A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 3 - Streams

Total Suspended Solids and Turbidity in Streams

Why Is It Important?

Total suspended solids (TSS) concentrations and turbidity both indicate the amount of solids suspended in the water, whether mineral (e.g., soil particles) or organic (e.g., algae). However, the TSS test measures an actual weight of material per volume of water, while turbidity measures the amount of light scattered from a sample (more suspended particles cause greater scattering). This difference becomes important when trying to calculate total quantities of material within or entering a stream. Such calculations are possible with TSS values but not with turbidity readings.

High concentrations of particulate matter can cause increased sedimentation and siltation in a stream, which in turn can ruin important habitat areas for fish and other aquatic life. Suspended particles also provide attachment places for other pollutants, such as metals and bacteria. High suspended solids or turbidity readings thus can be used as "indicators" of other potential pollutants.

Reasons for Natural Variation

TSS and turbidity values vary naturally for two main reasons – one physical, the other biological. Heavy rains and fast-moving water are erosive. They can pick up and carry enough dirt and debris to make any stream look dirty. So, heavy rainfall may cause higher TSS concentrations or turbidity, unless the additional particles are dispersed throughout large volumes of flood water. The native soils and geology of the watershed of course determine how easily erosion occurs.

A small part of the natural increase may be explained by seasonal changes in algae populations. It is the suspended forms of algae (i.e., those floating in the water column) that are measured by TSS and turbidity. If the original water source is a lake or wetland where algae populations can vary drastically with season, this may show up as changes in stream TSS or turbidity. However, in streams themselves, attached forms of algae (i.e., those attached to rocks, logs, or other substrate) are far more common. The change in these populations aren’t measured by TSS or turbidity until they wash off the substrate. Wash-off may not occur until the algal mass dies, is scoured off by large flows, or the mass becomes too large to remain on the substrate.

Expected Impact of Pollution

Land use is probably the greatest factor influencing changes in TSS or turbidity in streams. As watersheds develop, there is an increase in disturbed areas (e.g., cropland or construction sites), a decrease in vegetation, and increases in the rate of runoff. These all cause increases in erosion, particulate matter, and nutrients, which turn promote increased algal growth. For example, loss of vegetation due to urbanization exposes more soil to erosion, allows more runoff to form, and simultaneously reduces the watershed’s ability to filter runoff before in reaches the stream.

TSS concentrations are reported in units of milligrams of suspended solids per liters of water (mg/L). Turbidity is reported as nephelometric or Jackson turbidity units (NTU or JTU’s), depending on the instrument used to perform the measurement. There are no Washington State water quality standards for TSS. The water quality standard for turbidity is based on the amount of increase over background conditions. For Class AA and A streams, if background turbidity is 50 NTU or less, then the total amount of increase can not be more than 5 NTU. If the background is greater than 50 NTU, then the increase can not be above 10 percent of the background level. For Class B and C streams, if background turbidity is 50 NTU or less, then the total amount of increase can not be more than 10 NTU. If the background is greater than 50 NTU, then the increase can not be above 20 percent of the background level. The following tables provide summary information for both TSS and turbidity for three Western Washington streams.

The next section discusses fecal coliform bacteria in streams.

Chapter Four provides information about how to measure TSS and turbidity in streams.

Stream Temperature | Stream Dissolved Oxygen | Stream pH | Stream Nutrients | Stream TSS and Turbidity | Stream Fecal Coliforms | Return to Table of Contents