Distribution pattern of ambient cd in wetland ponds ditributed along an industrial complex

Distribution pattern of ambient cd in wetland ponds ditributed along an industrial complex

Distribution pattern of ambient cadmium in wetland ponds distributed along an industrial complex Shamik Das, B.B. Jana * Aquaculture and Applied Limnology Laboratory, Department of Zoology, University of Kalyani, Kalyani 741 235, West Bengal, India Received 27 February 2003; received in revised form 11 September 2003; accepted 7 October 2003 Abstract Water and sediment samples collected from 18 wetland ponds within and outside industrial areas were examined for cadmium concentration and water quality parameters during the period of January to July 1996. The Cd contents in gill, liver, mantle and shell of freshwater mussel (Lamellidens marginalis) as well as leaves and roots of water hyacinth Eichhornia those occurred in these ponds were also estimated. Cd concentration ranged from 0.006 to 0.7025 mg/l in water and from 7 to 77 lg/g dw in sediments of all the ponds investigated. The amount of Cd occurring in water and sediment was much higher in concentrations in the ponds located in Captain Bheri and Mudiali farm close to industrial areas, compared to remaining ponds located outside the industrial belt. Lamellidens marginalis procured from Mudiali and Captain Bheri ponds showed regardless of size, tissue and season of collection significantly higher Cd concentration than did those from other ponds. Likewise, tissue Cd in Eichhornia collected from Mudiali pond was as high as 125–152 lg/g dw in root and 21–63 lg/g dw in leaves compared to 40–108 lg/g dw in root and 9–43 lg/g dw in leaves in the remaining ponds. Seasonal variability of Cd was clear-cut; the concentration was relatively higher in water and sedi- ment in all ponds during summer than during monsoon season or winter. Size-wise, smaller groups showed the highest concentrations of Cd in all tissues of Lamellidens compared with medium and large size groups. Concentration fac- tor for all tissues of Lamellidens regardless of size and season, was inversely proportional with the ambient Cd con- centrations. Concentration factor estimated for all tissues in all ponds and all seasons was in the order: liver > gill > shell > mantle. As all ponds located outside the industrial belt showed Cd concentrations ranging from 0.006 to 0.049 mg/l, it is suggested that these wetlands do not pose serious risk to the environment. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Wetland ponds; Cadmium; Lamellidens marginalis; Eichhornia crassipes 1. Introduction Cd with an average concentration of 0.15 ppm (Weast, 1969–1970), ranks as the 67th element in order of abundance in the earth’s crust (Trotman-Dickenson, 1973). It normally occurs as an air-borne and aquatic contaminant associated with Pb-smelting and electro- plating processes. Increased concentrations of Cd deposition in impounded water occurs through atmo- spheric fallout and runoff (Borg et al., 1989). The global anthropogenic emission to the atmosphere is estimated at 7.6 · 10 6 kg Cd/year (Nriagu and Pacyna, 1988). Cd contamination of impounded waters has posed an important threat to human health because of its estab- lished harmful effect in the food chain of fishes and human health. * Corresponding author. Tel.: +91-033-5826-323; fax: +91- 033-5828-282. E-mail address: bbj@cal2.vsnl.net.in (B.B. Jana). 