6.4.2 Tier III - Pancake Column Leachate Testing (PCLT)

6.4.2 Tier III - Pancake Column Leachate Testing (PCLT)

A thin-layer, column leach test, called the PCLT, has been developed to

simulate contaminant leaching in CDFs (Myers, Brannon, and Tardy 1996). This

test is recommended for leachate testing of estuarine sediments that are dredged

and disposed in CDFs for which the primary source of water for leaching is low in

ionic strength (i.e., freshwater). Leaching of estuarine sediments and dredged

materials with low-ionic strength water results in destabilization of the colloidal

system as salt is washed out. Colloids and colloid-bound contaminants are

released.

The PCLT test is a column leaching test conducted with a column

configuration of 25 cm (10 in.) in diameter and 4.5 cm ( 1.77 in.) in height, a flat

shape resembling a pancake. The PCLT column device can be constructed in any

well equipped machine shop. The pancake design overcomes some of the

shortcomings of conventionally shaped columns. This design minimizes wall

effects by having a large column diameter-to-particle diameter ratio, minimizes

run time for obtaining elution curves by having a short column length, and

provides sufficient sample volume for chemical analysis since the flow-through

area is large (Myers and Brannon 1991).

The PCLT serves as a laboratory-scale physical model of contaminant elution

from dredged material that includes advection-dispersion, colloid release, and

other mass transfer effects. Contaminated sediment is mixed, weighed, and

loaded into the column leach apparatus. Deoxygenated, distilled-deionized water

is introduced into the loaded column over an extended time interval. Water flow

is controlled by a constant-volume pump. Leachate samples are collected at

specified time intervals and are analyzed for COCs. The PCLT results take the

form of an elution curve rather than an isotherm as for the SBLT. The elution

curve is then analyzed with a dispersion-advection model to derive partitioning

coefficients. For saline sediments, the results do not conform to a single

coefficient.

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

6.4.3 Tier III - SBLT or PCLT Adsorption or Challenge Testing

Adsorption or challenge testing can be performed to examine the attenuation

expected to occur when the leachate passes through cleaner materials and foundation

soils. The adsorption or challenge tests are performed in an identical manner as the

SBLT or PCLT with two exceptions:

1. Clean materials and foundation soils are used in the test instead of the

dredged material.

2. Leachate and/or water spiked with higher concentrations of the COC are

used as the leach test water.

The adsorption or challenge tests yield data on the adsorption of contaminants

on clean materials and attenuation for use in contaminant transport modeling.

6-12

Chapter 6

Leachate to Groundwater