Existing Monitoring Wells Research Articles

Delineation of a PFOA Plume and Assessment of Data Gaps in its Conceptual Model Using PlumeSeeker™.

An accurate conceptual site model (CSM) and plume-delineation at contamination sites are pre-requisites for successful remediation and for satisfying regulators and stakeholders. PlumeSeeker™ is well-suited for assessing data gaps in CSMs by using available site data and for identifying the optimal number and locations of sampling locations to delineate contaminant plumes. It is an enhancement of a university research code for plume delineation using geostatistical and stochastic modeling integrated with the groundwater modeling software MODFLOW-SURFACT™. PlumeSeeker™ increases the overall confidence in the location of the plume boundary through a variance-reduction approach that selects existing- or new monitoring wells for sampling based on minimizing the uncertainty in plume boundary and on new field information. Applicable at sites with or without existing monitoring wells, PlumeSeeker™ is particularly powerful for optimally allocating project resources (labor, well installation, and laboratory costs) between existing wells and sampling at new locations. An application of PlumeSeeker™ at Lakehurst, the naval component of Joint Base McGuire-Dix-Lakehurst in New Jersey, demonstrates how the cost of delineating the migration pathway of a perfluorooctanoic acid (PFOA) plume can be minimized by requiring only 9 new sampling locations in addition to samples from 2 existing wells for achieving a 70% reduction in plume uncertainty. In addition, the use of available site data in three different scenarios identified CSM data-gaps in the source area and in the interaction between Manapaqua Branch and groundwater, where the observed high concentration in this area could have resulted from a combination of groundwater migration and induced infiltration.

Optimization design of quality monitoring network of Urmia plain using genetic algorithm and vulnerability map

Contamination and seawater intrusion are esteemed as chief issues of coastal aquifers resulting from unscientific utilization, non-standard monitoring network and improper management. In the present study, an optimum monitoring network with appropriate numbers and standard spatial distribution was designed based on a vulnerability map of the Urmia coastal aquifer. A vulnerability map was extracted using a modified GALDIT-iP model and searching optimum network was done based on genetic algorithm (GA). The maximum value of the correlation between electrical conductivity (EC) and vulnerability index, the minimum number of monitoring wells and the highest value of Nash-Sutcliff were utilized for the simultaneous optimization model. The W-weighting coefficient was considered for economical goals and three targets were defined in a general objective function. The results showed that the W-weighting coefficient has a significant effect to determine optimal solution, and the best weighting was opted considering the most optimal response based on the precise of the monitoring network and vulnerability index. An acceptable optimization and validation process was obtained with the predictions of the validation results. For W = 1, the final value of the objective function was obtained 1.791 using 91 wells with a correlation coefficient of 0.935 and Nash-Sutcliff of 0.979, resulting in the appropriate spatial distribution of the wells and reduction of 18 wells from the existing monitoring wells.

Spatiotemporal optimization of groundwater monitoring networks using data-driven sparse sensing methods

Abstract. Groundwater monitoring and specific collection of data on the spatiotemporal dynamics of the aquifer are prerequisites for effective groundwater management and determine nearly all downstream management decisions. An optimally designed groundwater monitoring network (GMN) will provide the maximum information content at the minimum cost (Pareto optimum). In this study, PySensors, a Python package containing scalable, data-driven algorithms for sparse sensor selection and signal reconstruction with dimensionality reduction is applied to an existing GMN in 1D (hydrographs) and 2D (gridded groundwater contour maps). The algorithm first fits a basis object to the training data and then applies a computationally efficient QR algorithm that ranks existing monitoring wells (for 1D) or suitable sites for additional monitoring (for 2D) in order of importance, based on the state reconstruction of this tailored basis. This procedure enables a network to be reduced or extended along the Pareto front. Moreover, we investigate the effect of basis choice on reconstruction performance by comparing three types typically used for sparse sensor selection (i.e., identity, random projection, and SVD, respectively, PCA). We define a gridded cost function for the extension case that penalizes unsuitable locations. Our results show that the proposed approach performs better than the best randomly selected wells. The optimized reduction makes it possible to adequately reconstruct the removed hydrographs with a highly reduced subset with low loss. With a GMN reduced by 94 %, an average absolute reconstruction accuracy of 0.1 m is achieved, in addition to 0.05 m with a reduction by 69 % and 0.01 m with 18 %.