0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2003.10.016 Chemosphere 55 (2004) 175–185 www.elsevier.com/locate/chemosphere A large number of growing industrial complexes, such as a electroplating, cable, alloy, vehicle, plastic pigments, dyes in many parts of India, and especially in the vicinity of Calcutta city often have resulted in indiscriminate discharge of industrial effluents with high Cd load. These contaminants finally find their ways into the neighboring wetlands (Fig. 1) and thereby damage ecosystem health. According to the World Health Organisation (1971), the maximum permissible limit for Cd in drinking water is 0.01 mg/l. However, detected concentrations in many natural bodies of water are often much higher (Roth and Hornung, 1977; Murphy et al., 1978; Mathew and Menon, 1983). For example, in some fish inhabiting natural lakes, a concentration of 13.6 lg/g Cd has been detected (Murphy et al., 1978), while provisional toler- ance for humans range from 0.4 to 0.5 lg Cd/person/ week. In general, fishes growing even in sublethal contam- inated environments show high levels of Cd in their tissues due to bio-accumulation through the aquatic food chain. For example, Cd concentration in fish from Bombay fish markets ranges from 16 to 176 lg/g (Pillai, 1983), and is in the range of 2–417 lg/g from other parts of the world (Anand, 1978). Such concentration is potentially hazardous to human health as they exceed the tolerable Cd intake (0.4–0.5 mg/person/week). Thus, fish or animals living in Cd contaminated aquatic habi- tats pose hazard to human health if they are part of human food chain. Information pertaining to Cd distribution in fresh- water habitats in relation to Cd accumulation in animals is sparse in the Indian sub-continent. With respect to other regions of the world, data available on alpine lakes suggest a total anthropogenic discharge of 3.3 t Cd/year into Ontario waterways (Environment Canada and Health Canada, 1994). Fifty seven lakes in Central Ontario, Canada have a geometric mean Cd concen- tration of 10 ng/l (Stephenson and Mackie, 1988). Wavy Lake within the regional municipality of Sudbury, On- tario, Canada, had a water Cd concentration of 4780 ng/l in 1992 (Taylor et al., 1995). Fig. 1. General location of study ponds. 176 S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 The ability of certain freshwater plants and animals to accumulate metals above ambient water concentra- tion is well documented. Using these organisms as indicators, bio-availability of metals from the environ- ments can be monitored over extended period of time. However, environmental metal level is not the only factor which influences the metal content of mussels, as both the size of the animals and the seasons markedly affect these parameters (Penthreath, 1973; Boyden, 1977; Majori et al., 1978). Among molluscs, bivalve and gas- tropods are excellent bio-accumulators for a wide range of pollutants (Simkiss, 1983; Everaarts, 1990; Living- stone, 1991; Das and Jana, 1999) (Table 1). In general, they are filter feeder, herbivorous and have the potential to bio-accumulate contaminants that normally occur in the water or sediment at concentrations too low for detection by routine monitoring technique. Thus, they are considered ideal species for environmental moni- toring. Further more, the sedentary nature of these animal is helpful in the interpretation of bio-accumula- tion data (Short and Sharp, 1989; Livingstone, 1991). Information about the level of metal pollution and the distribution of bivalves in freshwater habitats of India is scarce. The purpose of this study was to examine the distribution of Cd concentrations of water and sedi- ment in relation to tissue Cd concentration in freshwater bivalve, Lamellidens marginalis in freshwater ponds along a Cd gradient from industrial complex to uncon- taminated areas. 