Dissertations / Theses on the topic 'Groundwater – Arsenic content – Vermont'

Volcanic tuffs and tuffaceous sediments are frequently associated with elevated As groundwater concentrations even though their bulk As contents (~ 5 mg kg-1; Savoie, 2013) are only marginally greater than the average crustal abundance of 4.8 mg g-1 (Rudnick & Gao, 2003). Thus, As mobilization must be facilitated by conditions particular to these rocks. Alkaline desorption, anionic competition, reactive glass dissolution, and reductive dissolution of iron oxides are proposed processes of As release from volcanic rocks. Geogenic As contamination of groundwater in the southern Willamette Valley in western Oregon has been well-documented since the early 1960s, and previous studies have identified the Little Butte Volcanics Series and Fisher and Eugene Formations as the source of As contamination. This study examines 19 samples from 10 units of ash flow tuffs and tuffaceous sediments within the Fisher Formation and Little Butte Volcanics Series, representing a range of weathering and devitrification, to determine conditions of mobilization and mineralogical constraints that control As release into solution. Leachate studies were conducted over a range of pH from 7 to 11, phosphate concentrations from 10 μM to 100 mM, and in time series from 4 to 196 hours. Results demonstrate that silicic volcanic tuffs are capable of mobilizing As in concentrations above regulatory limits at pH conditions produced naturally by the tuffs (pH 8-9) or with moderate concentrations of P (10-100 μM). Alteration products, e.g. zeolites and clays, appear to be the primary host phases for mobile As. Samples that do not contain these alteration products tend to produce concentrations of As well below regulatory limits and often below the instrument detection limits of this study. The type of alteration may influence As mobilization: tuffs containing more clays tend to mobilize As through surficial desorption, and tuffs containing more zeolites tend to mobilize As by dissolution or formation of colloids. Additionally, one volcaniclastic sample demonstrates that extremely elevated concentrations of As, up to 1000 μg/L are possible as a result of oxidative dissolution of As-bearing sulfide phases.

碩士
朝陽科技大學
環境工程與管理系
106
According to the EPA survey, most of the main causes of arsenic pollution in agricultural land in China are caused by long-term arsenic-based groundwater irrigation. The pollution range is mainly distributed in the Lanyang Plain, the Zhuoshuixi Alluvial Fan, the Jianan Plain and the Pingtung Plain, covering a wide area. In the face of so many arsenic-rich agricultural land caused by geological factors, the cost of rectification is quite large. If the geological factors are not improved, the safety of planting edible vegetables in arsenic-rich soils needs to be evaluated. Biochar has the function of changing soil properties and carbon sequestration, and is agronomic material that is currently actively promoted in agriculture and environmental protection. In this study, biochar was prepared from agricultural waste rice husks, and biochar was applied at a weight ratio of 0%, 5%, 10%, and 15% to investigate the effect of biochar application on the absorption of arsenic in vegetable crops. In addition, reducing the arsenic concentration of irrigation water can slow down the accumulation rate of arsenic concentration in agricultural land. In this study, twelve kinds of materials were selected for the adsorption test of arsenic-rich aqueous solution, and the adsorption capacity of different materials for arsenic was explored, thereby developing a treatment technology for reducing arsenic in irrigation water. The results showed that in addition to hydroponic water, the application of biochar in the other five experimental vegetable crops could increase the yield, and the arsenic content of the edible parts of the five vegetable crops was lower than the white rice inorganic arsenic consumption limit published by the Ministry of Health and Welfare. Mg/kg, showing that arsenic-rich agricultural soils are planted with vegetable crops, and the cumulative concentration of arsenic in edible parts is not high. The arsenic content of water amaranth is much higher than that of the other five vegetable plants. It is recommended that arsenic-rich agricultural land should not plant aquatic food crops as much as possible. The application of biochar has a tendency to increase the arsenic concentration in the roots and shoots of crops. If phyto-contaminated soil is improved by phytoremediation, the application of biochar has the potential to accelerate the rate of arsenic removal. The ultra-deep-burning ceramsite made in Taiwan has a very high adsorption capacity for arsenic in both bulk and powder form. For 0.25-1.0 ppm arsenic aqueous solution, the arsenic removal efficiency can reach more than 99%. The self-prepared calcium chloride modified biochar can also remove more than 60% of arsenic. In the future, it can continue to explore its feasibility as an arsenic removal technology for irrigation water.

