江汉平原高砷含水层沉积物地球化学特征

选取江汉平原典型地下水砷中毒区仙桃市沙湖原种场为研究区,对3个长50m的钻孔沉积物砷含量与赋存形态及其他化学组成进行了分析。结果表明区内沉积物砷质量分数为1.35~107.5mg/kg(平均值为12.8mg/kg)。黏土或亚黏土层中砷含量较高,这与细粒沉积物中铁锰氧化物、黏土矿物对砷的吸附有关。地下20m左右深度内含水层沉积物中砷含量最高,相应地下水中砷质量浓度高达到1 000μg/L。草酸-草酸铵选择性提取结果指示沉积物中10%~77%(平均38%)的As与无定形铁氢氧化物结合,表明无定形铁氢氧化物还原性溶解可能是控制砷释放与还原的主要地球化学过程,并且有机质生物氧化机制极大地促进了该过程。然而,沉积物中仅1.2%~23%的铁被草酸-草酸铵提取,含水层中砷浓度主要受铁的氢氧化物还原性溶解影响,但其他形式的铁、有机物的吸附作用亦控制着砷的含量。     

大同盆地浅层含水层沉积物中砷的生物可利用性评价

以采自大同盆地野外试验场的含水层沉积物为研究对象,调查高砷含水层中生物可利用性砷的含量及其影响因素。经实验室测定,沉积物总体呈碱性,Al、Fe等金属元素含量较高。利用连续提取法发现沉积物中砷结合形态主要为无定形和低结晶铁铝氧化物结合态,其次是残余态砷。其中铁铝氧化物形态的砷所占比例达53.54%,非专性吸附态和专性吸附态占14.33%,生物可利用形态的砷较少,这与体外模拟方法测定的生物可利用性砷含量较低吻合。体外模拟实验还发现：肠阶段生物可利用性砷含量比胃阶段低,生理学提取法(PBET)和体外消化法(IVG)测定的生物可利用性砷差异较小。     

沉积物岩性及水化学性质对水土界面氟迁移行为的影响

水土界面氟的迁移对高氟地下水的形成具有重要影响。以大同盆地典型氟中毒区为例,分析了不同岩性沉积物中的氟质量分数和形态分布特征,并通过室内静态实验,探究了水溶液的pH值、Ca2+、HCO-3以及H2PO-4质量浓度对水土界面氟迁移的影响。研究结果表明,沉积物中不同形态氟按含量由高到低的顺序依次为:残余态氟铁锰氧化态氟水溶态氟有机束缚态氟离子交换态氟;各形态氟在不同岩性沉积物中含量大小均表现为:黏土粉质黏土粉砂;在沉积物中,黏土矿物和方解石是主要的固氟矿物成分。溶液的pH值、Ca2+、HCO-3以及H2PO-4质量浓度与氟在沉积物表面的吸附-解吸和沉淀-溶解平衡密切相关,沉积物对氟的吸附量随溶液pH值和HCO-3质量浓度增大而降低,随Ca2+和H2PO-4质量浓度增大而增大。     

水流场及水化学过程对地下水中砷富集的影响

大同盆地是我国典型原生高砷地下水分布区,为精细刻画地下水系统中砷迁移富集的机理,在盆地高砷地下水分布区建立了面积为150m×250m的多水平试验监测场,开展了系统的水文地质与水文地球化学监测研究。结果表明,富砷沉积物是地下水中砷的直接来源,高pH值、强还原性的地下水环境及竞争吸附离子的存在是含水沉积物中砷向地下水迁移富集的主要控制因素。场地范围内地下水水化学组成及砷的空间分布特征明显受水流场影响,沿地下水径流方向,砷质量浓度逐渐升高。     

