6.4.1 Tier III - Sequential Batch Leachate Test (SBLT)

Evaluation of attenuation of contaminants in the foundation soils and

estimation of groundwater flow are also an integral part of the Tier III leachate

quality evaluations. Initial groundwater modeling using site data could improve

the estimates of attenuation and diffusion in the vadose zone and groundwater

between the CDF and the receptors. The SBLT and/or the PCLT provide better

long-term estimates of the leachate source strength. Adsorption tests on the

existing material in the CDF, on liner materials, on the foundation materials in the

vadose zone, and on dike materials would provide better estimates of attenuation.

Three-dimensional (3-D) groundwater and contaminant transport modelling could

improve the prediction of contaminant concentrations at the point of compliance

or exposed to the receptors as a function of time.

6.4.1 Tier III - Sequential Batch Leachate Test (SBLT)

The SBLT is recommended for leachate testing of freshwater sediments

(Brannon, Myers, and Tardy 1994). However, major differences in leaching

characteristics of freshwater and estuarine sediment make it difficult to predict

leachate quality for estuarine sediments using the SBLT, and it should not be used

for this purpose.

In the SBLT, sediment solids are challenged with successive aliquots of

distilled-deionized water in an agitated system. After the aqueous and solid phases

have reached steady-state, the phases are separated by centrifugation and

filtration, and the leachate is analyzed for contaminants of concern. The solid

phase is then reequilibrated with fresh distilled-deionized water, and the process

of phase separation and leachate analysis is repeated. Each cycle in the test

involves an equilibration step, a phase separation step, and a leachate analysis

step. A table of solid phase and aqueous phase concentrations is developed from

chemical analysis of the leachates, and these data are plotted to produce

desorption isotherms.1 From the desorption isotherms, contaminant-specific

equilibrium distribution coefficients are obtained (Myers and Brannon 1991).

Leaching of freshwater dredged materials in the SBLT usually yields a

classical desorption isotherm, but may also yield other types of partitioning

coefficients described in Section 6.4.4 for the HELPQ program. The key feature

of a classical desorption isotherm is a single-distribution coefficient that is

constant throughout the sequential leaching procedure. The constancy of

distribution coefficients during leaching of freshwater dredged materials is critical

to the prediction of leachate quality in CDFs from sequential batch leach test data.

Detailed guidance for conducting the SBLT is provided in Appendix D.

An isotherm is the measured equilibrium sorption (particle or solids-associated

concentration) as a function of the fluid phase concentration at a given temperature

(Rieble 1999). Isotherm is a term commonly used in the environmental engineering

literature and is derived from the fact that such relationships are developed under constant

temperature.

6-11

Chapter 6

Leachate to Groundwater