enters a stream, and actually traps some of the soil that the storm water was carrying. Maintaining a healthy riparian buffer zone along a waterway's edge can effectively prevent soil erosion and help trap sediments.

Farmers can further contribute to high turbidity when they choose poor tilling methods (not contour plowing). Late spring is a sensitive period because fields have been plowed and seeded, but crops have not yet emerged or are too small to provide much resistance to heavy rains and runoff. A heavy rain on an open field can carry away a valuable soil resource and wash it into the stream, especially if the farmer has not left any riparian zones intact.

Some land uses may result in a decrease in erosion and turbidity. This would not appear to be a problem, except if aquatic life had adapted to living in turbid water. Dams trap sediments on their upstream side, so that water flowing out of them is unnaturally clear. Along the Colorado River - a river that is naturally very turbid - dams have caused major changes in aquatic species. Perhaps an insect larva that used to thrive in the turbid water can no longer exist in the clear water because its predators (fish) are easily able to see them and eat them. In regions that are very heavily urbanized, covered with pavement, there may be less soil erosion than normal.

Water Quality Criteria

The Environmental Protection Agency (EPA) has set a general water quality standard that suspended solids should not reduce the depth of the compensation point for photosynthetic activity by more than 10 percent from the seasonably established norm for aquatic life. For drinking water, the turbidity should not exceed 5 JTUs (Jackson Turbidity Units) for two consecutive days or have a monthly average more than 1 JTU.