江汉平原第四系沉积物中砷的垂向分布规律及其对地下水中砷浓度的影响

含水层沉积物是江汉平原地下水中砷的主要来源,沉积物地球化学特征对地下水的水化学具有重要控制作用。为查明江汉平原第四系沉积物中砷的垂向分布及赋存环境,在典型高砷地下水分布区内选取2个深钻(JH002孔及YLW01孔,深度分别为230m和201m)采集沉积物样品进行了地球化学分析。结果表明全新统和上更新统含水层沉积物以黏土、粉土、淤泥质黏土、粉砂、细砂为主,指示着弱水动力的沉积环境;2个钻孔沉积物地球化学特征相似,w(As)=2.0~22.6mg/kg(平均9.0mg/kg),w(Fe)=11.8~55.0mg/g(平均37.8mg/g),w(S)=0.1~2.1mg/g(平均0.4mg/g)。中、下更新统沉积物岩相变化较大,以砂和砾石居多,局部含有黏土夹层,指示着沉积时较强的水动力沉积环境;其中JH002孔沉积物w(As)=2.7~160.5mg/kg(平均40.9mg/kg),w(Fe)=20.1~179.5mg/g(平均50.5mg/g),w(S)=0.1~17.7mg/g(平均4.9mg/g);YLW01孔沉积物砷、铁、硫质量分数均低于JH002孔,w(As)=5.2~56.1mg/kg(平均16.2mg/kg),w(Fe)=10.9~117.5 mg/g(平均36.4 mg/g),w(S)=0.3~7.8mg/g(平均1.8mg/g)。YLW01孔中、下更新统沉积物颗粒较JH002孔更细,所处的水动力条件更弱,砷、铁、硫质量分数均低于JH002孔,说明沉积历史环境影响着砷、铁、硫等元素的分布。沉积物地球化学数据聚类分析结果表明全新统和上更新统砷与铁具有显著的相关性,而中、下更新统沉积物砷与硫化物矿物紧密相关。结合不同深度含水层水化学特征差异指示上更新统含水层中含砷铁氧化物的还原性溶解导致浅层地下水中砷的富集,富硫的中、下更新统深层含水层中强还原环境下砷受到硫化物矿物的固定作用难以释放进入地下水中。     

江汉平原高砷地下水与含水层沉积物的地球化学特征

通过采集江汉平原沙湖原种场典型高砷地下水区的27个地下水样品、6个地表水样品和不同深度的沉积物样品,采用X射线荧光光谱、离子色谱、TOC分析仪以及沉积物平行提取方法对该区含水层的水化学特征以及沉积物的地球化学特征进行了分析。结果表明:该区地下水水化学类型主要为HCO_3-Ca·Mg型,属中性偏弱碱性水;氧化还原电位较负,NH+4-N、HS-和Fe(Ⅱ)质量浓度较高,指示地下水的强还原环境;地下水As质量浓度平均值为74.39μg/L,最大值达580.77μg/L,出现在地面以下25m位置,且As(Ⅲ)质量浓度普遍高于As(V)。随深度增加,沉积物中w(As)呈先增加后减小的趋势,在17.4m(黏土层)深度处达到最大(21.15mg/kg),且w(As)随深度的变化与沉积物中TOC、Fe_2O_3、Al_2O_3质量分数以及沉积物比表面积随深度的变化呈现相同的趋势,采用SPSS软件经相关分析亦表明As与TOC、Fe_2O_3、Al_2O_3和比表面积具有较强的相关关系,相关系数分别为0.70,0.68,0.70,0.67,揭示砷在富含有机质、铁铝氧化物及比表面积大的黏土层沉积物中更易富集。沉积物平行提取实验结果表明,江汉平原沉积物中的砷主要以强结晶态铁氧化物、溶解态硫化物和反应性硅酸盐的形式存在。     

