Shallow Groundwater Chemistry Research Articles

Hydrochemical evolution and hydrological zoning characteristics of a shallow groundwater system in Baiyangdian Wetland, North China Plain

AbstractA comprehensive understanding of the hydrochemical evolution and spatial patterns of shallow groundwater systems is essential for water resource management and wetland ecological restoration. The Baiyangdian Wetland is one of the most concerning areas because of the development of the Xiong'an New Area. The spatial characteristics of groundwater hydrochemistry and potential controlling factors associated with hydrochemical evolution remain unclear. In this study, hydrogeochemistry together with the hierarchical cluster analysis were used to elucidate the hydrochemical processes and hydrological zoning patterns of shallow groundwater systems in the Baiyangdian Wetland, North China Plain. The results showed that hydrochemical compositions of shallow groundwater had considerable spatial variations, which was closely related to the inflow rivers hydrochemistry and the dynamics of groundwater–surface water interactions. A significant increase in SO42− concentration occurring at the cone of the depression was related to extensive pumping caused by anthropogenic activities. Anthropogenic activities were also a major factor controlling the spatial distribution patterns of shallow groundwater hydrochemistry. Ca2+, Mg2+, and SO42− in the wetland and shallow groundwater were primarily derived from carbonate and gypsum dissolution, while Na+ and Cl− originated from halite and silicate dissolution. Rock weathering predominated the geochemical evolution of shallow groundwater in conjunction with carbonate precipitation and cation exchange. The hydrochemistry of the shallow groundwater system presented distinct spatial zonation patterns that were classified into four clusters corresponding to seven subzones. In Zones I–IV, water‐rock interaction was the dominant factor controlling shallow groundwater chemistry, which was driven by the positive groundwater–surface water exchange. The coupled effects of anthropogenic activities and river infiltration and mixing caused the high levels of dissolved components in Zones V–VII. This study contributes to have a better understanding of the water cycle and hydraulic connections among different bodies, and will benefit the rational evaluation of hydrochemical evolution and wetland ecological restoration in the Baiyangdian Wetland.

Identifying controlling factors for the evolution of shallow groundwater chemistry of Ellala catchment, northern Ethiopia

Identifying controlling factors for the evolution of shallow groundwater chemistry of Ellala catchment, northern Ethiopia

Characteristics and Driving Mechanisms of Shallow Groundwater Chemistry in Xining City

Groundwater is one of the major sources of water supply, especially in the western arid regions. However, with the deepening of the western development strategy, industrialization and urbanization have increased groundwater resource demands in Xining City. Overexploitation and utilization have led to a series of changes in the groundwater environment. Identifying the chemical evolution characteristics and formation mechanism of groundwater is crucial for preventing its deterioration and ensuring sustainable use. By combining hydrochemistry and multivariate statistical techniques, the chemical characteristics of groundwater in Xining City were analyzed, and the formation mechanism of groundwater and the influence of different factors were discussed. The results showed that there were as many as 36 chemical types of shallow groundwater in Xining City, mainly HCO3-Ca(Mg) (60.00%) and HCO3·SO4-Ca(Mg) (11.81%). There were 5-6 types of groundwater chemical types in bare land, grassland, and woodland. Groundwater chemical types in construction land and cultivated land were more complex, up to 21 types, indicating that they were strongly affected by human activities. The chemical evolution process of groundwater in the study area was mainly affected by rock weathering and leaching, evaporative crystallization, and cation exchange. The main controlling factors were water-rock interaction (contribution rate 27.56%) and industrial wastewater discharge (contribution rate 16.16%), acid-base environment (contribution rate 16.00%), excessive application of chemical fertilizers and pesticides (contribution rate 13.11%), and domestic sewage (contribution rate 8.82%). On account of the chemical characteristics of groundwater in Xining City and the influence of human activities, the management and control suggestions on the development and utilization of groundwater resources were put forward.