Sulphate-reducing bacteria (SRB) mediate the reduction of metals/metalloids directly or indirectly. Bioremediation of arsenic contaminated water could be a cost-effective process provided a cheap carbon source is used. To this end, molasses was tested as a possible source of carbon for the growth of sulphate-reducing bacteria (SRB). Its chemical composition and the tolerance of SRB toward different arsenic species [As (III) and As (V)] were also investigated. Batch culture studies were carried out to assess 1, 2.5 and 5 g l-1 molasses as suitable concentrations for SRB growth. The results indicate that molasses does support SRB growth, the level of response being dependent on the concentration; however, growth on molasses was not as good as that obtained when lactate, the usual carbon source for SRB, was used. The molasses used in this study contained several metals including Al, As, Cu, Fe, Mn and Zn in concentrations ranging from 0.54-19.7 ìg g-1, but these levels were not toxic to the SRB. Arsenic tolerance, growth response and sulphate-reducing activity of the SRB were investigated using arsenite and arsenate solutions at final concentrations of 1, 5 and 20 mg l-1 for each species. The results revealed that very little SRB growth occurred at concentrations of 20 mg l-1 As (III) or As (V). At lower concentrations, the SRB grew better in As (V) than in As (III). Batch cultures of sulphate-reducing bacteria (SRB) in flasks containing pine bark, sand and polystyrene as support matrices and Postgate medium B were used to study formation of biofilms. The effects of the support matrices on the growth of the organisms were evaluated on the basis of pH and redox potential change and the levels of sulphide production and sulphate reduction. Characterisation of the matrix surfaces was done by means of environmental scanning electron microscopy (ESEM). A consortium of SRB growing on polystyrene caused a 49% of original sulphate reduction whereas on sand a 36% reduction occurred. Polystyrene was further examined for its durability as a long-term support material for the growing of SRB in the presence of As(III) and/or As(V) at concentrations of 1, 5 and 20 mg l-1. Both sulphate reduction and sulphide production were greater in this immobilised system than in the matrix-free control cultures. With pine bark as support matrix no significant sulphate reduction was observed. The kinetics of sulphate reduction by the immobilised cells were compared with those of planktonic SRB and found to be superior. The leaching of organic compounds, particularly phenolic substances, from the pine bark had a detrimental effect on the growth of the SRB. Different proportions of pine bark extract were used to prepare media to investigate this problem. Growth of SRB was totally inhibited when 100% pine bark extract was used. Analysis of these extracts showed the concentration of phenolics increased from 0.33 mg l-1 to 7.36 mg l-1 over the extraction interval of 15 min to 5 days. Digested samples of pine bark also showed the presence of heavy metals. The effects of nitrate, iron and sulphate and combinations thereof were investigated on the growth of a mixed culture of sulphate-reducing bacteria (SRB). The addition of 30 mg l-1 nitrate does not inhibit the production of sulphide by SRB when either 50 or 150 mg l-1 sulphate was present. The redox potential was decreased from 204 to -239 mV at the end of the 14 day batch experiment in the presence of 150 mg l-1 sulphate and 30 mg l-1 nitrate. The sulphate reduction activity of the SRB in the presence of 30 mg l-1 nitrate and 100 mg l-1 iron was about 42% of original sulphate, while if no iron was added, the reduction was only 34%. In the presence of 20 mg l-1 either As(III) or As(V), but particularly the former, growth of the SRB was inhibited when the cells were cultured in modified Postgate medium in the presence of 30 mg l-1 nitrate. The bioremoval of arsenic species [As(III) or As(V)] in the presence of mixed cultures of sulphate-reducing bacteria was investigated. During growth of a mixed SRB culture adapted to 0.1 mg l-1 arsenic species through repeated sub-culturing, 1 mg l-1 of either As(III) or As(V) was reduced to 0.3 and 0.13 mg l-1, respectively. Sorption experiments on the precipitate produced by batch cultured sulphate-reducing bacteria (SRB-PP) indicated a removal of about 77% and 55% of As(V) and As(III) respectively under the following conditions: pH 6.9; biomass (2 g l-1); 24 h contact time; initial arsenic concentration,1 mg l-1 of either species. These results were compared with synthetic iron sulphide as adsorbent. The adsorption data were fitted to Langmuir and Freundlich isotherms. Energy dispersive x-ray (EDX) analysis showed the SRB-PP contained elements such as sulphur, iron, calcium and phosphorus. Biosorption