A numerical simulation of water quality in Tokyo Bay

Volume 23/1991/EMECS "90

A Numerical Simulation of Water Quality in Tokyo Bay N. HAMANO and Y. NAKAGAWA

Japan NUS Corp., 2-28 Dojima, 2-chome Kitaku, Osaka, 530Japan

In t h i s r e p o r t , t h e JATWEM model i s i n t r o d u c e d . T h i s model was d e v e l o p e d to s a t i s f y t h e n e e d f o r a model that can be a p p l i e d t o a l l k i n d s o f w a t e r b o d i e s , and i s e a s y t o u n d e r s t a n d and u s e in the e v a l u a t i o n o f primary production. The JATWEM model e n a b l e s water q u a l i t y to be c a l c u l a t e d by an i n d u c t i v e parameter m e t h o d b a s e d m a i n l y on i n f o r m a t i o n o b t a i n e d by f i e l d observation. A c a l u c u l a t i o n h a s b e e n e x e c u t e d a t Tokyo Bay w h i c h h a s s u c c e s s f u l l y e v a l u a t e d t h e water q u a l i t y o f t h e bay. T h i s r e p o r t e x p l a i n s t h e c o n c e p t and c a l u c u l a t i o n p r o c e s s o f t h e model. The D i f f u s i t i v e Equation of Nutrients. Total nitrogen (TN) a n d t o t a l phosphorus (TP) a r e e i t h e r in a soluble state or a p a r t i c u l a t e state. As p a r t i c l u l a t e movement i s d i f f e r n t than water, a n d s i n c e TN a n d TP r e a d i l y change from particulate to solubel state and from s o l u b l e to p a r t i l u l a t e state, the modeling aspect of the state o f TN a n d TP b e c o m e s i m p o r t a n t . T h e JATWEM m o d e l b a s i c a l l y consists o f two e q u a t i o n s : one f o r TN a n d TP i n a s o l u b l e state or as small-sized particulates w h i c h a c t t h e same a s w a t e r , a n d t h e o t h e r f o r large-sized particulates. Large-sized particulates were chosen to appoximate the vertical transportation i n t h e o c e a n w h i c h i s c a u s e d by giant-sized particulates c a l l e d Marine Snow. The s e t t l i n g mechanisum of giant-sized particulates are discussed by Tsunogai(1978), Nozaki(1981), et al., and many complex models to describe the reactions of various-sized particulates have been proposed based on t h i s mechanism. In this report, however, a simple one dimentional e q u a t i o n m o d e l was a p p l i e d f o r t h e s e t t l i n g of nutrients in s u r f a c e w a t e r , and i t was a s s u m e d t h a t o n c e s e t t l e d a certain percent of the particulates w i l l decompose n e a r t h e s e a f l o o r . For nutrients which act the same as water, soluble small-sized particulates, t h e f o l l o w i n g e q u a t i o n was u s e d : OC

OC

a-7= ux

OC

OC

02C

02C

02C

-U, Ty-U, Vz

For t h e s e t t l i n g __=

flux, the following aWM OM ( 1 - - r ) ' - - - ~ z +K,C

materials,

and

~ WM

o--U e q u a t i o n was u s e d :

Ot

note:

C Vx,Vy,Vz Kx,Ky,Kz K, M r

= = = = = =

c o n c e n t r a t i o n o f m a t e r i a l s which a c t t h e same as w a t e r v e l o c i t y in t h e x , y , and z d i r e c t i o n s diffusion coefficients f o r t h e x , y , and z d i r e c t i o n s removal coefficient with aggregation settling velocity r a t i o of d e c o m p o s i t i o n

P a r a m e t e r s u s e d i n t h e p r e s e n t model a r e shown i n T a b l e 1. K1 v a l u e s obtained by s e d i m e n t t r a p o b s e r v a t i o n , and r v a l u e s a r e c a l c u l a t e d Marine Pollution Bulletin, VoL 23, pp. 4 5 - 4 9 , 1991. Printed in Great Britain 0 0 2 5 - 3 2 6 X / 9 1 S3.[}0+00(J © I g g l Pergamon Press plc

