100,000 



90,000 



80,000 



70,000 



60,000 



50,000 



40,000 



30,000 



20,000 



10,000 







100,000 



90,000 



80,000 



70,000 



60,000 



50,000 



40,000 



30,000 



20,000 



10,000 







4 760 



4,740 



4 720 



4 700 



4 680 



4 660 



4 640 



4 620 



4 600 



4 580 



L 



Slope = 



Counti Pet Minute 



h 



slope = -27 



Counti Per M.nute 



sediment contained 



10 



20 30 



40 



50 60 



MINUTES 



Figure 13. — Rates of movement of zinc 65 from water to 

 sediments and walls of plastic cylinders. Graph A 

 represents movement of zinc 65 in "control" cylinder; 

 B represents movement of zinc 65 from water to sedi- 

 ment in typical sample from Newport River, N.C.; and 

 C shows a plot of log concentration of zinc 65 in the 

 water vs. time to be a straight line for first 90 minutes. 



d R 



The rate, ^ is assumed to be equal to the 



dt 

 slope of the R vs. t plot which is calculated 

 by inspection from the first 3 5 minutes of the 

 curve after the elimination of the first 5 min- 

 utes during which mixing occurred. This as- 

 sumption is permissible since the slope is 

 essentially constant over the first 35 minutes 

 after the addition of tracer (fig. 13). The rate 

 of exchange, p, is then computed from the 



value of 



d R 



w and that calculated for a at t = 0. 



dt 



p is the rate of exchange of the substance 

 between the water and sediment actually con- 

 tained in the experimental cylinders. To make 

 this rate applicable to the estuary, the rate is 

 divided by the exposed surface area of the 



in the cylinder, giving 

 dRw 



dt 



a ^ (surface area 

 of sediment) 



The exchangeable amount of element in the 

 sediment can be determined from the specific 

 activity in the water after the tracer 

 has equilibrated with the sediments. After the 

 cylinders are analyzed for 2 hours for the loss 

 of the tracer, they can be removed from the 

 shield, stored with aerating device in place, 

 and then periodically replaced in the shield and 

 analyzed for tracer content. At equilibrium, 

 i.e., when there is no further loss of isotope 

 from the water, the amount of exchangeable 

 element in the sediment can be calculated from 

 the expressions: 



Specific activity of the water = Specific ac- 

 tivity of sediment 



Rv 



S.c - 



Ss 

 (Sw) 



(Rs) 



R„ 



Application of Technique 



Cores collected June 1967 from the mouth of 

 the Newport River estuary in Beaufort, N.C, 

 were analyzed for rate of zinc exchange by this 

 technique. Cores consisted of 32 percent sand 

 (>0.05 mm.), 38 percent coarser silt (0.05 - 

 0.025 mm.), 18 percent coarse silt (0.025 - 

 0.005 mm.), 2 percent fine silt (0.005 - 0.002 

 mm.), and 10 percent total clay (<0.002 mm.). 

 Carrier-free zinc 65 as Zn65ci2 was used as 

 the tracer. Exchange rates were calculated as 

 described above, on the assumption that the 

 tracer when added mixed immediately and com- 

 pletely with the exchangeable zinc in the water 

 and that the concentration of stable zinc in the 

 water was constant during the 30-minute period 

 in which loss of the tracer was observed. The 

 average rate of exchange for 10 cores was 

 17 i 4//g. Zn/hour/m.2 This technique is ap- 

 plicable to studies of any other element with 

 a convenient gamma-emitting isotope, as long 

 as no isotopic effects occur. 



VARIATION IN CONCENTRATIONS OF IRON, 



MANGANESE, AND ZINC IN SEDIMENT 



COLLECTED FROM THE NEWPORT RIVER 



ESTUARY 



Thomas W. D\ike, James N. Willis, 

 Thomas J. Price, and Curtis W. Lewis 



The affinity of many radionuclides for par- 

 ticulate matter may produce significant reser- 

 voirs of radioactivity in estuarine sediments. 

 The amount of radioactivity accumulated by 



27 



