Sediment Dispersion During Placement

(i.e., submerged discharge) are necessary during placement. In addition,

the prediction indicates what portion of the contaminated material is dis-

persed during placement and is not capped.

Methods for evaluation of potential water-column contaminant release

are available ((USACE/EPA 1992). The contaminant release is predicted

by an elutriate test, and results are compared with applicable water-quality

criteria or standards as appropriate. In addition, acute water-column toxicity

bioassays considering initial mixing may be needed. The procedures to be

used in elutriate or water-column bioassays are provided in the MPRSA

and CWA testing manuals (EPA/USACE 1991; EPA/USACE 1998). For

disposal operations under the MPRSA, specific criteria for water quality

and water-column toxicity must be met, and specific allowances are speci-

fied for initial mixing (EPA/USACE 1991). For disposal operations under

CWA, water quality and water-column toxicity standards and allowances

for initial mixing are specified by the States as a part of the Section 401

water-quality certification requirements.

The physical development of a mound or deposit on the bottom due to

a number of barge or hopper releases or prolonged discharge from a pipe-

line is also of interest. Such information can be used to define the areal

extent of the mound or deposit for the contaminated material. This dic-

tates the required volume of capping material.

A computer model is available for evaluating the short-term fate of

dredged material discharges in open water from hoppers or barges. The

model is called the Short-Term FATE (STFATE) model (Johnson et al.

1993; Johnson and Fong 1995) and can be run on a personal computer

(PC). This model is available as a part of the Automated Dredging and

Disposal Alternatives Management System (ADDAMS) (Schroeder and

Palermo 1990). Versions of the model are also included in the Ocean and

Inland testing manuals (EPA/USACE 1991; EPA/USACE 1998). Appen-

dix D describes the STFATE model in greater detail.

Input data required to run the model include (a) description of the dis-

posal operation, (b) description of the disposal site, (c) description of the

dredged material, (d) model coefficients, and (e) controls for input, execu-

tion, and output. More detailed descriptions and guidance for selection

of values for many of the parameters are provided directly on-line in the

system software or default values may be used.

Model output includes a time history of the descent and collapse phases

of the discharge and suspended sediment concentrations for various parti-

cle size ranges as a function of depth and time. At the conclusion of the

model simulation, the thickness of the deposited material on the bottom is

given. Examples of model output are given in Figures 12 and 13. This al-

lows an estimate of the areal extent or "footprint" of contaminated mate-

rial as deposited on the bottom for a single disposal operation (i.e., a

single barge or hopper load of material).

Chapter 6 Sediment Dispersion and Mound Development and Site Geometry During Placement