Teng Ma

Geostatistical and geochemical analysis of surface water leakage into groundwater on a regional scale : a case study in the Liulin karst system, northwestern China

Abstract The Liulin karst system is typical of hydrogeological systems in northern China, with a group of springs as the dominant way of regional groundwater discharge. Surface water leakage into groundwater has been observed in six sections of the rivers in the study area. To extract hydrogeological information from hydrochemical data, 29 water samples were collected from the system. On a trilinear diagram, most of the groundwater samples are clustered around the surface waters, indicating the effect of leakage on their chemistry. R -mode factor analysis was made on seven variables (Na, Ca, Mg, SO 4 , Cl, HCO 3 , and NO 3 ) of the samples and three principal factors were obtained: the F 1 factor is composed of Ca, Mg and SO 4 , the F 2 of HCO 3 and NO 3 , and the F 3 of Na and Cl. These factors are then used as regionalized variables in ordinary Kriging for unbiased estimates of the spatial variations of their scores. Considering regional hydrogeological conditions, the hydrogeological implications of the spatial distribution of the factor scores as related to the effects of the surface leakage are discussed. To evaluate the geochemical processes, the geochemical modeling code NETPATH was employed. The modeling results show that mixing commonly occurs in the system and dolomite dissolution is more important than calcite dissolution. Dedolomitization (calcite precipitation and dolomite dissolution driven by anhydrite dissolution) is locally important, in the western flank of the system where the surface water leakage has the least effect.

Microbial communities involved in arsenic mobilization and release from the deep sediments into groundwater in Jianghan plain, Central China.

It was shown that groundwater in Jianghan Plain was severely contaminated by arsenic; however, little is known about the mechanism by which the mineral arsenic was mobilized and released into groundwater from the high-arsenic sediments in this area. Here, we collected sediment samples from the depths of 5-230m in Jianghan Plain. Although all of the samples contain high contents of total arsenic, the soluble arsenic was only detectable in few of the shallow sediments, but was readily detectable in all of the deep sediments at the depths of 190-230m. Analysis of the genes of arsenate-respiring reductases indicated that they were not present in all of the shallow sediments from the depths of 5-185m, but were detectable in all of the deep sediments from the depths of 190-230m; all of the identified reductase genes are new or new-type, and they display unique diversity. Microcosm assay indicated that the microbial communities from the deep sediments were able to reduce As(V) into As(III) using lactate, formate, pyruvate or acetate as an electron donor under anaerobic condition. Arsenic release assay demonstrated that these microbial communalities efficiently catalyzed the mobilization and release of the mineral arsenic into aqueous phase. We also isolated a novel cultivable dissimilatory As(V)-respiring bacterium Aeromonas sp. JH155 from the sediments. It is able to completely reduce 2.0mM As(V) into As(III) in 72h, and efficiently promote the reduction and release of the mineral arsenic into aqueous phase. Analysis of the 16S rRNA genes indicated that the deep sediments contain diversities of microbial communities, which were shaped by the environmental factors, such as As, SO42-, NO3-, Fe and pH value. These data suggest that the microorganisms in the deep sediments in Jianghan Plain played key roles in the mobilization and release of insoluble arsenic into the groundwater.

A cost-effective system for in-situ geological arsenic adsorption from groundwater

