USE OF TURBIDITY-TSS RELATIONSHIPS IN DREDGING

ERDC TN-DOER-E8

June 2000

Some states use a maximum turbidity standard, commonly 25 or 50 NTU, and some states allow

an arbitrary increment, commonly 50 NTU, above background (Lillycrop, Howell, and White 1996).

However, McCarthy, Pyle, and Griffin (1974) argue that this is not logical, because it would be too

restrictive in some cases and too lenient in others. In waters of nominal turbidity (10-15 NTU), an

increase of 50 NTU would allow a threefold to fivefold increase over natural levels. In extremely

clear waters (1-3 NTU), the increase could be up to two orders of magnitude. On the other hand,

in very turbid natural waters (100-1,000 NTU), an increment of 50 NTU may be extremely

restrictive. They argue for a turbidity limit of an increase of a certain percentage of normal

background, if turbidity is to be used as the standard, for convenience.

In general, waters over a sandy or gravelly substrate have low turbidity under normal conditions,

because the large particles do not stay in suspension very long. Consequently, dredging of coarse

substrates, which resuspends large amounts of solids, might not violate turbidity standards.

Conversely, dredging very fine sediments, which are usually overlaid by turbid waters because the

substrate is easily suspended and settles slowly, may cause a large increase in turbidity. Therefore,

from both an engineering and a legal standpoint, TSS is preferable as the variable to be measured

to enable resultant data to be interpreted in a meaningful manner.

However, because of the previously mentioned difficulties with using TSS in routine compliance

monitoring and the convenience of turbidity monitoring, many authorities are using turbidity.

Therefore, it is important that turbidity standards be set and used in a logical manner that reflects

reality, protects the environment, and facilitates dredging.

USE OF TURBIDITY-TSS RELATIONSHIPS IN DREDGING: Earhart (1984) described a

procedure designed to ensure compliance of three effluents from diked CDFs with a TSS standard

of 400 mg/L. The U.S. Army Engineer District, Baltimore, was concerned about potential violations

of this restrictive standard. The District was also concerned about the legal and economic

ramifications of requiring a dredging contractor to cease operations because of a potential violation

of the TSS standard that could not be verified until 24-48 hours later, when the results of a

gravimetric analysis became available. If the analysis did not reveal a permit violation, the recurring

damage claims of the contractor based on unnecessary delay could have been significant.

Earhart (1984) reported that samples of sediments and waters from the three dredging sites were

taken and mixed to form suspensions, from which samples were taken for gravimetric analysis for

TSS and turbidity measurement by a Model 16800 Hach Portable Turbidimeter. The suspensions

were then diluted back to the original volume with ambient site water and remixed, and the procedure

was repeated. In this way a series of increasingly dilute samples was analyzed for TSS and turbidity,

and the pairs of results were plotted to form a correlation curve. About 50 pairs of results were

plotted, so the 95 percent confidence limits were reasonably narrow. The correlation curves were

used to determine the turbidity at which the TSS would be 400 mg/L, indicating a violation. This

turbidity value was then routinely compared with the measured turbidity of the effluent from the

containment area as an operating control measure by an onsite inspector.

In order to check the accuracy of the control method, several samples of effluent from the

containment area were sampled for both TSS and turbidity, and the results were compared to the

laboratory correlation curve derived from in situ samples. The samples used for comparative