2. Materials and methods Eighteen wetland ponds were selected for the present investigation. These wetlands are located within a radius of 60 km of the University of Kalyani. Some rain-fed wetlands receive industrial effluents; others are situated in an uncontaminated area (Fig. 1). These wetlands are used for irrigation of agriculture crops, domestic use and fish culture. The selected wetlands were distributed along a pollution gradient ranging from very high level to low or uncontaminated areas. The pond area ranged Table 1 Cadmium accumulation in various tissues of bivalve molluscs under different ambient Cd concentrations Name and tissues of bivalves Cd accumulation Condition References Lamellidens marginalis Gill 15–258 lg/g dw Field experiment (Cd concentra- tion: 0.006–0.7025 ppm) Present study Liver 17–502 lg/g dw Shell 10–196 lg/g dw Mantle 5–67 lg/g dw Lamellidens marginalis Gill 536 lg/g dw Cd concentration: 30 ppm, exposure time: 40 days Das and Jana (1999) Liver 494 lg/g dw Shell 335 lg/g dw Mantle 211 lg/g dw Unio elongatus Foot 102 lg/g dw Cd concentration: 50 ppb, exposure time: 60 days Badino et al. (1991) Gills 196 lg/g dw Mantle 95 lg/g dw Crassostrea virginica Gill 275 lg/g dw – Zaroogian (1980) Gill 450 pmol Cd/g – Roesijadi and Unger (1993) Elliptio complanata Total soft tissue 20 lg/g dw/72 h Exposure time: 115 days Wang and Evans (1993) Mytilus edulis Kidney 300 lg/g dw Exposure time: 40 days Everaarts (1990) Gills 130 lg/g dw Hepatopancreas 170 lg/g dw Foot 110 lg/g dw Mantle 30 lg/g dw S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 177 from 0.1 to 6 ha with a depth range of 1.5–3 m (Table 2). Natural populations of floating macrophytes Eichhornia are common in about 39% the wetlands. Freshwater bivalve, Lamellidens marginalis is found in almost all ponds. Samples of water and surface sediment were collected from each pond during summer, monsoon and winter seasons in 1996. The samples from each pond were pooled into a composite sample. As ponds located in contaminated area had moderate water spread area and received industrial effluents from a point source, the collected sample represented the true picture of that body of water. Samples were collected in one liter polythene bottle, acidified with concentrated HNO 3 and were brought to the laboratory. Each one liter sample was then concen- trated to 10 ml volume by slow evaporation. 20 ml HNO 3 :H 2 SO 4 (1:3) was added to the sample, and the mixture was evaporated to near dryness. The residue was extracted with 50 ml double distilled water. Cd content of the sample was analysed by direct aspiration of the aqueous digest extract into atomic absorption spectrophotometer (Model UV 2201), following the method described in APHA (1995). The instrument was calibrated with metal standards,and Oyster Cd stan- dards procured from of National Research Council of Canada. The surface sediment (0–2.5 cm) of pond was col- lected by suitable bottom sampler (Van Raaphorst and Brinkman, 1984) from different sites in the pond and then mixed to make a homogenous sediment sample. One hundred ml sample was transferred to acid-washed 250 ml conical flask; concentrated HNO 3 (10 ml) was added and the mixture was digested to a volume of approximately 3 ml. Digests were allowed to cool to 55 °C, followed by addition of 10 ml concentrated HNO 3 and 30% (v/v) H 2 O 2 (1.0 ml). The flask was returned to hot plate and digested to approximately 3 ml. The di- gests were diluted to 25 ml with double distilled deion- ised water and transferred to a glass bottle prior to analysis. Cd concentration was analysed by direct aspi- ration of the aqueous digest into AAS as described by Walsh et al. (1994). Freshwater bivalves (Lamellidens marginalis) were collected from various sites in the pond using a 50 cm quadrate hand grab sampler (APHA, 1995). The speci- mens were washed thoroughly in tap water, blot-dried and their length and wet weights were recorded. Animals were sorted into small (12 ± 1.3 g; 3.8 ± 1.3 cm), medium (31 ± 2.5 g; 6 ± 1.5 cm) and large (55 ± 4 g; 9.5 ± 2.5 cm) classes, each comprising 12 animals. Water and sediment samples