Multilevel Monitoring Wells Research Articles

Estimation of pollution sources and hydraulic conductivity field in a coastal aquifer under tidal effects

In the past decades, most of the efforts have often been invested in the joint identification of contamination and hydraulic conductivity field in inland subsurface aquifers. However, the booming economy and dense population around the coast have brought a new challenge to the forefront: the serious pollution of coastal aquifers. Coastal aquifers are highly vulnerable ecological areas, which are affected by inland pollutants carried by groundwater during its entry into the sea and are also influenced by seawater intrusion. This article addresses the inverse problem of coastal aquifer pollution, which is subject to tidal effects, using the Iterative Local Updating Ensemble Smoother (ILUES) algorithm. The adopted ILUES algorithm is leveraged to efficiently extract pollutant concentration and salinity data for the identification of the source release history and characterization of the hydraulic conductivity field of a site with multiple pollutant sources. This study is based on a variant of the traditional benchmark problem, analyzing the influence of algorithm parameters, tidal effects, and heterogeneity on data assimilation. The article further demonstrates the great potential of ILUES to address new challenges. In addition, the significance of multi-level monitoring wells for the estimation of hydraulic conductivity in coastal multi-layered heterogeneous aquifers is explained.

Development of an integrated hydrochemical index for delineating livestock manure-derived groundwater plumes in agro-livestock farming areas

To delineate a livestock manure-derived groundwater plume (LDGP) in agro-livestock farming areas with the extensive use of chemical fertilizers, multilevel monitoring wells (MLWs) were installed for depth-specific sampling of groundwater, and then hydrochemical, nitrate N-O isotopic and microbiological data of shallow groundwater collected from the MLWs were evaluated with the help of multivariate statistical tools. The LDGP was distinguished from the pervasive agricultural contamination based on δ15Nnitrate (10‰). Fecal coliforms and Escherichia coli were not observed in the LDGP, whereas bovine enterovirus type 2 was detected at a sample collected at a depth of 18 m below ground level (bgl) downgradient manure piles, indicating a potential virus risk. Among hydrochemical parameters, Ca2+, Mg2+, Na+, Cl- and SiO2(aq) were shown to be effective indicators to trace the LDGP. On the other hand, nitrate was not effective to discriminate the LDGP because of denitrification in deep parts (>6 m bgl) within the LDGP, although nitrate contamination was serious at shallow parts (≤6 m bgl; 62.4 to 119.1 mg/L, median 97.9 mg/L). Thus, an integrated hydrochemical index consisting of Ca2+, Mg2+, Na+, Cl- and SiO2(aq) was suggested based on the result of principal component analysis with isometric log-ratio transformed data to delineate the LDGP in shallow unconsolidated aquifers overlying silicate bedrocks, and a threshold of the index was determined to be 1.17. The application of the index to three other agro-livestock farming areas with prevalent agricultural contamination successfully distinguished the groundwater samples within the LDGP (11 of 40 samples) that showed the index values exceeding the threshold and a median δ15Nnitrate of 12.5 ‰. The study results show that the hydrochemical index can be used for the quick evaluation of a LDGP in agro-livestock farming areas given its cost efficiency and easily accessible analysis devices for water chemistry compared to isotopes, although the broad use of the index to determine LDGPs needs to be further verified by application to various geology and land uses. Livestock manures should be carefully managed to protect groundwater resources, including impermeable covers above or beneath the manure piles, given the potential risk of virus at deep parts and nitrate at shallow parts within the LDGP. The methodology to select hydrochemical indicators for distinguishing a LDGP and the combination of the indicators to develop an index suggested in this study will be useful to build new indices adapted to local conditions.

