B.2.3 Chemical analyses

determine Td at the actual site under operational conditions at a previous time, with

the conditions similar to those for the operation under consideration. This

approach can be used only for existing sites.

Alternatively, the ratio Td/T = 1/HECF can be estimated from the equation:

T d = 0.9 1 - exp  - 0.3 L







(B-5)









where L/W is the length-to-width ratio of the proposed basin.

The L/W ratio can be increased greatly by the use of internal spur dikes,

resulting in a higher hydraulic efficiency and a lower required total area. In the

absence of dye tracer data or values obtained from other theoretical approaches, a

value for HECF of 2.25 may be used based on field studies conducted at several

sites (Montgomery, Thackston, and Parker 1983).

Step 5 - Sample extraction. After the appropriate period of quiescent settling,

an interface will usually be evident between the supernatant water, with a low

concentration of suspended solids above, and the more concentrated settled

material below the interface. Samples of the supernatant water should be extracted

from the cylinder at a point midway between the water surface and interface using

syringe and tubing. Care should be taken not to resuspend the settled material.

Step 6 - Sample preservation and analyses. The sample should be analyzed

as soon as possible after extraction. If applicable water quality standards are in

terms of dissolved concentrations, the elutriate samples should be analysed for

dissolved concentrations of COC. If applicable water quality standards are in

terms of total or whole water concentrations, the elutriate samples should be split

and analysed for both dissolved and total concentrations of COC, and for total

suspended solids in milligrams per liter. This will allow the calculation of the

fraction of analytes in the total suspended solids in milligrams per kilogram SS.

Filtration using 0.45-um filters should be used to obtain subsamples for analysis of

dissolved concentrations. Samples to be analyzed for dissolved pesticides or

polychlorinated biphenyls (PCBs) must be free of particles but should not be

filtered because of the tendency for these materials to adsorb on the filter.

However, particulate matter can be removed before analysis by high-speed

centrifugation at 10,000 times gravity using Teflon, glass, or aluminum centrifuge

tubes (Fulk, Gruber, and Wullschleger 1975). The total suspended solids

concentration can also be determined by filtration (0.45 um).

B.2.3 Chemical analyses

Chemical analyses of the effluent elutriate samples should be performed

according to the guidance in Chapter 9 of the ITM (USEPA/USACE 1998).

Appendix B Column Settling Test and Effluent Elutriate Procedures