studies indicated that SRB cell pellets removed about 6.6% of the As(III) and 10.5% of the As(V) from water containing an initial concentration of 1 mg l-1 of either arsenic species after 24 h contact. Arsenic species were precipitated out of synthetic arsenic-contaminated groundwater by reacting it with the gaseous biogenic hydrogen sulphide generated during the growth of SRB. The percentage removal of arsenic species was dependent on the initial arsenic concentration present. Lastly, laboratory scale bioreactors were used to investigate the treatment of arsenic species contaminated synthetic groundwater. A mixed culture of SRB with molasses as a carbon source was immobilised on a polystyrene support matrix. The synthetic groundwater contained either As(III) or As(V) at concentrations of 20, 10, 5, 1 or 0.1 mg l-1 as well as 0.1 mg l-1 of a mixture with As(III) accounting for 20, 30, 40, 60 and 80% of the total. More that 90% and 60% of the As(V) and As(III) respectively were removed by the end of the 14-day experiment. At an initial concentration of 0.1 mg l-1 total arsenic had been reduced to below the WHO acceptable level of 10 ìg l-1 when the proportion of As(III) was 20 and 30%, while at 40% As(III) this level was reached only when the treatment time was increased to 21 days. The efficiency of As(III) removal was increased by first oxidising it to As(V) using MnO2.
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.

Chronic exposure to naturally occurring arsenic (As) in groundwater threatens the health of >150 million villagers in S/SE Asia. In Bangladesh, low As aquifers offer the best hope of reducing the exposure of 35-40 million remain exposed to elevated levels of As in drinking water (>10 μg/L). These low As aquifers could be affected, however, by massive pumping from shallow (<30 m) depths for growing rice and overexploitation of deeper aquifer for municipal water supply. The goal of this dissertation is to assess the impacts of groundwater pumping on the distribution in groundwater of dissolved As, reactive carbon, and redox-sensitive elements in anoxic aquifers of Bangladesh based on long-term hydrologic measurements, geochemical analyses, and numerical flow modeling. In the second chapter, changes in the well-water As concentrations within a 25 sq. km area over a 10+ year timespan are assessed on the basis of continuous time series for 18 monitoring wells, a set of 271 wells resampled three times, and a large dataset obtained from blanket surveys of several thousand wells in the region. The two larger data sets both show a 10% decline in the initial areal mean As of 100 μg/L. This decline can be explained by flushing of As in the shallow aquifer by low-As recharge water, evidently compensated to some extent by the desorption of sediment-bound As. The presence of a large exchangeable pool of As in the sediment therefore seems to buffer changes in the distribution of As in the face of large perturbation in groundwater flow, albeit not enough to prevent some trends indicated by the detailed time series. The third chapter provides a complementary perspective on groundwater-sediment interactions by quantifying the rates of adsorption and desorption of As with column experiments conducted in the field for two different types of sediments: grey reduced Holocene sands and orange oxidized Pleistocene sands. The data show that, contrary to widely held beliefs, retardation of As transport by adsorption is quite similar in Holocene and Pleistocene sediments, even if Holocene sands initially contain a much larger pool of easily mobilizable As. The field column experiments also showed significant changes in solid phase speciation that affected As retention within a timespan of only a few weeks. Detailed field observations and flow modeling in the fourth chapter examine how perturbed flow paths can draw either As or reactive carbon into a Pleistocene aquifer. A groundwater flow model, constrained by head measurements and isotopic tracer data shows that certain portions of the aquifer are becoming increasingly contaminated with As as a result of municipal pumping, but against a background of redox transformation in the aquifer that probably preceded this perturbation. Overall, the research conducted for this thesis shows that alteration of the hydrological system due to local and regional forcing is affecting the distribution of As in groundwater. These changes do not affect all wells yet and, if they do, the increase in As concentrations observed so far are gradual because of the buffering capacity of the sediment. Lowering exposure by targeting low As aquifer should therefore definitely continue in Bangladesh, with particular attention paid to regular monitoring using vulnerability criteria this research has helped to identify.