Advantages of employing multilevel monitoring wells for design of tunnels subjected to multi-aquifer alluvial

For tunnels being excavated through multiple aquifer formations, having precise knowledge of the aquifers’ hydraulic head becomes essential for determining groundwater inflow into the tunnel and analyzing its stability, specifically using multilevel monitoring systems. In the multi-aquifer alluvial section of the Glas tunnel (Iran), since the hydraulic head calculations were based on the data obtained from single-piezometer boreholes, the excavation risk was assessed to be at high level and the tunnel seemed to be unstable, thus an incorrect conclusion was derived from the misleading data. To take cost mitigation measures into account, it was necessary to calculate the hydraulic head at tunnel level accurately. By installing nested and clustered wells the mean hydraulic head was measured to be 70 m, significantly different from the 90 m previously determined by boreholes. Considering the updated value, the groundwater inflow and bulkhead load, formerly calculated as 0.65 m3/s and 9.5 bars, were determined to be 0.49 m3/s and 7.5 bars, respectively.

Six pilot‐scale studies evaluating the in situ treatment of PFAS in groundwater

AbstractPer‐ and polyfluoroalkyl substances (PFAS) have been identified by many regulatory agencies as emerging contaminants of concern in a variety of media including groundwater. Currently, there are limited technologies available to treat PFAS in groundwater with the most frequently applied approach being extraction (i.e., pump and treat). While this approach can be effective in containing PFAS plumes, previous studies of pump and treat programs have met with limited remedial success. In situ treatment studies of PFAS have been limited to laboratory and a few field studies. Six pilot‐scale field studies were conducted in an unconfined sand aquifer coimpacted by petroleum hydrocarbon along with PFAS to determine if a variety of reagents could be used to attenuate dissolved phase PFAS in the presence of petroleum hydrocarbons. The six reagents consisted of two chemical oxidants, hydrogen peroxide (H2O2) and sodium persulfate (Na2S2O8), and four adsorbents, powdered activated carbon (PAC), colloidal activated carbon (CAC), ion‐exchange resin (IER), and biochar. The reagents were injected using direct push technology in six permeable reactive zone (PRZ) configurations. Groundwater concentrations of various PFAS entering the PRZs ranged up to 24,000 µg/L perfluoropentanoic acid, up to 6,200 µg/L pentafluorobenzoic acid, up to 16,100 µg/L perfluorohexanoic acid, up to 6,080 µg/L perfluoroheptanoic acid, up to 450 µg/L perfluorooctanoic acid, and up to 140 µg/L perfluorononanoic acid. Performance groundwater sampling within and downgradient of the PRZs occurred for up to 18 months using single and multilevel monitoring wells. Results of groundwater sampling indicated that the PFAS were not treated by either the persulfate nor the peroxide and, in some cases, the PFAS increased in concentration immediately following the injection of peroxide and persulfate. Concentrations of PFAS in groundwater sampled within the PAC, CAC, IER, and biochar PRZs immediately after the injection were determined to be less than the method detection limits. Analyses of groundwater samples over the 18‐month monitoring period, indicated that all the PRZs exhibited partial or complete breakthrough of the PFAS over the 18‐month monitoring period, except for the CAC PRZ which showed no PFAS breakthrough. Analysis of cores for the CAC, PAC, and biochar PRZs suggested that the CAC was uniformly distributed within the target injection zone, whereas the PAC and biochar showed preferential injection into a thin coarse‐sand seam. Similarly, analysis of the sand packs of monitoring wells installed before the injection of the CAC, PAC, and biochar indicated that the sand packs of the PAC and biochar preferentially accumulated the reagents compared with the reagent concentrations within the surrounding aquifer by up to 18 times.