Rapid Field Approach to Evaluating Natural Source Zone Depletion for a Range of Light Non‐Aqueous Phase Liquid Sites

AbstractMore rapid cost‐effective approaches are required to enable field evaluation of natural source zone depletion (NSZD) at petroleum LNAPL sites. Long and costly cleanups have continued as NSZD has not become widely acceptable as a remedial option. We have developed a low cost rapid field approach (RFA) and expanded the site conceptual models (SCMs) to include NSZD. Results from two sites are presented that illustrate the benefit of the RFA in supplementing existing data and updating SCMs. At present, most NSZD guidelines follow a single generic SCM (aerobic oxidation of methane) as the primary contributor to NSZD. Aerobic oxidation of LNAPL is also possible, and can be documented by subsurface measurements of soil‐gas composition and/or heat generated from biodegradation. The RFA was developed so that sites could be easily separated into those with and without a methane plume overlying LNAPL. This distinction is important as it defines the relative rates of oxygen transport into the soil, and methane generation and release from saturated zone anaerobic biodegradation. For the RFA, evidence for direct LNAPL oxidation is absence of methane (<0.1%‐v) in the unsaturated zone above LNAPL. This measurement can be readily made in the field using existing monitoring wells. The absence of a methane plume above LNAPL would indicate that NSZD is a result of direct biodegradation of LNAPL. This alternative SCM needs to be considered and evaluated prior to implementing more costly data collection or other remediation or site management options.

Retraction Note: Analysis of sponge city water pollution characteristics and urban music emotional expression based on GIS system

During the construction of the sponge city, the impact of rainwater facilities on the underground is very important, so it has received great attention. These rainwater facilities can affect groundwater quality and water level and are currently the focus of international personnel. This thesis focuses on the impact of concentrated rainwater infiltration on groundwater level and water quality and focuses on two methods of on-site monitoring and simulation prediction of rainwater infiltration wells in sponge cities. This study collected meteorological, hydrological, and geological conditions. From May to October 2018 and June to December 2020, the deep burying of some underground wells in sponge city was monitored, and groundwater samples were collected to determine the content of TN, NO3-N, NH4+-N, and TP. Then, the existing monitoring wells were used in sponge city to establish a conceptual model of groundwater, conduct sensitivity analysis on the generalization of hydrogeological parameters, and predict the depth of burial in the future water level. As the process of urbanization in China continues to accelerate, the exogenous indicators represented by the urban population are no longer sufficient to reflect the deep connotation of the process of urbanization in China. Therefore, as an endogenous motivation, the audience’s sense of belonging to the city and the identification of the city’s identity have increasingly become an important topic for scholars when discussing the process of urbanization in China. Urban music is just one of the cultural microcosms of the process of urbanization in China. It not only gives the city a field of expression outside the physical space and effectively connects the two subjects of the city and the audience but also enables the audience to listen to and interpret urban music, to continuously enhance the sense of belonging to the city and construct the identity of the city.

A comparison of three methods to assess natural source zone depletion at paved fuel retail sites

Natural source zone depletion (NSZD) encompasses all processes that result in petroleum hydrocarbon light non-aqueous phase liquid (LNAPL) mass loss. Vertical gas transport between the subsurface and atmosphere is a key component of NSZD. Gas exchange with the atmosphere may be restricted at sites with ground cover, which is typical for European fuel retail sites. This raises questions of whether, and to what extent, the generic NSZD conceptual model applies at these sites. Here, we present a study that evaluated how concrete and asphalt pavement affected NSZD processes and data interpretation for three NSZD assessment methods: soil gas concentration gradient, biogenic heat and carbon dioxide traps. All methods demonstrated that NSZD was occurring and NSZD rates were generally within the low end of values reported in the literature for unpaved sites. However, there was considerable variability in the rates, which highlights the need for careful examination of the conceptual site model and potential interferences for each method. The results demonstrate the viability of soil gas and temperature data collected from existing monitoring wells screened into the unsaturated zone without the need for additional, intrusive subsurface installations. The results also provide useful guidance for developing optimal long-term NSZD monitoring approaches, where necessary.

