Potential Ecological Risk Index FOR HEAVY METALS: Assay AT Guanabara Bay, Rio de Janeiro – Brazil

Campos, A. N.¹ and Bidone, E. D².

^{1, 2} Departamento de Geoquímica, Universidade Federal Fluminense, Niterói, Rio de Janeiro, Brazil; Outeiro São João Batista s/n^o, Centro, CEP 24020-150.

E-mails: ¹ alexcampos@gbl.com.br; ² geobida@vm.uff.br

Abstract

The Potential Ecological Risk Index (PERI) was proposed by Håkanson (1980, 1988) as a contamination control for lakes and coastal systems of Scandinavia. The objective of the present work is to evaluate the performance of PERI in tropical aquatic system. In order to achieve a satisfactory performance, we have introduced a few adaptations to PERI's constituent elements. Among them, the most important adaptation is concerning the estimate of the trophic state system by using the bioproduction number (BPN). The N/Organic Matter ratio of sediment, originally used for the calculation of BPN, was substituted by the evaluation of chlorophyll-a levels in the water column. Due high trophic state observed in Guanabara Bay, the Potential Ecological Risk Index obtained was low (PERI = 36). Hg was identified as being the priority contamination to be detailed: PER: Hg = 25,4> Cd = 5,7> Cu = 2,8> Pb = 1,2> Cr = 0,3> Zn = 0,2.

Introduction

The Potential Ecological Risk Index (PERI) is a diagnostic tool suggested by Håkanson (1980, 1988) for contamination control of lakes and coastal systems, it was originally developed for Scandinavian environments. PERI is formed by three basic modules: Degree of contamination (C_D); toxic-response factor (Tr¹); and potential ecological risk factor (Erⁱ). Its main function is to indicate the contaminant agents and where contamination studies should prioritized. In Brazil, the problems of contamination are increasing and control resources are scarce, therefore this indicator can be a very useful tool for aquatic contamination management, specially when it involves heavy metals. In this context, the assay in Guanabara Bay focused on two main objectives: (I) to evaluate the use of PERI in tropical coastal systems, identifying its potentialities and deficiencies; (ii) and to suggest necessary adaptations and/or changes so its general conception can be used in the development of an adapted index for Brazilian conditions.

Guanabara Bay (figure 1), located in Rio de Janeiro State (22º40' - 23º00' S and 43º00' - 43º20 W) is the most prominent coastal bay in Brazil. Its occupation began 4 centuries ago and became more intense in the early 70s when a large industrial development took place. Nowadays, there are 12 municipal districts, 7,8 million inhabitants and around 12,500 industries distributed unevenly over in the drainage basin area (4000 km²). As a result of this occupation we have currently not only heavy metals, phenols and cyanides but also 360t of DBO and 18t of oil thrown in the bay on a daily basis. This explain the poor quality of water and sediments observed mainly in the western part of the bay. The renewal time of 50% of the bay water volume is 11 days. The depth ranges from 1m to 30m (JICA 1994, Kjerve et. al. 1997).

Material and Methods

During the last two decades, several studies were carried out in the study area, providing relevant data about hydrobiogeochemistry of heavy metals. Essentially, the methodology approach applied in this paper was of collecting and reworking with available data set in a compatible manner. Through this data set, each module of PERI was evaluated separately, following three basic aspects: (i) up to dateness of theoretical concepts, from which PERI's approaches derived; (ii) performance of the diagnostic suggested by PERI, compared with results of main water, sediment and biota contamination studies in Guanabara Bay; (iii) identification of the main theoretical and practical elements that should become target of more detailed studies. As an alternative to the second topic (ii) PERI's estimates were accomplished for two compartments of the bay (A and B) with different impact level (figure 1 and table 2). The sediment data (JICA, 1994) used for PERI were from areas where accumulation prevails (water content > 75% and MO > 10%).

Results and Discussions

The first module of PERI corresponds to the estimate of the degree of contamination (C_D). The C_D is expressed by the sum of the contamination factor of each metal (Cf ⁱ): C_D = Σ Cf ⁱ, Cf ⁱ being defined as the mean metal concentration (C ⁱ), divided by it´s the pre-industrial concentration of the substance (C₀ ⁱ): Cf ⁱ = C ⁱ / C₀ ⁱ. Regional background levels were not used due to the range of values observed in the State of Rio de Janeiro (2,5 - 55 times) (Campos, 2000). We adopted local base levels obtained through dated (²¹⁰Pb) deep sediment core (JICA, 1994). The C_D (28) result suggests that contamination at Guanabara Bay is “considerable” for the cases of almost all metal, registering higher levels in compartment A (C_D = 42) than in compartment B (C_D = 16). The Cfs: Hg (9.0)> Cu (8.0)> Pb (3.5)> Zn(3.0)> Cd (2.7)> Cr (2.0) indicate that Hg and Cu are the main contaminants. These results are in compliance with literature data which shows the generalized character of contamination. From the management aspect, such approach allowed an overview, easing the problems regarding hierarchization.

