Leachate to Groundwater

Leachate to Groundwater

6.1 General Considerations

Leachate is the water with associated dissolved and colloidal materials that

seeps through dredged material in a CDF and subsequently through dikes or

foundation material. Solid particles are not generally transported with the leachate

and therefore the concerns for leachate quality are limited to the apparent

dissolved (including fine colloidal fraction) concentrations of contaminant. The

leachate pathway is perhaps the most technically complex to evaluate, yet it rarely

is of environmental concern for contaminant migration because of the physical

characteristics of most dredged materials, the nature of contamination, and the

isolation characteristics common to most CDFs. Prudent site selection for the

CDF will eliminate most concerns with leachate. For example, the CDF siting

process (USEPA/USACE 1992) should eliminate sites near wells for potable

water or over freshwater drinking water aquifers for CDFs intended for disposal

of dredged materials from a saltwater environment.

This chapter addresses leachate to groundwater as the primary migration

pathway for leachate. Water ponded over the dredged material that seeps through

porous dike sections is considered effluent rather than leachate because it does not

have the characteristics of passing through deposited dredged material. Leachate

that passes through dredged material and directly enters surface waters is not

generally a concern with regard to water column impacts, since the rate of flow of

leachate is so low and the leachate would be mixed and diluted to background

levels almost immediately. However, if this process is viewed as a concern for a

specific site, the procedures for prediction of leachate quality in this chapter are

applicable.

It is conceptually possible that leachate from a CDF may reach groundwater

that may resurface and enter surface water bodies. However, this occurring with

sufficient leachate concentration to be a concern is not a realistic possibility, and

is not addressed in the UTM. The character of the leachate would not be expected

to be significantly different from the effluent from the CDF. As such, if the

effluent does not pose a problem, the leachate is not likely to pose a problem. If

this process is viewed as a concern for a specific site, the procedures for

prediction of leachate quality are applicable.

Leachate from dredged material placed in a CDF is produced by three

potential sources: gravity drainage of the original pore water, inflow of

6-1

Chapter 6

Leachate to Groundwater