Leachate Screening Considerations

ERDC TN-DOER-C16

July 2000

Leachate Screening Considerations

PURPOSE: The purpose of this technical note is to present an approach for development of a

leachate screening protocol. The protocol will evaluate the acceptability of confined disposal of

dredged material and the need to perform laboratory testing and unsaturated and saturated ground-

water modeling. This technical note presents the main factors that will be considered during the

development of the leachate screening procedures. The impact of these factors on the leachate

quality exposed to receptors will be examined, and relationships among the dominant factors and

leachate quality will be developed for use in the screening procedures.

BACKGROUND: Contaminated dredged material is often placed in confined disposal facilities

(CDFs) designed and operated to control environmental impacts of the disposed material. A CDF

is a diked enclosure having structures that retain dredged material solids. CDFs can be upland,

nearshore (partially surrounded by water), and in-water (totally surrounded by water) (Figure 1).

When contaminated dredged material is placed in a CDF, contaminants may be mobilized to form

leachate that is transported to the site boundaries by seepage. Subsurface drainage and seepage through

dikes may reach adjacent surface water and groundwater and act as a source of contamination.

Leachate seeping into the groundwater from dredged material placed in a CDF can be produced by

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

groundwater, and infiltration of rainwater and snowmelt. Thus, leachate generation and transport

in a CDF depend on many disposal site-specific and sediment-specific factors. Immediately after

dredging and disposal, dredged material is saturated (all voids are filled with water). As evaporation

and seepage remove water from the voids, the amount of water stored and available for gravity

drainage decreases. Since the contaminants in dredged material are primarily adsorbed to sediment

particles, leaching by percolating site water from a CDF situated above the groundwater table is the

primary mechanism by which contaminant migration to groundwater takes place. If the site is

situated so that groundwater will flow through the material (typically, a nearshore CDF), percolating

groundwater may be the primary source of water through the material. If the CDF is a nearshore

or island facility, surface water may be in contact with the dredged material as a result of fluctuating

water levels and then transport contaminants from the CDF in a process termed tidal or wave

flushing.

INTRODUCTION: Section 404 of the Clean Water Act of 1972, as amended, the National

Environmental Policy Act of 1969, the U.S. Army Corps of Engineers (USACE) management

strategy for dredged material disposal (Francingues et al. 1985), and the USACE/U.S. Environ-

mental Protection Agency (USEPA) technical framework for evaluating the environmental effects

of dredged material management alternatives (USACE/USEPA 1992) require the evaluation of the

confined disposal alternative for dredged material to include groundwater impacts. Time-varying

leachate flow and leachate quality must be predicted to evaluate potential impacts. Because

contaminant mobility in dredged materials is variable and highly site specific, laboratory testing