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Table 7 Trace element concentrations (mg/l) of groundwater to wastewater may primarily be due to the replacement of these ions by bivalent Ca** and Mg’. been acting as the purification pathway of the perco- lating water, is likely to be favorable for groundwater environment. Most clay minerals have an excess of imbalanced negative charges in the crystal lattice. Adsorption processes in soils thus favor the adsorption of cations (Fetter 1994).

Table 7 Trace element concentrations (mg/l) of groundwater to wastewater may primarily be due to the replacement of these ions by bivalent Ca** and Mg’. been acting as the purification pathway of the perco- lating water, is likely to be favorable for groundwater environment. Most clay minerals have an excess of imbalanced negative charges in the crystal lattice. Adsorption processes in soils thus favor the adsorption of cations (Fetter 1994).

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Fig. 1 Lithologic cross-section of Dhaka city aquifer based on BWDB, DWASA logs (Sharif 1999) aquifer formation behaves as a single aquifer system in which the units are hydraulically connected. Transmis- sivity and permeability of the main aquifer varies be- tween 620 and 2,235 m?/day and 11 and 18 m/day, respectively. Recharge to the aquifer mainly occurs by the horizontal inflow of water from surrounding aquifers through and into the aquifers as well as by the direct infiltration of Buriganga water through river beds (Ah- med et al. 1998).
Fig. 1 Lithologic cross-section of Dhaka city aquifer based on BWDB, DWASA logs (Sharif 1999) aquifer formation behaves as a single aquifer system in which the units are hydraulically connected. Transmis- sivity and permeability of the main aquifer varies be- tween 620 and 2,235 m?/day and 11 and 18 m/day, respectively. Recharge to the aquifer mainly occurs by the horizontal inflow of water from surrounding aquifers through and into the aquifers as well as by the direct infiltration of Buriganga water through river beds (Ah- med et al. 1998).
Fluctuation of temperature of groundwater and sur- face water was observed. Water samples were collected in April when, generally, the temperature of ground water has its maximum value. The temperature o groundwater of the study area ranged between 26.7 and 27.0°C. The difference with depth is very negligible. This could be due to the thermal pollution at shallower depth influenced by point sources besides the warm climate o
Fluctuation of temperature of groundwater and sur- face water was observed. Water samples were collected in April when, generally, the temperature of ground water has its maximum value. The temperature o groundwater of the study area ranged between 26.7 and 27.0°C. The difference with depth is very negligible. This could be due to the thermal pollution at shallower depth influenced by point sources besides the warm climate o
Table 3 Trace element concentrations of wastewater and suspended materials
Table 3 Trace element concentrations of wastewater and suspended materials
Table 2 Concentrations of ions in wastewater samples (mg/l)
Table 2 Concentrations of ions in wastewater samples (mg/l)
Table 4 Concentrations of CaCO; and organic carbon in soil with depth
Table 4 Concentrations of CaCO; and organic carbon in soil with depth
In Hazaribagh area higher concentration of Cr in topsoils are found more hazardous to human health compared to other elements. Cr is generally present in trivalent oxidation state (Cr**) in sediment, but can also exist in the hexavalent (Cr°*) as well as tetravalent (Cr**) oxidation states. Among these three forms Cr(III) is least toxic. However, Cr(III) and Cr(VI) are stable in the pH—Eh range of most natural environ-
In Hazaribagh area higher concentration of Cr in topsoils are found more hazardous to human health compared to other elements. Cr is generally present in trivalent oxidation state (Cr**) in sediment, but can also exist in the hexavalent (Cr°*) as well as tetravalent (Cr**) oxidation states. Among these three forms Cr(III) is least toxic. However, Cr(III) and Cr(VI) are stable in the pH—Eh range of most natural environ-
Table 5 Concentrations of heavy metals in sediment with depth Under anoxic conditions, the Fe?* concentration in solution increases; under oxic condition, little Fe’* is released. Remarkably high concentration of Cr was found in the upper two samples of both the profiles (4,120—26,100 mg/kg) to 1.5 m depths, whereas total Cr content in natural soil ranges from | to 3,016 mg/kg, with a typical concentration of about 53 mg/kg (Richard and Bourg 1991). Below 1.5m the concentration de- creases sharply. A study carried out by the Geologica Survey of Bangladesh (unpublished data) supports the higher content of Cr (0.30-19.17 mg/gm) in topsoi (0.5-1.5 m samples) compared to lower soil samples (down to 4.5 m). Outside the embankment the conten of heavy metals decreased remarkably (UNIDO-Gov- ernment of Bangladesh 2000). each other, supports their distribution throughout the area. Concentrations of Cr, Pb, Zn which decrease with depth may be due to their adsorption nature by clay minerals as well as by iron oxyhydrates. Al, Fe, Mn showed increasing tendency to a depth of 4.5 m and then decreased downward. These three metals can be precipitated as hydroxides under oxidizing conditions. Therefore, they may accumulate in the interface between oxidation and reduction zones in sediments. Sorption of heavy metals onto inorganic or organic substances is often only a temporary condition. Changes in elevated salt concentration due to discharged tannery effluen could lead to replacement of adsorbed heavy metals by alkali and alkaline earth metals, particularly from clays. Because of the preferred adsorption by clays of Na relative to heavy metals, the latter are likely to be released.