Arsenic is a prevalent contaminant at a large fraction of US Superfund sites. Therefore, establishing techniques for accelerating As remediation could benefit many contaminated sites. Remediation of As contaminated groundwater by conventional methods, i.e. pump and treat (P&T), can be impeded by slow desorption of As from Fe and Al (hydr)oxides in aquifer solids. Through experimentation at different physical scales (grain, bench, and field scale), the potential for chemical additions to increase As release from sediments and possibly accelerate P&T remediation is examined. The work described here focuses on As contamination and remediation at the Vineland Chemical Co. Superfund site in southern NJ. The site is extensively contaminated with As resulting from decades of poor chemical storage and disposal practices by the Vineland Chemical Co., which manufactured As-based biocides from 1949-1994. Despite significant intervention, including groundwater remediation by P&T and treatment of solids via soil washing, sufficient site clean up could require many decades with current technologies. Chemical amendments that either compete with As for sorption sites or dissolve Fe and Al (hydr)oxides can increase As mobility and potentially improve P&T remediation efficiency. Simple extrapolations from bench scale column experiments based on pore volumes suggest that treatment with 10 mM oxalic acid could lower the time necessary for clean up at the Vineland site from 600 years (with current techniques involving just groundwater) to potentially on the order of 4 years. Small scale (<1 mm2) X-ray fluorescence maps from columns performed within the synchrotron beamline showed As release during oxalic acid treatment that was consistent with the bulk column materials and suggested that microscale processes can be predictive of the larger system. Finally, during a 3-month pilot study at the Vineland site, oxalic acid was injected into a section of the aquifer via an injection manifold system that was designed and built for the experiment. Groundwater samples indicate that introduction of oxalic acid led to increased As release at a sampling well and pump and treat recovery well in the study area. Addition of oxalic acid shows promise for accelerating treatment of a highly contaminated site.

M.Sc. (Geology)
Most rural communities in developing countries rely on borehole water as their only source of water. Since borehole water comes from underground, it is often considered pure and clean, but this is frequently not the case. Groundwater contains certain amounts of trace elements that may become deleterious to human health. The objectives of this investigation were to assess the concentration levels of Potential Harmful Trace Elements (PHTEs) and their spatial distribution patterns in borehole water in the Greater Giyani area of Limpopo, South Africa, and the potential human health risks associated with this. The method of research comprised two phases: (I) In the first phase, I assessed the occurrence and distribution patterns of PHTEs in the boreholes of the Giyani area. A total of 29 water samples were collected from boreholes (including 15 community boreholes and 14 primary school boreholes) in the Greater Giyani area during the dry season (July/August 2012), and for comparison another 27 samples (including 15 community boreholes and 12 schools boreholes) from the same localities during the wet season (March 2013). The samples were analysed for the trace elements arsenic (As), cadmium (Cd), chromium (Cr), selenium (Se) and lead (Pb) using the Inductively Coupled Plasma Mass-Spectrometry (ICPMS) technique. In order to assess the groundwater quality, PHTEs concentrations were compared with the South African National Standard of Drinking water (SANS 241-1:2011). (II) In the second phase, I evaluated the geographic variation between PHTEs and associated human health effects. This involved acquisition of data on a total of 100 cancer cases recorded during the period 2011-2014 at Nkhensani Hospital. ArcGIS Spatial analyst tool was used to create thematic maps illustrating spatial distribution of clinical data and arsenic concentrations in boreholes.

