Arsenic associations in sediments from shallow aquifers of northwestern Hetao Basin,Inner Mongolia

Understanding the mechanism of arsenic mobilization from sediments to groundwater is important for water quality management in areas of endemic arsenic poisoning, such as the Hetao Basin in Inner Mongolia, northern China. Aquifer geochemistry was characterized at three field sites (SH, HF, TYS) in Hangjinhouqi County of northwestern Hetao Basin. The results of bulk geochemistry analysis of sediment samples indicated that total As concentrations have a range of 6.8–58.5 mg/kg, with a median of 14.4 mg/kg. The highest As concentrations were found at 15–25 m depth. In the meanwhile, the range of As concentration in the sediments from background borehole is 3–21.8 mg/kg, with a median value of 9 mg/kg. The As sediments concentrations with depth from the SH borehole were correlated with the contents of Fe, Sb, B, V, total C and total S. Generally, the abundance of elements varied with grain size, with higher concentrations in finer fractions of the sediments. Distinct lithology profile and different geochemical characteristics of aquifer sediments indicate the sediments are associated with different sources and diverse sedimentary environments. Up to one third of arsenic in the sediments could be extracted by ammonium oxalate, suggesting that Fe oxyhydroxides may be the major sink of As in the aquifer. Sequential extraction results indicate that arsenic occurs as strongly adsorbed on and/or co-precipitated with amorphous Fe oxyhydroxides in sediments accounting for 35 and 20%, respectively, of the total contents of arsenic. The release of As into groundwater may occur by desorption from the mineral surface driven by reductive dissolution of the Fe oxide minerals. Furthermore, small proportions of As associated with iron sulfides occur in the reductive sediments.     

Human Influence on Heavy Metal Distribution in the Upper Lake Nansi Sediments, Shandong Province, China

Core and surface sediment samples were collected from three sub-lakes ( Lake Nanyang, Lake Dushan and Lake Zhaoyang) in the Lake Nansi Basin, Shandong Province. In order to reveal the characteristics of spatial and historical distribution of heavy metals in different sublakes of the Upper Lake Nansi, heavy metal (As, Cr, Cu, Hg, K, Mn, Ni, Pb, Zn, Al, Fe, Ti and V) concentrations of sediment samples were investigated. Based on the activity of^137Cs in the sediments, the modem accumulation rate of Lake Nansi sediments is 3.5 mm/a. Our results show that the whole Upper Lake Nansi has been already polluted by heavy metals, among which Lake Nanyang has been polluted seriously by mercury, as well as by lead and arsenic, while Lake Dushan has been most seriously polluted by lead and arsenic. Historical variation of heavy metal (Cr, Cu, K, Ni, Zn, A1, Fe, Ti and V) concentrations shows an abrupt shift in 1962, resuiting in a division of two periods: from 1957 to 1962 when metal enrichment increased with time, and from 1962 to 2000 when it decreased with time, while that of some anthropogenic elements such as Hg, Pb and Mn tend to increase toward the surface. However, the variation trend of As in the sediments is different from that of Hg, Pb and Mn, with its maximum value appearing in 1982. Since 1982 the concentrations of As have decreased due to the forbidden use of arsenite pesticides. This variation trend revealed changes in manner of human activity (coal combustion, waste discharges from both industries and urban sewage ) within the catchment during different periods.     

Geochemistry of surface sediments in tsunami-affected Sri Lankan lagoons regarding environmental implications