Fate and transport of sulfidated nano zerovalent iron (S-nZVI): A field study

Treatment of nano zerovalent iron (nZVI) with lower valent forms of sulfur compounds (sulfidation) has the potential to increase the selectivity and reactivity of nZVI with target contaminants and to decrease inter-particle aggregation for improving its mobility. These developments help in addressing some of the long-standing challenges associated with nZVI-based remediation treatments and are of great interest for in situ applications. Herein we report results from a field-scale project conducted at a contaminated site. Sulfidated nZVI (S-nZVI) was prepared on site by first synthesizing carboxymethyl cellulose (CMC) stabilized nZVI with sodium borohydride as a reductant and then sulfidating the nZVI suspension by adding sodium dithionite. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS) of CMC-S-nZVI, from synthesis barrels, confirms the presence of both discrete spherical nZVI-like particles (∼90 nm) as well as larger irregular structures (∼500 nm) comprising of iron sulfides. This CMC-S-nZVI suspension was gravity fed into a sandy material and monitored through multiple multi-level monitoring wells. Samples collected from upstream and downstream wells suggest very good radial and vertical iron distribution. TEM-EDS analysis from the recovered well samples also indicates the presence of both nZVI-like particles as well as the larger flake-like structures, similar to those found in the injected CMC-S-nZVI suspension. This study shows that S-nZVI stabilized with CMC can be safely synthesized on site and is highly mobile and stable in the subsurface, demonstrating for the first time the field applicability of S-nZVI.

Characterizing scales of hydrogeological heterogeneity in ice-marginal sediments in Wisconsin, USA

Creating a geological framework for hydrogeological investigations in ice-marginal settings is challenging, as the dynamic nature of ice margins results in heterogeneous sediments and complex geometries that are difficult to characterize geologically and hydraulically. In this study, a multi-scale and multi-tool approach was used to develop a robust and hydraulically calibrated conceptual model of a site in Wisconsin (USA), which constrains the subsurface geologic and hydraulic heterogeneity onsite and can be utilized offsite and in similar settings. Facies analysis, including detailed logging of sediment cores integrated with geophysical surveys, was used to reconstruct the glacial history of the site and to identify key sedimentation styles. Depth-discrete multilevel monitoring wells were used to measure hydraulic head and to explore the relationship between hydraulic conditions and stratigraphy. Results indicate that hydraulic and geological heterogeneity do not necessarily coincide, as hydraulic head profiles of the heterogeneous sediments showed little head change with depth. The minimal observed hydraulic heterogeneity in this complex geologic system is likely due to relatively small and spatially limited differences in hydraulic conductivities, limited water-table relief, and minimal natural or induced transience to enhance local head differentials. Hydraulic data were also used to provide new insights on the style and scale of geologic heterogeneity known to influence permeability within ice-marginal sediments; this is especially relevant at the study site where fine-scale bedding (millimeters to tens of centimeters scale) and lateral continuity of units (meters to hundreds of meters scale) influence water recharge and dense nonaqueous phase liquids migration and distribution.

Groundwater recharge, flow and stable isotope attenuation in sedimentary and crystalline fractured rocks: Spatiotemporal monitoring from multi-level wells

The use of stable isotopes from precipitation as tracers in unconsolidated porous media can be limited due to attenuation in the unsaturated zone, yet this process has been poorly constrained in fractured rock. The complexities of fractured rock systems necessitate the use of multi-level monitoring wells for the measurement of isotopic tracers with depth. These methods, however, are rarely employed in groundwater flow and recharge studies, particularly involving temporal isotope measurements in multiple rock types. To understand the attenuation of isotopic snowmelt tracers in fractured rock and explore surface connections and recharge, an investigation was undertaken at three geologically-distinct field sites in south-eastern Ontario, Canada. Results of constant head injection tests at 2-m intervals were used to identify and isolate high transmissivity zones (2–3 in each well) for the construction of eight multi-level wells (22 intervals) used for groundwater sampling. Deuterium and oxygen-18 were measured in rain (n = 64), snow (n = 67), soil water (n = 118) and groundwater (n = 594) on a bi-weekly to monthly basis over two seasonal snowmelt cycles (>1.5 years). In combination with stable isotopes, measurements of hydraulic head, groundwater temperature, specific conductance and two rounds of tritium sampling contributed to the development of a conceptual model for snowmelt recharge and flow in sandstone, limestone and crystalline rock. Seasonal variation of δ2H and δ18O was observed in all rock types indicating recent recharge and strong surface connections. The dampening and residence time of the snowmelt tracer increased with depth in crystalline rock. Tracer attenuation was highest in higher-porosity sedimentary rock and strong confining conditions were observed to inhibit recent recharge. Substantial isotopic attenuation measured in the overburden indicated that the strong snowmelt signal measured in groundwater stemmed from water infiltrating through areas with thin or no overburden. Identification of the periodic trends and distinct isotopic horizons in this study would not have been possible without the use of multi-level wells.

