Co-occurrence of arsenic and fluoride in groundwater of Guide basin in China: Genesis, mobility and enrichment mechanism.

Endemic arsenic poisoning and fluorosis caused by primary high arsenic (As) and high fluoride (F-) groundwater have become one of the most serious environmental geological problems faced by the international society. High As and high F- groundwater exists in Neogene confined aquifers in Guide basin, with concentrations of 355 µg/L and 5.67 mg/L, respectively, and showing a co-occurrence phenomenon of As and F- in the groundwater. This poses a double threat to the health of tens of thousands of local residents. In this study, based on the systematic collection of groundwater and borehole sediment samples, analysis of hydrochemistry and isotope indexes, combined with laboratory tests, purpose of this study is to reveal the migration rule and co-enrichment mechanism of As and F- in aquifers, and finally establish a hydrogeochemical conceptual model of the enrichment process of As and F-. The main conclusions are as follows: hydrochemical type of unconfined and confined groundwater in Guide basin is Ca-Na-HCO3 and Na-Cl-HCO3 type, respectively. Main minerals in sediments are quartz and plagioclase. Concentrations of As and F- are lower in unconfined groundwater, but higher in confined groundwater, and which show a gradual increasing trend along the groundwater flow path. The mineralization of natural organic matter in confined aquifer causes iron and manganese oxide minerals containing As to dissolve gradually, which leads to the gradual release of As into groundwater. Large amount of HCO3- produced by mineralization of organic matter precipitate with Ca2+ in groundwater, resulting in reduction of Ca2+ content, promoting the dissolution of fluoride-containing minerals such as fluorite (CaF2), and continuously releasing F- into groundwater. Meanwhile, competitive adsorption reactions in confined aquifers causes more As and F- to be released from mineral surface into groundwater, which gradually migrate and accumulate along groundwater flow. Finally, it is established that a conceptual model for the formation of high As and F- groundwater in the confined aquifer of Guide basin. The research results not only help to improve our understanding of the formation and evolution of groundwater with high As and F- with similar geological background, but also provide scientific basis for rational development and utilization of groundwater, and prevention and control of chronic As and F- poisoning in local and similar areas.

Recharge and residence times of groundwater in hyper arid areas: The confined aquifer of Calama, Loa River Basin, Atacama Desert, Chile.

Groundwater recharge in hyper arid areas often depends on surface water infiltration and diffuse recharge of highly evaporated precipitation only contribute under favorable conditions. This happens in the Calama basin two-aquifer system, in the Central Andean area of northern Chile. A conceptual model of the groundwater system and its relationship with the Loa River is defined. We focus on the confined aquifer of the Calama basin, combining hydrodynamic, hydrogeochemical and isotopic methods. Radiocarbon (14C) activity data of dissolved inorganic carbon (DIC), in conjunction with chemical data, are applied to evaluate groundwater residence time within the confined aquifer. The Loa River recharges the Calama basin aquifers in its northeastern part, with water that has chemical and isotopic characteristics inherited from the arid environment and volcanic rocks in its upper basin. In the central and northeastern part of the confined aquifer, minor variations in chloride concentration suggest that the deep aquifer is well confined. The δ18O and δ2H values in groundwater of the confined aquifer show an increasing isotopic fractionation from the recharge area (around -10‰ δ18O) to those in the discharge area (between -8.5‰ and -8‰) in the southwestern part of the aquifer. The 14C activity continuously decreases down flow from the recharge by the Loa River. Adjusted DIC radiocarbon ages indicate a groundwater travel time between 1500 and 4000 years in the confined aquifer of Calama. Despite the limitations and uncertainties of radiocarbon in DIC to estimate groundwater transit times for the confined aquifer and considering complementary chemical and isotopic constraints, the DIC 14C provides acceptable values. The approach may be applicable in other confined aquifers in hyper-arid climates in which the formation of aquifer systems linked to river damming by geological action took place. This information is needed for sound management of the scarce groundwater resources.

Long-term redox conditions in a landfill-leachate-contaminated groundwater.

