Estimation of effective cap thickness

reduces the cap thickness to 45 cm. The sediment also consolidates 5 cm as a

result of cap placement. Bioturbation is expected to influence the upper 10 cm

of sediment or cap. These and other problem parameters are collected in

Table B1. The calculation procedure is detailed below.

Table B1

Physico-Chemical Properties of Site Parameters for Example

Cap Properties

Initial cap thickness

50 cm

(L0)

Consolidation distance within cap

5 cm

( Lcap)

Consolidation distance of underlying sediment

5 cm

( Lsed)

Organic carbon content

0.01

(foc)

Porosity

0.5

()

1.25 g/cm3

Bulk density

( b)

Colloid concentration

10 mg/L

(Cc)

Effective cap thickness

35 cm

(Leff)

Pyrene Properties

Solubility

150 g/L

(s)

5 10-5 cm2/sec

Diffusivity in water

(Dw)

105 L/kg

Organic carbon partition coeff.

(Koc)

Mass transfer coeff. at air-water interface

7 cm/hr

(Ke)

Mass transfer coeff. at cap-water interface

1 cm/hr

(Kbl)

Site Properties

(Lbio)

Bioturbation depth

10 cm

10 cm2/year

Biodiffusion coefficient

(Dbio)

Seepage velocity in sediment (assume outflow)

10 cm/year

(U)

Pyrene sediment loading

100 mg/kg

( s)

Pore water concentration

200 g/L

(Cpw)

104 m2

Area of contaminated sediment

(As)

104 m2

Evaporative area

(Ae)

Benthic boundary layer velocity

10 cm/sec

(u)

1.7 x 1013 cm3/day

Basin flushing rate

(Q)

Thickness of contaminated region

35 cm (used in numerical

model only)

Estimation of effective cap thickness

The initial cap thickness is reduced by bioturbation (10 cm), consolidation of

the cap (5 cm), and penetration of pore water expressed by the consolidation of

the underlying sediment. Although the sediment consolidates a distance of 5 cm,

causing movement of pore water 10 cm into the cap (cap porosity of 50 percent),

the contaminant migration is retarded by sorption onto the organic carbon in the

cap. After estimation of the retardation factor associated with sorption onto the

cap materials, it is estimated that the chemical penetration into the cap as a result

of sediment consolidation is only about 80 m. Thus the effective cap thickness

50 cm

Leff

10 cm(bioturbation)

5 cm (consolidation of cap)

80 m (sediment consolidation)

35 cm

B21

Appendix B Model for Chemical Containment by a Cap