Source Evaluation

ERDC TN-DOER-C16

July 2000

C is not a one-to-one correspondence for DOC facilitated desorption isotherms, q as a function of

C cannot be developed from the isotherm.

The shear velocity at particle surfaces affects colloid release from sediment particles under the

influence of decreasing ionic strength. The shear velocities developed by agitation during batch

testing are infinitely large relative to the low shear velocities developed as water percolates through

dredged material in a CDF. Colloidal mass release in a batch test, therefore, is not representative

of colloidal mass release in a CDF under the influence of decreasing ionic strength. In addition,

batch testing requires a liquid-solids separation step that alters the size distribution of colloids

included in the dissolved phase. Thus, in a batch test, neither the mass nor the size distribution of

colloidal release to pore waters in a CDF is properly represented. For these reasons, it is difficult

to couple results from sequential batch leaching with porous media fluid mechanics (advection and

diffusion) and to predict leachate quality without column leaching results.

Source Evaluation. The initial contaminant concentration in the leachate is the contaminant

concentration in the pore water of the dredged material. This concentration can be estimated using

the partitioning relationships given previously. Under generation of the estimated leachate quality,

comparisons between drinking water, surface water, and risk-based standards and the initial

contaminant concentrations in the leachate can be made to provide an early indication of potential

leachate problems. If appropriate standards are not exceeded for some contaminants, such com-

parisons complete the leachate evaluation for these contaminants. If the appropriate standards for

some contaminants are exceeded, additional evaluations will be needed. Surface water standards

should be used only if the leachate is leaving the site and impacting surface waters.

CDF AND VADOSE ZONE CONSIDERATIONS: Two aspects of leachate generation from

CDFs are of particular concern: leachate quality and leachate quantity. Leachate generation depends

on site-specific hydrology and geohydrology, engineering controls at the disposal site, CDF

operation, dredged material hydraulic conductivity, initial water content, and nature of contami-

nants. Evaluation of potential leachate impacts will be greatly affected by the nature of the site and

the engineering controls in place. Varying the engineering controls and site operation during the

evaluation also allows selection of the optimum controls.

Transport Factors. CDF siting, design, and operation affect both leachate contaminant concen-

trations and leachate flow, but predominantly flow. Leachate flow in conjunction with leachate

contaminant concentration determines the mass of contaminant that can potentially leave the site

boundaries. Contaminant mass leaving site boundaries is particularly important when comparing

the effects of various CDF disposal options such as depth of fill, drainage of surface water, and

other leachate control measures such as liners, collection, and treatment. The main CDF factors

affecting leachate generation are climate, siting and foundation properties, dredging and disposal

methods, CDF design and operation, disposal sequence, and control features.

Climate. Climate influences the infiltration of precipitation into the CDF and the evapotranspiration

from materials in the CDF. Greater precipitation increases the potential infiltration and leachate

generation. Greater temperature, solar radiation, and wind and lower humidity decrease the

potential evapotranspiration and leachate generation. The distribution of precipitation throughout

the year also affects the potential infiltration and evapotranspiration. Higher precipitation during