Table 5 Concentrations of heavy metals in sediment with depth Under anoxic conditions, the Fe?* concentration in solution increases; under oxic condition, little Fe’* is released. Remarkably high concentration of Cr was found in the upper two samples of both the profiles (4,120—26,100 mg/kg) to 1.5 m depths, whereas total Cr content in natural soil ranges from | to 3,016 mg/kg, with a typical concentration of about 53 mg/kg (Richard and Bourg 1991). Below 1.5m the concentration de- creases sharply. A study carried out by the Geologica Survey of Bangladesh (unpublished data) supports the higher content of Cr (0.30-19.17 mg/gm) in topsoi (0.5-1.5 m samples) compared to lower soil samples (down to 4.5 m). Outside the embankment the conten of heavy metals decreased remarkably (UNIDO-Gov- ernment of Bangladesh 2000). each other, supports their distribution throughout the area. Concentrations of Cr, Pb, Zn which decrease with depth may be due to their adsorption nature by clay minerals as well as by iron oxyhydrates. Al, Fe, Mn showed increasing tendency to a depth of 4.5 m and then decreased downward. These three metals can be precipitated as hydroxides under oxidizing conditions. Therefore, they may accumulate in the interface between oxidation and reduction zones in sediments. Sorption of heavy metals onto inorganic or organic substances is often only a temporary condition. Changes in elevated salt concentration due to discharged tannery effluen could lead to replacement of adsorbed heavy metals by alkali and alkaline earth metals, particularly from clays. Because of the preferred adsorption by clays of Na relative to heavy metals, the latter are likely to be released.
Table 6 Ionic concentrations of groundwater (mg/l) ments. In the normal pH range of 4-9, Cr exists as insoluble Cr(IIJ) in a more reducing environment than more soluble Cr(VI) forms in oxidizing environments (Kotas and Stasicka 2000). The removal of Cr(VJ), dominating a near surface oxidizing environment, could be very significant in thick vadose zones (Oliver et al. 2003) and could approach almost 100% in Hazaribagh aquifer after long transport above water table. Once it has been reduced to Cr(IIJ), nearly all of the Cr should remain stable in this form (Oliver et al. 2003). Crd) sorption is enhanced by higher soil pH, more TIC, more clay and higher CEC. At higher pH values (>5.5) Cr precipitates to contaminated soils and adsorbs more Crd) on the A-horizon (Stewart et al. 2003). Reduc- tion of Cr(VI) by dissolved organic compounds at pH values from 4 to 8 is generally a slow process (Johnson et al. 1992). A soil microbial community can promote reduction of Cr(VI) to Cr(II) (Lovley 1993; Losi et al. 994) under both aerobic (Bopp and Ehrlich 1988; Shen and Wang 1993) and anaerobic (Wang et al. 1989) conditions. Concentrations of Zn and Mn are high in all he samples (84-190 and 562-960 mg/kg, respectively). Cr(VI) has been shown to be reduced by Mn(II) to Cr(II), while colloidal MnO, oxidized Cr(ID) to Cr(VI) Perez-Benito and Arias 2001). 7 Groundwater
Table 6 Ionic concentrations of groundwater (mg/l) ments. In the normal pH range of 4-9, Cr exists as insoluble Cr(IIJ) in a more reducing environment than more soluble Cr(VI) forms in oxidizing environments (Kotas and Stasicka 2000). The removal of Cr(VJ), dominating a near surface oxidizing environment, could be very significant in thick vadose zones (Oliver et al. 2003) and could approach almost 100% in Hazaribagh aquifer after long transport above water table. Once it has been reduced to Cr(IIJ), nearly all of the Cr should remain stable in this form (Oliver et al. 2003). Crd) sorption is enhanced by higher soil pH, more TIC, more clay and higher CEC. At higher pH values (>5.5) Cr precipitates to contaminated soils and adsorbs more Crd) on the A-horizon (Stewart et al. 2003). Reduc- tion of Cr(VI) by dissolved organic compounds at pH values from 4 to 8 is generally a slow process (Johnson et al. 1992). A soil microbial community can promote reduction of Cr(VI) to Cr(II) (Lovley 1993; Losi et al. 994) under both aerobic (Bopp and Ehrlich 1988; Shen and Wang 1993) and anaerobic (Wang et al. 1989) conditions. Concentrations of Zn and Mn are high in all he samples (84-190 and 562-960 mg/kg, respectively). Cr(VI) has been shown to be reduced by Mn(II) to Cr(II), while colloidal MnO, oxidized Cr(ID) to Cr(VI) Perez-Benito and Arias 2001). 7 Groundwater
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