of the pond were monitored for Cd concentration using standard AAS described by Walsh et al. (1994). Water samples from each pond were also monitored for other water para- meters (temperature, pH, dissolved oxygen, total hard- ness, total alkalinity) to specification given by APHA (1995). Changes in fresh weight in Lamellidens were recorded on each sampling day. Lamellidens were carefully dis- Table 2 Physico-chemical parameters of investigated ponds throughout the investigated period Ponds Area (ha) Depth (m) Temp. (°C) DO (mg/l) pH Total alkalinity (mg/l) Total hardness (mg/l) Ambient cadmium concentration Water (mg/l) Sediment (lg/g dw) Captain Bheri 8 1.5 21–34 3.5–4.8 6.2–6.45 120–142 150–165 0.458–0.6175 56–65 Mudiali 8 2.0 21–33 3.4–4.0 6.0–6.4 105–125 117–137 0.502–0.705 63–77 P3 0.2 2.0 20–34 4.9–6.5 6.9–7.3 130–147 153–160 0.032–0.049 20–26 P4 0.5 1.5 19–34 7.8–8.0 7.8–8.5 115–150 142–159 0.0235–0.016 14–20 P5 0.1 1.5 20–35 9.0–11.0 7.0–7.9 117–127 120–132 0.017–0.0135 10–13 P6 0.1 1.0 21–34 8.5–10.0 7.6–8.5 100–132 120–158 0.049–0.039 19–25 P7 0.2 1.0 20–33 8.0–11.0 7.0–7.9 107–150 130–142 0.0162–0.010 8–12 P8 0.5 1.0 21–35 7.8–9.3 7.6–8.5 127–135 130–147 0.0162–0.0115 1.5–11 P9 0.2 1.5 20–34 8.0–11.5 7.6–8.0 138–142 149–159 0.0125–0.0082 7–10 P10 1.0 1.0 21–35 7.3–10.5 6.9–7.8 152–160 150–167 0.009–0.006 7.5–10 P11 1.0 2.0 21–35 8.3–9.3 7.0–7.5 145–170 159–180 0.008–0.0067 8.0–9.5 P12 1.0 1.5 20–34 7.3–10.5 6.8–6.9 95–130 109–187 0.008–0.007 7.5–8.5 P13 1.0 1.5 20–35 7.5–10.0 8.0–8.5 95–117 107–125 0.0147–0.016 11–15 P14 0.5 1.5 20–35 6.9–10.0 7.3–8.1 105–119 119–137 0.032–0.017 13–18 P15 0.5 1.0 21–35 7.9–11.0 6.9–7.5 109–122 119–145 0.011–0.009 8.3–12 P16 0.3 2.0 20–33 8.0–11.5 7.5–7.8 109–129 130–149 0.035–0.025 15–19 P17 0.5 1.0 21–34 7.5–10.0 7.3–7.8 100–130 137–149 0.009–0.0061 27–35 P18 0.5 1.5 20–33 7.3–9.5 7.4–7.9 107–129 137–157 0.0081–0.006 7–8.9 178 S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 sected for gill, liver, mantle and shell. The shell valves were opened with a shell-valve opener. The mantle, liver and gill were carefully removed and placed on separate watch glasses on top of chipped ice in ice-buckets. The wet weight of tissues was recorded on an electrical bal- ance after blotting surface moisture with filter paper. In addition to Lamellidens, floating macrophyte, Eichhor- nia crassipes were collected manually and washed thor- oughly in tap water. The root and leaves were separated and dried for tissue Cd extraction and estimation by the method of Walsh et al. (1994). Bio-concentration factor (CF), reflecting the accu- mulation ability for Cd was calculated for each tissue using the formula given by Taylor (1983): CF ¼ TCd WCd or SCd where TCd ¼ cadmium content in animal tissue (lg/g dw), WCd or SCd ¼ average ambient cadmium concen- tration of water (mg/l) or sediment (lg/g dw) during the period of experiment. Mean Cd concentrations (SE±) for each pond was obtained. All pond, on the basis of their Cd concentra- tion were grouped in to (i) with values within the per- missible limit, 0.05 mg/l for drinking water imposed by Environmental Protection Agency (1986) and (ii) with value above permissible values or highly contaminated. One way analysis of variance and LSD test were used to determine Cd distribution among ponds during various seasons and tissues as well as the size-group of animals. The relationship between Cd concentration factor and ambient Cd concentration was determined by the use of the first order equation. The level of significance was accepted at P < 0:05. 