Lithologic Control of the Hydrochemistry of a Point-Bar Alluvial Aquifer at the Low Reach of the Nakdong River, South Korea: Implications for the Evaluation of Riverbank Filtration Potential

To assess the groundwater−river water interaction in a point-bar alluvial aquifer as a crucial step in site assessment for riverbank filtration, hydrochemical and hydrogeologic investigations were performed on a riverine island at the low reach of the Nakdong River, South Korea. The site was evaluated for the application of large-scale bank filtration. Unconsolidated sediments (~40 m thick) of the island comprise fine- to medium-grained sand (upper aquifer), silty sand with clay intercalations, and sandy gravel (lower aquifer) in descending order. The intermediate layer represents an impermeable aquitard and extends below the river bottom. A total of 66 water samples were collected for this study; groundwater (n = 57) was sampled from both preexisting irrigation wells, and three multi-level monitoring wells (each 35 m deep). Groundwater chemistry is highly variable, but it shows a distinct hydrochemical change with depth: shallow groundwater (<25 m deep) from the upper aquifer is characteristically enriched in NO3− and SO42−, due to agricultural contamination from the land surface, while deeper groundwater (>25 m deep) from the lower aquifer is generally free of NO3− and relatively rich in F. The lower aquifer groundwater is also higher in pH, and concentrations of K+, Mg2+, and HCO3−, indicating that the aquifer is likely fed by regional groundwater flow. Such separation of groundwater into two water bodies is the result of the existence of an impermeable layer at intermediate depth. In addition, the hyporheic flow of river water is locally recognized at the upstream part of the upper aquifer as the zone of low TDS (Total Dissolved Solids) values (<200 mg/L). This study shows that the study site does not seem to be promising for large-scale riverbank filtration because 1) the productive, lower aquifer is not directly connected to the bottom of the river channel, and 2) the upper aquifer is severely influenced by agricultural contamination. This study implies that the subsurface hydrogeologic environment should be carefully investigated for site assessment for riverbank filtration, which can be aided by a detailed survey of groundwater chemistry.

Security of Deep Groundwater in the Coastal Bengal Basin Revealed by Tracers

AbstractUncertainty persists regarding the vulnerability of deep groundwater across Asia's megadeltas. In the coastal Bengal Basin aquifer system, shallow groundwater (<100 m) commonly features high salinity or arsenic concentrations, and deep, better‐quality, groundwater supplies drinking water to >80 million people. Here we report new radiocarbon evidence from a network of nine dedicated, multilevel monitoring wells, which indicates residence times of between 103 and 104 years for groundwater at depths >150 m. Modern groundwater detected in some deep abstraction wells using anthropogenic tracers (SF6, CFCs) is attributed to short circuiting of shallow groundwater within wells. Age‐depth profiles and hydrochemical data in monitoring wells confirm the regional resilience of deep groundwater to ingress of shallow contaminated groundwater. Our results are consistent with high regional anisotropy in the aquifer and support continued use of deep groundwater though the potential for leakage of shallow contaminated groundwater in deep abstraction wells requires careful monitoring.

Controlled Release Test Facility to Develop Environmental Monitoring Techniques for Geologically Stored CO2 in Korea

Two sets of controlled release test facility (EIT) were constructed in unsaturated and saturated zones by the Korea CO2 Storage Environmental Management (K-COSEM) Research Center to develop efficient and accurate environmental monitoring techniques required for secure geologic carbon storage. The unsaturated zone EIT consists of a 51 m long horizontal well for artificial CO2 leakage at the depth of 2.5 m below the land surface, a surface injection control system, and diverse monitoring devices. Following baseline surveys, a preliminary CO2 release test was first performed for 4 days in October 2015, with the released amount of about 390 kg CO2. The saturated zone EIT to develop efficient monitoring techniques in intermediate depths was constructed in the end of 2015. It consists of an injection well at the depth of 24 m and 8 multi-level monitoring wells. In 2016, long-term CO2 release tests using EITs have been performed. The outlines of EITs of K-COSEM are summarized in this paper.