Source Analysis of Nitrate Nitrogen in Groundwater Based on Different Modes of Land use

Background: The study area is located in the Hebei province Luanxian section along the Luanhe River. The right of Luanhe River is arranged with 3 monitoring sections and is laid with 5 wells in the A-A section and B-B section and 1 well on the floodplain (inside the dike), the distance from the river level is about 10m. The distance between the 4 wells outside the dike is 20m, 50m, 200m and 1500m respectively. 2 wells are arranged on the C-C section, so there are 13 observation wells in the study area. 3# wells and 11# wells have used existing monitoring wells, so 11 wells have been set up actually. The monitoring wells are designed to monitor groundwater level, groundwater quality and isotope analysis. Methods: In the upstream of Luanhe’s A-A section of the No. 4 well near the edge of the woods and downstream of the B-B section of the No. 8 well near the edge of farmland (wheat), each point of soil is sampled to determine the content of nitrate nitrogen and 15N isotope ratio in soil profile. The depth of soil is sampled from surface to groundwater level, the total depth is about 2.5m. Taking about 1kg soil samples at 20cm every time from the ground surface and collect and seal in plastic bags and number. The sampling time was November 2015, March 2016, May 2016 and July 2016, taking 4 times. Results: The analysis showed that higher nitrate content in soil is within 0-60cm, and is same as the depth of fertilizing the crops (including animal manure), so when estimating the proportion of groundwater in nitrate, nitrate content in 0-60cm is assumed as the initial value; which is close to the underground water level in the soil and is regarded as the final value of 60cm, that is, the final value of nitrate into the groundwater, which can estimate the soil nitrate into groundwater ratio. Conclusion: The sources of nitrate in groundwater are complex and related to many factors, such as river water, soil and so on. According to the content of nitrogen and oxygen isotopes, it determines the main source of nitrate nitrogen:soil and Luanhe River. The results showed that the ratio of nitrate to groundwater in the forest soil was 20%~80.8%, and the average value was 56.3%. The proportion of nitrate in the soil in the wheat field was 6.8%~98.2%, with an average value of 48.3%. Nitrate nitrogen from water in proportion of large changes with the seasons, the upper reaches of the Luanhe River (near No. 2 wells) for 0~19.6%; the Luanhe River (near No. 7 wells) for 0~31.5%.

High Resolution Monitoring Above and Below the Groundwater Table Uncovers Small-Scale Hydrochemical Gradients.

Hydrochemical solute concentrations in the shallow subsurface can be spatially highly variable within small scales, particularly at interfaces. However, most monitoring systems fail to capture these small scale variations. Within this study, we developed a high resolution multilevel well (HR-MLW) with which we monitored water across the interface of the unsaturated and saturated zone with a vertical resolution of 0.05-0.5 m. We installed three of these 4 m deep HR-MLWs in the riparian zone of a third-order river and analyzed for hydrochemical parameters and stable water isotopes. The results showed three distinct vertical zones (unsaturated zone, upper saturated zone, lower saturated zone) within the alluvial aquifer. A 2 m thick layer influenced by river water (upper saturated zone) was not captured by existing monitoring wells with higher screen length. Hydrochemical data (isotopes, total ions) were consistent in all HR-MLWs and showed similar variation over time emphasizing the reliability of the installed monitoring system. Further, the depths zones were also reflected in the NO3-N concentrations; with high spatial variabilities between the three wells. The zonation was constant over time, with seasonal variability in the upper saturated zone due to the influence of river water. This study highlights the use of high resolution monitoring for identifying the spatial and temporal variability of hydrochemical parameters present in many aquifer systems. Possible applications range from riparian zones, agricultural field sites to contaminated site studies, wherever an improved understanding of biogeochemical turnover processes is necessary.

