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Turbidity and Total Solids in Surface Water

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Turbidity and total solids are related measurements used to quantify the clarity of surface waters.

Turbidity is an indirect measure of water clarity that determines the amount of light that can pass through the water. Total solids is a direct measurement, which records the total mass of solid particles suspended in water.

High levels of turbidity or total solids in water can be caused by many environmental factors. These include soil erosion, waste discharge, runoff, or changes in ecological communities including algal growth or abundance of benthic organisms that can disrupt sediments into the water column.

Higher turbidity and suspended solids can lower water quality by absorbing heat, causing increased water temperature and a corresponding decrease in oxygen levels, as warm water holds less oxygen. Photosynthesis may decline, as less sunlight is able to penetrate the water, making it unable to support some aquatic life. Additionally, suspended solids can clog gills, smother eggs, reduce growth rates, and disrupt the microhabitats of many aquatic organisms.

This video will illustrate how to quantify turbidity in a laboratory setting, and how to calculate the total solids in water samples.

Turbidity is caused by solid particles suspended in the water that scatter light rather than allowing it to be transmitted. The degree of the turbidity is determined by the concentration, size, and color of the particles. Larger particles scatter and concentrate light into a forward direction compared to smaller particles. Particle size can also affect light quality, with larger particles scattering longer wavelengths of light more than shorter wavelengths. Conversely, smaller particles scatter short wavelengths more intensely, whilst having relatively little effect on the longer wavelengths.

If particles are densely clustered in a sample, light will come into contact with an increased number of particles, and travel a shorter distance between them, resulting in multiple scattering events. Less dense solutions have a longer mean free path. Darker particles absorb more light, and lighter particles increase scattering, with both resulting in overall increased turbidity. Overall, darker particulates result in higher turbidity than lighter particulates due to the increased amount of light energy absorbed.

One method of measuring turbidity is with a Secchi disk. For freshwater, this is a metal disk 20 cm in diameter with alternating black and white quarters. For marine use a plain white disk of approximately 30 cm in diameter is standard. In both cases the disk is attached to a rope marked at known intervals, and dropped into the water until it can no longer be seen. The length of the rope at the point of disappearance is recorded as the Secchi depth, which is related to the turbidity of the water.

However, there are limitations to this field method. Ideal recording protocol requires sunny conditions and a shaded test area. Steep, wide, or loose riverbanks may pose difficulties or danger for disk operators, or access to a boat may be necessary. Using turbidity tubes to collect water to perform analysis back in the laboratory allows safer and more standardized measurements.

Once the water samples are back in the lab, they are compared to a reference sample. To do this, Standard Turbidity Reagent is added to deionized water in predetermined increments to increase cloudiness until the samples are matched, based on observation of a fixed point at the bottom of the two columns. The amount of reagent added to match the samples can then be recorded and converted to Jackson Turbidity Units or "JTU" using a reference table.

Total solids can be obtained by evaporating the water from the sample to isolate and weigh the solids.

Now that we are familiar with the principles behind the measurements of turbidity and total solids, let's take at look at how these are measured in a laboratory setting.

Once the sample water has been collected in the field, bring it into the laboratory for analysis. First, select a clean turbidity column and fill to the 50-mL line with the sample water.

Next, fill a second "blank" turbidity column with deionized water to the 50-mL line.

Place the two tubes side-by-side and observe the black dot at the base of the turbidity column. If the black dot is equally clear in both tubes, turbidity is zero. If the black dot in the sample tube is less visible, it will be necessary to add Standard Turbidity Reagent to match the turbidity of the reference sample to that of the test sample.

Shake the Standard Turbidity Reagent to re-suspend the particulates. Add 0.5 mL of the reagent to the distilled water tube, and use a stirring rod to mix the contents thoroughly.

Check the turbidity again by placing the sample tube and reference tube side-by-side, and looking down through the solutions at the black dots. If the turbidity of the sample water is still greater than that of the distilled water, continue to add Standard Turbidity Reagent in 0.5 mL increments, until the turbidity of the two tubes appears matched, recording the amount of reagent used and mixing after each addition. Finally, record the total amount of Standard Turbidity Reagent added. Use this value to convert to Jackson Turbidity Units.

In addition to measuring turbidity, the total solids contained in the sample can also be determined. With gloved hands, label abeaker with a grease pencil. A grease pencil is ideal, as these beakers will later be placed into an oven. Next, turn on the balance and tare it. Using gloved hands to avoid transferring body moisture and altering the weight of the beaker, place one of the empty beakers on the balance, and record the weight.

Ensure the sample water is well mixed by swirling it gently, then take a graduated cylinder and measure 100 mL of the water sample. Pour this into the beaker. Place the sample beaker in an oven set to 100 °C for 48 h in order to evaporate the liquid and dry the resulting residue. Remove the beaker from the oven with gloved hands, let it cool to room temperature, and reweigh the beaker containing the residue. To determine the weight of the residue, subtract the initial weight of the empty beaker from the weight of the beaker with residue. Next, convert the weight of the residue into mg/L using this calculation.

Turbidity samples with a JTU of less than 10 are classed as "Excellent"; a range of 11 to 20 JTU is classed as "Good", 21 to 90 JTU samples are "Fair", and in samples of greater than 90 JTU turbidity is classed as "Poor".

Total solids can be categorized using the Water Quality Monitoring Quantitative Analysis categories for total solids measurements. Here, a total solids measurement of less than 100 mg/L is classed as "Excellent", 101 to 250 as "Good", 251 to 400 are "Fair", and samples with greater than 400 mg/L are rated "Poor".

Measures of turbidity and total solids can be useful in a variety of situations, and other potential methods to collect and measure these data.

Another method to measure turbidity utilizes a sensor optimized to measure it directly. First, the sensor is calibrated using a sample of known turbidity and deionized water blank. Next, a water sample is placed in the turbidity sensor, and the handheld monitor will display a readout of turbidity. This method has benefits over the laboratory measurements in that it is faster, simpler, and can be carried out in the field, but does require the purchase of more expensive equipment.

Total dissolved solids can also be measured in the field using an automated device, which uses a conductivity probe to obtain a reading. Here, the probe is manually calibrated and set to record particulates in mg/L. The probe is submerged into the water sample and the total dissolved solids reading is displayed on the handheld monitor. Again, this method provides quicker and easier results than the laboratory method, but requires the purchase of a LabQuest meter and conductivity probe.

You've just watched JoVE's introduction to Turbidity and Total Solids in Surface Water. You should now understand the theory and principles underlying these two valuable measurements of water quality, how to measure them, and how to use these measurements to determine the quality of your water samples. Thanks for watching!

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