Machine learning and shallow groundwater chemistry to identify geothermal prospects in the Great Basin, USA

This study discovers various geothermal prospects in the Great Basin, USA based on shallow groundwater chemical (geochemical) data. The geochemical data are expected to include hidden (latent) information that is a proxy for geothermal prospectivity. We processed the sparse geochemical data in the Great Basin at 14,341 locations including 18 attributes. Next, a non-negative matrix factorization with customized k-means clustering is applied to the geochemical data matrix that automatically finds three hidden geothermal signatures representing modestly, moderately, and highly confident geothermal prospects. The algorithm also evaluated the probability of occurrence of these types of resources through the studied region. There is a consistency between regional geothermal prospectivity as estimated by our ML methodology and the traditional play fairway analysis conducted over a portion of the study area. We also identify the dominant data attributes associated with each signature. Finally, our ML analyses allow us to reconstruct attributes from sparse into continuous over the study domain. The predicted continuous attributes can be used for future detailed geothermal explorations in the Great Basin.

Hydrochemical Characteristics and the Genetic Mechanism of Low–Medium Temperature Geothermal Water in the Northwestern Songliao Basin

The geothermal resources in sedimentary basins have high potential for development and utilization, and have become an important research topic globally. This study focuses on the geothermal system in the northwestern Songliao Basin. Water chemistry and isotopic signatures of geothermal fluids and shallow groundwater are analyzed. Water–rock interactions, recharge sources, and the ages of geothermal fluids are revealed and recharge elevation, circulation depth, and the reservoir temperature of the geothermal fluids are estimated. This article proposes deep heat sources and genetic mechanism for geothermal system. The results are as follows: The hydrochemical types of geothermal water mainly included Cl·HCO3-Na, HCO3·Cl-Na, and Cl-Na, and the TDS gradually increased from the margin to the center of the basin and from anticlines to the depression on both sides. The geothermal water was recharged by paleo-atmospheric precipitation in the northwest mountainous area at an elevation of 300–700 m. The 14C ages showed that the geothermal water flowed at an extremely low rate (millennial scale) and had a low circulation rate. The temperature of the geothermal reservoirs was estimated to be 45.19–83 °C using a quartz geothermometer. The geothermal water had a genetic model of stratum-controlling geothermal reservoirs, lateral runoff recharge, and heat supply by terrestrial heat flow. The underlying reasons for the high geothermal gradient and terrestrial heat flow in the basin include the uplift of the Moho, the uplift of the upper mantle, and the presence of a high-electrical-conductivity layer in the crust.

Machine Learning and Shallow Groundwater Chemistry to Identify Geothermal Prospects in the Great Basin, USA

Machine Learning and Shallow Groundwater Chemistry to Identify Geothermal Prospects in the Great Basin, USA

Assessment of hydrological factors affecting shallow groundwater chemistry in the most urbanized area of the coastal sedimentary basin of Togo

Abstract. The project to reduce environmental and health risks related to the activity of boreholes water sellers in Lomé (in French PRRESAF) was implemented to contribute to the guarantee of public health. One of its objectives is to improve the hydrological understanding of the shallow Continental Terminal (CT) aquifer tapped by water sellers. It is a vital water resource in the coastal sedimentary extending from Ghana to Nigeria. The results presented here are part of this project and aim at assessing the hydrological factors controlling CT groundwater quality and chemistry using principal component analysis (PCA), hierarchical cluster analysis (HCA), and GIS. High-resolution sampling campaigns for major ions water quality analyses were performed to improve the accuracy of factors assessment. The results showed that groundwater is predominantly acidic with varying degrees of mineralization ranging from very soft to brackish waters. The integration of multivariate results to GIS allows classifying samples into five main groups. They revealed the probable recharge and discharge areas, associated hydrochemical evolution along flow paths, and areas under the high impact of land-use dynamics, wastewaters/sewage infiltration, and interactions with surface water systems. This study appears useful for managing boreholes' water sellers' activity relative to the integrity of groundwater resources and public health.