A spatiotemporal Bayesian maximum entropy-based methodology for dealing with sparse data in revising groundwater quality monitoring networks: the Tehran region experience

Data inadequacy is a common problem in designing or updating groundwater monitoring systems. The developed methodologies for the optimal design of groundwater monitoring systems usually assume that there is a complete set of data obtained from existing monitoring wells and provide a revised configuration for the system by analyzing the current data. These methodologies are not usually applicable when the current groundwater quantity and quality data are highly sparse. In this paper, a new simulation–optimization approach based on Bayesian maximum entropy theory (BME) is proposed for revising spatial and temporal monitoring frequencies in a sparsely monitored aquifer. The BME is used to simulate the spatial and spatiotemporal variations of groundwater indicators, incorporating the space/time uncertainties due to insufficient data. Comparing the obtained estimations with observations, the best BME model was selected to be linked with an optimization model. The main goal of optimization was to find out the spatial and temporal sampling characteristics of the monitoring stations using the concepts of Entropy theory and a groundwater vulnerability index. The results show the BME estimations are less biased and more accurate than Ordinary Kriging in both spatial and spatiotemporal analysis. The improvements in the BME estimates are mostly related to incorporating hard (accurate) and soft (uncertain) data in the estimation process. The applicability and efficiency of the proposed methodology have been evaluated by applying it to the Tehran aquifer in Iran which is suffering from high groundwater table fluctuations and nitrate pollution. Based on the results, in addition to the existing monitoring wells, seven new monitoring stations have been proposed. Few stations which potentially can be removed or combined with other stations have been identified and a monthly sampling frequency has been suggested.

Use of shallow groundwater temperature profiles to infer climate and land use change: interpretation and measurement challenges

AbstractTemperature is often used to infer the effect of land use and climate conditions on aquifers. Reliable data are needed to examine the temperature behaviour in the subsurface; thus, the use of robust acquisition techniques is unavoidable. Three temperature measurement techniques were applied to assess the sources of bias that could occur during temperature logging in a shallow Quaternary coastal aquifer in Ferrara (Northern Italy). Open borehole temperature logging, multilevel sampling straddle packers isolated temperature measurements within a flow cell above ground and multilevel sampling straddle packers isolated temperature measurements via an in‐well level logger (MLS‐IW) were compared for several coastal monitoring wells to gain insights on the limitations of each technique. Results show that the source of bias between the three applied techniques are different: (i) the open borehole temperature logging method tends to record heat convection through the open borehole and is not representative of the aquifer temperature distribution; (ii) the multilevel sampling straddle packers isolated temperature measurements within a flow cell above ground method is swayed by the air temperature and the heating of the submersible pump used to lift groundwater above ground; and (iii) the MLS‐IW provides the most reliable vertical thermal profiling both in summer and winter, because groundwater temperature is directly measured at the selected monitoring depth. The implementation of a 1D flow model demonstrates that if precise temperature profiles are needed to infer the influence that land use and climate changes have on groundwater, the MLS‐IW method is a reliable method that could be applied to existing monitoring wells. Copyright © 2016 John Wiley & Sons, Ltd.

Multi-Objective Based Approach for Groundwater Quality Monitoring Network Optimization

The process of designing a groundwater monitoring network is a very complicated task which helps to monitor the variation of contaminant factors in groundwater with a minimum number of sampling wells for saving resources and reducing monitoring cost. The uncertainties involved in selecting the initial existing monitoring wells, in absence of primary water quality data, was applied as a basic optimization criteria in this study, which could be quantified using geostatistical estimation error at all the potential monitoring locations. In the proposed methodology, natural contaminants factors (NCF) including; electrical conductivity, total dissolved solid, sodium, chloride, calcium and magnesium and anthropogenic contaminant factors (ACF) including total and fecal Coliforms, nitrate, biological oxygen demand and chemical oxygen demand were applied as selected parameters for optimal designing of monitoring wells based on principal component analysis. Three different optimal monitoring wells scenarios were defined depending on the maximum density of sampling wells for NCF and ACF cases. The efficiency of proposed scenarios was estimated using mass estimation errors and comparison between all the three scenarios of the two different cases. The novel approach was developed by integrating geostatistical and statistical techniques, hydrochemical factors, and potential risk of aquifer to contamination, where optimal number of sampling wells was achieved by eliminating the redundant wells. The results show that the contamination mass detection capacity of around 86 % can be estimated by 114 wells instead of sampling 154 wells in the initial existing monitoring well in the study area, for both natural and anthropogenic contaminations.

