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 The number of pore volumes required to reach the peak on contaminant
elution curves can be used to estimate the time to reach maximum contaminant
concentrations in a CDF. This time will depend on a number of site-specific
factors that govern hydraulic flux. These factors include dredged material
hydraulic conductivity, degree of saturation, and hydraulic gradients. A simple
method for estimating the field time to peak leachate concentrations is as
follows:
TfL
tp =
(D-1)
vf
where
tp = time to peak concentrations at bottom of a CDF, years
Tp = pore volumes eluted to reach peak in laboratory leaching column
L = depth of fill in CDF, m
vf = annual average pore water velocity in CDF, m/year
To use Equation F1, an estimate of the annual average pore water velocity is
needed. In some cases, the annual average pore water velocity is approximated
by the hydraulic conductivity of the dredged material. Better estimates can be
obtained by modeling water movement in the CDF. The Hydrologic Evaluation
of Leachate Production and Quality (HELPQ) model http://www.wes.army.mil/
el/elmodels/index.html#addams is applicable for some CDFs. Full groundwater
modeling using the GMS is an alternative but requires allocation of substantial
resources for model calibration.
In addition to modeling water movement, contaminant transport can be
modeled using the HELPQ or other groundwater and multi-media models.
Contaminant transport modeling usually requires more than estimates of peak
contaminant concentrations and pore volumes or time to peak concentrations. A
mathematical formulation of the source term (Equation 1 in Myers, Brannon, and
Tardy (1996) is required. Interim formulations for the source term are discussed
in detail in Myers, Brannon, and Tardy (1996) http://www.wes.army.mil/el/dots/
pdfs/trd961.pdf.
D.5 References
American Society of Testing and Materials. (1999). "Standard specification for
reagent water," D1193-99, West Conshohocken, PA.
Aziz, N. M., and Schroeder, P. R. (1999a). "Documentation of the hydrologic
evaluation of leachate production and quality (HELPQ) module," Dredging
Research Technical Note EEDP-06-20, U.S. Army Engineer Waterways
Experiment Station, Vicksburg, MS.
D21
Appendix D Leachate Testing Procedures
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