TRACE METALS IN
BENTHIC DIATOMS FROM SEDIMENTS – A CASE STUDY OF RIVER NARMADA, INDIA
Sunil
G. Bhand and Kamal K. Chaturvedi*
School
of Energy & Environmental Studies, Devi Ahilya University Indore, 452002 MP
INDIA.
E-mail:
sgbhand@hotmail.com,
sgbhand@cat.ernet.in
Abstract: To provide an accurate
assessment of the sediments of river Narmada (Madhya Pradesh, India) the
Sediment Quality Triad ‘SQT’ as proposed by Chapman & Long (1983) has been
adopted. The method is significantly useful for an integrated physical,
chemical and biological approach. The samples were collected from 15 sampling
stations, at an interval of 2Kms. downstream of Mortakka bridge at river Narmada
during June 1999. Samples were collected for sediments and associated benthic
diatoms and analyzed by Flame atomic absorption spectrometry (FAAS) and
inductively coupled plasma mass spectrometry (ICP-MS). Herein the first results
for an integrated approach to the development of quality criteria for the
sediments of river Narmada are presented.
INTRODUCTION
Sediments are sinks for a wide variety of chemicals. The chemicals sorbed onto the sediments are considerd a major concern to many commercial species and food chain organism, spending a major portion of their life cycle living in or on aquatic sediments. This provides a pathway for the chemicals (viz. Metals, organics such as PCB’s etc.) tobe consumed by higher aquatic life and subsequently by humans as well. Direct transfer of toxic chemicals from sediments to organisms is now considered tobe a major route of exposure for many species.
The contaminants are not necessarily fixed permanently into physico -chemical conditions. Bioavailability of toxic chemicals and food chain transfer may be strongly affected by such processes and by the type of chemical binding into the sediment particles (Calmano et al. 1996). In contaminated rivers, metal exposures of biota, determined by tissue residues in organisms such as benthic insects, can follow the downstream contamination trend as observed in fine grained bed sediments (Axtmann et al., 1997 and references therein). Thus, bioaccumulation appears tobe linked to some of the some processes that control the distribution of sediment bound metals.
Sediments contaminated with nutrients, metals, organics and oxygen demanding substances can be found in freshwater systems, while some of the contaminants are present in elevated concentrations as a result of natural processes, many are due to anthropogenic activities. Measuring the concentration of chemicals alone does not address the ultimate concern. Reynoldson et al. (1989) proposed series of bioassessment alongwith appropriate criteria are necessary to identify the type of stress being exerted and the bioaccumulation of contaminants present.
In the river Narmada, MP India, the flowstretch of the river selected for the assessment receives nonperiodic discharges from the various industries situated in the nearby areas as well as from the agriculture runoff. Preliminary analysis of bed sediments and benthic insects revealed that they are contaminated with metals (viz. lead, zinc, copper chromium cadmium iron etc.). Hence, in this project we have undertaken an intensive sampling and analysis of the study area of about 30Kms. length downstream of the river. The ‘SQT’ approach as proposed by Chapman & Long (1983) is applied in order to integrate physical, chemical and biological data. The first results for such an integrated approach to the development of quality criteria for the sediments of river Narmada are presented.
METHODOLOGY
Sampling Strategy: The flowstretch of 30 Kms. of river Narmada was selected for sampling and characterization. Samples were collected during the low flow season of June 1999. Sampling stations were fixed at an interval of 2Kms., in all constituting 15 sampling stations downstream of Mortakka bridge on the river Narmada. Habitat and accessibility to the river determined specific locations of sampling stations.
Sediment: Triplicate bed sediment samples were collected from both sides of the river from all the 15 sampling stations and were composite type. Samples were immediately sieved through 63mm nylon mesh, preserved in ice and transported to laboratory followed by drying at 600C. Samples were digested in concentrated HNO3, preconcentrated and aspirated to FAAS. A few selected samples (representing low level contaminant concentrations) were analyzed by ICP-MS in quantitative mode. Samples were decomposed using HNO3-HClO4-HF acid mixture and analyzed by standard addition method to take care of matrix effects. Results were calculated based on recoveries for each element. Recoveries for Standard Reference Material for sediment (NIES No. 2, India) ranges from 80 to 94%.