博士
國立成功大學
地球科學系
104
Arsenic (As), a toxic substance in the environment, is a major public health concern worldwide. High concentrations of As have also been linked to cancers of the nasal cavity, lung, liver, bladder, kidney, and prostate, and can lead to hypertension and diabetes. Peripheral vascular gangrene, also known as Blackfoot disease (BFD), was first reported in Chianan Plain of southwestern Taiwan in the 1960s. Since 1980, As-rich groundwater has no longer been consumed as drinking water in Taiwan, although it is still widely used for irrigation, aquacultural, and industrial purposes, and hence deserves our continued attention. Rice is the staple food for nearly one-half of world’s population including those living in Taiwan, China, Japan, Korea, and other Asian countries. However, rice uptakes As into grains are much more readily occurred than other cereal crops. The present study geochemically investigates As-rich groundwater, soil, and rice plants from paddy fields in Chianan Plain of southwestern Taiwan, an area which is mainly used for rice cultivation. The stable isotopes of oxygen-18 and deuterium were used to identify different sources of water in a soil layer in rice paddy during the rice growing season in 2014. This study can help us understand that the level of As in rice plants can be affected by the groundwater used for irrigation, type and concentration of As in the soil, and soil properties of paddy fields. In addition, experimental results by means of stable isotopes technique clarified the infiltration of rainfall in the complicated process that can be affected by soil structure, texture, moist and extent of heterogeneity. Results show that the total As concentrations in the groundwater used for irrigation of the sampled paddy fields at Hsuechia, Yichu, and Budai in the Chianan Plain are in the range of 32.9 to 175.7 μg L–1, which is higher than the permissible drink limit (10 μg L–1) recommended by the World Health Organization (WHO) and irrigation limit (50μg L–1) recommended by agricultural authorities of Taiwan. The percentages of As in different parts of the rice plants found in the current study are in the range of 88.3 to 92.8% in roots, 2.8 to 4% in shoots, 1.5 to 5.2% in leaves, 1 to 1.7% in husks, and 0.8 to 1.2% in grains. This study investigated the impacts of various types of irrigation practices with As-contaminated groundwater on the extent of As accumulation within rice plant parts during development and rice crop production at maturity. Three types of irrigation practices were applied to As-rich paddy fields: flooded irrigation, aerobic irrigation, and alternate wetting and drying irrigation (AWDI). Results show that the arsenic concentration in different parts of rice plants varied with growth stage and irrigation practices in both cropping seasons. Lower levels of As in rice were found in AWDI and aerobic irrigation than in flooded irrigation. Different irrigation practices can change the oxidation and reduction conditions of the paddy field, which lead to As release or absorption in the soil, thus influencing the uptake of As by plants. The chemical immobilization of As-rich soil by using ethylenediaminetetraacetic acid ferric sodium salt (EDTA-Fe) and calcium peroxide (CaO2) as stabilizing agents was investigated in Chianan Plain of southwestern Taiwan. The As-rich soil was amended with EDTA-Fe, at the rates of 0, 0.35, 0.7 and 1.4 Mg/ha, or with CaO2, at the rates of 0, 0.38, 0.76 and 1.52 Mg/ha, and grown with radish (Raphanus sativus L.), lettuce (Lactuca sativa), Chinese cabbage (Brassica rapa) and Arden lettuce (Lactuca sativa L.). Results showed that those amended with EDTA-Fe at 0.35 and 0.7 Mg ha−1 can significantly reduce As accumulation in vegetables. Moreover, the uptake of As in vegetables decreased more in soil amended with CaO2 relative to that amended with EDTA-Fe. As indicated, applications of EDTA-Fe at the rate of 0.7 Mg ha−1 and CaO2 at the rate of 1.52 Mg ha−1 can obtain optimal effect on suppressing As uptake by vegetables. Present study aimed to assess the presence and availability of As in paddy field of the Chianan Plain. Arsenic content was determined in soils and pore water sampled at 5 sampling depths (20, 40, 60, 80, 90 cm) in paddy field. The As concentrations in the experimental field soil varied slightly with sampling depths. Total As concentration positively and significantly (p 〈 0.05) correlated with the soil properties including free Fe (Fed), free Mn (Mnd), amorhous Fe (Feo), amorhous Mn (Mno), cation capacity exchange, and clay content. The sequential extraction of soil As showed that As was mainly fixed by both amorphous and crystalline Fe/Mn oxides, which may be ascribable to that As has a high affinity for Fe/Mn oxides under reduction regime of groundwater. As a consequence, As probably would become soluble due to the usage of groundwater and the change of redox regime. In addition, coarse texture of study soil, the flow path of irrigation water, and the gravity impact are factors affecting As movement in the soil layers. The soil As was concentrated on the surface soil and gradually decreased with soil depth. Therefore, total As in subsoils was lower than topsoil because As was adsorbed and accumulated by abundant Fe/Mn oxides in surface soil after irrigation. Based on the measured deuterium and oxygen-18 in soil water, rainwater and groundwater in the paddy field of the Chianan Plain of southwestern Taiwan, in the wet season, the relationship between δD and δ18O in soil water and groundwater recharge after typhoon rainfall (event water) was investigated in the present study. The soil water at different depths before and after event water varied in hydrogen and oxygen isotope ratios. The top soil layer (〈 30 cm depth) had more depleted isotopic compositions as a result of the higher rate of evaporation. Similar soil water isotope composition profiles were observed in shallow soil layers. More depleted fractions of isotopes were found in groundwater as those in rainwater, suggesting that the groundwater primarily came from the rainwater. However, the isotope compositions of hydrogen and oxygen in groundwater is still slightly deviated from the local meteoric water line in southwestern Taiwan. The proposed conceptual model for the interaction among soil, plant and arsenic in the study area can help understand the mechanism that arsenic was released in the subsurface environment. The remedial measure and strategy for reducing arsenic content in selected crops is also proposed in this research.