Characterization of shallow groundwater chemistry in the Yogyakarta basin, Central Java

Sufficient water supply can still be a big challenge in developing countries. A large number of households in Yogyakarta still relay on private dug wells to meet their daily demand. But water from shallow aquifers is prone to pollution from sewage systems, industry and agriculture. To assess the water quality in the basin of Yogyakarta, this study determines the chemical characteristics of the shallow groundwater in anthropogenically influenced and uninfluenced areas in and around the city of Yogyakarta. Therefore, a combination of on-site and laboratory analysis is used to distinguish areas of different water types. The observations give a major ion distribution for the whole research area and show significant anomalies in nitrate and phosphate concentrations especially in the city. Furthermore, high DOC values point out that the aquifer is widely polluted. There is a need for a better groundwater protection and remediation strategy to facilitate future use of those groundwaters.

ECOLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF SHALLOW GROUNDWATER IN SOUTHERN BUG AND SYNIUKHA INTERFLUVE AREA

Problem Statement and purpose. The study area encompassing the territory of about 150 km2, in August 2000, was declared as a zone of environmental emergency due to increasing pollution rates in shallow groundwater, in surface water and to health deterioration of the inhabitants of some settlements. Groundwater is widely used by the local population in numerous boreholes for domestic and agricultural purposes. In the zone of environmental emergency there are about 35 anthropogenic objects, such as: industrial and domestic landfills, pesticide and fertilizer storage, various livestock farm complexes, settling tanks, wastewater and solid waste discharges, etc. The insecurity of shallow groundwater, also its location close to the surface and in the lowest parts of the terrain leads to intensive pollution of groundwater. Besides, groundwater in relation to the fractured waters of the Precambrian crystalline rocks, which lie below and are used for drinking water supply, perform the functions of both a protective screen and a source of pollution. In this case, a multifaceted study of shallow groundwater chemical composition and their formation conditions is a priority task of this study for the environmental emergency zone and for the district as a whole. The purpose of this paper to study the chemical composition and to establish the processes and factors controlling the shallow groundwater chemistry in modern and Pleistocene sediments of the Southern Bug and Syniukha interfluve. Data and Methods. A total of 102 water samples were collected from groundwater wells and boreholes and were hydrochemically analyzed for their macrocomponent composition. Groundwater quality geochemical assessment was carried out using statistical methods in combination with conventional graphical methods by examining groundwater in the Boleslavchik village, where they are most polluted. Results. The dominant hydrochemical types of shallow groundwater are SO4- Na, HCO3-Na and mixed SO4-Mg-Na, SO4-Ca-Na types, sometimes NO3-HCO3 and NO3-Cl-Na types. These groundwater types are formed under the influence of natural and anthropogenic factors, such as water-rock interaction, evaporation-crystallization, precipitation and anthropogenic impact. In this case, water-rock interaction processes, such as silicate weathering, carbonate and sulfate dissolution, ion exchange, evaporation-crystallization and anthropogenic impact play major roles. The weathering of feldspars, chlorites and dissolution of calcites, dolomites are the primary sources for Na+, K+, Mg2+, Ca2+ ions, and dissolution of gypsum, sulfide mineralization for SO4 2– ions. High concentrations of Na+, Cl–, SO4 2–, NO3 – in shallow groundwater reflect agricultural activity as the main source of these ions.

Using isotopes and hydrogeochemistry to characterize groundwater flow systems within intensively pumped aquifers in an arid inland basin, Northwest China

The impacts of long-term pumping on groundwater flow patterns and groundwater chemistry are unclear in the Manas River Basin, an arid inland basin in Northwest China. In this study, hydraulic heads, hydrochemistry and environmental isotopic tracers were analyzed to reveal groundwater flow patterns, recharge sources and deduce hydrogeochemical processes in this highly exploited aquifer system. Spatial trends in the isotopic ratios δ18O and δ2H indicate that groundwater originates from glacial melting and precipitation in the mountainous area. In the piedmont plains, dissolution of calcite, gypsum and dolomite yields a groundwater type dominated by Ca-HCO3-SO4 with TDS of 300 mg/L. In this area, high 3H concentrations demarcate local flow systems and imply modern precipitation recharge. Downgradient in the lowlands, irrigation return flow recharges shallow groundwater through local flow paths. Groundwater pumping has intensified vertical hydraulic gradients causing vertical mixing between aquifers. Generally, across the basin, regional flow systems contain groundwater with 14C ages ranging from modern to 33,000 years, with the oldest waters found in wells with depths greater than 200 m. From high to low elevation groundwater chemistry transitions from Ca-HCO3-SO4 to Na-Cl-SO4 type as concentrations of Cl− and overall TDS increase. The replacement of Ca2+ with Na+ as the dominant cation suggests reverse ion exchange and carbonate precipitation occur along the intermediate and regional flow paths. Anthropogenic activities (i.e. pumping, irrigation return flow, application of fertilizers) impact shallow groundwater chemistry (<60 m). This research reveals the impacts of natural and human-induced hydrogeochemical processes impacting groundwater chemistry in this basin with implications for similar semi-arid to arid, inter-montane basins around the world.

