Methods Manual for Salt Lake Studies/Turbidity

Authors: PSJ Coleman,

Turbidity overview
Turbidity is a measure of brine/water clarity and reflects the suspended matter, disturbance and/or biomass content (and therefore nutrient level) of the brine or water. Turbidity is measured in the field with a secchi disk or turbidity meter, or in the laboratory using a spectrophotometer.

Method One - Secchi Disk


A secchi disk may be used in the field to measure the relative turbidity of brine or water. A secchi disk is a circular plate of metal with a diameter of 30-40 cm. The surface is painted in 4 segments, alternating black and white. The secchi disk is suspended from its centre point by a length of rope. It is helpful if the rope has distance indicators marked on it. The secchi disk is lowered into the lake by the rope until the quadrats can no longer be distinguished. The distance between the surface of the secchi disk and the surface of the brine is recorded in centimetres.

Challenges with this method
If the water surface is chopped by wind action, or if the day is very sunny or conversely, overcast, your readings will vary. Ideally choose a calm day for sampling. Also, reflections can be a problem. Methods of reducing the variation caused by weather include sampling on the shady side of the boat, wearing polarised sunglasses or inserting a glass bottomed bucket into the water to view the disk with. Whatever methods you use to reduce variability, use them every time you sample, and record the methods in your sampling plan.

Secchi disk readings do not really correlate to anything else, although several Australian state agencies have developed 'rules of thumb' that convert secchi depths to NTU (nephelomentric turbidity units). It is probably best to leave your secchi readings as secchi depth. If you want NTU's use a method that measures this.

Method Two - Spectrophotometer
Turbidity as Nephelometric Turbidity Units (NTU) may be measured, and corrected for the contribution of color, using a spectrophotometer. The spectrophotometer is prepared by turning it on. The machine may need calibration by zeroing against a blank. The sample cell is rinsed with sample, then filled with sample, carefully wiped with a tissue, and placed in the spectrophotometer. Readings of Absorbance are taken at 425nm and 580nm.

Record the absorbance of the sample at 425nm and 580nm. As color and turbidity affect each other, correct by plotting the readings on the Nomogram Chart below. The chart is derived from AT Palin (1955) "Photometric determination of the colour and turbidity of water." Water & Water Engineering, 59:341-345.

To plot the turbidity and color onto the nomogram, mark the long scale on the left side of the nomogram with the reading you obtained when you used a wavelength of 425nm. Mark the long scale on the right hand side with the reading you obtained when you used a wavelength of 580nm. Take a ruler and carefully rule a straight line that connects the two readings. Where this line intersects the color scale and the turbidity scale in the centre of the nomogram, the intersections mark the values to record for each parameter.

Record turbidity as NTU's.

Note: The nomogram is dependent on the path length of the spectrophotometer cell. This chart was developed for a 10mm cell path length. Should you have a different cell path length the chart will need adjusting.

Challenges with this method:
The sample may settle prior to return to laboratory. Address this by gently agitating the entire sample bottle prior to filling the sample cell.

Method Three - Turbidity Meter
A calibrated turbidity meter that measures the ratio of transmitted to scattered light using a light emitting diode may be used whether in the field or the laboratory. Turn the meter on and immerse the probe fully in the sample. Record the readings. Inherent uncertainties are present in all methods of turbidity measurement. Only report to the closeness specified in the table below (Eaton et al, 1995).