Groundwater arsenic contamination is currently a global problem, and also a concern at numerous former industrial sites, agricultural sites, landfill sites and mining operations in the U.S. This dissertation aims to develop improved strategies of remediating these arsenic contaminated aquifers. It focuses on two distinct approaches of remediation: (1) mobilizing arsenic from contaminated aquifer sediments to decrease the quantity of arsenic at the source of contamination; and (2) immobilizing arsenic in situ, to decrease the mobility and bioavailability of this arsenic. Optimal remediation may well involve combinations of these two approaches. Arsenic mobilization using oxalic acid is effective because oxalic acid dissolves arsenic host minerals and competes for sorption sites on those minerals. In this dissertation, oxalic acid treatment was tested using sediments with contrasting iron mineralogies and arsenic contents from the Dover Municipal Landfill and the Vineland Chemical Company Superfund sites. Oxalic acid mobilized arsenic from both sites and the residual sediment arsenic was less vulnerable to microbial reduction than before the treatment. Oxalic acid thus could improve the efficiency of widely used pump-and-treat remediation. Oxalic acid did not remove all of the reactive iron(III) minerals in Vineland sediment samples, and thus released significant quantities of arsenic into solution under reducing conditions than the Dover samples. Therefore, the efficacy of pump-and-treat must consider iron mineralogy when evaluating its overall potential for remediating groundwater arsenic. Arsenic immobilization occurs by changing the chemical state, or speciation, of arsenic and other elements in the system. Arsenic is often assumed to be immobile in sulfidic environments. In this dissertation, sulfate reduction was stimulated in sediments from the Vineland Superfund site and the Coeur d'Alene mining district. Sulfate reduction in the Coeur d'Alene sediments was more effective at removing arsenic from solution than the Vineland sediments. The Vineland sediments initially contained abundant reactive ferrihydrite, and underwent extensive sulfur cycling during incubation. As a result, arsenic in the Vineland sediments could not be effectively converted to immobile arsenic-bearing sulfides, but instead a part of the arsenic was probably converted to soluble thioarsenates. Therefore, coupling between the iron and sulfur redox cycles must be fully understood for arsenic immobilization by sulfate reduction to be successful. Arsenic can also be immobilized by retention on magnetite (Fe3O4). Magnetite is stable under a wide range of aquifer conditions including both oxic and iron(III)-reducing environments. In this dissertation, a series of experiments were performed with sediments from the Dover and Vineland Superfund sites, to examine the potential of magnetite for use in arsenic immobilization. Our data suggest that the formation of magnetite can be achieved by the microbial oxidation of ferrous iron with nitrate. Magnetite can incorporate arsenic into its structure during formation, forming a stable arsenic sink. Magnetite, once formed, can also immobilize arsenic by surface adsorption, and thus serve as a reactive filter when contaminated groundwater migrates through the treatment zone. Reactive transport modeling is used for investigating the magnetite based arsenic immobilization strategy and for scaling laboratory results to field environments. Such modeling suggests that the ratio between iron(II) and nitrate in the injectant regulates the formations of magnetite and ferrihydrite, and thus regulates the long-term evolution of the effectiveness of the strategy. The results from field-scale models favor scenarios that rely on the chromatographic mixing of iron(II) and nitrate after injection. The studies in this dissertation demonstrate that the environmental fate of arsenic depends on the biogeochemical cycling of arsenic, iron, and to a lesser extent, sulfur. The development of effective groundwater arsenic remediation strategies depends on a good understanding of each of the involved processes, and their combinations.