Hydro-Geochemical Mechanisms of Brackish Shallow Groundwater Development- Coastal Greater Accra Region, Ghana

This study investigated the processes influencing the chemistry of surface and shallow groundwater in tropical coastal environments - south east of greater Accra region of Ghana using GIS models and combination of geological and hydro geochemical techniques for sustainable management of freshwater resources and abundantly available brackish water resources. A total of 37 shallow groundwater and 11 surface water samples were collected and analysed for their physico-chemical constituents.&#x0D; The samples were adjusted to room temperature after which the hydrogen ion concentration (pH), Total Dissolve Solids (TDS) and Electrical Conductivity were measured with a precision of 0.01 for both parameters using a La Motte, USA in unfiltered samples. The analysis of major and minor ions were performed using ion chromatography (DIONEX-ICS-1000 Series). The geographical locations of the samples were recorded with the aid of a handheld Global Position System (GPS).&#x0D; The analysed shallow groundwater indicates minimum salinity values of 70.2 psu/ppm, maximum salinity of 4398.3 psu/ppm with an average of 1571.4 psu/ppm whilst surface water has minimum salinity of 33.8 psu/ppm, maximum of 43574 psu/ppm with an average of 4972.6 psu/ppm therefore highly saline. The total minimum dissolved solid (TDS) concentrations is 87.5 ppm, maximum of 5160 ppm with an average of 1911.96 ppm for shallow groundwater and for surface water minimum of 38.4 ppm, maximum of 27100 ppm and average 3938.12 ppm.

Effects of Prescribed Burn on Nutrient and Dissolved Organic Matter Characteristics in Peatland Shallow Groundwater

Wildfires are a common disturbance in boreal regions and have the potential to affect the waterborne export of organic matter and nutrients from burned catchments. To understand the effect of fire on shallow groundwater chemistry in a forested peatland in northern Alberta, Canada, shallow groundwater monitoring wells were sampled before and after a prescribed burn. The samples were collected from control and treatment wells between May and August 2019. The results indicate no differences in dissolved organic matter concentration and chemical composition between wells in burned and unburned sections but substantially increased nutrient concentrations were found in the burned section. Here, the levels of phosphorus increased and did not return to pre-fire levels at the end of the monitoring period, while the levels of inorganic nitrogen increased and returned to pre-fire levels within a few months. With increasing wildfire activity, or as a result of prescribed burns in the Boreal Plains, we may see implications for downstream water quality, including lake trophic status.

From on high: Geochemistry of alpine springs, Niwot Ridge, Colorado Front Range, USA

AbstractSnowmelt‐fed springs and small (0.5 km2) upland catchments in alpine areas of the western United States contribute significantly to the quantity and inorganic chemistry of water delivered to downstream basins but have not been studied extensively. Mineral weathering, transit time, and hydrologic mixing control the solute chemistry of waters that drain the upland zone of Niwot Ridge, Colorado Front Range, and adjacent areas in the granitic core of the Southern Rocky Mountains. Water in 37 springs sampled in this study flows in generally short steep paths (~0.3 km) through shallow regolith with mean transit times (MTT) of weeks to months, producing solutions dominated by Si, Ca2+, Na+, and HCO3−, locally SO42−. Rock type is a significant control on spring, surface, and shallow groundwater chemistry, and plagioclase (oligoclase) is the major source of dissolved Na+ and Si. Concentrations of Ca2+ exceed stoichiometric predictions of oligoclase weathering by ~3.5×; excess Ca2+ likely represents weathering of aeolian material, vein calcite, or trace minerals. Concentrations of base cations and Si increase slowly with estimated MTT of 0.2 years for Niwot Ridge spring waters, and several years for shallow groundwater sampled by wells. Chemical weathering of silicate minerals is slow with estimated rates of ~2.0 and 0.2 pmol·m−2·s−1 for oligoclase and microcline, respectively; the most mineralized spring waters are saturated only with respect to kaolinite and montmorillonite. More than 50% of the dissolved base cations + Si measured in Boulder Creek at Orodell (~25 km downstream) accumulate before water emerges from alpine springs on Niwot Ridge. Warming global temperatures are shifting more high‐elevation precipitation to rain, potentially changing run‐off patterns, transit time, and solute loads. Acquisition of solutes by alpine waters thus has implications far beyond small upland catchments.

