Indian Journal of Marine Sciences Vol. 28, December 1999, pp. 355-359

Effect of mining rejects on the nutrient chemistry of Mandovi estuary, Goa

S N de Sousa

National Institute of Oceanography, Dona Paula, Goa 403004, India

Received /9 June 1998, revised 24 August /999

Nutrient chemistry in Mandovi estuary during premonsoon is affected by the discharge of mining rejects. Concentrations of nitrate and phosphate, in general, show low levels in this season (nitrate 0.3-3.3JlM, phosphate 0.11-0.S6JlM) while silicate show wide range in concentration (1.25-130JlM). Elevated concentrations of nitrate (6. 1-9. IIlM ) associated with reduced levels of phosphate «0.1 JlM) and silicate (50-60JlM) in the mid-estuary, near the discharge point of mining rejects, suggest that the suspended sediments derived from the mining rejects adsorb phosphate and silicate, and remove them from the water column while the mining rejects act as a source of nitrate to the estuary. Large quantities (10 tonnes/month ) of ammonium nitrate used in explosives for blasting the hard rock at the mining site seem to be the source of nitrate in mining rejects. The discharged nitrate is carried both upstream and downstream of the outfall by the tidal currents. Its di strihuti on in the estuary is studied with respect to mixing processes. A plot of predicted nitrate resembled the observed nitrate distribution over -70 km. However, the observed nitrate levels were always lower than the predicted ones suggesting biological uptake of this nutrient.

The Mandovi estuary is a tropical estuary on the west coast ofIndia (Fig. I). Its hydrological characteristics are governed by the monsoon regime. Together with the Zuari estuary, they constitute the lifeline of Goa's economy because a) they provide amenities such as fishing round the year and recreation, and b) offer excellent waterways-an efficient means of transport for passengers and goods. About 90% of Goa's mineral ore is transported through these estuaries from stacking points situated upstream, to Mormugao Port from where it is loaded on to ships for export. An iron ore beneficiation plant situated on the left bank of Mandovi estuary uses river water to wash the iron ore, and discharges the waste water directly into the estuary . Consequently this results in high turbidity and enrichment of iron 1.2. This paper studies the effect of mine waste water on the nutrient chemistry of the estuary.

Materials and Methods

Figure 1 shows the locations of sampling points in the estuary. The sampling was carried out during premonsoon of 1981 (February 18th and March 31 st) and repeated in premonsoon of 1982 (March 25th) . The sampling was done always at low tide starting from the freshwater-end of the estuary and ending at the sea-end, covering the entire salinity range. At each sampling point water samples were collected from mid-depth using a 5-liter Niskin sampler fixed

on a nylon rope. The samples of source ri ver water at the extreme freshwater-end were collected by road, as the point is not accessible by boat. The samples after collection were immediately filtered on Whatman No.1 filter paper and frozen, and analyzed within 24 hours. Chlorinity was measured by titration with silver nitrate, phosphate by the method of Murphy & Rile/ and silicate by the method given by FA04. Nitrate in this discussion refers to the sum of nitrate and nitrite. It was estimated by the modified method of Morris & Riley (Grasshoffl

Fig. I-Map showing the sampling positions

356 INDIAN 1. MAR SCI., VOL. 29, DECEMBER 1999

Results and Discussion Distribution of nitrate in Mandovi estuary during

the three observations indicates the presence of <in internal source of nitrate in the low chlorinity region 3-lOxlO-3 (Fig. 2). To determine the exact location of this possible source, the nitrate concentrations from a single observation (March, 1981) were plotted against distance from the freshwater-end station-Colem (Fig. 3). Figure 3 also shows a curve of predicted nitrate if it had behaved conservatively. The predicted nitrate values Cp were computed using the relation 2 (below) which is derived from equation 1 given by Yenstch6 for a two end-member mixing series of a non-conservative property:

C - Fx%(C) Sx%(C) C x --- r +-- s - u 100 100

... (1)

where Fx% and Sx% are the percentages of freshwater and seawater in the estuarine sample, Cr and c., are the concentrations of a non-conservative property in the river water and seawater end-members, respectively, of the mixing series, and Cu is the net loss or gain of the property in the estuary due to some removal or addition mechanism, the sign of Cu changing from negative to positive in case of net gain. The sum of the first two terms on the right hand side ofEq. (1), i.e.,