The worldwide natural occurrence of high levels of arsenic (As) in groundwater and its deleterious effects on human health have inspired a great amount of related research in public health and geosciences internationally. With >100 million people in South and Southeast Asia exposed to >10 µg/L As in shallow groundwater that they use for drinking, the installation of deeper, low-As wells has emerged as a major strategy for lowering the exposure. As the magnitude of deep pumping continues to increase, this work focuses on the geochemical and hydrologic questions surrounding the vulnerability and sustainability of low-As aquifers in Bangladesh, the country most affected by As crisis. In an effort to better understand the residence time of groundwater in low-As aquifers at depth, radiocarbon (14C) and 13C in dissolved inorganic carbon, tritium (3H), stable isotopes of hydrogen (2H) and oxygen (18O), and noble gas concentrations were measured across a ~25 km2 area of Araihazar, ~30 km east of Dhaka. Groundwater from >120 m depth is shown to be ~10,000 years old and its isotopic signatures indicate that recharge occurred at the time of changing climate from the late Pleistocene to early Holocene, with little recharge occurring since. In contrast, the intermediate depth low-As aquifers (<120 m) have a heterogeneous distribution of groundwater chemistry and ages, and contain groundwater recharged <60 years ago in certain locations. In one such area surrounding a small village, the effects that subsurface clay layer distribution has on recharge patterns and redox status of the intermediate aquifer was investigated. The relevant hydrogeologic and geochemical processes that led to documented failures of a community well at the site were assessed using a combination of solid and water phase geochemistry with tritium-helium (3H/3He) dating, hydraulic head monitoring, and pumping tests. Organic matter seeping from a compressible clay layer, which is subject to a pumping-induced, downward hydraulic gradient, reduces iron oxides and helps release As in the grey, upper part of the intermediate aquifer. No recent recharge was detected by 3H measurements in the upper, grey sand layer, however a layer of orange sand beneath it contains groundwater that was recharged 10-60 years ago. This groundwater laterally bypasses the confining clay layer to recharge the middle of the aquifer and contains dissolved As levels of <10 µg/L. In this particular case, the pore water that leaches from clay layers contributes to As contamination, whereas the lateral recharge with shallow groundwater coincides with the low-As depth. Thus, clay layers may not always protect the low-As aquifers from As contamination, even if they can block direct vertical recharge with shallow groundwater enriched in As and organics. Finally, the adsorption of As to aquifer sediments, as a natural mechanism of the low-As aquifer defense against contamination, was assessed in the field via a column study. The column experiments were conducted by pumping shallow, high-As groundwater through freshly collected sediment cores to quantify the retardation of As transport through the aquifer. This study demonstrated an elegant method of assessing contaminant transport under nearly in situ conditions that resulted in sorption estimates similar to those made by field studies using more challenging methods or located at hard-to-find sites with convenient flow patterns. My work, therefore, contributed to a better understanding of low-As aquifers in Bangladesh from the perspectives of both the groundwater flow and water-sediment interactions on various scales, and it integrated methods that can be employed elsewhere to characterize aquifers and study contaminant transport.

PhD (Environmental Sciences)
Department of Ecology and Resource Management
The presence of arsenic in groundwater has drawn worldwide attention from researchers and public health officials due to its effects on human health such as, cancer, skin thickening, neurological disorders, muscular weakness, loss of appetite and nausea. World Health Organisation (WHO) has set the limit of 10 μg/L for arsenic in drinking water in trying to reduce the effects of arsenic. This was further adopted by South African National Standard (SANS). The present study aims at evaluating arsenic concentration in selected groundwater sources around Greater Giyani Municipality in Limpopo Province and further synthesize clay based adsorbents for arsenic removal using Fe3+ and Mn2+ oxides and hexadecylammonium bromide (HDTMA-Br) cationic surfactant as modifying agents. The first section of the work presented the hydrogeochemical characteristics of groundwater in the Greater Giyani Municipality. The results showed that the pH of the samples ranges from neutral to weakly alkaline. The dominance of major anionic and cationic species was found to be in the order: HCO3 ->Cl->SO4 2->NO3 - and Na+>Mg2+>Ca2+>K+>Si4+, respectively. Hydrogeochemical facies identified in the study area include CaHCO3 (90%) and mixed CaNaHCO3 (10%) which shows the dominance of water-rock interaction. About 60% of the tested samples contains arsenic concentration above 10 μg/L as recommended by SANS and WHO. Concentration of arsenic was found to be ranging between 0.1 