Processes Affecting the Vertical Distribution of Trace Components in

10

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Processes Affecting the Vertical Distribution of Trace Components in the Chesapeake Bay RICHARD L. HARRIS and GEORGE R. HELZ University of Maryland, Department of Chemistry, College Park, Md. 20742 ROBERT L. CORY U.S. Geological Survey, Edgewater, Md.

The study of trace components is of considerable interest since certain ones are essential to l i f e while others are highly toxic. It is within the scope of human endeavor to alter the rates at which some of these components are delivered to estuaries and coastal waters (1 - 3) and, consequently, the question of the fate of these substances is important. To answer this question, we must f i r s t understand the processes which control their transport, and ultimately their deposition. These transport processes are conveniently separated into horizontal and vertical components. In most estuaries, horizontal transport is dominated by simple advective flow. However vertical transport, especially of trace components, is influenced by a number of physical, biological and chemical processes. This paper, which explores the relative importance of some of the vertical transport processes in estuaries, is based upon observations made during an ongoing study in the central Chesapeake Bay, Maryland. To date, the vertical distribution of phosphate and five trace metals (Cu, Fe, Mn, Pb, and Zn) have been investigated over the period of a year. Samples were collected in the deep waters off Bloody Point in the Chesapeake Bay at latitude 3 8 ° 5 0 ' , longitude 7 6 ° 2 4 ' . This station lies in the northernmost part of a trough having depths to 60 m, that extends about thirty miles south. During late summer, the bottom waters in this trough frequently become anoxic and sulfide-bearing. Water samples were collected through polyethylene tubing connected to a gasoline powered peristaltic pump. We also obtained surface sediment samples. The iodometric method (4) was used to determine sulfide, while phosphate was found colorimetrically (5). Total trace metals were analyzed by atomic absorption, and 176

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10.

Vertical Distribution of Trace Components

HARRIS E T A L .

177

n i t r i c a c i d d i g e s t i o n s o f the p a r t i c u l a t e s by flameless atomization w i t h a carbon r o d . A detailed discussion o f the s a m p l i n g and a n a l y t i c a l p r o c e d u r e s and a f u l l p r e s e n t a t i o n o f t h e d a t a w i l l b e g i v e n e l s e w h e r e (6) .


The C o n s e r v a t i v e

Model

The s i m p l e s t d i s t r i b u t i o n p a t t e r n t h a t m i g h t be e x p e c t e d i n an e s t u a r y w o u l d b e one e s t a b l i s h e d b y conservative mixing. A c o n s e r v a t i v e component v a r i e s l i n e a r l y w i t h s a l i n i t y i n b o t h the h o r i z o n t a l and the vertical directions. Such a d i s t r i b u t i o n i s indeed o b s e r v e d f o r some t r a c e c o m p o n e n t s . F o r example Warner (2) s h o w e d t h a t F " i n t h e C h e s a p e a k e B a y was d i s t r i b uted conservatively. On t h e o t h e r h a n d , i t i s w e l l known t h a t c o n s e r v a t i v e m i x i n g c a n n o t a c c o u n t f o r t h e d i s t r i b u t i o n o f many t r a c e m e t a l s . A s an e x a m p l e , i n F i g u r e 1, we show t o t a l F e ( u g / 1 ) p l o t t e d v e r s u s salinity ( ° / )F o r r e f e r e n c e , we h a v e p l o t t e d t h e l i n e a r d i s t r i b u t i o n s w h i c h would be e x p e c t e d under c o n s e r v a t i v e c o n d i t i o n s assuming t h a t r i v e r water e n t e r i n g t h e Bay c o n t a i n e d , r e s p e c t i v e l y , 100, 300 o r 1000 u g / 1 F e . S c h u b e l (8) c i t e s C a r p e n t e r a s f i n d i n g the average annual c o n c e n t r a t i o n i n the Susquehanna R i v e r , the p r i n c i p a l t r i b u t a r y t o the n o r t h e r n Bay, t o b e 300 u g / 1 . Note t h a t t o t a l i r o n o b s e r v e d at our s t a t i o n d i f f e r s from c o n s e r v a t i v e d i l u t i o n models i n two w a y s : (a) t h e i r o n c o n c e n t r a t i o n s a r e s y s t e m a t i c a l l y lower t h a n w o u l d be e x p e c t e d i f i r o n were c o n s e r v a t i v e ; (b) t h e t r e n d i s w r o n g ; s a l i n i t y c o n s i s t e n t l y i n c r e a s e s d o w n w a r d , b u t on some o c c a s i o n s F e was o b s e r v e d t o p a s s t h r o u g h a m i n i m u m a t i n t e r m e d i a t e depth. A s i m i l a r phenomenon i s s o m e t i m e s o b s e r v e d i n the v e r t i c a l d i s t r i b u t i o n of phosphate. I n F i g u r e 2, we show a c a s e i n w h i c h t o t a l p h o s p h a t e p a s s e s t h r o u g h a maximum a t i n t e r m e d i a t e d e p t h w h i l e s o l u b l e p h o s p h a t e , t h a t i s , t h e f r a c t i o n w h i c h c a n go t h r o u g h a 0.45 urn M i l l i p o r e f i l t e r , p a s s e s t h r o u g h a m i n i m u m . No c o n s e r v a t i v e m i x i n g model c o u l d account f o r t h i s type of v e r t i c a l distribution. U s i n g average abundances o f Mn i n r i v e r a n d o c e a n w a t e r s (9) , s i m i l a r c o n s i d e r a t i o n s r e v e a l t h a t manganese a l s o i s u s u a l l y p r e s e n t a t our s t a t i o n i n q u a n t i t i e s l e s s t h a n would be e x p e c t e d i f i t were c o n s e r v e d d u r i n g m i x i n g . On t h e o t h e r h a n d , C u , Z n , a n d Pb a r e t y p i c a l l y p r e s e n t i n e x c e s s o f t h e c o n s e r v a t i v e v a l u e s , although the data vary c o n s i d e r ably. 0