3. Results 3.1. Ambient water cadmium The concentration of ambient Cd in water was highly variable according to the season and the location of the ponds. Cd concentration ranged from 0.006 to 0.7025 mg/l (in or for) all ponds throughout the investigation period. Cd concentration was higher (0.63–0.7025 mg/l) in ponds designated as numbers 1 and 2 and located in contaminated areas. Uncontaminated ponds designated as numbers 3–18 had a Cd concentration of 0.006–0.049 mg/l. Cd concentration in each pond was significantly higher during the summer than during the monsoon and winter (Fig. 2). There was about 86 fold variability in ambient Cd (0.0081 and 0.7025 mg/l) among all ponds during the summer (ANOVA, P < 0:05). Ponds (11%) located in industrial areas showed as high as 0.617– 0.7025 Cd mg/l, implying direct impact of industrial effluents. Cd concentrations in the remaining 16 ponds (89%) ranged from 0.0081 to 0.049 mg/l, but, remained within the permissible Cd limit (0.05 mg/l) for drinking water as per standards set by Environmental Protection Agency––US (1986) for the USA, and therefore posed no potential threat to the environment under local conditions. Spatial distribution of Cd was also variable (P < 0:05) during winter and monsoon. Similar to the scenario found for the summer, two industrial ponds (11%) showed higher values than the remaining 16 ponds (89%) during winter and monsoon. In general, Cd concentration in water varied in three seasons of the year (ANOVA, P < 0:05) depending upon the leaching, runoff water or dilution by rainfall. Winter values (0.007–0.6175 mg/l) were lower than those of summer (0.0081–0.7025 mg/l), but higher than those of monsoon (0.0062–0.502 mg/l). In these ponds, summer values were 12% and 31% higher than those observed during winter and monsoon, respectively. 3.2. Ambient sediment cadmium The Cd content in the sediment ranged from 7 to 77 lg/g dw Cd concentration was distinctly higher in pond- 1 (63–77 lg/g dw) and in pond-2 (55–65 lg/g dw) than in remaining ponds located at uncontaminated sites (7–26 lg/g dw). Summer Cd values varied between 8.5 and 77 lg/g dw, indicating a significant difference (ANOVA, P < 0:05) among the 18 ponds. Only about 11% of ponds were characterized by extremely high Cd con- centration that ranged from 65 to 77 lg/g dw in summer. Remaining 89% ponds showed concentration ranging from 8.9 to 35 lg/g dw (Fig. 2). The spatial distribution of sediment Cd varied (AN- OVA, P < 0:05) during winter and monsoon. Two ponds (11%) had higher values than remaining 89% ponds during winter and monsoon. The remaining ponds showed winter (7–30 lg/g dw) and monsoon (7–27 lg/g dw) values that were 64–90% lower than the former two ponds. Seasonal variability (ANOVA, P < 0:05) in sediment Cd remained was identical to that of water. The values were significantly higher (ANOVA, P < 0:05) in the summer (5.8–9% higher than winter and 11.7–18% higher than monsoon). During the summer, Cd concentrations in Captain Bheri pond (pond-2) was 13.8% higher than during monsoon and 6.2% higher than in winter. Similar trend was found in the rest of the ponds. 3.3. Cadmium in Lamellidens Cd was detected in the shell and liver of Lamellidens collected from all 18 ponds. However, the gills were analysed from only six and mantle from only seven ponds. S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 179 Lamellidens from Mudiali (pond-1) and Captain Bheri (pond-2) regardless of their size, tissue and col- lection season showed significantly higher (P < 0:05) Cd concentration than those from other ponds. Cd con- centration in smaller animal ranged between 254 and 502 lg/g dw in liver, 189–258 lg/g dw in gill, 160–196 lg/ g dw in shell and 41–67 lg/g dw in mantle. In other ponds it compared to ranged from 10 to 189 lg/g dw in liver, 9–128 lg/g dw in gill, 4.7–100 lg/g dw in shell and 2–23 lg/g dw in the mantle (Figs. 3–5). Cd concentrations were highest in small animals (10– 502 lg/g dw in liver, 9–258 lg/g dw in gill, 4.7–196 lg/ g dw in shell and 2–67 lg/g dw in mantle) and lowest in large