大同盆地地下水高砷、氟、碘分布规律与成因分析及质量区划

为了对大同盆地地下水中砷、氟、碘等的分布和成因进行分析,开展地下水质量区划,依据地下水污染调查取得的最新系列测试数据,结合以往水文地质和水文地球化学研究成果,编制大同盆地浅层和中深层地下水砷含量、氟含量、碘含量等水化学特征分布图,以直观反映大同盆地地下水高砷、高氟、高碘区的空间分布规律; 通过分析pH值、硫酸根含量、硝酸根含量、铁含量、锰含量与砷的关系,探讨高砷水的形成原因; 根据pH值、钙离子、重碳酸氢根离子与氟的关系,分析氟超标原因; 指出高碘区与高氟区分布的相似性和成因的相似性。研究结果表明,盆地周边高砷、高氟岩层是地下水砷、氟的原生来源,特定的河湖相沉积环境则为砷、碘的富集提供了原生地质条件; 北部地区氟增高与地下水位下降致使黏性土中的氟离子进入含水层有关,中部地区高氟与土壤盐渍化有关; 中部富含淤泥质黏土的湖相地层是碘富集的原生地质因素,冲积洼地地下水径流条件滞缓是碘富集的水动力因素; 干旱气候条件下强烈的蒸发浓缩作用亦是高氟、高碘地下水形成的重要因素。依据砷、氟、碘、硝酸盐、亚硝酸盐、总含盐量(total dissolved salt,TDS)、总硬度、氨氮等单组分含量分布,利用GIS空间分析功能,进行了大同盆地浅层和中深层地下水质量区划,可为当地地下水开发利用提供地学依据。     

江汉平原高砷地下水原位微生物的铁还原及其对砷释放的影响

微生物参与铁氧化物矿物的还原性溶解是高砷地下水形成的关键过程,其中具有砷还原功能的微生物如何参与含水层砷释放的生物地球化学过程亟待研究.利用从江汉平原典型高砷含水层中厌氧条件下分离出的四株细菌(Citrobacter sp.JH-1、Clostridium sp.JH-6、Exiguobacterium sp.JH-13、Paenibacillus sp.JH-33),通过室内厌氧模拟培养实验,查明其砷、铁还原能力,并通过分别与铁氧化物矿物及原位沉积物共同培养,探究原位含水层微生物参与的砷释放机理.结果表明:四株细菌均具有厌氧条件下砷、铁还原功能,Citrobacter sp.JH-1砷还原能力最强,96 h内还原的As(Ⅴ)浓度为2.22 μmol/L.其中Citrobacter sp.JH-1不仅可在厌氧和有氧条件下还原溶液中的As(Ⅴ),还可在厌氧条件下还原溶液中的Fe(Ⅲ)和无定型的水铁矿,在与含水层沉积物共培养12 d后,沉积物中铁与砷的释放量分别为510 mg/kg及1 150 μg/kg.江汉平原含水层中的原位微生物兼具砷/铁还原功能,在厌氧条件下可还原沉积物中的铁氧化物矿物并促进砷的释放,为深入揭示高砷地下水成因机理与地下水砷污染的防控提供重要科学依据.      

江汉平原浅层含水层中土著硫酸盐还原菌对砷迁移释放的影响

硫酸盐还原菌是厌氧环境中参与砷形态转化的重要微生物种群,其介导的生物地球化学循环过程对铁氧化物表面吸附态砷迁移转化的影响亟待深入研究.选取江汉平原典型高砷含水层原位沉积物分离纯化出一株严格厌氧硫酸盐还原菌Desulfovibrio JH-S1,对其进行砷和铁还原能力鉴定,并通过模拟培养实验探究硫酸盐还原菌参与下的铁矿物相转化对吸附态砷迁移的影响.Desulfovibrio JH-S1具有Fe(III)还原能力,无硫和有硫体系中Fe(III)均能被还原,但在硫酸盐充足条件下铁还原量显著增加;该菌株不具备As(V)还原能力,但添加硫酸盐的培养体系中As(V)去除率可达96%以上.Desulfovibrio JH-S1能够还原硫酸盐从而促进载砷的水铁矿还原转化为纤铁矿,并导致吸附的砷释放.江汉平原高砷含水层土著硫酸盐还原菌兼具硫酸盐/铁还原功能,参与了高砷含水层系统中砷-铁-硫耦合循环,对高砷地下水的形成具有重要作用.      