Indicators of redox conditions; oxygen, sulphate, nitrate, ammonium, iron and manganese, and in addition, bicarbonate and total organic carbon were studied in groundwater samples contaminated by leachate emanating from Revdalen Landfill (Norway). Based on these variables, the study aimed to deduce the redox conditions in the aquifer. Literature on landfill leachate contamination of confined aquifers is scarce and to the best of our knowledge, this study, which describes long-term analysis of redox chemistry, is the first of its kind in such an environment. Groundwater samples were monitored for a period of 24 years, enabling us to describe redox conditions on both short-term and long-term bases. Levels of measured parameters in the contaminated aquifer varied spatially and with time, but were generally elevated except oxygen; pH (4.9-8.8), oxygen (0-11.3 mg/L), sulphate (0-28 mg/L), nitrate (0-16 mg N/L), ammonium (0.02-40 mg/L), iron (0-99 mg/L), manganese (0.06-16 mg/L), bicarbonate (22-616 mg/L) and total organic carbon (1.3-47 mg/L). From the result, levels of iron, manganese, nitrate and ammonium violated the Norwegian drinking water norms. However, iron, ammonium, total organic carbon and bicarbonate showed strong attenuation along the groundwater flow path. By contrast, oxygen, nitrate and sulphate increased farther out in the plume. The redox conditions that developed in the aquifer were similar to those previously reported for phreatic aquifers, structuring by proximity to the landfill as sulphate-reducing, iron-reducing, manganese-reducing, nitrate-reducing, and finally aerobic condition. Eventually, there was an apparent breakdown of this system due to ecosystem shift in the landfill when leachable reduced ions were depleted and the landfill became aerobic. Overall, the redox framework provided remarkable attenuation to contaminants, and thus prevented potential degradation of ecological health due to the landfill leachate.

Assessment of confined aquifer response to recharge variations and water inflow distributions using analytical approach.

Quantification of the amount of the exchanged water between the surface water and a confined aquifer is a basic step in water balance and environmental hydraulics. The hydraulic connection between a surface water and a confined aquifer may occur through different recharge variations. The current research presents new analytical solutions for confined aquifer response to recharge variations and different inflow distributions. Different cases were studied, where a constant piezometric head is applied at the right boundary of the 2D confined aquifer plane and various distributions of water inflow through the recharging windows are considered on a part and/or parts of the left boundary. Finally, a uniform water inflow distribution on parts of the left boundary and a uniform distribution of water outflow at the right boundary was considered. Both steady and unsteady state problems can be solved using proposed equations. The performance of developed analytical solutions was examined compared to the numerical finite difference modeling. The results show reasonable precision of the developed analytical solutions. The developed solutions can be used as a benchmark to verify numerical approaches with similar boundary conditions.

Assessing groundwater pollution and potential remediation processes in a multi-layer aquifer system.

Pumping-induced leakage across aquitards may induce a deterioration of water quality in multi-layer aquifer systems. It is critical to understand long-term trends of water quality parameters when assessing the sustainability of groundwater abstraction. Daily drinking water needs of 2.2 million people in Yinchuan region of northwest China are solely met by groundwater resources, but long-term groundwater withdrawal has created an extensive cone of depression (294 km2 in area) in confined aquifer causing increased vertical recharge. In this study, a model was established and calibrated with head data, then was incorporated with field tracer tests to provide key information on the hydro-dispersive characteristics of the contaminant for assessing both the current and future state of the aquifer system. The results confirmed a close association between water quality deterioration and high downward fluxes of high chloride groundwater, most notably near the center of the cone of depression. On a temporal scale, water quality degradation remains slow, largely due to the high, pre-existing storage of good quality water. Modeling suggests that the water quality in the upper confined aquifer will lose its potability over a 25 km2 and 50 km2 area within 200 years under the current and intensified pumping conditions, respectively. Elevated chloride values were also detected toward the east of the cone, highlighting the impact of hydrological settings on the vertical groundwater flow. Modeling of potential aquifer remediation shows an even slower response with a further 250 years or more required for potability to be restored in affected areas. The findings can provide valuable guidance to for decision makers and support the sustainable management of aquifer exploitation.

Processes and characteristics of hydrogeochemical variations between unconfined and confined aquifer systems: a case study of the Nakdong River Basin in Busan City, Korea.

This study is to assess the hydrogeochemical characteristics of groundwater at the deltaic region of the Nakdong River Basin in the Busan Metropolitan City of Korea. The study area is covered by the Quaternary sedimentary deposits and the Cretaceous granites associated with unconformity. The thick sedimentary deposits consists of two aquifers, i.e., unconfined and confined aquifers on the basis of clay deposit. Groundwater samples were collected from seven boreholes: two from unconfined aquifer and five from confined aquifer systems during the wet season of 2017 year. ORP and DO indicates that the groundwater of the unconfined aquifer exists in the oxidization condition and that of the confined aquifer pertains in the reduction condition. Piper's trilinear diagram shows CaSO4 type for groundwater of the unconfined aquifer, and NaCl type for that of the confined aquifer. Ionic concentrations of groundwater increase in the confined aquifer because of direct and reverse ion exchange processes. Carbonate weathering and evaporation are other mechanisms in the water-rock interaction. Saturation indices of dolomite and calcite are observed as oversaturated, while halite reveals undersaturation. Hierarchical cluster analysis (HCA) exhibits that cluster 1 and cluster 2 represents the properties of groundwater in unconfined and confined aquifers, respectively. Factor analysis shows that groundwater of the confined aquifer is much influenced by seawater, and includes heavy metals of iron and aluminum. Groundwater samples in unconfined and confined aquifers are located at the rock weathering and evaporation zones in the Gibbs diagram. Inverse geochemical modeling of PHREEQC code suggests that carbonate dissolution and ion exchange of major ions are the prevailing geochemical processes. This comprehensive research provides the distinguished hydrogeochemical characteristics of groundwater in confined and unconfined aquifer systems of the Nakdong River Basin in Busan City, Korea.