Temperature as a Tool to Evaluate Aerobic Biodegradation in Hydrocarbon Contaminated Soil

AbstractThis study evaluates the theory, and some practical aspects of using temperature measurements to assess aerobic biodegradation in hydrocarbon contaminated soil. The method provides an easily applicable alternative for quantifying the rate of biodegradation and/or evaluating the performance of in situ remediation systems. The method involves two nonintrusive procedures for measuring vertical temperature profiles down existing monitoring wells; one using a thermistor on a cable for one‐time measurements and the other using compact temperature data loggers deployed for 3‐month to 1‐year period. These vertical temperature profile measurements are used to identify the depth and lateral extent of biodegradation as well as to monitor seasonal temperature changes throughout the year. The basic theory for using temperature measurements to estimate the minimum rate of biodegradation will be developed, and used to evaluate field measurements from sites in California where biodegradation of spilled petroleum hydrocarbons is due to natural processes. Following, temperature data will be used to evaluate the relative rates of biodegradation due to natural processes and soil vapor extraction (SVE) at a former refinery site in the North‐Central United States. The results from this study show that the temperature method can be a simple, cost effective tool for assessing biodegradation in the soil, and optimizing remediation systems at a wide variety of hydrocarbon spill sites.

Mapping groundwater conditions in different geological environments in the northern area of UAE using 2D earth resistivity imaging survey

A water resources database was developed and used to characterize the geological and hydrogeological settings of groundwater aquifers in the northern part of the United Arab Emirates. An intensive 2D Earth Resistivity Imaging (ERI) survey was conducted in selected areas to assess the available groundwater resources and delineate the salt-water intrusion. Drilling information of the existing monitoring wells as well as their records of water table elevations and groundwater salinity were used to measure the horizontal and vertical variations in lithology, degree of saturation, and groundwater salinity and thus to improve the interpretation of ERI data. The results of the chemical analyses of water samples collected from the wells along with the 2D ERI survey profiles were used to obtain an empirical relationship between the inferred earth resistivity and the amount of total dissolved solids. This relationship was used along with the true resistivity sections resulting from the inversion of 2D ERI data to identify and map three zones of water-bearing formation (fresh, brackish, and salt-water zones) in the coastal areas. The results indicated that the depth to the fresh-brackish interface exceeded 50 m at the upstream of the wadies and was in the order of 10 m or less in the vicinity of shoreline. Because of the high resistivity contrast between dry and water saturated fractured rocks; this method was very successful in detecting water-filled fractures and cavities in the carbonate aquifer. The application of this method was unsuccessful in clayey aquifers as it was not possible to isolate lenses of gravel and/or saturated with saline or brackish water from the surrounding clayey materials.

Characterisation of microbial activity in the framework of natural attenuation without groundwater monitoring wells?: a new Direct-Push probe

At many contaminated field sites in Europe, monitored natural attenuation is a feasible site remediation option. Natural attenuation includes several processes but only the microbial degradation leads to real contaminant removal and very few methods are accepted by the authorities providing real evidence of microbial contaminant degradation activity. One of those methods is the recently developed in situ microcosm approach (BACTRAP®). These in situ microcosms consist of perforated stainless steel cages or PTFE tubes filled with an activated carbon matrix that is amended with 13C-labelled contaminants; the microcosms are then exposed within groundwater monitoring wells. Based on this approach, natural attenuation was accepted by authorities as a site remediation option for the BTEX-polluted site Zeitz in Germany. Currently, the in situ microcosms are restricted to the use inside groundwater monitoring wells at the level of the aquifer. The (classical) system therefore is only applicable on field sites with a network of monitoring wells, and only microbial activity inside the monitoring wells at the level of the aquifer can be assessed. In order to overcome these limitations, a new Direct-Push BACTRAP probe was developed on the basis of the Geoprobe® equipment. With respect to the mechanical boundary conditions of the DP technique, these new probes were constructed in a rugged and segmented manner and are adaptable to various sampling concepts. With this new probe, the approach can be extended to field sites without existing monitoring wells, and microbial activity was demonstrated to be measureable even under very dry conditions inside the vadose zone above the aquifer. In a field test, classical and Direct-Push BACTRAPs were applied in the BTEX-contaminated aquifer at the ModelPROBE reference site Zeitz (Germany). Both types of BACTRAPs were incubated in the centre and at the fringe of the BTEX plume. Analysis of phospholipid fatty acid (PLFA) patterns showed that the bacterial communities on DP-BACTRAPs were more similar to the soil than those found on classical BACTRAPs. During microbial degradation of the (13)C-labelled substrate on the carrier material of the microcosms, the label was only slightly incorporated into bacterial biomass, as determined by PLFA analysis. This provides clear indication for decreased in situ natural attenuation potential in comparison to earlier sampling campaigns, which is presumably caused by a large-scale source remediation measure in the meantime. In conclusion, Direct-Push-based BACTRAPs offer a promising way to monitor natural attenuation or remediation success at field sites which are currently inaccessible by the technique due to the lack of monitoring wells or due to a main contamination present within the vadose zone.