高砷含水层沉积物矿物学特征及砷的活化

利用X射线衍射和X射线荧光分析、沉积物序列提取试验及矿物饱和度的计算,对采自江汉平原中部沙湖地区典型高砷含水层钻孔沉积物样品矿物学进行了分析,并讨论了控制含水层中砷释放和迁移的地球化学机制.对矿物在沉积物与土壤中的分布及组成的对比分析,在一定程度上指示了矿物赋存环境和/或高砷水形成的环境背景:土壤与沉积物中高岭石以低于其他3种粘土矿物的含量普遍存在,指示了含水层沉积物形成过程江汉平原存在一定的湿热古气候环境;沉积物绿泥石含量低于土壤中绿泥石,恰恰反映了土壤比沉积物略强的碱性环境;沉积物中黄铁矿的存在,显示了含水层局部的强还原性环境,指示地下水中广泛存在的Fe2+容易与二价硫发生沉淀并结合砷.砷主要以无定形铁锰氧化物结合态(平均在31％以上)形式存在,其次以碳酸盐和有机质结合态存在.无定形铁锰氧化物的还原溶解可能是控制砷迁移到地下水中主要的地球化学机制.相对高含量的绿泥石容易在含水层中发生风化,其溶解过程可以将铁释放到地下水中,从而成为影响地下水中砷活化的潜在因素.     

Characterization on arsenic sorption and mobility of the sediments of Chia-Nan Plain, where Blackfoot disease occurred

Higher levels of arsenic in the aquifers of Chia-Nan Plain in southwestern Taiwan were attributed to the cause of Blackfoot disease in the area half a century ago. Although extensive studies were conducted on the occurrence, speciation, mobility, and transport of arsenic in the region, the relationship between arsenic adsorption by different aquifer materials and the concentration and speciation of arsenic in these aquifers was poorly delineated. This study focused on characterization of sediments in the region, as well as determination of relations between arsenic adsorption and other geochemical and clay mineralogical properties of the sediments. The arsenic adsorption capacity was positively related to the clay minerals, Fe, and Mn contents in the sediments. The higher arsenic adsorption capacity of the sediments served as a sink for the arsenic during its transport from the central mountains to the coast and as a source for its release under reduced environment and in the presence of humic substances.     

Geochemical mobility of arsenic in the surficial waters from Argentina

The presence of arsenic (As) in surface water constitutes an important environmental risk, where mobility and adsorption processes are responsible for its behavior in the sediment–water interface. Therefore, the assessment of adsorption, mobility and water availability of arsenic in freshwater sediments, with agricultural, livestock and urban soil uses was performed. Arsenic concentrations in sediments ranged from 5.4 to 15.9 mg kg^?1 (total) and 2.8 to 6.5 mg kg^?1 (labile), and those of iron and manganese were 11,563–23,500 and 140.6–662.1 mg kg^?1, respectively. The As levels in water were significantly lower than those of sediments. Results would suggest that As co-precipitation and adsorption on Fe oxides are probably the major route of immobilization, determining its low lability. Manganese did not present an outstanding contribution to the retention, and cation-exchange capacity, pH and organic matter of sediments did not show an influence on the mobility of As.     

The behavior of arsenic and antimony at Pezinok mining site,southwestern part of the Slovak Republic