are to 45

Marine Pollution Bulletin

meet mass b a l a n c e o f n u t r i e n t s b e t w e e n s e d i m e n t and w a t e r , on t h e b a s e s o f the difference between settling rate of nutrients in w a t e r and their sedimentation rate, and also the seasonal variation o f N/P r a t i o in w a t e r . V a l u e s in T a b l e 1 r e p r e s e n t summer c o n d i t i o n in Tokyo Bay. E v a l u a t i o n o f COD O r i g i n a t e d in P r i m r y P r o d u c t i o n In t h e JATWEM model, p r i m a r y p r o d u c t i o n i s n o t c a l c u l a t e d d i r e c t l y , estimated from simulated v a l u e s o f TN and TP. The r a t i o between nitrogen or organic phosphorus is determined from field data. originating from p r i m a r y p r o d u c t i o n can be e s t i m a t e d . D i r e c t i n f l o w is simulated with another diffusion equation. The two s i m u l a t e d for primary production and f o r d i r e c t i n f l o w o f COD, a r e a d d e d t o g i v e a f i n a l v a l u e f o r COD c o n c e n t r a t i o n . Table. nutrient TP TN

1 Parameters level 1 , 2 3 1 , 2 3

but is organic Then COD o f COD value, together

in t h e model K1 0.07 day-' 0.00 0.016 0.00

r 0.0 1.1 0.0 0.95

The r e s u l t s o f t h e S i m u l a t i o n o f TN, TP, and COD The m o d e l s i m u l a t i o n was a c c o m p l i s h e d u s i n g a 4 x 4 km mesh h o r i z o n t a l l y and a d o p t i n g a 3 l e v e l s y s t e m (0 t o 2 m, 2 t o 10 m, and 10 m t o b o t t o m ) . The current data was modelled using the resultus of the current caluclation w i t h t h e L e v e l model r e p o r t e d by N a k a t a ( u n p u b l i s h e d ) . Inflow load data from the summer season o f 1980 was u s e d . The r e s u l t s are indicated in Figures 1 t o 3. TN a n d TP v a l u e s simulated by t h e JATWEM model , in g e n e r a l , c o r r e l a t e d w e l l w i t h t h e o b s e r v e d v a l u e s in t h e f i e l d . Correlation coefficients b e t w e e n s i m u l a t e d and o b s e r v e d v a l u e s a r e f a i r l y high, i.e., correlation coefficient f o r TP a t l e v e l 1 was 0 . 8 7 and l e l e l 3 , 0 . 6 9 , f o r TN a t l e v e l 1 , 0 . 8 8 and a t l e v e l 3 0 . 6 6 . C o n c e n t r a t i o n s of TP and TN show n e a r l y a 1:1 a g r e e m e n t . Mass b a l a n c e s o f TN and TP show t h a t t h e i m p o r t a n t fluxes for nitrogen and phosphorus are the settling flux (Figure 4 to 5 ). Especially for phosphorus, the role of the settling flux is important. Except by the mouth of the bay, downward fluxes to the bottom levels are almost equivalent to the upward fluxes. One c o n d i t i o n of the simulation is that all of the settling fluxes of phosphorus are converted to a soluble state by d e c o m p o s i t i o n and a f t e r being added to phosphorus from sediments by ellution, are again transported to the surface. Thus, the residual time for phosphorus gets longer than the residual time for water. Consequently, p h o s p h o r u s a c c u m m u l a t e s in t h e b a y . T h i s i s t h e r e a s o n t h a t the concentration of the bottom level becames nearly equal to the surface for phosphorus. B a s e d on t h e o b t a i n e d TN and TP v a l u e s , the ratio between nitrogen and phosphrus can be calculated. Being affected by t h e d i f f e r e n c e of the removal coefficient between phosphorus and n i t r o g e n , where phosphorus dissolves out from sediment and nitrogen is added to sediments, our investigation shows the differences of the ratio of nitrogen to phosphorus in t h e u p p e r l e v e l and b o t t o m l e v e l . That is, at the upper level all the phosphorus ratios are more than the Redfield ratio by 7.2:1. Thus, phosphorus becomes the limiting factor of primary production 46

