Dredging and disposal methods

ERDC TN-DOER-C16

July 2000

winter months when potential evapotranspiration is lowest produces conditions for greater infiltra-

tion and leachate generation. Precipitation from large, intense storms produces greater runoff and

therefore less infiltration and leachate than gentle rainfall for precipitation events of equal magni-

tude. Precipitation on frozen soil also produces greater runoff and therefore less infiltration and

leachate than precipitation on unfrozen soil.

Siting. Several siting factors influence the leachate evaluation. Among the more important factors

are foundation properties, foundation thickness, location of receptors, and geohydrology. Founda-

tion soils that are in a reduced state and have high pH, organic matter, mineral oxides, and acid

volatile sulfides retard contaminant mobility by increasing contaminant retention. Foundation soils

with low permeability restrict leachate flow. These properties are more common in fine-grained

soils. Thicker foundations of fine-grained soils provide greater retention of contaminants. The

location of receptors and geohydrology are important because greater distance from the CDF and

the path of leachate flow reduces the contaminant concentration exposed to the receptor. Similarly,

siting a CDF for saltwater dredged material over a saltwater aquifer reduces the potential for

contaminating a freshwater aquifer. Areas with high groundwater velocities provide greater dilution

of the leachate plume, but spread the leachate plume more quickly.

Dredging and disposal methods. Hydraulic dredging or disposal as opposed to mechanical

dredging and disposal greatly increases the initial water content of the dredged material, providing

a greater short-term source of leachate. As such, the required storage volume for a given quantity

of in situ sediment is much larger, requiring a larger CDF in depth or area. Increasing the area

increases the leachate volumetric flow rate. Increasing the depth of dredged material increases the

pressure head driving leachate production as can be seen in Figure 1, especially for an upland CDF.

Additionally, hydraulic operations greatly increase the short-term hydraulic conductivity of the

dredged material, increasing the rate of leachate production. In the long term (after several years)

the leachate flux for the hydraulically dredged or placed material will approach the flux for

mechanically dredged and placed material as the material consolidates from the dewatering.

Hydraulic dredging or disposal also separates the material into a mound of predominantly coarse-

grained material and a layer of fine-grained material. This process serves to concentrate the

contaminants in the layer of fine-grained material. This may change both the leachate flow rate and

quality in the short and long term. Leaching from the sand mound may increase the leachate

production due to its low contaminant retardation and high permeability; however, the concentration

of contaminants in the sand mound would be expected to be much lower than in the fine-grained

layer.

CDF design and operation. CDF design and operation can affect both leachate quantity and

quality. Leachate quantity increases with the area of the CDF. Leachate quantity may decrease

with increasing dewatering efforts and promotion of runoff. Dewatering and consolidation of the

dredged material decrease the pressure head driving drainage through the CDF and decrease the

hydraulic conductivity of the dredged material, both serving to decrease leachate production.

Desiccation of the dredged material will cause volatile and semivolatile organic contaminants to

volatilize, reducing their concentrations in the leachate. If the entire thickness of dredged material

in the CDF is fully desiccated, the material will become oxidized and the pH may drop if the sediment