Large water-table response to rainfall in a shallow bedrock aquifer having minimal overburden cover

Rapid recharge events manifested as significant increases in hydraulic head have been observed in many fractured bedrock aquifers around the world. Often the response in hydraulic head exceeds what would be observed in an equivalent porous media by more than an order of magnitude. As the mechanisms that cause these events are poorly understood particularly under highly-transient conditions, a detailed investigation was conducted at a well-characterized field site in eastern Canada. During the spring and summer of 2012, frequent measurements of hydraulic head were obtained in gneissic terrain covered by a thin veneer of drift materials using 21 multi-level monitoring wells installed in the bedrock. Each of the wells was hydraulically tested from the water table to total depth using a straddle-packer system and fractures intersecting the wells were identified using a borehole camera prior to the construction of the multi-level piezometers. Rainfall and weather data were also collected over the same time period. A piezometer located on a bedrock outcrop which responded rapidly to rainfall was identified and used as a focus for numerical simulations. To determine the properties of the drift materials in the vicinity of the outcrop, a ground penetrating radar (GPR) survey was conducted over a 40×40m area to map depth to bedrock and five in-situ permeameter tests were performed to estimate the hydraulic conductivity. Three-dimensional numerical simulations were conducted to reproduce the response in the piezometer for both short (24h) and long (one month) timescales. The numerical simulations were used to determine what parameters have the greatest impact on controlling rapid recharge. Based on this study it was concluded that the large magnitude head rises recorded in this piezometer are a result of recharge to steeply inclined fractures exposed on or immediately adjacent to the outcrop. The hydraulic head responds rapidly because of the low specific yield of the rock to which the transmissive features are connected. The modelling also showed that as little as 0.4m of drift material can completely eliminate the response in the well especially during times when evapotranspiration is high.

Use of above-zone pressure data to locate and quantify leaks during carbon storage operations

We determine the potential accuracy in estimating the size and location of leaks in the caprock by history-matching above-zone pressure monitoring data to leakage models. The number of monitoring wells and the total monitoring time is varied in order to determine minimum pressure-monitoring requirements for effective leak characterization. The history-matching procedure uses particle swarm optimization to minimize the error between leakage-model solutions and synthetic observation data by adjusting model parameters governing the size and location of the leak, as well as the heterogeneous permeability field throughout the model. The method applies to storage projects with uncertain heterogeneous geology that may be described by a variogram model. Results from several examples indicate that as little as 6–12 months of above-zone pressure monitoring data, collected from at least 3 or 4 wells, may be sufficient to locate and estimate the size of a leak to inform mitigation and remediation strategies. No significant benefit is seen when using multilevel monitoring wells versus single-level monitoring wells. We also find that adding white noise to the synthetic observation data, with magnitude consistent with current pressure monitoring techniques, generally reduces error in solutions, likely due to a regularization effect. Taken in total, the results and procedures introduced in this study should be of use in designing monitoring strategies in large-scale carbon storage projects.