Arsenic and antimony contamination is found at the Pezinok mining site in the southwest of the Slovak Republic. Investigation of this site included sampling and analysis of water, mineralogical analyses, sequential extraction, in addition to flow and geochemical modeling. The highest dissolved arsenic concentrations correspond to mine tailings (up to 90,000 μg/L) and the arsenic is present predominately as As(V). The primary source of the arsenic is the dissolution of arsenopyrite. Concentration of antimony reaches 7,500 μg/L and its primary source is the dissolution of stibnite. Pore water in mine tailings is well-buffered by the dissolution of carbonates (pH values between 6.6 and 7.0) and arsenopyrite grains are surrounded by reaction rims composed of ferric iron minerals. Based on sequential extraction results, most solid phase arsenic is in the reducible fraction (i.e. ferric oxyhydroxides), sulfidic fraction, and residual fraction. Distribution of antimony in the solid phase is similar, but contents are lower. The principal attenuation mechanism for As(V) is adsorption to ferric oxide and hydroxides, but the adsorption seems to be limited by the competition with Sb(V) produced by the oxidation of stibnite for adsorption sites. Water in mine tailings is at equilibrium with gypsum and calcite, but far from equilibrium with any arsenic and antimony minerals. The concentrations of arsenic and antimony in the surrounding aquifer are much lower, with maximum values of 215 and 426 μg/L, respectively. Arsenic and antimony are transported by ground water flow towards the Blatina Creek, but their loading from ground water to the creek is much lower compared with the input from the mine adits. In the Blatina Creek, arsenic and antimony are attenuated by dilution and by adsorption on ferric iron minerals in stream sediments with resulting respective concentrations of 93 and 45 μg/L at the site boundary south of mine tailing ponds.     

金矿区河水和底泥中重金属含量分布与耦合关系

河流中河水与底泥中重金属含量之间的关系对于河流污染防治意义重大。对某金矿区3种典型河流的河水与底泥中的重金属含量进行分析,发现4条河流均有超过国家标准限值的情况,河水中7种重金属元素均出现超标,Cr、As元素超标不严重;底泥中Hg、Pb、Cr、Cu、Zn超标,其中Hg超标最严重,河水和底泥中Hg最大超标倍数分别达到3099倍和244倍。河流中重金属主要赋存形态为沉积态,底泥的吸附解吸作用是河流底泥和河水中重金属沿程变化的主控因素;金矿区区域上河水和底泥中重金属很好地服从Langmuir等温吸附模式。矿业活动、地层岩性均会影响底泥对重金属的平均最大吸附容量,流径黄土区的双桥河平均最大吸附容量最大。研究结果为矿山河流污染防治与预警提供了参考依据。     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Arsenic mobilization in shallow aquifers of Datong Basin: Hydrochemical and mineralogical evidences

To better understand the sources and mobilization processes responsible for arsenic enrichment in groundwater in the central part of Datong Basin where serious arsenic poisoning cases have been reported, hydrochemical characteristics of the groundwater and the geochemical and mineralogical features of the aquifer sediments were studied. The aqueous arsenic levels are strongly depth-dependent in the study area and the high arsenic concentrations are found at depths between 15 m and 60 m, with a maximum up to 1820 μg/L. The hydrochemical characteristics of high arsenic groundwater from the Datong Basin indicate that the mobilization of arsenic is related to reductive dissolution of Fe oxides/oxyhydroxides and/or desorption from the Fe oxides/oxyhydroxides at high pH (above 8.0). The bulk chemical results of sediments show the arsenic and iron are moderately correlated, suggesting that arsenic is associated with iron-bearing minerals. Results of sequential-extraction experiment show that solid-phase arsenic is similarly distributed among the different pools of reservoir in the aquifer sediments. Strongly adsorbed arsenic and co-precipitated arsenic are its dominant species in the solid-phase. Geochemical studies using chemical analysis, X-ray diffraction and scanning electron microscopy on magnetically separated fractions demonstrate that iron oxides/oxyhydroxides with residual magnetite and chlorite, illite, iron oxides/oxyhydroxides-coated quartz and feldspar, and ankerite are the dominant carriers of arsenic in the sediments. The major processes of arsenic mobilization are probably linked to desorption of As from Fe oxides/oxyhydroxides and reductive dissolution of Fe-rich phases in the aquifer sediments under reducing and alkaline conditions.     