Assessment of shallow groundwater quality and its suitability for drinking purpose near the Béni-Mellal wastewater treatment lagoon (Morocco)

The present study aimed to evaluate the suitability for drinking purpose of shallow groundwater near the Béni-Mellal wastewater treatment lagoon based on various physicochemical, heavy metals, and bacteriological parameter analyses. The physicochemical results revealed that some of the samples do not comply with the Moroccan and/or WHO standards for drinking water. Parameters including turbidity, TH, Na+, Li+, Ba2+, Ca2+ (∼47.1% of samples), Cd (∼52.9% of samples), Fe (∼82.4% of samples), Pb (∼58.8% of samples), T. coliforms, and E. coli exceeded the drinking limits. The statistical analyses revealed that the shallow groundwater chemistry is mainly controlled by geogenic and anthropogenic sources. For quality assessment, using the Moroccan groundwater assessment grid, the values of EC and Cl–, NO3–, NH4+, oxidability, and E. coli, fixed as pollution indicators, showed that most of the wells showed medium-to-poor quality, 14% of them have a very poor water quality, and 20% of them belong to the bad water quality. According to geometric and arithmetic DWQI values, the groundwater quality was frequently fair to good, needing treatment or at least disinfection before public consumption. A sensitivity analysis results indicated that Fe, Cd, Cr, Pb, and E. coli have an important impact on the DWQI computing.

Nitrate contamination and subsequent hydrogeochemical processes of shallow groundwater in agro-livestock farming districts in South Korea

In last several decades, the nitrogen cycle has been significantly perturbed, largely due to intensification of agricultural activities throughout the world. In this study, we examined the impact of agricultural N inputs on the quality and chemistry of shallow groundwater, based on a large hydrochemical dataset (n ≈ 4000) collected from 100 agro-livestock farming districts in South Korea. The South Korean groundwater, studied mostly in silicate aquifers, shows very high nitrate concentrations (median NO3− = 22.2 mg/L) and acidification (median pH = 5.6). The groundwater nitrate levels tend to increase with the estimated N loadings, and the groundwater pH generally decreases with increasing nitrate levels. The relationship between the concentrations of Ca2+ + Mg2+ and HCO3- has a moderate adjusted R2 value (0.4), and the molar (Ca2+ + Mg2+) / HCO3- ratios (r) tend to increase with nitrate concentrations. This implies that the chemical composition of the groundwater is controlled by multiple hydrogeochemical processes, which include weathering of silicates and carbonates induced by carbonic acid (r = 0.5), nitrification (r ≥ 1) and denitrification (mostly, r < 0.5 in this study). In particular, undrinkable (NO3− > 44.3 mg/L) groundwater (n = 988) shows an average molar ratio of 1.04, indicating that such highly contaminated groundwater experiences the enhanced geochemical weathering of aquifer materials by an anthropogenic process (e.g., addition of N fertilizers). The results of principal component (PC) analysis also support our explanation: the first PC shows significant negative correlations with major components including NO3− while there is a weak positive correlation with pH, indicating anthropogenically enhanced weathering that includes the buffering process by agricultural liming materials, while the second PC shows negative correlations with Eh, DO and NO3− but positive correlations with pH and HCO3−, suggesting the denitrification process. A few groundwater samples (about 4%) experienced heterotrophic denitrification, and they have molar ratios (r) of less than 0.5. The results of this study indicate that 1) geochemical weathering of aquifer materials in shallow groundwater systems is enhanced by nitrate contamination caused by high N loadings in agro-livestock farming districts, causing increased salinity, and 2) in silicate aquifers, the prevailing hydrochemical process can be interpreted by the molar (Ca2+ + Mg2+) / HCO3- ratios of groundwater.