individuals (7.5–451 lg/g dw in liver, 3.7–189 lg/ g dw in gill, 3–196 lg/g dw in shell and 2–57 lg/g dw in mantle). With the exception of Captain Bheri and Mu- diali, variability in tissue Cd in smaller animals from 16 ponds was highest in liver (99.8%) followed by shell (90%), gill (80%) and mantle (78%). In general, summer values for Cd were about 11–28% higher than they were (A) WATER 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 123456789101112131415161718 PONDS PONDS Cadmium content of water (mg/l) Summer Winter Monsoon (B) SEDIMENT 0 10 20 30 40 50 60 70 80 90 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Cadmium content of sediment (µg/g dw.) Summer Winter Monsoon Fig. 2. Mean (±SE) concentration of water (A) and sediment (B) cadmium in 18 investigated ponds during three different seasons. Note the clear-cut differences in water and sediment cadmium distribution. 180 S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 GILL 0 100 200 300 400 500 PONDS PONDS PONDS PONDS Gill Cd content (µg/g dw.)Liver Cd content (µg/g dw.) Shell Cd content (µg/g dw.) Mantle Cd content (µg/g dw.) Summer Winter Monsoon LIVER 0 100 200 300 400 500 123456789101112131415161718 Summer Winter Monsoon SHELL 0 100 200 300 400 500 123456789101112131415161718 Summer Winter Monsoon MANTLE 0 100 200 300 400 500 1 2 3 4 5 6 7 8 9 101112131415161718 Summer Winter Monsoon 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Fig. 4. Cadmium accumulation in four different tissues of medium group Lamellidens procured from 18 ponds in three different seasons. Note the clear-cut variability of tissue cadmium in different seasons and investigated ponds. GILL 0 100 200 300 400 500 123456789101112131415161718 PONDS PONDS PONDS PONDS Gill Cd content (µg/g dw.) Liver Cd content (µg/g dw.) Mantle Cd content (µg/g dw.) Shell Cd content (µg/g dw.) Summer Winter Monsoon Summer Winter Monsoon LIVER 0 100 200 300 400 500 123456789101112131415161718 Summer Winter Monsoon SHELL 0 100 200 300 400 500 123456789101112131415161718 MANTLE 0 100 200 300 400 500 123456789101112131415161718 Summer Winter Monsoon Fig. 3. Cadmium accumulation in four different tissues of small group Lamellidens procured from 18 ponds in three different seasons. Note the clear-cut variability of tissue cadmium in different seasons and investigated ponds. S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 181 in winter (liver––20–492 lg/g dw, gill––12–221 lg/g dw, shell––9–177 lg/g dw, and mantle––2–55 lg/g dw) and 5–15% higher than monsoon (17–389 lg/g dw in liver, 9–198 lg/g dw in gill, 4–16 lg/g dw in shell and 1.5–44 lg/g dw in mantle). 4. Concentration factor (CF) CF for all tissues of Lamellidens regardless of size and season, were higher in those ponds that had low Cd concentration and low in those with high ambient Cd concentration. CF estimated for all tissue in all ponds during all seasons was in the order: liver > gill > shell > mantle. The Cd concentrations in all tissues were less in the summer than in the winter and monsoon. CF among smaller animals were 9–15% higher than those for medium-sized animals and 14–37% higher than the larger animals (Table 3). 5. Cadmium in Eichhornia The natural population of Eichhornia was observed in seven out of 18 ponds. Eichhornia collected from Mudiali (pond-1) and Captain Bheri (pond-2) had sig- nificantly higher (P < 0:05) Cd concentrations than did the remaining ponds. In general, tissue Cd in Eichhornia collected from Mudiali pond was as high as 125–152 lg/g dw in root and 21–63 lg/g dw in leaves compared to Table 3 Range of the values of concentration factor for different tissues of Lamellidens marginalis procured from 18 ponds during the period of investigation Tissue Summer Winter Monsoon Small Medium Large Small Medium Large Small Medium Large Liver 600–4050 550–3095 