The December 26, 2004 Indian Ocean tsunami was one of the largest in human history, devastating the coastal wetlands of surrounding countries. This study present the chemical analyses of tsunamigenic and pre-tsunami sediments from Hikkaduwa and Hambantota lagoons in southern Sri Lanka, to assess their geochemical composition, their source, and subsequent environmental impacts. Principal component analysis of the tsunami sediments shows that 42% of the total variance is accounted for calcium oxide and Sr. That is, the tsunami deposits are rich in biogenic phases derived from shallow marine sediments. High organic matter contents of the tsunami sediments of up to 80 wt% also support this interpretation. The association of chlorine (<9.4 wt%), brome (<170?mg/kg), arsenic (<17?mg/kg), iron (III) oxide (<12.9 wt%) and sulfur (<7.6 wt%) accounts for 33% of the variance, reflecting higher salinity. This further suggests that the sediments were mainly derived from a marine environment, rather than from non-marine sands and/or soils. Immobile element contents and relations (thorium, scandium and zirconium) suggest that the tsunami sediment source was mostly felsic in composition, with some mafic component, and mixed with predominantly shallow marine shelf or slope sediments. Additional compositional variations in the tsunami sediments in both lagoons may be associated with variations of wave strength along the coast and by the morphology of the continental shelf. Lower elemental abundances in Hambantota lagoon sediments compared to Hikkaduwa equivalents may thus reflect a greater non-marine component in the former, and greater shelf sediment component in the latter.     

The landscape-scale relationship between lake sediment geochemistry and catchment bedrock composition from the Temagami and Gowganda areas of Northeastern Ontario,Canada

Lake sediments are integrators of watershed wide environmental information that includes bedrock geology, glacial overburden, vegetation, hydrology and land use. Lake sediments were collected from an area covering approximately 13,600 km² from the Gowganda and Temagami regions to determine the geochemical background conditions representing different bedrock types. For the entire dataset, the median value for arsenic in lake sediment was 2.6 mg/kg, well within the prescribed limits indicating that the Canadian national objective for arsenic in sediments for the protection of aquatic life (17 mg/kg). Overall, the vast majority of lake sediment samples (97.2%) were below the 17 mg/kg Canadian objective for the protection of aquatic life, however, catchments with Nipissing Diabase had the highest background levels for arsenic (10 mg/kg) with some lakes ranging up to 30 mg/kg. The major geological controls influencing changes in lake sediment geochemistry were determined using random forest classification and principal component analysis (PCA). Random forest classification was able to identify which geological province the samples were derived from and the dominant rock types in the sample catchment with respectable accuracy. PCA revealed strong spatial relationships between lake sediment geochemistry and bedrock geology, particularly a strong relationship between Nipissing Diabase in the watershed of the lake and the cobalt-type mineralization indicator elements in the lake sediment including naturally higher levels of arsenic. Within the samples that exceeded regulatory standards, over half the samples that exceeded the regulatory objectives were located in watersheds that contained Nipissing Diabase in their catchment. This study demonstrates that unique geochemical assemblages can be associated with specific geological areas which is of interest to exploration geologists, for target generation and prospecting, and to environmental policy makers, for site-specific restoration targets.     

天鹅洲长江故道湿地地下水砷的时空分布特征及控制机理

高砷地下水不仅直接危害供水安全,还可通过与湿地之间的交互作用,影响湿地水质进而威胁湿地生态安全.长江中游河湖平原已被报道广泛分布有高砷地下水,而位于长江中游故道区域的天鹅洲湿地地下水中砷的空间分布特征尚不明确,湿地与地下水的交互作用对地下水中砷季节性动态的控制机理尚不明确.本研究在天鹅洲湿地采集2个水文地质钻孔的35件沉积物样品、12个分层监测井不同季节的共72组地下水样和18组地表水样,通过水位-水化学监测、沉积物地球化学组成分析和砷、铁形态表征探究天鹅洲湿地地下水中砷的时空分布规律及控制机理.研究发现天鹅洲湿地地下水砷含量为1.08~147 μg/L,牛轭湖外侧浅井（10 m）地下水砷含量普遍高于深井（25 m）和牛轭湖内侧浅井（10 m）、深井（25 m）地下水,枯水期和平水期的砷含量高于丰水期.牛轭湖外侧浅层地下水系统具有更厚的粘土、亚粘土沉积,沉积物中总砷、强吸附态砷和易还原的铁氧化物的含量更多,吸附砷的水铁矿等无定形铁氧化物还原性溶解导致砷释放进入地下水中.枯水期天鹅洲湿地底部向牛轭湖外侧浅层含水层输送不稳定的有机质,使天鹅洲湿地地下水-地表水界面成为砷释放的热点区域.丰水期时牛轭湖外侧含水层受长江补给的影响,还原环境发生改变使地下水中的砷和铁被氧化固定从而不利于砷向地下水释放.      

