Three-Dimensional Effluent Tracing Techniques For Pollutant Dispersion Monitoring

INSTRUMENTATION Coup l ing a V a r i o s e n s i n s i t u f l u o r o m e t e r and G u i l d l i n e CTD w i t h a compute r i zed d a t a a c q u i s i t i o n and p o s i i t i o n i n g s y s t e m h a s p r o v i d e d a new i n s t r u m e n t f o r m a p p i n g e f f l u e n t p l u m e s i n 3 d i m e n s i o n s u s i n g d y e t r a c i n g methods. The pr irnary s a m p l i n g s t r a t e g y i n v o l v e s t r a n s e c t i n g t h e p l u m e a t s p e e d s o f 3 t o 5 k n o t s w h i l e c y c l i n g t h e i n s t r u m e n t v e r t i c a l l y t h r o u g h t h e d y e c l o u d , t h u s d e l i n e a t i n g b o t h v e r t i c a l a n d h o r i z o n t a l boundar i e s . Grouping a l l t r a n s e c t d a t a i n t o s p e c i f i c t i m e p e r i o d s a n d c o n t o u r i n g d y e c o n c e n t r a t i o n a t s e l e c t e d t h r e s h o l d s y i e l d s t h e d e s i r e d 3-D r e p r e s e n t a t i on . The sys t em (named TRACER) h a s been s u c c e s s f u l l y used t o mon i to r s eabed c o n t a c t a r e a s , t h e n e t d i r e c t i o n a n d r a t e o f d i s p e r s i o n , a n d e f f e c t i v e d i l u t i o n of d i f f u s e r p lumes €or sewage and p u l p m i l l e f f l u e n t d i s p o s a l i n m a r i n e a r e a s . INTRODUCTION One o f t h e main p rob lems i n a s s e s s i n g e f f l u e n t impac t s i n v o l v e s o b t a i n i n g a n a c c u r a t e 3 -d imens iona l mapping oE t h e s u b s t a n c e a s i t d i s p e r s e s i n t h e r e c e i v i n g waters . A mapping o f t h i s n a t u r e p r o v i d e s q u a n t i t a t i v e informat i o n on s e a b e d / s h o r e l i n e c o n t a c t a r e a s , t h e n e t d i r e c t i o n and r a t e o f a d v e c t i o n , t h e s h a p e a n d p o s i t i o n o f t h e e f f l u e n t plume f o l l o w i n g d i s c h a r g e , and e s t i m a t e s o f t h e e f f e c t i v e d i l u t i o n . These a r e i m p o r t a n t pa ra m e t e r s f o r p l a n n i n g and c a r r y i n g o u t w a t e r q u a l i t y and b e n t h i c s a m p l i n g p r o g r a m s , a n d f o r a s s e s s i n g t h e pe r fo rmance of submar ine o u t f a l l s . E f f l u e n t d i s p e r s i o n s t u d i e s u s i n g f l u o r e s c e n t d y e s , s u c h a s Rhodamine WT, a r e o f t e n u s e d i n a n a t t e m p t t o m e a s u r e t h e s e p a r a m e t e r s . However , p r o f i l e m e a s u r e m e n t s made w i t h pumped s a m p l i n g s y s t e m s a r e s l o w a n d l a b o r i o u s to c a r r y o u t , g e n e r a l l y y i e l d too few d a t a t o d e l i n e a t e t h e p l u m e a s i t d i s p e r s e s , a n d a r e c o n s t r a i n e d to s h a l l o w sampl ing . To overcome t h e s e l i m i t a t i o n s , a r a p i d s a m p l i n g i n s i t u f l u o r o m e t e r a n d a G u i l d l i n e CTD i n a V-wing t o w f i s h was c o u p l e d w i t h a r e a l time d a t a a c q u i s i t i o n and p o s i t i o n i n g sys t em named T R A C E R . H i g h s a m p l i n g r a t e s o f 2 0 tIz o n t h e f l u o r o m e t e r a n d 25 Hz o n t h e CTD p r o v i d e c e n t i m e t e r s c a l e r e s o l u t i o n o f s a l i n i t y , t e m p e r a t u r e , f Luorescence and s e n s o r dep th . The i n s t r u m e n t i s o p e r a t e d i n a t o w e d mode, e i t h e r h o l d i n g t h e f i s h a t c o n s t a n t d e p t h , o r c y c l i n g i t v e r t i c a l l y w h i l e underway. A l l d a t a a r e a c q u i r e d on a p o r t a b l e PC computer , c a l i b r a t e d and d i s p l a y e d i n r e a l t ime. BY t a k i n a a d v a n t a a e of t h e r a p i d s a m p l i n g r a t e s , and t h e c a p a b i l i t y o f computer t o p r o c e s s and s t o r e t h e l a r g e number o f o b s e r v a t i o n s g e n e r a t e d a t towing s p e e d s between 3 and 5 k n o t s , i t i s now p o s s i b l e to a c c u r a t e l y d e l i n e a t e t h e v e r t i c a l and l a t e r a l b o u n d a r i e s o f subs u r f a c e p lumes f o r many h o u r s a f t e r d i s c h a r g e . I n a d d i t i o n t o mapping e f f l u e n t c o n c e n t r a t i o n , s i m u l t a n e o u s d a t a on t e m p e r a t u r e and d e n s i t y a l l o w a c c u r a t e c a l i b r a t i o n o r v a l i d a t i o n o f s u b s u r f a c e buoyant plume models. T h i s c a p a b i l i t y combined wi th a c c u r a t e d e p t h d a t a j u s t i f i e s t h e u s e o f a n i n s t r u m e n t w i t h t h e p r e c i s i o n o f a G u i l d l i n e CTD w i t h t h e f l u o r o m e t e r . A V a r i o s e n s I1 m a n u f a c t u r e d by I m p u l s p h y s i k G M B H o f Hamburg, Germany, i s u s e d t o m e a s u r e f l u o r e s c e n c e . T h i s i n s t r u m e n t , d e s c r i b e d by F r u n g e l and Koch (1980), e x c i t e s f l u o r e s c e n t r a d i a t i o n w i t h a 1 m i c r o s e c o n d Xenon s p a r k i l l u m i n a t e d a t a f r e q u e n c y o f 1 0 Hz. The s o u r c e l i g h t is o p t i c a l l y f i l t e r e d to a r a n g e between 400 and 750 nanometres . F l u o r e s c e n t i n t e n s i t y , p a s s e d t h r o u g h a n a r r o w b a n d o p t i c a l f i l t e r , i s s e n s e d o n a p h o t o d i o d e . The r e c e i v e f i l t e r s a r e m a t c h e d t o t h e f l u o r e s c i n g compound, e i t h e r a d y e l i k e Rhodamine o r n a t u r a l l y o c c u r i n g C h l o r o p h y l l a . The r e s p o n s e i s l i n e a r o v e r f o u r d e c a d e s , w h i c h p r o v i d e s a s t a n d a r d R5odamine measurement r a n g e of 10-1 mg/L t o mg/L. A t y p i c a l c a l i b r a t i o n c u r v e is shown i n Fig. 1. loglo Concentration (ppbl -2 -1 0 I 2 0.001 0.01 0.1 1 10 10