Biological

0

Processes



178

MARINE CHEMISTRY

Most oceanographers and marine b i o l o g i s t s w i l l n o t be s u r p r i s e d t h a t the t r a c e components we have mentioned so f a r a r e n o t c o n s e r v e d d u r i n g m i x i n g i n estuaries. These s u b s t a n c e s a r e known t o be taken i n t o l i v i n g organisms, s o t h e i r d i s t r i b u t i o n i n an e s t u a r y might be e x p e c t e d t o be i n f l u e n c e d by b i o l o g i c a l processes. F o r example, u s i n g the g r o s s p r i m a r y p r o d u c t i v i t y d a t a o f F l e m e r (10) and the e m p i r i c a l f o r m u l a o f R i c h a r d s (11) f o r marine o r g a n i c m a t t e r , we e s t i m a t e t h a t t y p i c a l l y 0.6 um/l/day of phosphate i s f i x e d i n the e u p h o t i c zone o f the c e n t r a l Chesapeake Bay i n the summer. T h i s r e p r e s e n t s about t w i c e the t o t a l amount p r e s e n t i n s u r f a c e w a t e r s , i n d i c a t i n g t h a t much o f t h i s phosphate i s r a p i d l y r e c y c l e d , an o b s e r v a t i o n which i s c o n s i s t e n t w i t h other p u b l i s h e d r e p o r t s (12,13). I t seems l i k e l y t h a t t h e l a r g e p a r t i c u l a t e phosphate c o n c e n t r a t i o n shown i n F i g u r e 2 a t i n t e r m e d i a t e depths i s due t o s e t t l i n g p l a n k t o n d e b r i s which has become t r a p p e d by buoyancy a t the top o f the h i g h e r d e n s i t y bottom w a t e r . A l t h o u g h b i o l o g i c a l p r o c e s s e s such as those d e s c r i b e d above can p l a y a major r o l e i n the d i s t r i b u t i o n o f phosphorus i n e s t u a r i e s , they are p r o b a b l y much l e s s e f f e c t i v e f o r t r a c e m e t a l s . In t h i s r e g a r d e s t u a r i e s may d i f f e r from the deep ocean where l o n g e r p e r i o d s o f time a r e a v a i l a b l e . E s t i m a t e s o f the time needed f o r p h y t o p l a n k t o n t o s t r i p 100% o f t h e phosphorus and the f i v e t r a c e m e t a l s , assuming no r e g e n e r a t i o n , are compared i n T a b l e I (bottom l i n e ) . These e s t i m a t e s are b a s e d on a p l a n k t o n summertime g r o s s p r o d u c t i v i t y r a t e o f 1.6 g C/m /day (10) and on compos i t i o n d a t a f o r marine p h y t o p l a n k t o n (14). We f e e l t h a t the use o f marine p h y t o p l a n k t o n d a t a i n t h i s environment i s j u s t i f i e d because t r a c e element concent r a t i o n s are n o t g r e a t l y d i f f e r e n t from those i n t h e oceans. Furthermore, R i l e y and Roth (15) showed t h a t t r a c e m e t a l c o n c e n t r a t i o n s i n p h y t o p l a n k t o n do n o t depend c r i t i c a l l y on p l a n k t o n s p e c i e s . Mstal r a t e s a r e much l o n g e r than r a d i o a c t i v e i s o t o p e exchange r a t e s o b s e r v e d by a number o f a u t h o r s (16,17). The removal time i s q u i t e s h o r t f o r phosphorus b u t l o n g f o r the trace metals. These d a t a , a l t h o u g h s u b j e c t t o some u n c e r t a i n t y , c a s t doubt on the w i d e l y h e l d view t h a t b i o l o g i c a l p r o c e s s e s a r e p r i m a r i l y r e s p o n s i b l e f o r the n o n - c o n s e r v a t i v e b e h a v i o r o f t r a c e elements i n estuaries. 2