Spatial variations and distributions of phosphorus and nitrogen in bottom sediments from a typical north-temperate lake,China

Concentrations and vertical distributions of total nitrogen (TN), total phosphorus (TP) and their different forms in sediments obtained from nine locations of Lake Dalinouer in September 2008 were analyzed. The results demonstrated that TP in surface sediments ranged from 0.493 to 0.904 g/kg, and inorganic phosphorus was the main fraction of total phosphorus, ranging from 335 to 738 mg/kg. Simultaneously, the autogenetic calcium phosphorus (ACa-P) was the main fraction of inorganic phosphorus, ranging from 145.4 to 543.2 mg/kg. Vertical distribution of different phosphorus forms in different sediment cores was distinguishing, and most of them tended to increase toward the surface sediment, indicated that the phosphorus concentration was related to the humanity with a certain extent. The relationships between TP and occluded phosphorus and ACa-P were significant. Nitrogen in the sediment was composed mainly of organic nitrogen, accounting for grater than 80 % of TN. NO₃ ^?-N was the dominate fraction of inorganic nitrogen in the surface sediment, ranging between 51 and 346 mg/kg (151.1 ± 104.4 mg/kg), and accounting for between 2.2 and 17.7 % of total sediment nitrogen (6.2 ± 5.6 %). The ratio of organic carbon and TN in sediment was in range of 6.0–25.8 and presented a tendency of lake centre >lake sides, indicating that nitrogen accumulated in the sediments from lake sides came mainly from terrestrial source and nitrogen was mainly autogenetic in lake centre. Ratio of N:P in all sampling sites was below 14, indicated that N was the limiting nutrient for algal growth in this lake.     

The research of phosphorus of Xiangxi River nearby the Three Gorges, China

The Xiangxi River is the first middling tributary of the Changjiang River near the Three Gorges Dam. The River is subject to phosphorus pollution mainly from industrial wastewater. As the water quality of the Xiangxi River could directly influence the water quality of the Three Gorges Reservoir, the research on phosphorus levels and its change in the sediment profile of the Xiangxi River could provide useful information in the dynamic changes in the system, thereby offering options for mitigative measures. Water and sediment samples from lower reaches of Xiangxi River were collected and the different forms of phosphorus in sediments of the Xiangxi River were studied. The concentrations of total phosphorus in sediment ranged from 757.67 to 1438.54 mg/kg. Inorganic phosphorus concentrations ranged from 684.63 to 1055.58 mg/kg. Phosphorus contamination was serious in some parts of the Xiangxi River. With an average concentration of 635.17 mg/kg, calcium-bound phosphorus is the main form among different inorganic phosphorus forms. Labile phosphorus and iron/aluminum-bound phosphorus measured 3.40, 0.05and 35.28 mg/kg, respectively. The mobilization potential of phosphorus of sediments was studied through adsorption and release experiments. The equilibrium concentration of phosphorus adsorption and release was around 0.1 mg/L. The initial concentrations of phosphorus in the overlying water and the sediments have obvious effect on phosphorus mobilization potential. In addition, the release rate of phosphorus in sediment increased with water depth.     