Fx% (Cr) + Sx% (Cs) = C 100 100 P

... (2)

gives the concentration Cp of a conservative property in estuarine water as predicted by simple dilution. Equation (2) was used to calculate the predicted values of nitrate and plotted in Fig. 3. It is evident that the suspected nitrate source is located at about 35 km from Colem. The only possible source in this area could be the iron ore beneficiation plant which uses about 1.6x 1 06 litres/day river water7 to wash the iron ore and discharges the 'muddy' waste water directly into the estuary. Analysis of waste water showed a nitrate concentration of 80 flM, which is suspected to be the source of nitrate enrichment in estuarine waters. The source of nitrate in iron ore is the ammonium nitrate used as explosive to blast the hard rock . About 10 tonnes/month ammonium nitrate is being used in the mining site7

. Polling & Ellis8 have reported use of ammonium nitrate along with fuel oil in the Black Angel Lead and Zinc Mine, Greenland. Elevated concentrations of lead and zinc were observed in organisms (mussels, seaweeds) in the

a e

6 6 i 6 l a

6 6 a .= z 06 a 0 Ir ,. '0.,. ,4 .

""6 z rf' a

2 66 a

6 a a 8' t;O ~6'l:06 a

0 10 12 14 16 18 2

CI X 1()3

Fig. 2-Variation of nitrate with chlorinity (0---0 Feb. 198 1; ~---~ Mar., 1981; --- Mar., 1982)

0 .0 l---':--~-"'7n:----;;~-~-60-7( o

Ols1once " fro m Colem IknJ}

Fig. 3-Variation of nitrate with distance from Colem (. ----. predicted values, x---x observed values)

vicinity of the tailings disposal site of this mine8,

however they have not studied the effect of nutrients. The present data shows that the nitrate added is carried both upstream and downstream of the outfall by tidal currents (Fig.3). Further, the observed nitrate concentrations are much higher than those predicted by simple dilution of river water and seawater. Towards the mouth of the estuary a net loss of nitrate is evident. However, from Fig.3 it is not easy to discern any other processes in the estuary since the high concentrations of nitrate coming from the outfall mask the effect of any other removal or supply mechanism that might be operating in this area. For this purpose a different approach has been adopted by considering the outfall as the third source of nitrate, and dividing the estuary into two zones, i.e., upstream and downstream of the outfall. The nitrate

)

DESOUSA: NUTRIENT CHEMISTRY OF MANDOVI ESTUARY 357

concentration at any point in the estuary is calculated using the relations given by Officer9

• For any point X upstream of the outfall, there are two point sources of nitrate-river water flowing downstream, and the nitrate rich water at the outfall carried upstream by tide. For this part of the estuary the nitrate concentration is given by

fx Sx Cx = Cr-+ Co- ... (3)

fr So

• Similarly, for any point X downstream of the outfall, again there are two point sources of nitrate-the nitrate enriched water at the outfall flowing downstream, and the coastal seawater flowing upstream at high tide. In this case, the nitrate concentration is given by

fx Sx Cx= Co-+Cs-

fo Ss ... (4)

where C,., C" and C are the nitrate concentrations in the river water, at the outfall and in the seawater; S .. SO and S, are the salinities at point X, at the outfall and of the seawater, and fn i, and J,J are the fractions of freshwater in the river water, at point X and at the outfall respectively. The fraction of freshwater at any point in the estuary is calculated from the relation

£ex = (Ss - Sx) j " ... (5)

Ss

Equations (3) and (4) were used to compute the predicted nitrate concentrations resulting from mixing of waters from three point sources. The computed values of nitrate and the observed values of the same were plotted against distance (Fig. 4) . The two curves show striking similarities with the observed nitrate always being lower than the expected nitrate. This indicates that nitrate is removed in the estuary . Nitrate loss due to denitrification may be ruled out as the estuary is very shallow and well oxygenated 10.

Also, removal of nitrate by adsorbtion on to the suspended particles, if at all , will be insignificant. That leaves biological uptake as the only removal mechanism. Maximum loss of nitrate, however, occurred in the lower estuary. The primary productivity in this estuary is reported to be the highest during the premonsoon season 1 1.12. Highest phytoplankton values (12,040; 12,000 and 26,000 cells/I) were reported for this estuary in the months of February, March and April, respectivelyl2. The

zooplankton biomass also was high during thi s season l2

. This could have led to higher biological uptake of nitrate in the lower estuary where the waters are clear as compared to that in the turbid waters of the upper estuary. It may be perti nent to mention that during all the three sampling sessions, the water in the upper estuary was visibly very turbid, the. turbidity being caused by the mining rejects7

discharged from the iron ore beneficiation plant and by the spill over of ore during the loading operations .