An effective and low-cost in-situ geological filtration system was developed to treat arsenic-contaminated groundwater in remote rural areas. Hangjinhouqi in western Hetao Plain of Inner Mongolia, China, where groundwater contains a high arsenic concentration, was selected as the study area. Fe-mineral and limestone widely distributed in the study area were used as filter materials. Batch and column experiments as well as field tests were performed to determine optimal filtration parameters and to evaluate the effectiveness of the technology for arsenic removal under different hydrogeochemical conditions. A mixture containing natural Fe-mineral (hematite and goethite) and limestone at a mass ratio of 2:1 was found to be the most effective for arsenic removal. The results indicated that Fe-mineral in the mixture played a major role for arsenic removal. Meanwhile, limestone buffered groundwater pH to be conducive for the optimal arsenic removal. As(III) adsorption and oxidation by iron mineral, and the formation of Ca-As(V) precipitation with Ca contributed from limestone dissolution were likely mechanisms leading to the As removal. Field demonstrations revealed that a geological filter bed filled with the proposed mineral mixture reduced groundwater arsenic concentration from 400 μg/L to below 10 μg/L. The filtration system was continuously operated for a total volume of 365,000L, which is sufficient for drinking water supplying a rural household of 5 persons for 5 years at a rate of 40 L per person per day.

Remediation of arsenic-contaminated groundwater by in-situ stimulating biogenic precipitation of iron sulfides

Severe health problems due to elevated arsenic (As) in groundwater have made it urgent to develop cost-effective technologies for As removal. This field experimental study tested the feasibility of in-situ As immobilization via As incorporation into newly formed biogenic Fe(II) sulfides in a typical As-affected strongly reducing aquifer at the central part of Datong Basin, China. After periodic supply of FeSO4 into the aquifer for 25xa0d to stimulate microbial sulfate reduction, dissolved sulfide concentrations increased during the experiment, but the supplied Fe(II) reacted quickly with sulfide to form Fe(II)-sulfides existing majorly as mackinawite as well as a small amount of pyrite-like minerals in sediments, thereby restricting sulfide build-up in groundwater. After the completion of field experiment, groundwater As concentration decreased from an initial average value of 593xa0μg/L to 159xa0μg/L, with an overall As removal rate of 73%, and it further declined to 136xa0μg/L adding the removal rate up to 77% in 30xa0d after the experiment. The arsenite/Astotal ratio gradually increased over time, making arsenite to be the predominant species in groundwater residual As. The good correlations between dissolved Fe(II), sulfide and As concentrations, the increased abundance of As in newly-formed Fe sulfides as well as the reactive-transport modeling results all indicate that As could have been adsorbed onto and co-precipitated with Fe(II)-sulfide coatings once microbial sulfate reduction was stimulated after FeSO4 supply. Under the strongly reducing conditions, sulfide may facilitate arsenate reduction into arsenite and promote As incorporation into pyrite or arsenopyrite. Therefore, the major mechanisms for the in-situ As-contaminated groundwater remediation can be As surface-adsorption on and co-precipitation with Fe(II) sulfides produced during the experimental period.

Using isotopic, hydrogeochemical-tracer and temperature data to characterize recharge and flow paths in a complex karst groundwater flow system in northern China