Chemical

Processes

Chemical

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MARINE CHEMISTRY

t o a f f e c t the d i s t r i b u t i o n o f t r a c e components i n e s t u a r i e s and i n the oceans a r e p r e c i p i t a t i o n and adsorption. K r a u s k o p f ( 1 8 ) i n v e s t i g a t e d t h e s e mechanisms and c o n c l u d e d that~Tn t h e oceans, l o c a l p r e c i p i t a t i o n of s u l f i d e s c o n s t i t u t e d a p o s s i b l e c o n t r o l f o r some m e t a l s . S u b s e q u e n t l y , however, a number o f a u t h o r s ( 1 9 , 2 0 ) have found t h a t s u l f i d e - r i c h marine waters a p p a r e n t l y f a i l t o p r e c i p i t a t e t r a c e m e t a l sulfides. We have c a l c u l a t e d the degree o f s a t u r a t i o n o f the t r a c e m e t a l s i n our samples w i t h r e s p e c t t o s u l f i d e s , h y d r o x i d e s , phosphates and c a r b o n a t e s u s i n g c o m p u t a t i o n a l p r o c e d u r e s s i m i l a r t o those d e s c r i b e d i n the l i t e r a t u r e ( 2 1 ) . In g e n e r a l , t h e waters o f the c e n t r a l Chesapeake Bay a r e u n d e r s a t u r a t e d w i t h r e s p e c t t o t h e s e phases. Where the c a l c u l a t i o n s s u g g e s t t h a t the waters a r e s u p e r s a t u r a t e d , t h e r e i s no e v i d e n c e t h a t d i s s o l v e d m e t a l c o n c e n t r a t i o n s i n t h e water a r e responding. F o r example, the h i g h e s t d i s s o l v e d Pb c o n c e n t r a t i o n s which we o b s e r v e d i n the bottom waters d u r i n g our y e a r - l o n g s a m p l i n g p e r i o d were o b s e r v e d i n June 1 9 7 4 when t h e bottom w a t e r c o n t a i n e d 0 . 3 mg/1 sulfide. Thus p r e c i p i t a t i o n appears t o p l a y no r o l e i n t r a c e m e t a l c y c l i n g i n the Chesapeake Bay. Possib l e e x c e p t i o n s t o t h i s r u l e , based on s o l u b i l i t y c a l c u l a t i o n s , a r e the o x y h y d r o x i d e s o f Fe and Mn. Because b o t h b i o l o g i c a l a s s i m i l a t i o n and p r e c i p i t a t i o n appear t o be i n s i g n i f i c a n t as v e r t i c a l t r a n s p o r t mechanisms f o r t r a c e m e t a l s o t h e r than Fe and Mn, we f e e l t h a t a d s o r p t i o n - d e s o r p t i o n p r o c e s s e s must be o f major importance. Other w o r k e r s ( 2 2 , 2 3 ) have d i s c u s s e d t h e s e p r o c e s s e s p l a c i n g emphasis on the d e s o r p t i o n which o c c u r s as r i v e r - b o r n e p a r t i c l e s e n c o u n t e r the i n c r e a s i n g s a l i n i t y o f e s t u a r i n e w a t e r . However the work o f S c h u b e l (%) and B i g g s ( 2 4 ) r e v e a l t h a t most o f the p a r t i c l e s b r o u g h t i n t o Chesapeake Bay by the Susquehanna R i v e r , the major t r i b u t a r y t o the n o r t h e r n s e c t i o n , a r e d e p o s i t e d n e a r the head o f the Bay. Consequently a d s o r p t i o n - d e s o r p t i o n r e a c t i o n s i n v o l v i n g r i v e r - b o r n e p a r t i c l e s can n o t account f o r the n o n - c o n s e r v a t i v e v e r t i c a l p r o f i l e s which we observe i n the c e n t r a l p a r t o f the Bay. Based upon the sediment mass b a l a n c e d a t a which B i g g s ( 2 4 ) c o m p i l e d and which we show i n F i g u r e 3 , the p r i n c i p a l sources of p a r t i c u l a t e matter i n the c e n t r a l p a r t o f the Chesapeake Bay are s h o r e e r o s i o n and b i o l o g i c a l p r o d u c t i o n . Data on t h e c h e m i c a l and m i n e r a l composition of source m a t e r i a l i s not a v a i l a b l e , b u t we have e s t i m a t e d i t s c o m p o s i t i o n u s i n g d a t a o b t a i n e d from the d e p o s i t e d sediments. T a b l e I I shows the average c o n c e n t r a t i o n s o f c l a y m i n e r a l s , i r o n and




