Improved Methods for Correlating Turbidity and Suspended Solids for Monitoring

ERDC TN-DOER-E8

June 2000

Improved Methods for Correlating Turbidity

and Suspended Solids for Monitoring

PURPOSE: This technical note describes techniques normally used to measure turbidity and

suspended solids in waters, how the two parameters relate to each other and to various environmental

impacts, and why one cannot be routinely substituted for the other. It further outlines a technique

whereby simple and quick turbidity measurements can be used as an operational aid in monitoring

dredging and dredged material disposal operations as an adjunct to more costly and time-consuming

suspended solids measurements.

BACKGROUND: Resuspension or discharge of suspended solids during dredging and disposal of

sediments can cause significant environmental impacts. Therefore, water quality regulatory agen-

cies often impose permit conditions limiting the concentration of suspended solids that can be

present at specified locations near the operating dredge, point of open-water disposal, or point of

effluent discharge from a confined disposal facility (CDF). Suspended solids concentrations cannot

be determined easily and quickly in the field, however, and transportation to a laboratory and

analysis are time-consuming and costly. Therefore, suspended solids measurements cannot be

easily used to detect and correct short-term problems or permit violations.

Because of these problems, turbidity measurements are often substituted for suspended solids.

Turbidity is easy to measure quickly, but there is no universal relationship between it and suspended

solids, nor among turbidity measurements made on different water-sediment suspensions, nor even

among turbidity measurements made on the same suspension with different instruments. In

addition, turbidity does not correlate well with many categories of environmental impact. However,

turbidity can be used to indicate suspended solids concentration on a site-specific basis, if certain

specific techniques are used.

DEFINITIONS, UNITS, AND MEASUREMENT METHODS: The term total suspended solids

(TSS), sometimes referred to simply as suspended solids (SS), encompasses both inorganic solids

such as clay, silt, and sand, and organic solids such as algae and detritus. It is a measure of the dry

weight of suspended solids per unit volume of water, and is reported in milligrams of solids per liter

(mg/L) of water. The analysis is performed in four specific steps (American Public Health

Association (APHA) et al. 1992), summarized as follows:

1. Dry a glass fiber filter in an aluminum foil dish to constant weight at 103 C, cool it in a

desiccator, and weigh it and the dish (all weights typically measured to the nearest 0.1 mg).

2. Filter a known volume of sample water through the filter.

3. Place the wet filter and its trapped solids in the dish again, dry it in the oven to constant

weight at 103 C again, and weigh it again.

4. Subtract the first weight from the second weight and divide by the volume of sample in

liters to get the suspended solids in milligrams per liter.