Sustainable wind‐driven bioventing at a petroleum hydrocarbon–impacted site

AbstractBioventing—the injection of air into the vadose zone to increase microbial activity—is a commonly used, proven technology for remediating volatile organic compounds present in the vadose zone. Passive systems driven by wind or solar power are both more cost‐effective and sustainable than conventional systems. Such a passive system is being applied successfully to remediate a site impacted with total petroleum hydrocarbons (TPH) and benzene, toluene, ethylbenzene, and xylenes (BTEX) in soil. Bioventing technology was approved by the regulatory agency as an interim remedial action to remove chemicals of concern (COCs) in the vadose zone. A bioventing pilot study was conducted to evaluate the effectiveness of COC removal and collect parameters for full‐scale design and implementation. To evaluate the potential to use wind‐driven bioventing technology, two mobile weather stations were installed at the site and monitored for one month for a wind speed study. Based on the pilot‐test data and wind speed research, 12‐inch diameter funnel/vane 360‐degree wind collectors were designed as passive wind‐driven air‐injection devices and connected to existing monitoring wells. The measured air velocity ranged from 20 to 110 feet per minute during the start‐up and the first three months of operation and maintenance. Monitoring indicated a 20 percent oxygen delivery and greater than 90 percent reduction in COC concentrations, demonstrating a successful sustainable remediation with no power requirement and minimal operation and maintenance. © 2012 Wiley Periodicals, Inc.

Upgrading a regional groundwater level monitoring network for Beijing Plain, China

Monitoring of regional groundwater levels provides important information for quantifying groundwater depletion and assessing impacts on the environment. Historically, groundwater level monitoring wells in Beijing Plain, China, were installed for assessing groundwater resources and for monitoring the cone of depression. Monitoring wells are clustered around well fields and urban areas. There is urgent need to upgrade the existing monitoring wells to a regional groundwater level monitoring network to acquire information for integrated water resources management. A new method was proposed for designing a regional groundwater level monitoring network. The method is based on groundwater regime zone mapping. Groundwater regime zone map delineates distinct areas of possible different groundwater level variations and is useful for locating groundwater monitoring wells. This method was applied to Beijing Plain to upgrade a regional groundwater level monitoring network.

Dynamic Factor Analysis for Estimating Ground Water Arsenic Trends

Drinking ground water containing high arsenic (As) concentrations has been associated with blackfoot disease and the occurrence of cancer along the southwestern coast of Taiwan. As a result, 28 ground water observation wells were installed to monitor the ground water quality in this area. Dynamic factor analysis (DFA) is used to identify common trends that represent unexplained variability in ground water As concentrations of decommissioned wells and to investigate whether explanatory variables (total organic carbon [TOC], As, alkalinity, ground water elevation, and rainfall) affect the temporal variation in ground water As concentration. The results of the DFA show that rainfall dilutes As concentration in areas under aquacultural and agricultural use. Different combinations of geochemical variables (As, alkalinity, and TOC) of nearby monitoring wells affected the As concentrations of the most decommissioned wells. Model performance was acceptable for 11 wells (coefficient of efficiency >0.50), which represents 52% (11/21) of the decommissioned wells. Based on DFA results, we infer that surface water recharge may be effective for diluting the As concentration, especially in the areas that are relatively far from the coastline. We demonstrate that DFA can effectively identify the important factors and common effects representing unexplained variability common to decommissioned wells on As variation in ground water and extrapolate information from existing monitoring wells to the nearby decommissioned wells.