Benthos: Benthic diatoms were harvested from five muddy sites of the river in the downstream during June 1999. The epipelic diatoms (mobile diatoms, migrating mainly in the upper few cms. of sediments) were harvested employing the combination of lens cleaning tissue and plankton gauze method (Stronkhorst et al., 1994). The benthos were transformed into the sealable plastic bags, filled with ambient river water and kept in ice followed by deep freezing in the laboratory. Samples were dried at 800C, weighed and acid digested with concentrated HNO3. The acid evaporated and dry residue reconstituted in 0.6N HCl, filtered (0.4mm) and analyzed by FAAS. For the five sites selected for harvesting diatoms, bioconcentration factors (BCFs) were calculated.
RESULTS AND DISCUSSION
The sediment samples collected from the 15 stations were characterized for the physical and chemical characteristics presented in Table 1. The particle size of the different samples was determined. The results of the physical variables and their mean values are clustered into the five different groups namely A, B, C, D and E. The results are presented in Table 2.
The distribution of sediments is found tobe complex and variable for this river system. The fine sediments (group A and B) were found approximately at 22 and 30 Kms. downstream of first sampling station. The major contaminant metals present in the sediments were found tobe in the range Cr (12-26 µg g –1 dry wt.), Cd (4-10 µg g –1 dry wt), Pb (24-160 µg g –1 dry wt), Zn (82-600 µg g –1 dry wt), Cu (36-180 µg g –1 dry wt) and iron (2-8% dry wt.).
The longitudinal variation for the contaminant metals (Pb, Zn and Cu) was determined and is depicted in Figure 1. Chromium, copper and lead are contributed mainly from anthropogenic activities in the nearby areas, while zinc represented the concentrations comparable to levels observed in upstream sediment samples. From fig.1 it can be observed that contaminant concentrations increased slightly at few downstream stations. This can be correlated with the higher concentrations of organic carbon present, which might resulted in increased sorption of metals onto the sediments.
The samples of benthic diatoms had a percentage dry wt. of 7.2 ± 2.4%. Samples collected for each group (A to E) weighing 8, 5, 1.6, 2 and 1.4g respectively. The basic parameters, mean concentrations of trace metals in benthic diatoms and sediments are presented in Table 3.
In general the concentration of lead, zinc
and copper are some what higher and there is need to evaluate the biological
stress in terms of the sediment toxicity. Chromium is also present but the
current levels are not alarming. Concentrations for all the metals accumulated
in benthic diatoms are lower than those in sediments. For all the metals
identified the BCFs are below one (Table 3), that might be due to less
bioavailability of these micro contaminants. Minimum BCFs are all found for the
first few Kms. (1 to 6, group D & E) of the study area. While the maximum
BCFs were calculated for group A & B. Since the bioavailability of trace
metals depends on organic content, concentrations for organic carbon were
determined for the samples.
Thus from the analysis of the data obtained from this study, it can be concluded that sediments are contaminated but are not polluted at this stage. However for the sites with higher contamination (group A & B), further studies addressing the bioassessment of specific species of diatoms be undertaken.
Acknowledgement: The authors whishes to acknowledge University Grants Commission (N.Delhi, India) for financial assistance under the project F.12–78/97 (SR–I) and associateship to SGB.
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Axtmann EV, Cain DJ and Luoma SN (1997), Environ. Sci. Technol. 31:750-758.
Calmano W and Förstner U (1996), Sediments and Toxic Substances, Springer Heidelberg.
Chaman PM and Long ER (1983), Mar. Pollut. Bull.14:81-84.
Reynoldson TB and Zarull MA (1989), Hydrobiologia 188/189:463-476.
Stronkhorst J, Vos PC and Misdorp R (1994),Bull. Environ. Contam. Toxicol. 52:818-824.