Data interpretation and analyses

achieve the salinity of the proposed disposal area. For a freshwater simulation,

10 L of either distilled or reverse osmosis water is used. The water is added as

gently as possible to each small-scale unit and allowed to equilibrate for 3 days

while being aerated. Aeration will ensure that the DO concentration in all units

is at or near saturation (within *0.5 mg/L) at the start of the test.

After 3 days of aeration, the airstone is removed, and a plunger and mineral

oil are added. The plunger is used for daily mixing to prevent the establishment

of concentration gradients in the water column and to ensure a well-mixed

column. Mineral oil is used to seal the surface of the water column from the

atmosphere to allow the development of anaerobic conditions in the water

column. The plunger is suspended between the sediment and the mineral oil.

Mixing should be done in a manner that will not disturb the sediment in the

bottom of the unit or breach the mineral oil on the surface of the water. After

mixing, the plunger is left suspended in the water column.

Step 4 - DO measurements. Water samples are taken immediately after

aeration for initial DO determination. Dissolved oxygen is measured daily until

the DO is depleted in the water column of the uncapped contaminated sediment.

The consequences of reducing the volume of the water column by taking DO

samples is accounted for by multiplying the DO concentration (milligrams per

liter) by the volume of water remaining in the unit after a given sampling. (See

the Calculations section that follows.)

Step 5 - Water sampling and preservation. Water samples to be analyzed

for ammonium-nitrogen and orthophosphate-phosphorus are taken immediately

after the DO is depleted (Day 0) and subsequently on Days 15 and 30. These

water samples should be cleared of particulate matter by passing through a

0.45-m membrane filter, preserved by acidification with concentrated hydro-

chloric acid (HCI) to pH 2, then stored at 4 C. After the water column is

sampled on Day 30, all water samples (Days 0, 15, and 30) are analyzed. Results

from previous small-scale studies (Brannon et al. 1985, 1986; Gunnison et al.

1987; Environmental Laboratory 1987) have shown that complete anaerobic

conditions are achieved in the water column within 30 days.

Data interpretation and analyses

The results from these laboratory tests indicate which of the thicknesses

tested reduce overlying-water oxygen demand and transfer of ammonium-

nitrogen and orthophosphate-phosphorus from the contaminated sediment to the

level of the cap material alone.

Oxygen-depletion rates and ammonium-nitrogen and orthophosphate-

phosphorus release rates are determined by performing linear regression analyses

of mass uptake or release per unit area (milligrams per square meter) versus

time. Means and standard deviations are determined for the triplicates, and t-tests

are conducted to determine the statistical significance of differences between the

Appendix C Capping Effectiveness Tests