445–3000 620–4150 555–3295 490–2100 631–4612 565–3193 500–2193 Gill 288–2072 205–1901 195–1702 290–2575 225–2057 200–1780 302–3750 250–2666 175–1765 Shell 285–2067 271–1950 262–1900 290–2170 280–1970 295–1970 330–2795 302–2009 165–195 Mantle 88–490 60–415 55–350 85–450 65–420 63–365 302–2001 165–195 60–370 Each value represents the data calculated from six animals. LIVER 0 100 200 300 400 500 123456789101112131415161718 Summer Winter Monsoon SHELL 0 100 200 300 400 500 123456789101112131415161718 MANTLE 0 100 200 300 400 500 123456789101112131415161718 Summer Winter Monsoon Summer Winter Monsoon GILL 0 100 200 300 400 500 123456789101112131415161718 PONDS PONDS PONDS PONDS Gill Cd content (µg/g dw.) Liver Cd content (µg/g dw.) Shell Cd content (µg/g dw.) Mantle Cd content (µg/g dw.) Summer Winter Monsoon Fig. 5. Cadmium accumulation in four different tissues of large group Lamellidens procured from 18 ponds in three different seasons. Note the clear-cut variability of tissue cadmium in different seasons and investigated ponds. 182 S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 40–108 lg/g dw in root and 9–43 lg/g dw in leaves in the remaining ponds (Fig. 6). Pond variability of tissue Cd was as high as 65–81% for leaves and 74–85% for roots of Eichhornia from the seven ponds. Seasonal variability of tissue Cd showed higher val- ues during summer winter and monsoon. Summer Cd concentration (root––55–214 lg/g dw, and leaves–– 11.75–63.8 lg/g dw) were 5–27% and 11–46% higher than monsoon (29–168 lg/g dw in root, and 5–34 lg/g dw in leaves) and winter (39–187 lg/g dw in root, and 5.9–48 lg/g dw in leaves). 6. Water quality Water pH (6.0–6.5) and dissolved oxygen (3.5–4.8 mg/l) regardless of the seasons were significantly lower in Mudiali (pond-1) and Captain Bheri (pond-2) ponds than 16 ponds (pH––6.9–8.5; DO––7.5–11.5 mg/l). There was no marked difference of total alkalinity and total hardness among the 18 ponds investigated. In general, pH was higher during monsoon (7.7–8.5) followed by winter (6.5–7.6) and summer (6.0–6.8). Values of DO were higher in winter (11.5–4.9 mg/l) and lower in summer (7.6–3.5 mg/l). Total alkalinity (115– 175 mg/l) and total hardness (130–175 mg/l) were highest in summer and lowest in monsoon (total alkalinity–– 109–125 mg/l, total hardness––115–130 mg/l) (Table 2). 7. Discussion Spatial Cd distribution in wetlands ponds depended upon their degree of contamination. About 80–86 fold increase in Cd concentration in two ponds situated in the industrial belt of Calcutta (Captain Bheri and Mu- diali farm) over remaining ponds outside the city industrial complex may be due by the discharge of high anthropogenic Cd through wastewater effluents. As all ponds located outside the industrial belt show Cd concentration within range of 0.006–0.049 mg/l, (the permissible limit, Environmental Protection Agency–– US, 1986) it is suggested that these wetlands do not pose serious environmental threat. Wide range Cd variability among the ponds located outside the city industrial complex with low Cd con- centration (water––0.006–0.049 mg/l; sediment––7–35 lg/g dw) perhaps caused large variability of tissue Cd of the test animal. This implied that the ambient Cd of uncontaminated ponds remained far below the accu- mulating potentials of test animal and, hence, are quite useful as bio-filter. On the other side, less variability of Cd at higher concentrations of Cd (0.458–0.7025 mg/l) indicated that Lamellidens population occurring in these habitats, were almost at the plateau, and might not be considered suitable for bio-removal of Cd from the environments. As observed by other investigators of various lakes (Stephenson and Mackie, 1988; Stephenson et al., 1996), Cd concentration in sediment was significantly higher than that in the water column in all