Hydrogeochemical Characterization and Quality Assessment of Groundwater in a Long-Term Reclaimed Water Irrigation Area, North China Plain

Water scarcity has led to wide use of reclaimed water for irrigation worldwide, which may threaten groundwater quality. To understand the status of groundwater in the reclaimed water irrigation area in Beijing, 87 samples from both shallow and deep aquifers were collected to determine the factors affecting groundwater chemistry and to assess groundwater quality for drinking and irrigation purposes. The results show that groundwater in both shallow and deep aquifers in the study area is weakly alkaline freshwater with hydrogeochemical faces dominated by HCO3-Na·Mg·Ca, HCO3-Mg·Ca·Na, HCO3-Ca·Na, and HCO3-Na. The chemical composition of groundwater in both shallow and deep aquifers is dominantly controlled by the dissolution of halite, gypsum, anhydrite, and silicates weathering, as well as ion exchange. Geogenic processes (rock weathering and ion exchange) are the only mechanisms controlling groundwater chemistry in deep aquifers. Besides geogenic processes, evaporation and anthropogenic activities also affect the chemistry of shallow groundwater. Quality assessment reveals that both shallow and deep groundwater are generally suitable for drinking and irrigation purposes. The quality of deep groundwater is more excellent for drinking than shallow groundwater. However, long-term use of deep groundwater for irrigation exhibits higher potential risks to deteriorate soil property due to the relative higher permeability indexes (PI). Therefore, it is recommended that deep groundwater is preferentially used for drinking and domestic purpose, and shallow groundwater for agricultural irrigation.

Assessment of Nitrate Occurrence in the Shallow Groundwater of Merimandroso Area, Analamanga Region, Madagascar Using Multivariate Analysis

Knowing the groundwater quality is important for the drinking water supply in the highland area of Madagascar, including Merimandroso Commune insofar that groundwater is the main source of drinking water for a large number of Malagasy people. In this way, this study assessed the shallow groundwater quality with special focus on nitrate occurrence using multivariate statistical techniques such as cluster analysis (CA) and principal component analysis (PCA). That was to determine the similarities among the water samples in terms of hydrochemical features and to identify the different mechanisms involved in the shallow groundwater hydrochemistry. The study was conducted on twenty-one water samples collected from dug wells. Cluster analysis grouped the water-sampling points into two main clusters: a highly nitrate polluted group (concentration greater than 50 mg/l) and a non-nitrate polluted group. The results showed a spatial variation of the groundwater chemistry processes, while no such variability was found temporally for water samples collected at different periods. Principal component analysis extracted three principal components accounted for over 82% of the total variance. It attributed the hydrochemical features of the water samples of high nitrate content to the nitrate pollution mechanisms along with the weathering of feldspar and ferromagnesian minerals. For some of the latter water samples, the water chemistry is likely affected by igneous rock weathering. This study confirmed both the usefulness and powerfulness of multivariate statistical techniques in water quality assessment, since they helped get a proper understanding of processes controlling the shallow groundwater chemistry.

Assessing major factors affecting shallow groundwater geochemical evolution in a highly urbanized coastal area of Shenzhen City, China