Resistivity and induced polarization monitoring of biogas combined with microbial ecology at a brownfield site

The accumulation of biogenic greenhouse gases (methane, carbon dioxide) in organic sediments is an important factor in the redevelopment and risk management of many brownfield sites. Good practice with brownfield site characterization requires the identification of free-gas phases and pathways that allow its migration and release at the ground surface. Gas pockets trapped in the subsurface have contrasting properties with the surrounding porous media that favor their detection using geophysical methods. We have developed a case study in which pockets of gas were intercepted with multilevel monitoring wells, and their lateral continuity was monitored over time using resistivity. We have developed a novel interpretation procedure based on Archie’s law to evaluate changes in water and gas content with respect to a mean background medium. We have used induced polarization data to account for errors in applying Archie’s law due to the contribution of surface conductivity effects. Mosaics defined by changes in water saturation allowed the recognition of gas migration and groundwater infiltration routes and the association of gas and groundwater fluxes. The inference on flux patterns was analyzed by taking into account pressure measurements in trapped gas reservoirs and by metagenomic analysis of the microbiological content, which was retrieved from suspended sediments in groundwater sampled in multilevel monitoring wells. A conceptual model combining physical and microbiological subsurface processes suggested that biogas trapped at depth may have the ability to quickly travel to the surface.

Effects of Glacial Sediment Type and Land Use on Nitrate Patterns in Groundwater

AbstractGrowing population centers such as those in southern Ontario rely on fractured bedrock aquifers for drinking water. A threat to these aquifers is posed by surficial nonpoint‐source pollution infiltrating with rainwater and moving through the overlying Quaternary glacial deposits. Investigation of local unconsolidated sediments, and the factors affecting contaminant transport through these, is needed to assess risks to the quality of underlying groundwater resources. In this study, sites with a variety of agricultural land management practices and glacial geologic settings were investigated by employing high‐resolution data collection methods. Geologic data from continuous sediment cores were combined with depth‐discrete hydrogeologic and geochemical parameter measurements using high‐resolution multilevel monitoring wells. Within a relatively small geographic area with three distinct glacially derived sediment types, the three sites exhibited greatly disparate vulnerability to nitrate contamination. The geologic setting, including surface topography and architecture and heterogeneity of sediment types at depth, influenced groundwater flow paths and water geochemistry, and subsequently, nitrate distribution. Although management practices influence the quantity of pollutants leaching to groundwater resources, the physical and chemical properties of the subsurface related to the geologic setting ultimately determine the persistence or attenuation of nitrate, and therefore become important to characterize when evaluating best nutrient and waste management practices.

Evaluating TCE Abiotic and Biotic Degradation Pathways in a Permeable Reactive Barrier Using Compound Specific Isotope Analysis

AbstractA pilot‐scale zero valent iron (ZVI) Permeable Reactive Barrier (PRB) was installed using an azimuth‐controlled ‐vertical hydrofracturing at an industrial facility to treat a chlorinated Volatile Organic Compound (VOC) plume. Following ZVI injection, no significant reduction in concentration was observed to occur with the exception of some multilevel monitoring wells, which also showed high levels of total organic carbon (TOC). These patterns suggested that the zero valent iron was not well distributed in the PRB creating leaky conditions. The geochemical data indicated reducing conditions in these areas where VOC reduction was observed, suggesting that biotic processes, associated to the guar used in the injection of the iron, could be a major mechanism of VOC degradation. Compound‐Specific Isotope Analysis (CSIA) using both carbon and chlorine stable isotopes were used as a complementary tool for evaluating the contribution of abiotic and biotic processes to VOC trends in the vicinity of the PRB. The isotopic data showed enriched isotope values around the PRB compared to the isotope composition of the VOC source confirming that VOC degradation is occurring along the PRB. A batch experiment using site groundwater collected near the VOC source and the ZVI used in the PRB was performed to evaluate the site‐specific abiotic isotopic fractionation patterns. Field isotopic trends, typical of biodegradations were observed at the site and were different from those obtained during the batch abiotic experiment. These differences in isotopic trends combined with changes in VOC concentrations and redox parameters suggested that biotic processes are the predominant pathways involved in the degradation of VOCs in the vicinity of the PRB.