Volume 23/1991/EMECS '90

in Tokyo Bay. On t h e o t h e r hand, a t t h e m i d d l e and l o w e r l e v e l s , t h e r a t i o o f n i t r o g e n t o p h o s p h o r u s d e c r e a s e s compared w i t h t h e u p p e r l e v e l . In c e r t a i n w a t e r s , t h e r e a p p e a r e s a r e a s where t h e r a t i o of n i t r o g e n to p h o s p h o r u s i s l e s s t h a n 5:1. T h i s t e n d e n c y c o i n c i d e s w i t h t h e f i e l d o b s e r v a t i o n d a t a . I t can be s a i d t h a t b a s i c a l l y the theoretical framework for the actions of n i t r o g e n and p h o s p h o r u s i s a d e q u a t e . C o m p a r i n g s i m u l a t e d COD v a l u e s by t h e JATWEM modes w i t h o b s e r v e d v a l u e s , r e s u l t s c o r r e l a t e w e l l w i t h o b s e r v e d d a t a e x c e p t n e a r Tokyo Harbor ( n o r t h w e s t s i d e o f t h e B a y ) . N e a r Tokyo Harbor o b s e r v e d d a t a i s l o w e r t h a n model results. Correlation coefficient b e t w e e n s i m u l a t e d v a l u e s by JATWEM and observed values are high, i.e., 0.85 a t t h e l e v e l 1 and 0.71 a t l e v e l 2, not i n c l u d i n g v a l u e s a t Tokyo Harbor.

level

I

Fig.

level

1

2

S i m u l a t e d Values of TP f o r 1980

level

3

(mg/1)

47

Marine Pollution Bulletin

level

1

Fig.

level

2

2

level

S i m u l a t e d Values of TN f o r 1980

3

(mg/1)

•° ~;~o !~o~, r~

Ii

?

".." .1"" __. I o.~'N

//"

~2.02

level

I

Fig.

48

level

3

~"

I 2.02

2

S i m u l a t e d Values of COD f o r 1980

level

3

(mg/1)

Volume 23/1991/EMECS

'90

5.8

i6.5

. I0.6

184

.~___

8

54

169

~o~,,oo? I--~- ~-,0oo6 82 105 05

0.95 4.4 0.2 ~175 "~0.03

505 ft.1 ~ 3

2.4

3O 5

0.05

-~I-~0fl7 26.3 ~

?

17.4

33.1 -, 0.03

~ 131.5 - I - ' 0.01

140,9 .

I . I

105 001

-~1326

~'333 -, 0.05

02

"1~ 0 t 52 252 |

"--3--.$. -,007 4(k9

7546 +0.09

+ o.(i~) T

38.9 2.7 0 . 0 4 /

1 ;/ ,007 1763

~P--I

09-t-,0.3

[ T ~

1391

176

232

r

:

39 0~9

--0.03 --397

12

24 0.5

~'0.2 -1775

I7

--.% -*flow flux -*settling flux =*sedimentation (elusion) Fig. 4

Mass Balance o f TP

unit

(ton/day)

Fig. 5 Mass Balance of TN

Discussion In a s i m u l a t i o n of the water quality o f Tokyo Bay, i t h a s b e e n f o u n d t h a t t h e JATWEM model, whose p a r a m e t e r s has been o b t a i n e d t h r o u g h f i e l d observations, shows good r e s u l t s as mentiond b e f o r e . I t was proven t h a t a s i m p l e s t r u c t u r e d model u s i n g e a s i l y a t t a i n a b l e i n f o r m a t i o n can d e s c r i b e behaviers of nutrients and t h e phenomena o f p r i m a r y p r o d u c t i o n o f COD. There were, however s e v e r a l problems. In Tokyo Harbor, the c a l c u l a t e d COD value significantly exceeded the observed value, even though the concentration of nutrients ( b a s e s o f COD e s t i m a t i o n ) have been consistant with observed values. This indicates that the model considering only n i t r o g e n and phosphorus a s l i m i t t i n g f a c t o r s f o r primary p r o d u c t i o n can not e v a l u a t e c o n c i s e l y t h e a c t u a l primary p r o d u c t i o n near i n f l o w load a r e a s . References E.Matshumoto ( 1 9 8 3 ) : The s e d i m e n t a r t y e i r o n m e n t in t h e Tokyo Bay. Chikyukagaku ( G e o c h e m i s t r y ) . 17,27 2 S . T s u n o g a i and M. Uemstu (1978) : S e t t l i n g odel for the remlval fo insouble chemical e l e m e n s t s in s e a w a t e r . Geocemical. J . 1 2 , 4 4 - 5 6 . Y . N o z a k i , H.S.Yang. and a. Tsubota (1981) : S c a v e n g i n g f o thorium in t h e ocean. J.Groephys. R e s . , 9 2 , 7 7 2 - 7 7 8 K. Miyata and A . H a t t o r i (1986) : U i s t r i b u on and Seasonal V a r i a t i o n o f Phosphorus in Tokyo Bay. J. Oceanogr Soc. Japan,42,255-265

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