to 172.53 μg/L with the average of 32.21 μg/L. In the second part of this work, arsenic removal efficiency of locally available smectite rich and kaolin clay was evaluated. Results showed that the percentage As(V) removal by kaolin clay was optimum at pH 2 while the percentage As(III) removal was greater than 60% at pH 2 to 12. For smectite rich clay soils, the percentage of As(III) and As(V) removal was found to be optimum at pH between 6 and 8. The adsorption isotherm data for As(III) and As(V) removal by both clays fitted better to Freundlich isotherm. Adsorption of both species of arsenic onto the clay mineral occurred via electrostatic attraction and ion exchange mechanisms. Both clay soils could be regenerated twice using Na2CO3 as a regenerant. Kaolin clay showed a better performance and was selected for further modification. In the third section of this work, Fe-Mn bimetal oxide modified kaolin clay was successfully synthesized by precipitating Fe3+ and Mn2+ metal oxides to the interlayer surface of kaolin clay. Modification of kaolin clay increased the surface area from 19.2 m2/g to 29.8 m2/g and further v decreased the pore diameter from 9.54 to 8.5 nm. The adsorption data fitted to the pseudo second order of reaction kinetics indicating that adsorption of As(III) and As(V) occurred via chemisorption. The adsorption isotherm data was described by Langmuir isotherm models showing a maximum As(III) and As(V) adsorption capacities of 2.16 and 1.56 mg/g, respectively at a temperature of 289 K. Synthesized adsorbent was successfully reused for 6 adsorptiondesorption cycles using K2SO4 as a regenerant. Column experiments showed that maximum breakthrough volume of ≈2 L could be treated after 6 hours using 5 g adsorbent dosage. Furthermore, the concentration of Fe and Mn were within the WHO permissible limit. In the fourth part of the work kaolin clay was functionalized with hexadecyltrimethylamonium bromide (HDTMA-Br) cationic surfactant and its application in arsenic removal from groundwater was investigated. The results revealed that adsorption of As(III) and As(V) is optimum at pH range 4-8. The maximum As(III) and As(V) adsorption capacities were found 2.33 and 2.88 mg/g, respectively after 60 min contact time. Pseudo first order model of reaction kinetics described the adsorption data for As(V) better while pseudo second order model described As(III) adsorption data. The adsorption isotherm data for As(III) and As(V) fitted well to Langmuir model indicating that adsorption of both species occurred on a mono-layered surface. Adsorption thermodynamics model revealed that adsorption of As(III) and As(V) was spontaneous and exothermic. The As(III)/As(V) adsorption mechanism was ascribed to electrostatic attraction and ion exchange. The regeneration study showed that synthesized adsorbent can be used for up to 5 times. In the firth part of the work inorgano-organo modified kaolin clay was successfully synthesized through intercalation of Fe3+ and Mn2+ metal oxides and HDTMA-Br surfactant onto the interlayers of the clay mineral. The batch experiments showed that As(III) removal was optimum at pH range of 4-6, while the As(V) removal was optimum at pH range 4-8. The adsorption data for both species of arsenic showed a better fit to pseudo second order of reaction kinetics which suggest that the dominant mechanism of adsorption was chemisorption. The isotherm studies showed better fit to Langmuir isotherm model as compared to Freundlich model. The maximum adsorption capacity As(III) and As(V) at room temperature as determined by Langmuir model were found to be 7.99 mg/g and 7.32 mg/g, respectively. The thermodynamic studies for sorption of As(III) and As(V) showed negative value of ΔGᴼ and ΔHᴼ indicating that adsorption process occurred spontaneously and is exothermic in nature. The regeneration study showed that the vi inorgano-organo modified kaolin clay can be reused for up 7 adsorption-regeneration cycles using 0.01 M HCl as a regenerant. Thomas kinetic model and Yoon-Nelson model showed that the rate of adsorption increases with increasing flow rate and initial concentration and decreases with increasing of the bed mass. In conclusions, adsorbents synthesized from this work showed a better performance as compared to other adsorbents available in the literature. Among the synthesized adsorbents, inorgano-organo modified clay showed highest adsorption capacity as compared to surfactant functionalized and Fe-Mn bimetal oxides modified kaolin clay. However, all adsorbents were recommended for use in arsenic remediation from groundwater. The following recommendations were made following the findings from this study: 1) routine monitoring of arsenic in groundwater of Greater Giyani Municipality, 2) evaluating the possible link between arsenic exposure and arsenic related diseases within Giyani in order to find the extent of the problem in order to establish the population at risk, 3) The toxicity assessment for HDTMA-Br modified kaolin clay should be carried out, 4) Materials developed in the present study should be modeled and tested at the point of use for arsenic removal, and lastly, 5) this study further encourage the development of other arsenic removal materials that can be used at household level.
NRF