南四湖及主要入湖河流沉积物中磷的赋存形态研究

应用沉积物磷形态SMT提取方法,分析了南四湖及主要入湖河流丰水季节表层沉积物、独山湖及微山湖区沉积岩芯中磷的含量及形态组成变化,研究了磷的埋藏演化过程与人为污染特征.结果表明,老运河表层沉积物中磷的含量最高,为13068.3 mg/kg,以OP及NaOH-P为主,其富集系数分别为6.5和9.0,主要是与来自济宁市的废水污染有关.其余表层沉积物以HCl-P为主;受人为污染的影响,NaOH-P、OP产生一定程度的富集,其富集系数分别为1.0.3.7、1.3～6.0;部分沉积物中高含量的有机质加重了OP的富集.独山湖及微山湖沉积岩芯中以HCl-P和OP为主;20世纪60年代末期以来,随着湖泊生产力的提高及早期成岩过程中有机质的矿化分解,独山湖及微山湖沉积岩芯中TOC含量自下而上呈指数增加,这也导致了沉积物中OP的增加;独山湖及微山湖沉积岩芯6～0 cm和5～0 cm较高的OP及NaOH-P含量,指示了20世纪80年代中期以来磷的人为污染过程.     

广东省普宁市土壤硒的分布特征及影响因素研究

开展了广东省普宁市区域土壤硒调查研究,采集了413个表层土壤样品(0～20 cm)和103个深层土壤样品(> 150 cm),测定了土壤全硒含量,据此研究土壤硒分布特征及其影响因素。结果表明,普宁市土壤全硒含量变化于0. 16～2. 01 mg/kg,平均值为0. 63 mg/kg,总体上处于中硒及高硒水平,不存在缺硒和硒过剩土壤。砂页岩风化形成的赤红壤全硒含量较高,平均值达0. 86 mg/kg,以侏罗系页岩母质发育的土壤全硒含量最高,平均值达0. 89 mg/kg;三角洲第四系沉积物发育形成的水稻土全硒含量最低,平均值为0. 41 mg/kg。回归分析表明,土壤全硒含量与铁铝含量、有机碳含量具有极显著正相关,与p H呈极显著负相关。影响普宁市土壤硒含量的主要因素是成土母质,土壤p H、有机碳和铁铝含量及土地利用方式对土壤全硒含量分布与富集也有一定的影响。     

湖泊沉积物中蛋白质和氨基酸的动态变化

测定了阿哈湖和百花湖沉积物孔隙水中蛋白质和氨基酸的含量。蛋白质含量分别在0．32－1．71mg/ml和0．22－1．38mg/ml之间，氨基酸含量分别在1．15－5．01μg/ml（以含N量计）和1．41－4．16μg/ml之间。两湖沉积物氨基酸含量逐渐增高，表明扩散作用的存在。随着时间的推移，蛋白质在沉积物各层中得到了很好的保存，说明它的分解比较有限。沉积物孔隙水中氨基酸含量和微生物活动有关，氨基酸在27cm以后的阿哈湖沉积物中明显积累，可能指示微生物活动的减弱。     

Arsenic exposure through groundwater in the middle Ganga plain in the Varanasi environs,India: A future threat

The study area covers an about 100 km² of the middle Ganga plain in Uttar Pradesh, experiencing intensive groundwater extraction. In order to recognize the arsenic contamination zones of the Varanasi environs, sixty eight groundwater samples have been collected and analyzed for major ions, iron and arsenic. Twenty one sediment samples in the four boreholes were also collected to deduce the source of arsenic in the groundwater. The preliminary survey reports for the first time indicates that part of rural and urban population of Varanasi environs are drinking and using for irrigation arsenic contaminated water mostly from hand tube wells (<70 m). The study area is a part of middle Ganga plain which comprises of Quaternary alluvium consists of an alternating succession of clay, clayey silt and sand deposits. The high arsenic content in groundwater samples of the study area indicates that 14% of the samples are exceeding the 10 μg/l and 5% of the samples are exceeding 50 μg/l. The high arsenic concentration is found in the villages such as Bahadurpur, Madhiya, Bhojpur, Ratanpur, Semra, Jalilpur, Kateswar, Bhakhara and Kodupur (eastern side of Ganga River in Varanasi), situated within the newer alluvium deposited during middle Holocene to Recent. The older alluvial aquifers situated in the western side of the Ganga River are arsenic safe (maximum As concentration of 9 μg/l) though the borehole sediments shows high arsenic (mean 5.2 mg/kg) and iron content (529 mg/kg) in shallow and medium depths. This may be due to lack of reducing conditions (i.e organic content) for releasing arsenic into the groundwater. Rainfall infiltration, organic matter from recently accumulated biomass from flood prone belt in the newer alluvium plays a critical role in releasing arsenic and iron present in sediments. The main mechanism for the release of As into groundwater in the Holocene sandy aquifer sediments of Varanasi environs may be due to the reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica content. The high societal problems of this study will help to mitigate the severity of arsenic contamination by providing alternate drinking water resources to the people in middle Ganga plain and to arrange permanent arsenic safe drinking water source by the authorities.     

