Turbidity

Turbidity refers to the clarity of the water. The greater the amount of total suspended solids (TSS) in the water, the murkier it appears and the higher the measured turbidity. The major source of turbidity in the open water zone of most lakes is typically phytoplankton, but closer to shore, particulates may also be clays and silts from shoreline erosion, resuspended bottom sediments, and organic detritus from stream and/or wastewater discharges. Dredging operations, channelization, increased flow rates, floods, or even too many bottom-feeding fish (such as carp) may stir up bottom sediments and increase the cloudiness of the water (AECOS, Inc. 2000).

There are several ways of quantifying cloudiness in water; the most direct is some measure of attenuation (that is, reduction in strength) of light as it passes through a sample column of water. The now little-used Jackson Candle method (units = Jackson Turbidity Unit or JTU) is essentially the inverse measure of the length of a column of water needed to completely obscure a candle viewed through it (Figure 33). The more water needed (the longer the water column), the clearer the water (AECOS, Inc. 2000).

Figure 33: The Jackson Candle Method of Measuring Turbidity (AECOS, Inc. 2000)

Of course water alone produces some attenuation, and any substances dissolved in the water that produce color can attenuate some wavelengths. Therefore, a property of the particles - that they will scatter a light beam focused on them - was selected as a more meaningful measure of turbidity. Turbidity is measured this way in an instrument called a nephelometer. A light detector is set up to the side of a (source) light beam; more light reaches the detector if there are lots of small particles reflecting the source beam than if there are few (Figure 34). The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). To some extent, how much light reflects for a given amount of particulates is dependent upon properties of the particles like their shape, color, and reflectivity. For this reason (and the reason that heavier particles settle quickly and do not contribute to a turbidity reading), a correlation between turbidity and TSS is somewhat unique for each location or situation (AECOS, Inc. 2000).

Figure 34: The Nephelometric Method of Measuring Turbidity (AECOS, Inc. 2000)

A “low tech” method of measuring turbidity in lakes is the Secchi disk. The depth at which the disk can be seen when lowered into a body of water indicates the clarity of a lake. A Secchi disk is a circular plate divided into quarters painted alternately black and white (Figure 35, below). The disk is attached to a rope and lowered into the water until it is no longer visible. Secchi disk depth, then, is a measure of water clarity. Higher Secchi readings mean more rope was let out before the disk disappeared from sight and indicates clearer water. Lower readings indicate turbid or colored water. Clear water lets light penetrate more deeply into the lake than does murky water. This light allows photosynthesis to occur and oxygen to be produced. The rule of thumb is that light can penetrate to a depth of about 2 - 3 times the Secchi disk depth. Clarity is affected by algae, soil particles, and other materials suspended in the water. However, Secchi disk depth is primarily used as an indicator of algal abundance and general lake productivity. Although it is only an indicator, Secchi disk depth is the simplest and one of the most effective tools for estimating a lake’s productivity (Lake Access 2005).

A modified Secchi disk is used to measure turbidity in streams. The Secchi disk is attached to the bottom of a turbidity tube. Water is poured into the tube until the image of the Secchi disk on the bottom is no longer visible.

Figure 35: Secchi Disk - Another Measure of Turbidity (Lake Access 2005)