Variations of soil–gas composition around the active Chihshang Fault in a plate suture zone, eastern Taiwan

The active Chihshang Fault in the Longitudinal Valley of eastern Taiwan is part of the suture boundary between the Eurasian plate and Philippine Sea plate. Soil–gases collected systematically across the Chihshang Fault reveal anomalies at specific positions that coincide with the geological, geophysical and topographical characteristics of faults/fractures in studied area. An automatic soil radon monitoring station was set up at the fault zone, at the same place as existing monitoring wells and creep meter monitoring. Some anomalous high soil radon concentrations were observed, usually a few days before local earthquakes. However, other anomalies could not be related to any earthquake event, and may be related to surface creeping of the fault as revealed from the creep meter monitoring in the studied area. In addition, heavy rainfall induced significant increase of soil radon concentrations which may be related to the rapid surface creep that was triggered by the heavy rain in a short time.

Oxyanion flux characterization using passive flux meters: Development and field testing of surfactant-modified granular activated carbon

We report here on the extension of Passive Flux Meter (PFM) applications for measuring fluxes of oxyanions in groundwater, and present results for laboratory and field studies. Granular activated carbon, with and without impregnated silver (GAC and SI-GAC, respectively), was modified with a cationic surfactant, hexadecyltrimethylammonium (HDTMA), to enhance the anion exchange capacity (AEC). Langmuir isotherm sorption maxima for oxyanions measured in batch experiments were in the following order: perchlorate >> chromate > selenate, consistent with their selectivity. Linear sorption isotherms for several alcohols suggest that surfactant modification of GAC and SI-GAC reduced (∼ 30–45%) sorption of alcohols by GAC. Water and oxyanion fluxes (perchlorate and chromate) measured by deploying PFMs packed with surfactant-modified GAC (SM-GAC) or surfactant-modified, silver-impregnated GAC (SM-SI-GAC) in laboratory flow chambers were in close agreement with the imposed fluxes. The use of SM-SI-GAC as a PFM sorbent was evaluated at a field site with perchlorate contamination of a shallow unconfined aquifer. PFMs packed with SM-SI-GAC were deployed in three existing monitoring wells with a perchlorate concentration range of ∼ 2.5 to 190 mg/L. PFM-measured, depth-averaged, groundwater fluxes ranged from 1.8 to 7.6 cm/day, while depth-averaged perchlorate fluxes varied from 0.22 to 1.7 g/m 2/day. Groundwater and perchlorate flux distributions measured in two PFM deployments closely matched each other. Depth-averaged Darcy fluxes measured with PFMs were in line with an estimate from a borehole dilution test, but much smaller than those based on hydraulic conductivity and head gradients; this is likely due to flow divergence caused by well-screen clogging. Flux-averaged perchlorate concentrations measured with PFM deployments matched concentrations in groundwater samples taken from one well, but not in two other wells, pointing to the need for additional field testing. Use of the surfactant-modified GACs for measuring fluxes of other anions of environmental interest is discussed.

Geoelectrical and hydrogeochemical studies for delineating seawater intrusion in the outlet of Wadi Ham, UAE

The Quaternary aquifer of Wadi Ham, UAE has been overexploited during the last two decades to meet the increasing water demands. As a result, the dynamic balance between freshwater and seawater has been disturbed and the quality of the groundwater has deteriorated. In this paper, a 2D earth resistivity survey was conducted in Wadi Ham in the area between Fujairah and Kalba to delineate the seawater intrusion. Existing monitoring wells were used to measure the horizontal and vertical variations in water salinity and thus to improve the interpretation of earth resistivity imaging data. Results of vertical electrical soundings and chemical analyses of collected water samples were used to obtain an empirical relationship between the inferred earth resistivity and the amount of total dissolved solids. This relationship was used along with the true resistivity sections resulting from the inversion of 2D resistivity data to identify three zones of water-bearing formation (fresh, brackish, and salt-water zones). Along the four 2D resistivity profiles, the depth to the fresh-brackish interface exceeded 50 m at the western part of the area and was in the order of 10 m or less in the eastern side near the shoreline. Depth to the brackish-saline water interface reached about 70 m in the western side and was in the order of 20 m in the eastern side. The thickness of the fresh water zone decreases considerably in the farming areas toward Kalba and thus the degree of seawater intrusion increases.