黔西南高砷煤开发现状与污染调查

贵州省黔西南州的兴仁县是典型的高砷煤矿区,经化验,该区煤中砷含量普遍偏高,最大可达21 000mg/kg,为Ⅱ型的高砷矿区;水体和土壤砷含量亦高,土壤中砷的最高含量为234.141mg/kg,最低含量5.279mg/kg,平均值37.735mg/kg;水中砷的含量相对较低,最高含量为0.004 3mg/L,平均值为0.000 5mg/L;河流沉积物中砷的最高含量为219.140mg/kg,最低值为20.676mg/kg,平均值为60.700mg/kg。分析认为土壤中砷与受矿业活动影响的灌溉水体有关,矿业开采已造成了当地水环境、土壤环境、大气环境的严重污染。     

Arsenic in geothermal sources at the north-central Andean region of Ecuador: concentrations and mechanisms of mobility

The arsenic content of geothermal hot springs and their sediments in the north-central Andean region of Ecuador has been investigated. The area of study is located between parallels 1°11′N and 1°30′S and includes five provinces. The area is rich in geothermal surface manifestations that are mainly used for medicinal baths in recreational complexes. Unfortunately, water residuals without treatment are released from the recreational facilities to surrounding water bodies. The results indicate that total arsenic in geothermal waters in this region has a range of 2–969 μg As/L, and sediments contain arsenic ranging from 1.6 to 717.6 mg/kg. Chemical analyses of sediment samples show the presence of sulfur, iron, aluminum and calcium. A high concentration of natural organic matter was also found in some samples (20–29.5%); thus sorption and coprecipitation can be the main mechanisms of As immobilization on mineral phases and natural organic matter.     

Vertical distribution and mobilization of arsenic in shallow alluvial aquifers of Chapai-Nawabganj district, Northwestern Bangladesh

Core sediments from two boreholes and groundwater from fifty four As-contaminated well waters were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10 to 40 m of depth) range from 2.76?C315.15 mg/l (average 48.81 mg/l). Arsenic concentration in sediments ranges from 3.26?C10 mg/kg. Vertical distribution of arsenic in both groundwater and sediments shows that maximum As concentration (462 mg/l in groundwater and 10 mg/kg in sediments) occurs at a depth of 24 m. In January 2008, 2009 and 2010, maximum As concentration occurs at the same depth. Environmental scanning electron microscope (ESEM) with EDAX was used to investigate the presence of major and trace elements in the sediments. The dominant groundwater type is Ca-HCO₃ with high concentrations of As and Fe, but with low levels of NO₃ ^? and SO₃ ^?2. Statistical analysis clearly shows that As is closely associated with Fe (R² = 0.64) and Mn (R² = 0.91) in sediments while As is not correlated with Fe and Mn in groundwater samples. Comparatively low Fe and Mn concentrations in some groundwater, suggest that probably siderite and/or rhodochrosite precipitated as secondary mineral on the surface of the sediment particles. The correlations along with results of sequential leaching experiments suggest that reductive dissolution of FeOOH and MnOOH mediated by anaerobic bacteria represents mechanism for releasing arsenic into the groundwater.     