The second module of PERI is a toxic-response factor (Trⁱ) which is composed by the sedimentological toxic factor (Stⁱ) and sensitivity factor (S) of the system: Trⁱ = Stⁱ/S. Sti is an estimate of metal toxic degrees in sediments: Hg = 40> Cd = 30> Pb = 5 >Cu = 5> Cr = 2> Zn = 1. The values estimated by Trⁱ are in compliance with the approach of sediment quality criteria (SQC) (Webster and Ridgway 1996). The sensitivity factor (S) incorporates to the index the fact that different systems have different sensibility to different metals. In this case, we used the trophic state system admitting that eutrophic systems are less sensitive than oligotrophic systems (Håkanson, 1988). The PERI trophic state is estimated using the bioproductivity number (BPN). This is a result of the slope coefficient (x10) of the regression line between organic matter (IG) and nitrogen (Kjeldahl) content in sediment. BPN defined by Håkanson (1984) was not capable of indicating the trophic state of Guanabara Bay. This is due to different budget C/N between the systems of Scandinavia and Guanabara Bay. In the latter, there is low organic carbon content of sediments compared to the seston of overlaying water. This is a result of high respiration rates in the water column that decompose labil carbon before reaching the sediments located in the bottom (Leal & Wagner, 1993). To establish BPN we have adopted an indirect alternative approach based on correlation analysis among NBP and chlorophyll-a values (NBP = 33 x 23Cl-a, p < 0.05, r = 0.82, n = 12) which was obtained from datum in Swedish lakes (Håkanson, 1984). That strategy allowed the establishment of a more suitable NBP to the trophic state of Guanabara Bay and a comparison with the original scale of PERI classification.

The third module of PERI is the potential ecological risk index: PERI = Σ PERi where the potential ecological risk associated to each metal (REPi) is given by PERI - Trⁱ x Cfi. The results suggested that Hg is the prioritary metal in Guanabara Bay (PER: Hg = 25,4 > Cd = 5.7 > Cu = 2.8 > Pb = 1.2 > Cr = 0.3 > Zn = 0.2). The calculated PERI value (36), suggests that all metals present a low potential ecological risk (table 2). This indication is strongly reinforced by data included in specific literature. In every study of Guanabara Bay biota, the heavy metal observed was of low concentration, similar to non polluted areas (Carvalho and Lacerda, 1996, Lima, 1997).

Conclusions

The PERI adopted for the Guanabara Bay conditions presented a satisfactory performance. Additional efforts should be accomplished with the implementation of a regional background level for heavy metal concentrations in sediments, as well as an index to express the trophic state of tropical coastal systems based on the sediment analysis.

References

Carvalho CEV., Lacerda LD (1992) Ciência e Cultura, v.44, n.213, p.184-186.

Campos AN (2000) Dissertação de mestrado, Departamento de Geoquímica, Universidade Federal Fluminenses, Niterói, Brasil.

Håkanson L. (1988) In: Metals in Coastal of Latin America. ( U. Seeliger, L.D. Lacerda., S.R: Patchineelam,.Editors) Springer-Verlag. pp.240-257.

Håkanson L. (1984) Water Research, v.18, n.3, pp.303-314.

Håkanson L. (1980) Water Research. v.4, pp.975-1001.

JICA (1994). Kokusai Kogyo Co. Ltd., Tokio. v.6.

Kjerfve B et. al. (1997) Continental Shelf Research.v.17, n.3, pp.1609-1643.

Leal MLF., Wagner A L. (1993) Chemical Speciation & Bioavalability. Connecticut, v.5, n.1, pp. 31-42

Lima (1997) Dissertação de mestrado, Departamento de Química. Potífice Universidade Católica (PUC), Rio de janeiro – Brasil.

Ribeiro C. (1996) Dissertação de mestrado, Departamento de Geoquímica, Universidade Federal Fluminenses, Niterói, Brasil.

Webster J., Ridgway I. (1994) Marine Pollution Bulletin, 28, n. 11. pp. 653-661

Figure 1 – Drainage basin of Guanabara Bay with compartments A and B.

Table 1 – Environmental characteristics of Guanabara Bay compartments (A and B).

Characteristics	Comp A	Comp B
Area (km^2)	76	252
Volume (m^3)	2,3 . 10^8	1,64 . 10^9
Mean depth (m)	3	6,5
Chlorophyll-a (µg.L^-1)	94,1	26,9
Total N (mg.L^-1)	2,31	0,31
Total P (mg.L^-1)	0,92	0,08
Drainage basin area (km^2)	1299	2782
Inhabitants in drainage basin (million)	6	1,6
Trophic state	hipertrophic	Eutrophic

Sources: Ribeiro (1996); JICA (1994).

Table 2 – The potential ecological risk (PER) and potential ecological risk index (PERI) for the whole Guanabara Bay.

Metal	C^i	Co^i	Cf ^i = C^i / Co^i	Tr^i = St^i / NBP	PER = Cf ^ix Tr^i
Hg	0,45	0,05	9,0	2,8	25,4
Cd	3	1,1	2,7	2,1	5,7
Pb	69	20	3,5	0,4	1,2
Cu	48	6	8,0	0,4	2,8
Cr	80	40	2,0	0,1	0,3
Zn	180	30	3,0	0,1	0,2
		CD = S Cf^i	28	IREP = SPER^i	36

Cⁱ – Mean metal concentration: ppm dry weight (n = 15, JICA 1994); Coi – Local background level of metals (ppm dry weight) obtained in sediment core (JICA, 1994); Cf – Concentration factor (high >6; considerable, 6-3; moderate, 3-1; low <1); C_D – Degree of contamination (high >36; considerable, 36-18; moderate, 18-6; low <6); Tri – Toxic-response factor; St – Sedimentological toxic factor; NBP – Bioproduction number based in annual mean chlorophyll-a values (57 µg/L) (Kjerfve et. al. 1997); PER classification: high > 160; 80-160 considerable; 40-80 moderate; < 40 low.