Isotopic and hydrogeochemical analysis, combined with temperature investigation, was conducted to characterize the flow system in the carbonate aquifer at Taiyuan, northern China. The previous division of karst subsystems in Taiyuan, i.e. the Xishan (XMK), Dongshan (DMK) and Beishan (BMK) mountain systems, were also examined. The measured δD, δ18O and 3He/4He in water indicate that both thermal and cold groundwaters have a meteoric origin rather than deep crustal origin. Age dating using 3H and 14C shows that groundwater samples from discharge zones along faults located at the margin of mountains in the XMK and DMK are a mixture of paleometeoric thermal waters and younger cold waters from local flow systems. 14C data suggest that the average age was about 10,000xa0years and 4,000xa0years for thermal and cold groundwater in discharge zones, respectively. Based on the data of temperature, water solute chemical properties, 14C, δ34SSO4, 87Sr/86Sr and δ18O, different flow paths in the XMK and DMK were distinguished. Shallow groundwater passes through the upper Ordovician formations, producing younger waters at the discharge zone (low temperature and ionic concentration and enriched D and 18O). Deep groundwater flows through the lower Ordovician and Cambrian formations, producing older waters at the discharge zone (high ionic concentration and temperature and depleted D and 18O). At the margin of mountains, groundwater in deep systems flows vertically up along faults and mixes with groundwater from shallow flow systems. By contrast, only a single flow system through the entire Cambrian to Ordovician formations occurs in the BMK.RésuméL’analyse isotopique et hydrogéochimique, combinée avec examen des données de température, a été effectuée pour caractériser le système d’écoulement dans l’aquifère carbonaté de Taiyuan, dans le nord de la Chine. La subdivision précédente des sous-systèmes karstiques du Taiyuan, à savoir les systèmes montagneux du Xishan (XMK), du Dongshan (DMK) et du Beishan (BMK), ont également été examinées. Les analyses de δD, δ18O et du rapport 3He/4He dans l’eau indiquent que les eaux souterraines thermales et froides ont toutes les deux une origine météorique plutôt qu’une origine de croûte profonde. La détermination de l’âge de l’eau utilisant 3H et 14C montre que les échantillons d’eau souterraine des zones de décharge le long des failles localisées sur la bordure des montagnes dans les systèmes XMK et DMK résultent d’un mélange entre des eaux thermales paléométéoriques et des eaux froides plus jeunes des systèmes d’écoulement locaux. Les données de 14C suggèrent que l’âge moyen était de 10,000 ans et de 4,000 ans pour les eaux souterraines thermales et froides dans les zones de décharge, respectivement. A partir des données de température, des propriétés chimiques du soluté, de 14C, δ34SSO4, 87Sr/86Sr et δ18O, différents chemins d’écoulements dans les systèmes XMK et DMK ont été identifiés. Les eaux souterraines peu profondes traversent les formations de l’Ordovicien supérieur, fournissant les eaux les plus jeunes au niveau de la zone de décharge (basse température et concentration ionique et D enrichi et 18O). En bordure des montagnes, les eaux souterraines des systèmes profonds s’écoulent verticalement le long de failles et se mélangent avec les eaux souterraines des systèmes d’écoulement peu profonds. En revanche, seul un système d’écoulement unique à travers l’ensemble des formations du Cambrien à l’Ordovicien existe au sein BMK.ResumenSe llevó a cabo el análisis isotópico e hidrogeoquímico, combinado con la investigación de la temperatura, para caracterizar el sistema de flujo en el acuífero carbonático en Taiyuan, el norte de China. Se examinó la división previa de los subsistemas de karst en Taiyuan, es decir, los de Xishan (XMK) Dongshan (DMK) y los sistemas de montaña Beishan (BMK). Las medidas de δD, δ18O y 3He/4He