Hydrogen sulfide and organic carbon at the sediment–water interface in coastal brackish Lake Nakaumi, SW Japan

The relationship among H₂S, total organic carbon (TOC), total sulfur (TS) and total nitrogen contents of surface sediments (0–1 cm) was examined to quantify the relationship between H₂S concentrations and TOC content at the sediment water interface in a coastal brackish lake, Nakaumi, southwest Japan. In this lake, bottom water becomes anoxic during summer due to a strong halocline. Lake water has ample dissolved SO₄ ^2? and the surface sediments are rich in planktic organic matter (C/N 7–9), which is highly reactive in terms of sulfate reduction. In this setting the amount of TOC should be a critical factor regulating the activity of sulfate reduction and H₂S production. In portions of the lake where sediment TOC content is less than 3.5 %, H₂S was very low or absent in both bottom and pore waters. However, in areas with TOC >3.5 %, H₂S was correlated with TOC content (pore water H₂S (ppm) = 13.9 × TOC (%) ? 52.1, correlation coefficient: 0.72). H₂S was also present in areas with sediment TS above 1.2 % (present as iron sulfide), which suggests that iron sulfide formation is tied to the amount of TOC. Based on this relationship, H₂S production has progressively increased after the initiation of land reclamation projects in Lake Nakaumi, as the area of sapropel sediments has significantly increased. This TOC–H₂S relationship at sediment–water interface might be used to infer H₂S production in brackish–lagoonal systems similar to Lake Nakaumi, with readily available SO₄ ^2? and reactive organic matter.     

乌梁素海沉积物中全磷的分布特征

开展了浅水草型湖泊乌梁素海沉积物中TP(全磷)的分布特征研究。结果表明,乌梁素海表层沉积物中TP含量在0.37~0.99 g/kg之间变化,平均值为0.75 g/kg;水平分布上,TP含量呈现从湖区四周向湖心递减的变化趋势,大部分湖区已具有生态毒性效应,并对湖区底栖生物群落和生态环境构成了较大威胁;垂向上,尽管沉积物中TP含量有所波动,但0~10 cm的表层沉积物中TP含量均随深度增加而递减,揭示了近年来人类活动的加剧对乌梁素海沉积物中磷累积的重大影响;实验得出了粉砂、粘土粒级>极细砂粒级>细砂粒级>中砂以上粒级的TP含量序列,各粒级对TP的吸附贡献率由吸附能力和质量百分比共同控制;磷的沉积对乌梁素海富营养化进程有深远影响。     

Mobilisation of traffic-derived trace metals from road corridors into coastal stream and estuarine sediments,Cairns, northern Australia

This investigation revealed the presence of traffic-derived metals within road, stream and estuarine sediments collected from a coastal catchment, northern Australia. Studied road sediments displayed variable total metal concentrations (median Cd, Cu, Pb, Pd, Pt, Ni and Zn values: 0.19, 42.6, 67.5, 0.064, 0.104, 36.7 and 698 mg/kg, respectively). The distinctly elevated Zn values are due to abundant tyre rubber shreds (as verified by SEM-EDS and correlation analysis). By comparison to the road sediments, background stream sediments taken upstream from roads have relatively low median Pb, Pd, Pt and Zn concentrations (7.3 mg/kg Pb, 0.01 mg/kg Pd, 0.012 mg/kg Pt, 62 mg/kg Zn). Stream and estuarine sediment samples collected below roads have median values of 21.8 mg/kg Pb, 0.014 mg/kg Pd, 0.021 mg/kg Pt and 71 mg/kg Zn, and exhibit ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios that appear on a mixing line between the isotopically distinct background stream sediments and the road sediments. Thus, mobilisation of dusts and sediments from road surfaces has resulted in relatively elevated Pb, Pd, Pt and Zn concentrations and non-radiogenic Pb isotope ratios in local coastal stream and estuarine sediments. The investigation demonstrates that traffic-derived metals enter coastal stream and estuary sediments at the fringe of the Great Barrier Reef lagoon.     

