RECOMMENDED PROCEDURES

ERDC TN-DOER-E8

June 2000

analysis contained both larger and smaller particles, and the larger particles were resuspended each

time the diluted samples were remixed. A larger particle contributes much more to the TSS than a

small particle because of its much larger volume and weight, but does not contribute much more to

the turbidity than a smaller particle.

In the effluent from the containment area, larger particles are not usually present, having settled in

the containment area. Only the smallest particles remain. The removal of larger particles reduces

the TSS, based on the mass of the particles, which is a function of the particle diameter cubed, much

more than it reduces the turbidity, based on the number of particles or their surface area, which is

a function of the diameter squared.

Based on this rationale and an analysis of the physics of light scattering and turbidity, one concludes

that any samples used to produce a correlation curve between TSS and turbidity must be suspension-

specific, not just site-specific. The sample must approximate the suspension to be represented in

the size, number, shape, and type of particles. If the suspension to be represented is the effluent

from a containment area, in which a sediment suspension is modified by settling, the best way to

approximate the effluent suspension is by subjecting sediment samples to a comparable period of

settling. A recommended procedure for conducting such a test is described in the next section.

RECOMMENDED PROCEDURES: There are three general situations for which one might wish

to use a TSS-turbidity correlation curve as an aid in routine monitoring of a dredging operation for

which TSS standards or operating guidelines have been set:

1. Monitoring resuspension of solids in the immediate vicinity of the dredge.

2. Monitoring for TSS in the effluent discharge from a CDF containment area or sedimenta-

tion pond.

3. Monitoring for TSS during open-water dredged material placement.

Solids resuspension near dredge. For this case, in the immediate vicinity of the dredge, most

solids, large and small, will be present, because they will be continuously replenished by the action

of the dredge. However, these high TSS concentrations usually do not persist more than about

20-50 m from the dredge, if that far (LaSalle et al. 1991; McLellan et al. 1989). Therefore, the TSS

and turbidity measurements should be made on dilutions of the whole sediment samples, similar to

the method of Earhart (1984). The method used by Earhart (1984) is summarized as follows:

1. Prepare a 1-L mixture of wet sediment and site water by adding 10 g of wet sediment to a

1-L graduated cylinder, and then filling the cylinder with site water.

2. After agitating the mixture, extract 20 mL for TSS measurement and 40 mL for turbidity

measurement with a pipette.

3. Add 60 mL of site water to the cylinder to increase the volume of water to 1 L. Repeat the

extraction for TSS and turbidity measurement.

4. Repeat the process, developing a series of approximately 50 serial dilutions.