Groundwater is the most important alternative drinking water source in the coastal urban area of Shenzhen City, China. Understanding the main geochemical factors affecting groundwater chemistry is vital for sustainable water resource management in the long term. Seawater intrusion has been a serious problem, but the situation seems to have recently improved with the establishment of a water supply system and land reclamation. However, total dissolved solids (TDS) concentrations in the groundwater have increased. To determine major factors controlling groundwater geochemical evolution, studies were carried out in the area in Shenzhen most vulnerable to seawater intrusion. Along the groundwater flow path from the bedrock outcrop area to the coastal plain, groundwater evolution processes were studied using multiple hydrogeochemical analyses. Piper plots and hydrochemical facies evolution diagrams show the aquifer vulnerable to seawater intrusion is currently dominated by fresh groundwater of Ca-HCO3 or Ca-Na-HCO3-Cl type; only a few locations close to Shenzhen Bay appear to be Na-Cl type, indicating the threat of seawater intrusion. Anthropogenic pollution is mainly characterized by elevated NO3− and NH4+ in the form of point sources. Correlation coefficient analysis to evaluate water-rock interactions during granite weathering indicates this process can contribute significant amounts of Ca2+, Mg2+, Na+, K+, and HCO3– to groundwater. Evaluation of the aggressiveness index (AI), Langelier saturation index (LSI), and Ryznar stability index (RSI) suggest all groundwaters are corrosive and the dissolution of concrete material foundations in urban areas contributes significant amounts of Ca2+ and SO42− to groundwater. Principal components analysis (PCA) demonstrates water-rock interactions and concrete material dissolution are more important factors than seawater intrusion in terms of affecting shallow groundwater chemistry. This study demonstrates that hydrogeochemical tools are effective in assessing groundwater evolution under the influence of multiple factors in a highly urbanized coastal area.

Hydraulic Fracturing in the Upper Humboldt River Basin, Nevada, USA

Water-quality and isotopic data were collected in central Nevada, USA in an exploration hydraulic fracturing area with no previous oil or gas production. The target shales of the Elko Formation are unique fresh-water hydrocarbon reservoirs with relatively dilute source water (8.5 g/L total dissolved solids [TDS]). Additionally, the Elko Formation is underlain by a fresh-water carbonate aquifer (0.2-0.3 g/L TDS) that outcrops downgradient of the exploration area. The water quality and isotopic data were used to evaluate pre-hydraulic fracturing conditions in this undeveloped area. The same data were also collected for groundwater and surface-water sites about two months and one year after exploration hydraulic fracturing. No systematic differences in water chemistry were observed between pre- and post-hydraulic fracturing samples. Based on water chemistry of shallow groundwater, surface water, and from the production zone, the most useful constituents identified for monitoring for potential future incursion of reservoir-associated fluids into the near-surface environment are TDS (or electrical conductivity), chloride, propane, methanol, ethanol, and 2-butoxyethanol. Groundwater flow and transport models were developed to evaluate the potential movement of hydrocarbons and hydraulic fracturing fluids from the targeted zones, which are about 1800 to 3600 m beneath the land surface, to shallow groundwater (<300 m below land surface). Model simulations indicate that hydraulic fracturing fluid remains contained within the target shales for at least 1,000 years for most development scenarios.

The impact of river infiltration on the chemistry of shallow groundwater in a reclaimed water irrigation area

Reclaimed water reuse is an effective method of alleviating agricultural water shortages, which entails some potential risks for groundwater. In this study, the impacts of wastewater reuse on groundwater were evaluated by combination of groundwater chemistry and isotopes. In reclaimed water infiltration, salt composition was affected not only by ion exchange and dissolution equilibrium but also by carbonic acid equilibrium. The dissolution and precipitation of calcites and dolomites as well as exchange and adsorption between Na and Ca/Mg were simultaneous, leading to significant changes in Na/Cl, (Ca+Mg)/Cl, electrical conductivity (EC) and sodium adsorption ratio (SAR). The reclaimed water was of the Na-Mg-Ca-HCO3-Cl type, and groundwater recharged by reclaimed water was of the Na-Mg-HCO3 and Mg-Na-HCO3 types. The hydrogeological conditions characterized by sand-clay alternation led to both total nitrogen (TN) and total phosphorus (TP) removal efficiencies >95%, and there was no significant difference in those contents between aquifers recharged by precipitation and reclamation water. >40years of long-term infiltration and recharge from sewage and reclaimed water did not cause groundwater contamination by nitrogen, phosphorus and heavy metals. These results indicate that characteristics of the study area, such as the lithologic structure with sand-clay alternation, relatively thick clay layer, and relatively large groundwater depth have a significant role in the high vulnerability.