en agua indican que tanto el agua subterránea termal como la fría tienen un origen meteórico en vez de un origen de la corteza profunda. La datación de las edades utilizando δD, δ18O y 3He/4He muestra que las muestras de agua subterránea de las zonas de descarga a lo largo de las fallas ubicadas en el margen de las montañas en el XML y DMK son una mezcla de aguas termales paleo meteóricas y aguas frías más jóvenes procedentes de los sistemas de flujo local. Los datos de 14C sugieren que la edad promedio fue de alrededor de 10,000 años y 4,000 años en el caso del agua subterránea termal y fría en las zonas de descarga, respectivamente. Basándose en los datos de la temperatura, de las propiedades químicas del soluto agua y de 14C, δ34SSO4, 87Sr/86Sr y δ18O, se distinguieron diferentes trayectorias de flujo en el XML y DMK. El agua subterráneas poco profunda pasa a través de las formaciones del Ordovícico superior, produciendo aguas más jóvenes en la zona de descarga (baja temperatura y concentración iónica y D y 18O enriquecidos). El agua subterránea profunda fluye a través del Ordovícico inferior y formaciones del Cámbrico, generando aguas más viejas en la zona de descarga (alta concentración iónica y temperatura y D y 18O empobrecidos). En el margen de las montañas, el agua subterránea en los sistemas profundos fluye verticalmente hacia arriba a lo largo de las fallas y se mezcla con el agua subterránea con el flujo de los sistemas de poca profundidad. Por el contrario, sólo un sistema de flujo único se produce a través de todas las formaciones del Cámbrico al Ordovícico en el BMK.摘要进行了同位素和水文地球化学分析以及温度调查,以描述中国北方太原碳酸盐含水层中的水流系统。同时也对太原先前的岩溶亚系统的划分,如西山、东山和北山山系进行了调查。测量的水中的δD, δ18O 和 3He/4He结果表明,热地下水和冷地下水是大气来源的,而非深部的地壳来源。使用3H和 14C测年显示,从沿位于西山和东山山脉边缘断层排泄带获取的地下水样是本地水流系统中古大气热水和年轻冷水的混合水。14C资料表明,排泄带热地下水和冷地下水的年龄分别平均为大约10000年和4000年。根据温度、水溶质化学特性、14C、 δ34SSO4、87Sr/86Sr 和 δ18O等资料,对西山和东山不同的水流通道进行了区分。浅层地下水流经晚奥陶系地层,在排泄带产生较年轻的水(低温和低离子浓度及D和 18O富集)。深层地下水流经早奥陶系和寒武系地层,在排泄带产生较老的水(高离子浓度和高温及D 和 18O耗尽)。在山脉边缘,深层系统的地下水流沿断层垂直向上流动与浅层水流系统的地下水混合。相比之下,在北山整个寒武系到奥陶系地层只有一个单一的水流系统。ResumoA análise isotópica e hidrogeoquímica, combinada com a investigação de temperatura, foi conduzida para caracterizar o sistema de fluxo no aquífero carbonático de Taiyuan, norte da China. A divisão anterior dos subsistemas cársticos em Taiyuan, ou seja, o Xishan (XMK), Dongshan (DMK) e o sistema de montanhas Beishan (BMK), também foram examinados. A medida de δD, δ18O e 3He/4He na água indica que ambas as águas subterrâneas, termal e fria, possuem origem meteórica ao invés de origem crustal profunda. A idade datada usando 3H e 14C demonstra que as amostras de água subterrânea vindas das zonas de descarga ao longo das falhas localizadas na margem de montanhas em XML e DMK são uma mistura de águas termais paleometeóricas e águas frias mais jovens de sistemas de fluxo local. Dados de 14C sugerem que a idade média foi de cerca de 10.000 anos e 4.000 anos para as águas subterrâneas termais e frias nas zonas de descarga, respectivamente. Com base nos dados de temperatura, as propriedades químicas do soluto água, 14C, δ34SSO4, 87Sr/86Sr e δ18O, foram distinguidos caminhos de fluxo diferentes no XML e DMK. Águas subterrâneas rasas passam através das formações do Ordoviciano superior, produzindo águas mais jovens na zona de descarga (baixa temperatura e concentração iônica e enriquecimento de D e 18O). Águas subterrâneas profundas fluem através do Ordoviciano inferior e das formações Cambrianas, produzindo águas mais velhos na zona de descarga (alta concentração iônica e temperatura e empobrecimento de D e 18O). Na margem das montanhas, sistemas profundos de água subterrânea fluem verticalmente para cima ao longo de falhas e se misturam com as águas subterrâneas com sistemas de fluxo rasos. Entretanto, no BMK ocorre apenas um sistema de fluxo único através de todo o Cambriano até às formações do Ordoviciano.