Understanding the impacts of groundwater abstraction through long-term trends in water quality.

It is vital to understand long-term trends in water quality parameters when assessing the sustainability of groundwater abstraction. Withdrawal-to-availability analysis is still used widely in groundwater management considering quantities and utilization rates based on in- and outflows. In this study, we took a step further than the water balance approach and carried out a detailed investigation of trends in long-term time series of groundwater quality, in order to analyze the sustainability of groundwater abstraction. We assessed trends, links, and patterns in abstraction, potentiometric surface, and water quality parameters for 28 well fields around Copenhagen, Denmark. Groundwater monitoring data from 1900 until 2014 were investigated for each well field. During this period, the well fields experienced a 2-14 m decrease in the nearby potentiometric surface compared to the first-or pre-pumping-potentiometric surface recordings. Sulfate concentrations increased in 25 out of 27 well fields after the maximum abstraction period, compared to the earliest water quality measurements. The results indicate that in the 1980s, when water consumption (abstraction) and drawdown were at their highest, water abstraction caused a steady increase in sulfate and calcium, which we consider unsustainable. In contradiction, the abstraction in 24 well fields show almost steady sulfate levels in the aquifer after decreased water consumption since 1995. Only four well fields showed more than a 20 mg/L increase in sulfate concentration, which indicates that the recent abstraction does not interfere with sulfate levels in the aquifer. Our method and results show how long-term water quality trends can support the management of aquifer exploitation and evaluate sustainability on the well field scale.

Estimating confined aquifer parameters using a simple derivative-based method.

The confined aquifer parameters, transmissivity and storage coefficient, are commonly determined using the pumping tests. Several methods have been developed to estimate confined aquifer parameters using pumping tests, but different methods suffer from different drawbacks. Those methods that use the truncated Theis well function w ( u ) , apply just early or late drawdowns, depending on the case, to estimate the aquifer parameters. Those methods, such as Theis (1935), that use non-truncated well function w ( u ) , can apply all drawdown data for aquifer parameter estimation but may still suffer from subjectivities such as personsal judgment in curve matching, time-consuming procedure and requiring values for Theis well function, w ( u ) , and its argument u ( u = r 2 S 4 T t ) . The aim of this study is to present a new method to overcome the aforementioned drawbacks and subjectivities involved in available published methods. In this paper, a simple derivative-based method is presented to estimate confined aquifer parameters applicable for entire drawdowns during the pumping period. The time derivative of drawdowns relate non-linearly with pumping time t , and therefore, aquifer parameters are estimated using developed equations based on the least squares optimization approach. The method is applied to three sets of synthetic, published and field data and results show that the estimation accuracy is acceptable. The drawdown time interval measurement has a marginal effect on parameters estimation due to the analytical basis of derivative calculations. The method does not require construction of graphs, and numerical calculations may be performed on a calculator to determine the aquifer parameters on site. It does not require curve matching, initial guess of the parameters and values of w ( u ) and u .

High arsenic groundwater in the Guide basin, northwestern China: Distribution and genesis mechanisms.

High arsenic (As) groundwater has been found in Pliocene confined aquifers at depths from 100 to 300 m of the Guide basin, but little is known on the main hydrogeochemical processes leading to its elevated concentrations. Ninety-seven water samples and fifty-three sediment samples were collected for chemical and/or isotopic analysis. Concentrations of As in groundwater of confined aquifer range from 9.9 to 377 µg/L (average 109 µg/L), which generally show a sharply increasing trend along with NH4+, HCO3-, CO32- and TOC along the inferred flow path, while NO3-, SO42-/Cl- and redox potential (Eh) have decreasing trends. Results of sequential extraction show that As bound to amorphous and crystalline Fe oxide minerals are the main As forms, accounting for around 50% of total As in sediments. Reductive dissolution of As-bearing Fe(III) oxide minerals under reducing conditions in confined aquifers lead to the mobilization of As in groundwater. In addition, alkaline environment and high concentrations of HCO3- and CO32- may make contributions to As enrichment in groundwater. High As groundwater in confined aquifer continuously flows out on the ground surface through tens of artesian wells, which may potentially contaminate low As groundwater in unconfined aquifer. Thus, further investigation is needed to evaluate long-term variations of water chemistry of low As groundwater and assess vulnerability of unconfined aquifer to As contamination.