Distribution of silicate in the estuary (Fig. 5) relative to chlorinity-a conservative property­shows that the observed silicate concentrations were always lower than those predicted by simple mixing of river water and seawater (TDL) . This suggests that silicate is removed from the water column during mixing; the highest removal occurring in the chlorinity range 4-IOxI0-3

. Two possible mechanisms are suggested for the removal of dissolved silicate

6.0

5.0

0.0,L--~--c---=----:c---":--X:---:; o ~ m m ~ ~ 00 ro 01sfonc:e from Colem Ikml

Fig. 4-Variation of nitrate with distance taking the outrall as the third source of nitrate (. ----. predicted values . x-----x observed values)

130,

\'\,,~~

10

-..~

~.~

o 2 4 8 10 12 14 16 18 20

CI X 10- 3

Fig. 5-Variation of silicate with ehlorinity

358 INDIAN 1. MAR SCI., VOL. 29, DECEMBER 1999

1.0

:;: 8. ~

2 \

4

TOL

i of 6 . o CI X 10-3

Fig. 6-Variation of phosphate with chlorinity

during estuarine I)1ixing-uptake by diatoms, and abiological removal by adsorbtion on to the suspended sediment I3,14, Both the mechanisms may be operating in this estuary but at different levels I5 ,16, Although, the primary productivity in this estuary is the highest during the premonsoon 11 ,12, the high turbidity present in the upstream regions is expected to hinder the productivity through impairment of light penetration. Consequently, removal of silicate through uptake by diatoms will be relatively lower in the upstream regions compared to that in the lower reaches. On the other hand, presence of large amounts of suspended sediments, in association with sea salts, is favourable for abiological removal of silicate through adsorbtion I3,14. This implies that while biological uptake is the main mechanism operating in the relatively clearer waters of the lower estuary, abiological removal could be the dominating process in the turbid waters of the upper estuary, This probably explains the maximum removal of silicate observed in the upper estuary.

The concentrations of phosphate were high in the freshwater zone of the estuary varying between 0.5 and 0.611M at chlorinities <3.5xlO-3 (Fig. 6). However, it showed a sudden decrease to <0.1 11M in chlorinity range 3.75-lOx 10-3 followed by a gradual increase to 0.39 11M towards the mouth, This suggests removal of phosphate from the water column in the chlorinity range 3,75-10xlO-3 which incidentally, is the region where the rejects of the iron ore beneficiation plant are discharged causing high turbidity in water. Like silicate, both biological and abiological processes could be responsible for the removal of phosphate, The concentration of iron is very high in this estuary (dissolved iron 0,3-0.7 mg/I, particulate iron 5-44 mg/I)I ,2. Such high concentrations of iron together with high turbidity in well oxygenated low salinity waters 10 favour

abiological removal of phosphate through adsorbtion on to the suspended sediment I7

.l s

. Another mechanism by which dissolved phosphate may be removed IS by precIpitation of carbonate fluoroapatite l9

, While studying the profiles of ammonia and phosphate in pore waters of cores at the . site of submarine tailings di sposal of a copper mine at Rupert Inlet, which uses lime as additive, Ellis et al. 20

observed that regenerative phosphate was absent in the cores affected by the tailings despite the presence of significant concentrations of ammonia, This he attributed to the precipitation of phosphate as carbonate fluorapatite, Whether phosphate is precipitated in Mandovi estuary is not known, and future studies can throw light in this aspect. Al so, biological uptake may not be significant in the turbid waters of upper estuary, Hence it is suggested that, similar to silicate, the biological removal is probably, the dominant factor in the lower estuary where the water is clear, while abiological removal may be the main process in the upper reaches of the estuary.

Conclusion The discharge of mining rejects from the iron ore

beneficiation plant besides increasing the turbidity and metal content in Mandovi estuary al so results in

• abiological removal of phosphate and silicate from the water column in the low salinity region of the estuary by adsorbtion on to the suspended sediment particles, This may adversely affect the biological production in the estuary by limiting the availability of nutrients, especially phosphate, to primary producers.

• supply of nitrate to the estuary which is carried both upstream and downstream of the outfall , and reduced biological uptake of the three nutrients in the upper reaches of the estuary due to high turbidity.

)

DESOUSA: NUTRIENT CHEMISTRY OF MANDOVI ESTUARY 359

Acknowledgement The author is grateful to Dr. S. Z. Qasim, former

Director and to Dr.E. Desa, present Director, for the support and encouragement.

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