Turbidity and Total Solids in Surface Water

Turbidity is a relative measurement determined by measuring how much light can pass through the water sample. The higher the turbidity, the less light will pass through the sample and the “cloudier” the water will appear. Higher turbidity levels are caused by solid particles suspended in the water that scatter light rather than allow it to be transmitted through the water. The physical characteristics of suspended particles can have an effect on overall turbidity. Larger sized particles can scatter light and concentrate it a forward direction, increasing turbidity by creating interference with light transmission through the water. Particle size can also affect the quality of light; larger particle sizes tend to scatter longer wavelengths of light more than shorter wavelengths, whereas smaller particles have more scattering effect on shorter wavelengths. Increased particle concentration can also lower light transmission when light comes into contact with an increased number of particles and travels a shorter distance in between the particles, causing multiple scatterings with each particle. Darker colored particles absorb more light, whereas light colored particles can increase light scattering, and both result in increased turbidity measurements. Overall Darker particulates will result in higher turbidity than lighter colored particulates due to the increased amount of light energy absorbed by the color. The collected unknown water sample is compared to a de-ionized (DI) water blank sample that represents a turbidity value of zero. A purchased standard turbidity reagent (<1% kaolin, <0.1% magnesium nitrate, <0.1% magnesium chloride, < 0.1% 2-methyl-4-isothiazolin-3-one, < 0.1% 5-chloro-2-methyl-4-isothiazolin-3-one) is added to the blank test column in predetermined measurements to increase cloudiness in known increments until the blank and unknown sample match in turbidity based on observation of a fixed point on the bottom of two test columns. The amount of reagent required to achieve matching samples can then be converted with a table into Jackson Turbidity Units (JTUs), named after the original method of holding a long glass "Jackson" tube over a lit candle.

Total Solids is a direct measurement of the suspended solid material in the water sample. The mass of the solids is determined by using an oven to evaporate the water from the sample and isolate and weigh the solids.

The table below is used to convert amount of reagents into the turbidity units (JTU). (Table 1)
TURBIDITY
Excellent < 10 JTUs
Good 11 – 20 JTUs
Fair 21 – 90 JTUs
Poor > 90 JTUs

Total solids can be assessed using the Water Quality Monitoring Quantitative Analysis categories for total solids measurements.
TOTAL SOLIDS (mg/L)
Excellent <100
Good 101 – 250
Fair 251 – 400
Poor > 400

Table 1. Turbidity Test Results Table to convert number of drops (turbidity reagent) to turbidity units (JTU) and the Water Quality Monitoring Quantitative Analysis categories for turbidity.

Turbidity and total solids are important measurements of water quality because they are the most visible indicators of how “clean” a water-source is. High turbidity levels and total solids can indicate the presence of water pollutants that have an adverse effect on human, animal and plant life including bacteria, protozoa, nutrients (e.g. nitrates and phosphorus), pesticides, mercury, lead and other metals. Increased turbidity and total solids in surface water make water unpalatable for human use aesthetically and can also provide surfaces in the water for disease-causing microorganisms to grow harboring water-borne pathogens such as cryptosporidiosis, cholera and giardiasis. High amounts of suspended solids can also become be a problem to other species living in the water if particles become lodged into gills of oxygen-breathing animals in the water. Suspended particles can also disrupt light cycles and photosynthesis, altering the concentration of oxygen in the water and disturbing the aquatic system food web. Turbidity and total solids both increase at times when algal growth is high or when sediment is lifted up into the water during a storm. Both can also increase in response to human activity such as water pollution including industrial, agricultural, and residential runoff. Wastewater from sewage systems, urban runoff, and soil erosion from development can also contribute to high levels of turbidity and total solids. Easy to conduct at the site of water collection, these two simple measurements are broad indicators for a wide range of threats to water quality, all of which render surface water less useful for human purposes and also less able to support itself as an aquatic ecosystem.

Total solids are important to as use as a monitoring test for discharges from sewage treatment plants, industrial plants, or extensive crop irrigation. Areas where freshwater levels are low tend to have higher rates of evaporation and more vulnerable to higher concentrations of solids. Turbidity and total solids concentrations also tend to increase during rainfall events, especially in more highly developed areas with increased amounts of impervious surfaces and urban runoff.