Geologically controlled agricultural contamination and water–rock interaction in an alluvial aquifer: results from a hydrochemical study

Hydrogeochemistry data collected from three multi-level monitoring wells in a sandy alluvial aquifer located in the Keum River watershed, South Korea, are used in this study to evaluate groundwater chemistry change in relation to geologic controls of groundwater recharge, redox condition, water–rock interaction and contamination susceptibility. A silt layer at a depth of about 10 m is a hydrologic barrier which prohibits the continued downflow of groundwater and divides the groundwater into two distinct masses (i.e., shallow ‘oxic’ groundwater and deeper ‘sub-oxic’ groundwater) with different redox states and ion chemistry. Compared to ‘sub-oxic’ water, ‘oxic’ water has lower SiO2/(Na + K) and higher Ca, NO3 and SO4, indicating significant contamination from agrochemicals and manures. The higher Ca and NO3 but lower HCO3 concentrations in ‘oxic’ water result from lime dissolution and nitrification. In contrast, ‘sub-oxic’ water shows increased SiO2 and HCO3, suggesting the effect of water–rock interaction in the aquifer. Potential sources of cations are Na from albite, Mg from chlorite and illite, and K from illite. Vertical fluctuations of cationic composition in ‘sub-oxic’ water suggest heterogeneity of aquifer mineralogy with depth. Mineral stability calculations show that the chemistry of ‘oxic’ water plots in the kaolinite field, while that of ‘sub-oxic’ water plots in the Ca-montmorillonite field and near the kaolinite/montmorillonite equilibrium boundary. Concentrations of dissolved inorganic carbon (DIC) in ‘sub-oxic’ water are much higher than total alkalinity (HCO3 + CO3), suggesting that CO2 generated by redox reactions such as denitrification, iron reduction and sulfate reduction causes increasing DIC, which also affects the dissolution of silicate minerals in an aquifer free of carbonate minerals.

Geochemistry of natural chromium occurrence in a sandstone aquifer in Bauru Basin, São Paulo State, Brazil

Anomalous concentrations of Cr(VI) occur in groundwaters of the Adamantina Aquifer, in a large region in the western state of São Paulo, sometimes exceeding the potability limit (0.05 mg L−1). To identify the possible geochemical reactions responsible for the occurrence of Cr in groundwater in Urânia, borehole rock samples were collected in order to carry out mineralogical and chemical analyses. In addition, multilevel monitoring wells were installed and groundwater samples were analyzed. Analyses of the borehole rock samples show the occurrence of a geochemical anomaly of Cr in the quartzose sandstones (average concentrations of 221 ppm). Chrome-diopside is one of the main minerals contributing to this anomaly, having an average Cr content of 1505 ppm. Sequential extraction experiments indicated weakly adsorbed Cr in the order of 0.54 ppm, and this quantity is enough to provide the Cr concentrations observed in groundwater. Groundwaters from the monitoring wells proved to be stratified, with the highest concentrations of Cr(VI) (0.13 mg L−1) being associated with high redox and pH values (over 10) and high concentrations of Na. Geochemical reactions that may explain the release of Cr from the solid phase to groundwater involve the release of Cr(III) from minerals (like chrome-diopside and Cr–Fe hydroxide), followed by oxidation of Cr(III) to Cr(VI), probably related to the reduction of Mn oxides present in the aquifer. Then cation exchange occurs and dissolution of carbonates which increases the pH of groundwater, resulting in the desorption and mobilization of Cr(VI) into groundwater.

Influence of nanoscale zero-valent iron on geochemical properties of groundwater and vinyl chloride degradation: A field case study

A 200m2 pilot-scale field test successfully demonstrated the use of nanoscale zero-valent iron (NZVI) for effective remediation of groundwater contaminated with chlorinated organic compounds in Taiwan within six months. Both commercially available and on-site synthesized NZVI were used. A well-defined monitoring program allowing to collect three-dimensional spatial data from 13 nested multi-level monitoring wells was conducted to monitor geochemical parameters in groundwater. The degradation efficiency of vinyl chloride (VC) determined at most of monitoring wells was 50–99%. It was found that the injection of NZVI caused a significant change in total iron, total solid (TS) and suspended solid (SS) concentrations in groundwater. Total iron concentration showed a moderate and weak correlation with SS and TS, respectively, suggesting that SS may be used to indicate the NZVI distribution in groundwater. A decrease in oxidation–reduction potential (ORP) values from about −100 to −400mV after NZVI injection was observed. This revealed that NZVI is an effective means of achieving highly reducing conditions in the subsurface environment. Both VC degradation efficiency and ORP showed a correlative tendency as an increase in VC degradation efficiency corresponded to a decrease of ORP. This is in agreement with the previous studies suggesting that ORP can serve as an indicator for the NZVI reactivity.