water bodies. It has been shown that Cd was rapidly lost from the water column to suspended particles (Stephenson et al., 1996). The loss may also be due to the presence of humic substances and the organic content of the sediment (Stephenson and Mackie, 1988; Pempkowiak and Kozuch, 1994). Cd tissue distribution in Lamellidens was in the order: liver > gill > shell > mantle. This order was found to be true, regardless of the size group, ponds and sea- son. As hepatopancreas acts as a sink for metal ions (Rajalekshmi and Mohandas, 1993), it is possible that it bears the Cd load of the main body and thus shows the highest Cd concentration among all tissues examined. Similar results were reported by other investigators (Merigomez, 1989; Merigomez and Ireland, 1989). Al- though the ability to regulate the internal concentration LEAVES 0 10 20 30 40 50 60 70 80 Mudiali C. Bhery P4 P6 P14 P16 P17 PONDS PONDS Cd content (µg/g dw.) Cd content (µg/g dw.) Summer Winter Monsoon ROOT 0 50 100 150 200 250 300 Mudiali C. Bhery P4 P6 P14 P16 P17 Summer Winter Monsoon Fig. 6. Cadmium accumulation in leaves and root of Eichhornia in investigated ponds. S. Das, B.B. Jana / Chemosphere 55 (2004) 175–185 183 of Cu and Zn over a wide range of dissolved concen- trations have been demonstrated in intertidal shrimp Palaemon elegans, the Cd concentration, on the other hand, is not regulated resulting in body concentration of metal directly proportional to external metal concen- tration of the environment (White and Rainbow, 1986). Cd uptake by Lamellidens gill was appreciably higher than the uptake by the shell and mantle. Gill was the primary site for Cd accumulation because of its rela- tively large surface area and filtration activity (V-Balogh and Salanki, 1984; Holwerda et al., 1989; Salanki and V- Balogh, 1989; Everaarts, 1990). It was estimated that about 90% of Cd uptake occurred through absorption from solution, is facilitated by diffusion of CdCl 2 across the gill or by complexation with a high molecular weight-compound present on the gill surface (Carpene and George, 1981). It was proposed that large surface area and the gill mucous, which might act in ion ex- change, contributed to high metal concentration found in gill tissue (Brooks and Rumsby, 1967; Pringle et al., 1968). Relatively low Cd accumulation in mantle and shell might be due to the shell acting as a safe storage matrix for toxic contaminants resistant to soft tissue detoxifi- cation mechanism (Walsh et al., 1995). Computation of correlation coefficient between tissue Cd and ambient Cd revealed no significant relationship between the Cd content of different tissues of Lamelli- dens and water hardness (r ¼À0:1721–0.0115, P > 0:05) or total alkalinity (r ¼À0:024–0.026, P > 0:05) of water. The Ca content of water was, however reported to have an inverse relationship with tissue Cd (Bjerreg- aard and Depledge, 1994; Jana and Das, 1997). Though Eichhornia was known to be an important bio-filter for the removal of metal in many experimental studies (Nir et al., 1990; Xiang et al., 1994), their occurrence in these wetlands did not exert any clear-cut relationship either between tissue Cd of Lamellidens and the ambient Cd concentration or between the tissue Cd of Eichhornia and the latter. This is perhaps due to the fact that the Eichhornia population was quantitatively less exerting any substantial bio-filter effect on pond wetlands. Acknowledgements This research was supported by a grant Department of Environment and Forests, Government of India (to B.B. Jana). Shamik Das is grateful to DoEn for pro- viding him with a Junior Research Fellowship. 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Information pertaining to Cd distribution in fresh- water habitats in relation to Cd accumulation in animals is sparse in the Indian