STANDARDS AND COMMON PRACTICES

ERDC TN-DOER-E8

June 2000

imprecise. It varies from a meter or less in highly turbid waters to 30 to 40 m in clear ocean water.

It has limited use in monitoring dredging operations.

Numerous authors have published data that show good correlations between TSS and one or more

types of more easily performed turbidity measurements, or among various types of turbidity and

absorbance measurements, at a specific site (Jones and Wills 1956; Postma 1961; McCarthy, Pyle,

and Griffin 1974; Duchrow and Everhart 1971; Ohira, Nikaido, and Takagi 1987; JBF Scientific

Corporation 1978; Austin 1974). Austin (1974) found that Secchi disk visibility is inversely

proportional to the total surface area of the particles in the water, instead of their weight, but that

organic particles, such as algae, scatter light less than inorganic particles of the same size. Jones

and Wills (1956) found a linear relationship between TSS concentration C and the light attenuation

coefficient α for artificial suspensions, but not for sea water, except at TSS < 10 mg/L. They found

a linear relationship between α and 1/D (where D is the depth of Secchi disk visibility) for sea water

up to α of 3, TSS of 12 mg/L, and D down to 2 m, which implies a linear relationship between C

and 1/D, but only over a limited range. However, all of those relationships are specific to a particular

place, suspension, and time.

The theoretical considerations mentioned earlier prevent any simple, universal relationship between

TSS and turbidity from ever being developed, because they measure different things, and their values

are functions of different variables (Lillycrop, Howell, and White 1996). TSS depends on the total

weight of particles in suspension, and is a direct function of number, size, and specific gravity of

the particles, while turbidity is a direct function of the number, surface area, and refractive index of

the particles, but is an inverse function of their size (for constant TSS).

STANDARDS AND COMMON PRACTICES: The measurement of turbidity is most common

in the water treatment industry (drinking water), because consumers want clear water. Turbidity

standards (0.5 NTU) and goals (0.1 NTU) are primarily for aesthetic reasons, although NTU values

above 1.0 indicate that enough solids may be present to harbor bacteria and impair disinfection.

The primary reason for wanting to use turbidity measurements instead of suspended solids is that

turbidity measurements are quick. Nephelometric turbidity readings can be done in a matter of

minutes. On the other hand, taking a sample, transporting it to the laboratory, filtering it, drying it,

weighing it, and calculating the TSS value can take from 6 to 24 hours. In the meantime, the TSS

of the discharge or water body of interest will have changed.

Most discharge or monitoring permits connected with dredging operations are based on TSS rather

than turbidity, because TSS correlates well with environmental impact and is at least roughly

comparable from site to site and sediment to sediment. A TSS of 100 mg/L at one site would have

approximately the same potential for impact as a TSS of 100 mg/L at another site with very different

sediment size characteristics, because it indicates that approximately the same volume of material

is present. However, a turbidity of 50 NTU with one suspension is not comparable in TSS or

potential for environmental impact to another suspension with a turbidity of 50 NTU, but with a

different size distribution of particles. This comparison assumes that, in either case, no toxic or

otherwise hazardous chemical constituents are adsorbed on the sediment particles. In fact, if such

chemicals are present, they tend to adsorb more on fine particles with high specific surfaces.