Relações Água-Rocha e a Hidrogeoquímica do Cromo na Água Subterrânea de Poços de Monitoramento Multiníveis de Urânia, SP, Brasil

Teores anômalos e naturais de cromo ocorrem nas águas subterrâneas do Aquífero Adamantina no município de Urânia (SP) e em uma ampla região do oeste do Estado de São Paulo, algumas vezes ultrapassando o limite de potabilidade (0,05 mg.L-1). Visando identificar as possíveis reações geoquímicas que justificam a ocorrência do cromo na água subterrânea em Urânia, foram realizadas perfurações com coleta de amostras contínuas de rocha para a condução de análises mineralógicas e químicas, construídos poços de monitoramento multiníveis e realizadas coletas e análises de amostras estratificadas de água subterrânea. Análises das amostras de testemunhos de sondagem indicaram a ocorrência de uma anomalia geoquímica de cromo (concentrações médias de 221 ppm), sendo o diopsídio cromífero provavelmente o mineral geoquimicamente mais reativo que contribui para esta anomalia, apresentando concentrações de cromo de 1.000 a 6.000 ppm. As análises químicas de amostras de água coletadas dos poços de monitoramento indicaram uma estratificação hidroquímica do aquífero: águas na base do aquífero apresentam pH anomalamente alcalino (superior a 10), enquanto águas mais rasas possuem pH neutro a ligeiramente ácido. O cromo ocorre predominantemente na forma hexavalente e alcança concentrações máximas de 0,13 mg.L-1. As reações geoquímicas que explicam a passagem do cromo da fase sólida para a água provavelmente envolvem a dissolução de minerais contendo Cr3+ (diopsídios), seguida de uma reação redox que oxida o Cr3+ para o Cr6+, provavelmente relacionada com a redução de óxidos de manganês presentes no aquífero. Adicionalmente, devem também ocorrer reações de adsorção, sendo que os ambientes de pH elevados favorecem a dessorção e mobilização do Cr6+ para a água

Results of geodetic and geotechnical monitoring of subsidence for Taiwan High Speed Rail operation

Taiwan High Speed Rail (THSR), which began operations in January 2007, passes through an area in Yunlin County where the largest cumulative subsidence measured during 1992–2006 exceeds 100 cm. Leveling benchmarks, GPS pillars and multi-level monitoring wells were deployed in this area to collect detailed subsidence data from October 2003 to 2006. Leveling is carried out on both ground benchmarks and survey bolts attached to THSR columns. Minimum constraint solutions of leveling networks produce estimated heights accurate to a few mm. Special attention is paid to code smoothing, ionospheric, tropospheric and ocean tidal loading (OTL) effects, so that height estimates from GPS are optimal. Leveling and GPS-derived height changes are consistent to 1 cm, and show that from Stations 210 to 240K of TSHR, the subsidence is bowl shaped. Measurements of sediment compaction in specific depth intervals at three monitoring wells indicate that most of the subsidence is caused by sediment compaction at depths from 50 to 300 m. The major compaction occurs in the interval 220–300 m and is attributed to ground water withdrawal. Large angular deflections as determined from subsidence measurements are detected at some columns, but are below the upper bound (1/1,000) of tolerance specified in the safety code. With the current subsidence and sediment compaction, no significantly reduced loading capacity of the columns is expected to occur. For a safe THSR operation, subsidence and sediment-compaction monitoring should be continued, and current ground water withdrawal in Yunlin must be reduced or stopped.