HAD SHIELD SlIT, CYLINDER 



TO RECORDERi 



:x_ 



DETECTOR 



LEAD SHIELD 



-STIRRING DEVICE 



■ SEA WATER 



-LEAD SHIELD 



■ SEDIMENT 



BOTTOM PLUG 



Figure 12. --Apparatus for measuring loss of radionuclide from water to sediment instantaneously and continuously. 

 The cylinder is 72 mm. in /liameter and the slit is 2 cm. high. 



tube at a rate that does not disturb the sediment 

 surface but does ensure circulation and mixing 

 of water above the sediment. Preliminary 

 studies showed that when the cylinder is in 

 position, radiation from the sediment cannot be 

 detected. Thus, only loss of the tracer from 

 the water is recorded. A "blank" cylinder con- 

 taining only filtered water is "spiked" with 

 tracer and analyzed so that a curve repre- 

 senting movement of the tracer due to processes 

 other than exchange with the sediment can be 

 determined. During the analysis, the top of the 

 cylinder is covered with plastic wrap to ensure 

 that radioactivity is not lost in the spray from 

 bursting air bubbles. 



In the exchange of any element between water 

 and sediments, at equilibrium, when the con- 



centration of the element in the water remains 

 constant, the rate of movement of the element 

 from water to sediment equals the rate of 

 movement from sediment to water. Thus 



Pw-*s = Ps->-w - P (the net rate of exchange) 



Or written in the notation of compartmental 

 analysis for the general case of n water com- 

 partments and m sediment compartments 



(Pw,->S, + PWi-»S2 ■■•"'" PW I ->$„,) ■*■ (Pw^-^i ^PWj-'Sj ■ ■ ■ 



+ Pw2-^i„) • • ■ ■'■(Pwn-^s, ■'"Pwn-1^2 ■ ■ ■ * '^Wn"*Sn, ) " (PS|->w, 



■*■ PS,-W2- ■ ■ +Ps,-I-Wn) ^ <PS2-Wi + PS2->-W2- ■ ' + PS2 -Wn • • ■ ■ 



+ (Psm-'w, + Ps„,^w2 ■■■* "Sm^Wn) 



25 