Geochemical records of the sediments and their significance in Dongping Lake Area,the lower reach of Yellow River,North China

Dongping Lake area, located in the lower reaches of Yellow River, is an ideal place to study the changes of modern river and lake sedimentary environment. The sediment samples of Dawen River, Yellow River, and Dongping Lake were collected, and the major elements, trace elements and organic matter geochemical composition of the samples were analyzed. Cluster analysis, characteristic element ratio method and graphic method were used to explore the geochemical characteristics of sediments and their environmental implication. The results show that the contents of SiO_2, Na_2O, TiO_2 and Zr in sediments of Dawen River and Yellow River are relatively high, and the contents of iron and manganese oxides, organic matter, CaO, P_2O_5 and Sr in lake sediments are relatively high. That reveals the differences of sedimentary environments between the rivers and the lake. The contents of Sr and Zr in Dawen River are affected by the rapid migration of clastic materials in the upstream carbonate source area during the flood season; the δCe,ΣREE and REE's ratios in the sediments of the Yellow River reflect the influence of the Loess source; and the distribution of elements changes along the flow direction during the flood season. The characteristics of p H, element composition and LREE HREE fractionation of the lake sediments indicate that the sediment source is complex, and the lake environment is affected by the flood season. The study shows that the geochemical content and its variation characteristics of sediments effectively reveal the sedimentary environment, material composition and characteristics of flood season of rivers and the lake in the study area.     

Arsenic Levels in Groundwater from Quaternary Alluvium in the Ganga Plain and the Bengal Basin, Indian Subcontinent: Insights into Influence of Stratigraphy

Late Quaternary stratigraphy and sedimentation in the Ganga Alluvial Plain and the Bengal Basin have influenced arsenic contamination of groundwater. Arsenic contaminated aquifers are pervasive within lowland organic rich, clayey deltaic sediments in the Bengal Basin and locally within similar facies in narrow, entrenched river valleys within the Ganga Alluvial Plain. These were mainly deposited during early-mid Holocene sea level rise. Arsenic was transported from disseminated sources as adsorbed on dispersed phases of hydrated-iron-oxide. These were preferentially entrapped as sediment coatings on organic-rich, fine-grained deltaic and floodplain sediments. Arsenic was released later to groundwater mainly by reductive dissolution of hydrated-iron-oxide and corresponding oxidation of sediment organic matter. Strong reducing nature of groundwater in the Bengal Basin and parts of affected middle Ganga floodplains is indicated by high concentration of dissolved iron (maximum 9-35 mg/l). Groundwater being virtually stagnant under these settings, released arsenic accumulates and contaminates groundwater. The upland terraces in the Bengal Basin and in the Central Ganga Alluvial Plain, made up of the Pleistocene sediments are free of arsenic contamination in groundwater. These sediments are weakly oxidised in nature and associated groundwater is mildly reducing in general with low concentration of iron (<1 mg/l), and thus incapable to release arsenic. These sediments are also flushed free of arsenic, released if any, by groundwater flow due to high hydraulic head, because of their initial low-stand setting and later upland terraced position.     

平水期洞庭湖不同形态磷赋存特征

通过现场调查和室内实验,对平水期洞庭湖24个断面上覆水-沉积物磷的污染程度、分布特征进行研究;同时,分析了各形态磷间的相关性,探讨了磷的赋存形态对水体富营养化的影响、沉积物释放量和释放风险。结果表明,水体内总磷分布规律为南洞庭西洞庭东洞庭大通湖,东洞庭湖以颗粒态磷为主,西洞庭湖、南洞庭湖、大通湖均以溶解态磷、无机磷为主。沉积物中总磷的分布规律为南洞庭西洞庭东洞庭大通湖。东洞庭湖、西洞庭湖、南洞庭湖则以Ca-P为主,大通湖以Fe/Al-P为主。Ca-P与Fe/Al-P呈现较显著的负相关关系。大通湖释放风险较大,可释放量大;东洞庭湖有一定的可释放量,应对这两个湖泊给予较多的关注。与国内其他湖泊比较,洞庭湖沉积物中TP处于中下水平,IP在沉积物中占据较高比例。湖区沉积物中Fe/Al-P与水体中DIP、DTP、TP均具有较显著的负相关关系,存在磷的内源释放。