西南表层岩溶带土壤中砷的迁移规律实验与模拟

岩溶地下河是我国西南地区的重要水源,工业生产过程中产生的砷污染物,除通过落水洞等直接进入并污染地下水外,还会在表层岩溶带溶缝、溶隙内吸附、滞留及富集,并在特定条件下再次迁移,成为"稳定次生污染源".以广西某砷污染事件为例,采用窄缝槽物理模型装置进行砷的动态吸附、解吸实验,并结合地球化学模拟研究砷污染物在表层岩溶带土壤中的迁移规律.实验结果显示表层岩溶带对砷的吸附以物理吸附（扩散过程）为主,相比吸附过程而言解吸速率则显得缓慢,而酸溶液相比去离子水可促进砷的解吸过程.地球化学模拟结果表明土壤矿物中以针铁矿对砷的吸附贡献最大,而酸溶液通过溶蚀针铁矿等矿物削弱对砷的吸附能力.因此在西南岩溶地区,表层岩溶带系统一旦纳入砷污染物,则解吸过程缓慢,易形成砷污染物的滞留、富集;而酸雨作用下砷的解吸、迁移过程加快,则会提高地下水系统的污染风险.      

Distribution and Abundance of Arsenic in the Soils and Sediments of O‘ahu,Hawai‘i

Lack of high-spatial-resolution soil and sediment arsenic data for Hawai‘i has generated substantial disagreement between researchers and regulators regarding the magnitude of natural levels of arsenic in Hawai‘i and rendered difficult the defining of areas of anthropogenically elevated arsenic. Our earlier research into the occurrence of arsenic in terrestrial and marine environments revealed widely disparate concentrations of arsenic with no apparent spatial pattern. To better understand the distribution and abundance of arsenic in soils and sediments of O‘ahu, we collected an additional 64 samples at locations chosen to represent different environments with varying degrees of human impact. We found surface arsenic values that ranged from 0.28 to 740 ppm with a median concentration of 8.1 ppm, which is above the global median of 5 ppm and US soil median of 5.2 ppm. Higher concentrations of arsenic (up to 913 ppm) were encountered at depth in soil cores. The median arsenic in streambed sediments from one of our earlier studies of 6.1 ppm was comparable to the conterminous US median of 6.3 ppm; however, we encountered arsenic concentrations as high as 43.9 ppm (median = 8.60 ppm, n = 75) in marine sediments in recent work off the leeward coast of O‘ahu. Overall, arsenic in the soils and sediments of O‘ahu is elevated relative to world and national values, but there still is no readily discernible pattern in the distribution of arsenic to explain these elevated values.     

Arsenic solubilization and redistribution under anoxic conditions in three aquifer sediments from a basin-fill aquifer in Northern Utah: The role of natural organic carbon and carbonate minerals

The basin-fill aquifers of the Western U.S. contain elevated concentrations of arsenic in the groundwater due to ancient volcanic deposits that host arsenic minerals. Microcosms were constructed using two oxidized sediments and, by contrast, a reduced sediment collected from a shallow basin-fill aquifer in the Cache Valley Basin, Northern Utah to evaluate the fate of geologic arsenic under anoxic conditions. Sequential extractions indicated the primary arsenic host mineral was amorphous iron oxides, but 13%–17% of the total arsenic was associated with carbonate minerals. Arsenic was solubilized from the sediments when incubated with groundwater in the presence of native organic carbon. Arsenic solubilization occurred prior to iron reduction rather than the commonly observed co-reactivity. Arsenic(V) associated with carbonate minerals was the main source of arsenic released to solution and redistributed onto less soluble minerals, including FeS and siderite as defined by chemical extraction. Arsenic reduction occurred only in the site-oxidized sediments. The addition of a carbon and energy source, glucose, resulted in enhanced arsenic solubilization, which was coupled with iron reduction from the site-oxidized sediments. Adding glucose promoted iron reduction that masked the role of carbonate minerals in arsenic solubilization and retention as observed with incubation with groundwater only.