189 



4.2.3 Effect of changes in the application rate of 

 oil and dispcrsant 



To dcicrmine the effect on limpets of changing the 

 application rates used in the test, each of the 

 reference dispersanls and the standard oil were tested 

 at three different application rates using the same 

 b;tch of limpets. The results, presented in Table 12, 

 show that a 75% reduction of the application rale of 

 oil caused no significant reduction in the mortality of 

 limpets in the test, consistent with the suggestion 

 earlier that Limpet mortality from oil is primarily due 

 to physical rather than chemical processes. Dispersant 

 to-xiciiy does however, decrease as the application 

 rale is lowered; thus the number of dispersants pass- 

 ing the test (Section 1 .4) was dependent on the 

 application rate used in the test. In the standard 

 tests this was set at the average rate used in practice 

 (0.4 1m'") (Section 1.3.2). These test results, how- 

 ever, show that the danger of damage to intertidal 

 organisms can be reduced if dispersants are used at 

 low rates of application. To avofd increasing the 

 damage already being caused by the oil spOt on the 

 shores, dispersant should always be used at the mini- 



mum application rale which results in effective re- 

 moval of the oil. 



4.2.4 Effect of applying dispersant to oiled limpets 



Although the standard test is based on a comparison 

 of the toxicities of oil and dispersant to limpets, in 

 a clean-up operation already-oiled organisms will be 

 sprayed with dispersant. The additional effect of dis- 

 persants on oiled limpets was thus determined using 

 the four reference dispersants (Table 13). In each 

 case the mortality of oiled limpets sprayed by dispers- 

 ant was significantly higher than that of limpets ex- 

 posed to oil or dispersant alone; this suggests that 

 under most circumstances, the toxic effects of oil 

 and dispersant are additive. A comparison of the 

 effect of dispersant on oiled limpets with the effect 

 of oil alone could thus not be readOy used for 

 regulatory purposes. Nevertheless, of the reference 

 dispersants, those with a lower toxicit>' to unoiled 

 limpets also cause the smallest increase in mortality 

 when sprayed on oiled limpets. Thus the effect of 

 restricting licences to products having a low toxi- 

 city under the standard beach test conditions also 

 minimises the adverse effects of oil-dispersant mix- 

 tures on organisms. 



Table 1 2 - Effect of different application rates on results of beach tests using four dispersants 



Application rate of 

 oil and dispersant 

 (lm-=) 



04 + 



Dispersant 



BP llOOX 

 S\nperonic OSD 20 

 Shell Dispersant Cone. 

 Corexit 9527 



Mean mortality {Vc) 



Oil 



35 

 40 

 SO 

 45 



Dispersant 



15 

 50 

 15 

 70* 



Mean 



BPIIOOX 

 Synperonic OSD 20 

 Shell Dispersant Cone. 

 Corexit 9527 



40 

 40 

 25 

 50 



.Mean 



BP llOOX 

 Synperonic OSD 20 

 Shell Dispersant Cone. 

 Corexit 9527 



Mean 



•Mortaliiy significantly creaier than oil control (Siude 

 ■"■Rate used in standard lest (Section 1.3.2) 



esI;P = <D.05) 



