STATION 2 
M4=X—— 
2,000 MIO-OoO—— 
ue 
(a. 
(a. 
= 
= 10004 
a 
>I000 1000-500 500-210 210-105 105-60 60-30 30-10 lO-| <| 
SEDIMENT - SIZE CLASSES, IN ym 
FIGURE 11B.—Concentrations of barium in different size fractions of bottom sediments collected on cruises 4 and 10 at control 
station 2 on Georges Bank. 
smaller area is that data on Ba in the fine fraction 
are available from most of these 20 stations and 
so permit an additional rate calculation. The 
changes in inventory of Ba as BaSO, in the 0.5- 
2 -km area are shown in figure 19. 
For cruise 5, we assigned a value of 1 to the net 
barite inventory (total barite minus background 
barite)for the 0.5-2 -km area and calculated the net 
barite for each successive cruise relative to cruise 
5. A semilog plot of the data appears to have two 
relatively straight line segments (fig. 20) and 
approximates the mathematical model for radioac- 
tive decay for two isotopes having different half- 
lives. If we use this model and least squares regres- 
sion to describe the removal of barite from the 
surface sediments, the initial half-life or half-time 
of barite within this area of the site-specific survey 
is 0.34 year. For cruises 5, 6, and 8, r=-0.99. 
A second slower rate is more arbitrarily defined 
by the data at the end of the monitoring program. 
The linear regression of cruises 8-12 gives a half 
life of 3.4 years (r=-0.40) as shown in figure 20, 
but cruises 9-12 did not predict a decrease with 
time. The average inventory of barite between 
0.5-2 km from the drill site during cruises 9-12 
is 5.5+0.1 (X104) lb. This value is 12 percent 
higher than the average predrilling inventory of 
4.9+0.1 (X104) Ib calculated for cruises 1 and 2 
(fig. 19). The total inventory (fig. 19) should be 
monitored again at annual intervals to determine 
the slower rate of removal. 
A similar calculation of the change in net barite 
inventory of the fine fraction (fig. 21) yields a rapid 
initial half-time of 0.25 year. The secondary rate 
after cruise 8 appears much slower, or zero; how- 
ever, there are insufficient data for an estimate. 
24 
This model is certainly oversimplified but 
retains some merit. One basic assumption of this 
exponential decay model is that each particle has 
the same probability of escaping. In this case, the 
mechanism of escape is thought to be suspended 
or bed-load transport away from the drill rig or 
downward mixing and exchange with uncon- 
taminated sediments below. Although larger 
barite particles generally would have a lower pro- 
bability of eroding than finer particles, there are 
relatively few barite particles large enough to 
resist sediment transport by resuspension. Bot- 
tom stresses on Georges Bank are frequently 
greater than 1.7 dynes/cm2, the stress required to 
resuspend barite particles 63 um in diameter (But- 
man and Moody, 1983). Since about 96 percent of 
the barite used in drilling is finer than 63 pm, we 
assume that most of the whole size range of barite 
particles will be moved frequently by resuspen- 
sion. Therefore, we attribute the initial rapid rate 
of Ba removal to the effects of resuspension and 
transport within the water column. 
The slower rate of barite disappearance from the 
upper 2 cm measured after cruise 8 is probably 
related to the effect of downward mixing into the 
sediment. This process decreases the concentra- 
tion in the surface 0- to 2-cm layer by dilution 
rather than by actual removal. Consequently, the 
removal process is actually slowed by mixing pro- 
cesses, since larger storms (which are less frequent) 
are necessary to erode sediments to the increas- 
ing depths of Ba penetration. 
The slower rate of barite disappearance after 
cruise 8 also could be caused by sediments hav- 
ing lower Ba concentration accumulating on top 
of sediments having higher Ba concentrations. 