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182

MARINE CHEMISTRY

TABLE

I - ESTIMATED BIOLOGICAL ASSIMILATION


Ρ0 TYPICAL

CONCENTRATION

TYPICAL

CONCENTRATION

MN

FE

CU

ZN

PB

30

11

20

3

6

7

2.7X13^

5

200

4

20

5

Α

IN EUPHOTIC

ZONE OF CHESAPEAKE BAY (SUMMER) (UG/L)

PHYTOPLANKTON

RATES IN SURFACE WATER

IN IIARINE

(UG/G DRY WEIGHT)

PHYTOPLANKTON FIXATION RATE

IN

CHESAPEAKE BAY (UG/L/DAY)

60

0.02

TURNOVER RATE (DAYS)

0.5

600

TABLE

0.8 30

0.01

0.06

0.01

300

100

700

II

MINIMUM CONCENTRATIONS AND DEPOSITION

RATES FOR

POTENTIAL ADSORBING AGENTS IN CHESAPEAKE BAY SEDIMENTS

COMPONENT

AVERAGE

CONCENTRATION

DEPOSITION

(2) CHLORITE

(109

1

12

10

30

KAOLINΙ TE

7

21

MoNTMORILLONITE IRON

6 if

21

ORGANIC MATTER

5

15

36

117

ILLITE

117 χ 1 0

9

(G/YR) TOTAL

1 5

18

DEPOSITION

-ζ 9 χ 10

= (L/YR)

RATE

G/YR)

1.3

m/i

TOTAL WATER FLOW



10.

HARRIS E T A L .