Hydrogeochemistry of co-occurring geogenic arsenic, fluoride and iodine in groundwater at Datong Basin, northern China.

Abnormal levels of co-occurring arsenic (As), fluorine (F) and iodine (I) in groundwater at Datong Basin, northern China are geochemically unique. Hydrochemical, (18)O and (2)H characteristics of groundwater were analyzed to elucidate their mobilization processes. Aqueous As, F and I ranged from 5.6 to 2680 μg/L, 0.40 to 3.32 mg/L and 10.1 to 186 μg/L, respectively. High As, F and I groundwater was characterized by moderately alkaline, high HCO3(-), Fe(II), HS(-) and DOC concentrations with H3AsO3, F(-) and I(-) as the dominant species. The plots of δ(18)O values and Cl/Br ratios versus Cl(-) concentration demonstrate build-up of more oxidizing conditions and precipitation of carbonate minerals induced by vertical recharge and intensive evaporation facilitate As retention to Fe (hydr) oxides, but enhance F and I mobilization from host minerals. Under reducing conditions, As and I can be simultaneously released via reductive dissolution of Fe (hydr) oxides and reduction of As(V) and I(V) while F migration may be retarded due to effects of dissolution-precipitation equilibria between carbonate minerals and fluorite. With the prevalence of sulfate-reducing condition and lowering of HCO3(-) concentration, As and I may be sequestered by Fe(II) sulfides and F is retained to fluorite and on clay mineral surfaces.

Arsenic behavior in different biogeochemical zonations approximately along the groundwater flow path in Datong Basin, northern China

Studies have shown that arsenic is desorbed/released into groundwater as a result of bacterial reduction of As(V) and Fe(III). However, bacterial activities like sulfate reduction process can also reduce the content of arsenic in groundwater. In this study, we examined the effects of different biogeochemical processes (e.g. NO3- and SO42- reduction) on arsenic, by investigating the chemical characteristics and bacterial community structure of groundwater in the Datong Basin, northern China. Along the groundwater flow path, arsenic concentration increased from <1 to 947.6μg/L with dominant bacteria change from aerobic (Fluviicola, Rhodococcus) to denitrifying bacteria (Thauera, Gallionella), and then to sulfate reducing bacteria (Desulfosporosinus). According to the groundwater redox sensitive indicators (Eh, NO3-, SO42-/Cl- and Fe2+) concentrations (or ratios), the sampling points were approximately divided into three zones (I, I and II). Variation in features of these indicators suggested that the groundwater evolved from a weakly oxidizing environment (Zone I, Eh average 93.3mV, respectively) to strong reducing environment (Zone II, Eh average -101.8mV). In Zone I, bacteria mainly consuming O2 or NO3- were found which inhibits Fe(III) and As(V) reduction reaction, resulting in a low As zone (<1 to 3.3μg/L). However, in Zone II, where O2 and NO3- have been depleted, SO42- reduction appears to be the dominant process, and the Fe(III) and As(V) reduction processes are also occurring and hence, enrichment of As in the groundwater (2.8 to 947.6μg/L, average 285.6μg/L). Besides, bacterial Fe(III) reduction process was retarded due to the weakly alkaline conditions (pH7.60-8.11, average 7.83), but abiotic Fe(III) reduction by HS- may be continued. Therefore, we conclude that the Fe(III) and As(V) reduction processes contributed to arsenic enrichment in the groundwater, and the reductive desorption of arsenate is the main occurring process especially in the weakly alkaline environment. Moreover, NO3- reduction process can significantly restrain the release of arsenic, but the process of SO42- reduction is insignificant for arsenic concentration decline in natural groundwater.

Arsenic releasing characteristics during the compaction of muddy sediments

Muddy sediments are abundant in pore water and capable of preserving a large amount of chemicals, such as arsenic. Muddy sediments would transform into aquicludes or aquitards during long-term compaction and burial. It remains unclear whether the release of arsenic from muddy sediments poses a potential contamination risk to groundwater in the adjacent aquifer. An indoor compaction simulation experiment was conducted, coupled with an investigation on vertical geochemical profiles of muddy sediments in one actual borehole. In this experiment, aqueous arsenic in released pore water ranged from 17.5 to 21.3 μg L-1 and the accumulated content of the released arsenic was 17.576 μg during the compaction. As(iii) was the main As species in released pore water and had good correlations with Fe2+ and Mn. The analysis of the solid phase showed a remarkable depletion of Fe-Mn oxide bound arsenic during the compaction. In the profiles of the actual borehole, the contents of Fe-Mn oxide bound arsenic also exhibited a gradual decreasing trend from shallow to deep. Based on both the indoor experiment and the field profile, it can be concluded that the reductive dissolution of Fe-Mn oxides took place in arsenic-rich muddy sediments and Fe-Mn oxide-bound arsenic transformed into soluble arsenic, then soluble arsenic was released into the adjacent aquifer along with the pore water in the long-term compaction and burial.

