Finite element modeling of contaminant transport through confined disposal facilities

The US Army Corps of Engineers is responsible for regular dredging of shipping channels which produces about 300 million yd³ of dredged sediments annually. Many of these sediments have to be contained within confined disposal facilities (CDFs) due to the presence of heavy metals, PCB’s and other harmful constituents within the pore water of the dredge soils. However, these contaminants frequently seep back into the water from which the dredge was removed. The primary objective of this research was to modify the existing finite element program POLUT2D to evaluate the rate and quantity of contaminant transport through CDFs. Two actual field problems were evaluated using the modified program. One of these problems was a new CDF to be located along the US coast and the other was the existing Buffalo Harbor Dike facility located on Lake Erie in Buffalo, New York. The analyses of the coastal facility indicated that a cumulative quantity of about 43 kilograms of arsenic will seep back into the bay at the end of 50 years following filling of the CDF with arsenic contaminated dredge. Analyses of the Buffalo Harbor facility indicated that about 45 kilograms of chlorobenzene seeps annually into Lake Erie from the dredge material contained within this structure.

Sensitivity analyses were also performed to evaluate the effect of soil properties, boundary conditions, etc. on contaminant transport through CDFs. The results indicated that some soil properties such as unit weight, molecular diffusion, and transverse dispersivity have little impact on contaminant transport. Other properties, such as the distribution coefficient and the longitudinal dispersivity, have only a slight to moderate impact on contaminant transport, while the coefficient of hydraulic conductivity can have a significant impact on contaminant transport though CDFs. Analyses also indicated that tidal fluctuations and infiltration from precipitation impact contaminant transport and must be modeled. Additional studies indicated that a slurry trench may provide better containment than a soil liner, and that a combination of a slurry trench and soil cover can reduce contaminant loading by a factor of about 4 depending on the thickness of the soil cover.