A groundwater, sediment and soil chemistry and mineralogical study has been performed to investigate the sources and mobilization process of Arsenic (As) in shallow aquifers of Bangladesh. The groundwater from the shallow aquifers is characterized by high concentrations of Arsenic (47.5-216.8 µg/L), iron (0.85-5.83 mg/L), and phosphate, along with high electrical conductivity (EC). The groundwater has both very low oxidation-reduction potential (Eh) and dissolved oxygen (DO) values indicating reducing conditions. By contrast, the deep aquifers and surface waters (pond, canal) have very low concentrations of Arsenic (<6 µg/L), iron (0.12-0.39 mg/L), and phosphate along with a relatively low EC. Furthermore, the values of Eh and DO are high, indicating oxic to suboxic conditions. Arsenic is inversely correlated with Eh values in the upper aquifer, whereas no relationship in the deeper aquifer is observed. These results suggest that As mobilization is clearly linked to the development of reducing conditions. The clayey silt, OPEN ACCESSWater 2011, 3 1051 enriched in Fe, Mn, Al oxides and organic matter, and deposited in the middle unit of shallow aquifers, contains moderately high concentrations of As, whereas the sediments of deep aquifers and silty mud surface soils from paddy fields and ponds contain a low content of As (Daudkandi area). Arsenic is strongly correlated with the concentrations of Fe, Mn and Al oxides in the core samples from the Daudkandi and Marua areas. Arsenic is present in the oxide phase of Fe and Mn, phyllosilicate minerals and in organic matter in sediments. This study suggests that adsorption or precipitation of As-rich Fe oxyhydroxide on the surface or inner sites of biotite might be responsible for As concentrations found in altered biotite minerals by Seddique et al. Microbially or geochemically mediated reductive dissolution of Fe oxyhydroxides is the main mechanism for As release. The reducing conditions are caused by respiratory decomposition of organic matter, either sedimentary or labile organic C. The process can be accelerated by agricultural activity and domestic organic wastes. An agricultural fertilizer can directly contribute As to groundwater as well as promote As mobilization by ion-exchange with phosphorus.

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Many studies have investigated the contamination level, spatial distribution, sources, chiral signals, and potential ecological and public health risks of dichlorodiphenyltrichloroethanes, its metabolites and the isomers of hexachlorocyclohexane in the environment. This study presents a critical review to provide updated knowledge about the fate, persistence, long-range transport and toxicity effects of dichlorodiphenyltrichloroethanes and hexachlorocyclohexanes in the environment on the basis of analytical data between 1990 and 2014. Highest levels were found for dichlorodiphenyltrichloroethanes (200-9300 ng/ L) and hexachlorocyclohexanes (20-36,000 ng/L) with mean values of 1000 and 5600 ng/L, which were 10 and 56 times higher than the European Community allowable residual limit of 100 ng/L, respectively, in Lake Taihu water of China obtained between 1999 and 2000. Levels of dichlorodiphenyltrichloroethanes in sediments and fish species were remarkably higher than hexachlorocyclohexanes. The highest levels of dichlorodiphenyltrichloroethanes in sediments were found up to 780-227,000 ng/g near a production factory in China and 20,000-5,463,000 ng/g in cattle dips disposal areas in Australia. Out of 32 selected locations for common fish species, tilapia had the highest mean concentration of dichlorodiphenyltrichloroethanes up to 3800 ng/g in Noha River of Japan in 2006. Dichlorodiphenyltrichloroethanes and hexachlorocyclohexanes derive from agricultural runoff, industrial and urban wastewater, etc., that ultimately associate with soil and sediments. The carbon-carbon and carbon-chlorine bonds in dichlorodiphenyltrichloroethanes and hexachlorocyclohexanes provide them persistence, lipophilicity and high binding affinity leading to bioaccumulation in the receptor protein. High accumulation of dichlorodiphenyltrichloroethanes and hexachlorocyclohexanes causes mutagenic, carcinogenic and endocrine disrupting toxicity effects to the humans and wildlife.

Industrial wastewater discharged into aquatic ecosystems either directly or because of inadequate treatment of process water can increase the concentrations of pollutants such as toxic metals and others, and subsequently deteriorate water quality, environmental ecology and human health in the Dhaka Export Processing Zone (DEPZ), the largest industrial belt of 6-EPZ in Bangladesh. Therefore, in order to monitor the contamination levels, this study collected water samples from composite effluent points inside DEPZ and the surrounding surface water body connected to effluent disposal sites and determined the environmental hazards by chemical analysis and statistical approach. The water samples were analysed by inductively coupled plasma mass spectrometry to determine 12 trace metals such as As, Ag, Cr, Co, Cu, Li, Ni, Pb, Se, Sr, V and Zn in order to assess the influence of multi-industrial activities on metal concentrations. The composite effluents and surface waters from lagoons were characterized by a strong colour and high concentrations of biochemical oxygen demand, chemical oxygen demand, electrical conductivity, pH, total alkalinity, total hardness, total organic carbon, Turb., Cl(-), total suspended solids and total dissolved solids, which were above the limit of Bangladesh industrial effluent standards, but dissolved oxygen concentration was lower than the standard value. The measurement of skewness and kurtosis values showed asymmetric and abnormal distribution of the elements in the respective phases. The mean trend of variation was found in a decreasing order: Zn > Cu > Sr > Pb > Ni > Cr > Li > Co > V > Se > As > Ag in composite industrial effluents and Zn > Cu > Sr > Pb > Ni > Cr > Li > V > As > Ag > Co > Se in surface waters near the DEPZ. The strong correlations between effluent and surface water metal contents indicate that industrial wastewaters discharged from DEPZ have a strong influence on the contamination of the surrounding water bodies by toxic metals. The average contamination factors were reported to be 0.70-96.57 and 2.85-1,462 for industrial effluents and surface waters, respectively. The results reveal that the surface water in the area is highly contaminated with very high concentrations of some heavy/toxic metals like Zn, Pb, Cu, Ni and Cr; their average contamination factors are 1,460, 860, 136, 74.71 and 4.9, respectively. The concentrations of the metals in effluent and surface water were much higher than the permissible limits for drinking water and the world average concentrations in surface water. Therefore, the discharged effluent and surface water may create health hazards especially for people working and living inside and in the surrounding area of DEPZ.

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