Effect of microbially mediated iron mineral transformation on temporal variation of arsenic in the Pleistocene aquifers of the central Yangtze River basin

Significant seasonal variation of groundwater arsenic (As) concentrations in shallow aquifers of the Jianghan Plain, central Yangtze River Basin has been reported recently, but the underlying mechanisms remain not well understood. To elaborate biogeochemical processes responsible for the observed As concentration variation, 42-day incubation experiments were done using sediment samples collected respectively from the depth of 26, 36 and 60m of the As-affected aquifer which were labeled respectively as JH26, JH36, JH60. Where JH denotes Jianghan Plain, and the number indicates the depth of the sediment sample. The results indicated that As could be mobilized from the sediments of 26m and 36m depth under the stimulation of exogenous organic carbon, with the maximum As release amount of 1.60 and 1.03mgkg-1, respectively, while the sediments at 60m depth did not show As mobilization. The microbially mediated reductive dissolution of amorphous iron oxides and reduction of As(V) to As(III) could account for the observed As mobilization. The 16S rRNA high-throughput sequencing results indicated that the variation of microbial community correlated with the released As concentration (R=0.7, P<0.05) and the iron-reducing bacteria, including Pseudomonas, Clostridium and Geobacter, were the main drivers for the As mobilization from the sediments at 26m and 36m depth. The increase of arsC gene abundance (up to 1.4×105 copies g-1) during As release suggested that As reduction was mediated by the resistant reduction mechanism. By contrast, in the 60m sediments where the Fe and As release was absent, the iron-reducing bacteria accounted for a very minor proportion and sulfate-reducing bacteria were predominant in the microbial community. In addition, after 30days of incubation, the released As in the 26m sediments was immobilized via co-precipitation with or adsorption onto the Fe-sulfide mineral newly-formed by the bacterial sulfate reduction. These results are consistent with the results of our previous field monitoring, indicating that the bacterial sulfate reduction could lead to the temporal decrease in groundwater As concentrations. This study provides insights into the mechanism for As mobilization and seasonal As concentration variation in the Pleistocene aquifers from alluvial plains.

In-situ arsenic remediation by aquifer iron coating: Field trial in the Datong basin, China.

An aquifer Fe-coating technology was evaluated for in-situ As remediation. The groundwater in the aimed aquifer has low dissolved Fe(II) concentration and high As(III) concentration, which has a low affinity toward Fe-oxides/hydroxides. To overcome these challenges, dissolved Fe(II) (5.0 mM) and NaClO (2.6 mM) were injected into the studied aquifer to promote the formation of Fe oxides/hydroxides and to oxidize As(III) into As(V), thus removing aqueous As via adsorption and/or co-precipitation. During field experiment, As concentration in groundwater from the pumping well significantly decreased. Fe and As speciation calculations indicate that incorporation of negatively charged As(V) into goethite was the probable mechanism for As removal. Both chemical sequential extraction results and spectroscopic data also support that alternating injection of Fe(II) and NaClO can achieve aquifer Fe coating and immobilize As via adsorption onto Fe oxides/hydroxides. Geochemical modelling further confirms that although competition for sorption sites between As and other dissolved species is expected in the natural groundwater system, high surface area of the Fe oxides/hydroxides can provide sufficient sites for As retention. The ability to effectively decrease As concentration of in-situ aquifer Fe-coating technology indicates that this approach should have extensive applicability to similar high As groundwater occurred worldwide.