183

o r g a n i c m a t t e r i n sediments b a s e d on the work o f Ryan (25) , Owens, e t a l (26) , and our own work. The p e r c e n t a g e f i g u r e s i n the m i d d l e column a r e g i v e n on a dry weight b a s i s . Only t h e c l a y m i n e r a l s found i n the c l a y s i z e d f r a c t i o n , which makes up 27% o f t h e average sediment, a r e i n c l u d e d . In t h e r i g h t hand column, the a n n u a l d e p o s i t i o n r a t e f o r each o f t h e s e components i s shown. These a r e b a s e d on the mass b a l a n c e d a t a o f F i g u r e 3. A t the bottom o f T a b l e I I we have computed the average c o n c e n t r a t i o n o f t h e s e components i n Bay w a t e r by d i s t r i b u t i n g the t o t a l amount d e p o s i t e d a n n u a l l y o v e r the t o t a l amount o f w a t e r which f l u s h e s through t h i s s e c t i o n o f the Bay. This figure i s a minimum v a l u e because the computation assumes t h a t sediment p a s s e s once through the s e c t i o n o f the Bay under s t u d y . A c t u a l l y , because o f t h e s m a l l s i z e o f t h e c l a y m i n e r a l s and the low d e n s i t y o f the o r g a n i c m a t t e r , t h e s e p a r t i c l e s have s m a l l s e t t l i n g v e l o c i t i e s and thus may be r e c y c l e d by t u r b u l e n c e s e v e r a l times b e f o r e f i n a l l y b e i n g d e p o s i t e d . Thus t h e t r a c e conc e n t r a t i o n o f p o t e n t i a l a b s o r b i n g agents i n Bay water might be s e v e r a l times h i g h e r than the 1.3 mg/1 shown i n T a b l e I I . B i g g s (2 4) has found t o t a l suspended m a t t e r up t o 20 mg/1 I n the Chesapeake Bay. I f we assume t h a t the a d s o r p t i v e c a p a c i t y o f t h i s m a t e r i a l i s on the o r d e r o f 0.1 - 1%, as i s u s u a l l y o b s e r v e d (27, 2 8 ) , then a d s o r p t i o n - d e s o r p t i o n p r o c e s s e s c o u l d be i m p o r t a n t f o r t h o s e t r a c e m e t a l s , such as Cu, Zn and Pb, which are t y p i c a l l y p r e s e n t i n d i s s o l v e d form a t c o n c e n t r a t i o n s below 10 y g / l . T h i s w i l l be t r u e , however, o n l y i f the t r a c e m e t a l s a r e a b l e t o dominate the a d s o r p t i o n s i t e s i n c o m p e t i t i o n w i t h t h e much more abundant Na, Mg and Ca. Conclusions V e r t i c a l d i s t r i b u t i o n s o f phosphate, Fe, Mn, Cu, Zn, and Pb i n c e n t r a l Chesapeake Bay are n o t e x p l a i n a b l e by c o n s e r v a t i v e m i x i n g , s o a c t i v e v e r t i c a l t r a n s p o r t p r o c e s s e s must be o p e r a t i n g . F o r phosphate, b i o l o g i c a l p r o c e s s e s a r e undoubtedly i n v o l v e d , b u t f o r the m e t a l s , b i o l o g i c a l a s s i m i l a t i o n appears t o be t o o slow. S o l u b i l i t y r e a c t i o n s a l s o appear t o be unimport a n t , e x c e p t p o s s i b l y f o r Fe and Mn. G e n e r a l l y the waters are u n d e r s a t u r a t e d w i t h r e s p e c t t o pure m i n e r a l phases o f Cu, Zn and Pb, b u t i n summer when s u l f i d e appears i n t h e bottom waters and s u p e r s a t u r a t i o n i s r e a c h e d w i t h r e s p e c t t o CuS, ZnS and PbS, t h e r e i s s t i l l no e v i d e n c e t h a t t h e s e m e t a l s are b e i n g removed. Because b i o l o g i c a l a s s i m i l a t i o n and s o l u b i l i t y



184

MARINE CHEMISTRY

reactions do not appear capable of explaining the nonconservative vertical distribution of Cu, Zn and Pb, we feel that adsorption-desorption processes must control vertical transport of these metals. Estimates of the adsorption capacity available on the kinds of particulate matter which must be present in the central part of Chesapeake Bay suggest that this mechanism is feasible provided adsorption is selective for trace metals. Abstract A one-year, single station study has been com pleted in the central Chesapeake Bay, Maryland to investigate seasonal changes and vertical transport mechanisms of phosphate and five trace metals (Cu, Fe, Mn, Pb, Zn). Vertical distributions are not explain able by conservative mixing. Biological processes can account for phosphate distributions, but appear to be too slow to affect vertical metal transport. Mid-Bay waters never reach saturation with respect to carbon ate, phosphate, or sulfide minerals. Precipitation of oxyhydroxides may be the controlling process for Fe and Μn, while adsorption appears to be the only reasonable mechanism for Cu, Pb, and Zn. Literature Cited (1) (2)

(3) (4)

(5) (6) (7) (8)

Bertine, K.K. and Goldberg, E . D . , Science (1971) 173, p. 233-235. Young, D.R., Young, C.S., and Hlavka, G . E . , in Curry, M.G. and G i g l i o t t i , G.M. (Ed.), "Cycling and Control of Metals; Proceedings of an Environ mental Resources Conference", p. 21-39, U.S. Environmental Protection Agency, 1973. Helz, G.R. (1975), in press. Brown, E., Skougstad, M.W., and Fishman, M . J . , "Techniques of Water Resources Investigations of the United States Geological Survey", p. 154, Department of the Interior, Washington, D.C., 1970. Strickland, J.D.H. and Parsons, T.R., "A Practical Handbook of Seawater Analysis", p. 49, Fisheries Research Board of Canada, Ottawa, 1972. Harris, R.L. (1975), Ph.D. Thesis, Univ. Md. Warner, T . B . , Deen-Sea Research (1971) 18, p. 1255-1263. Schubel, J.R., "The Physical and Chemical Condi tions of Chesapeake Bay; An Evaluation", p. 59, Chesapeake Bay Institute, John Hopkins University, Baltimore, 1972.


10. HARRIS ET AL. (9) (10)


(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)


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Processes Affecting the Vertical Distribution of Trace Components in

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