Groundwater Geochemistry Research Articles

Geochemistry of fresh groundwater in the Altai–Sayan folded area and adjacent areas of the West Siberian plate

This study investigates the regional geochemical features of groundwater in the hypergenesis zone of the Altai–Sayan fold zone and in the adjacent structures of the West Siberian plate (hereafter simply called the Region). The hypergenesis zone of the Region contains widely spread freshwater and (to a lesser extent) low-saline waters. The patterns of changes in the water composition, the migration forms of various macro- and micro-components, and the equilibria of waters with various minerals, are analysed. The hypergenesis zones of the Altai–Sayan folded area and the adjacent structures of the West Siberian plate are characterised by high-altitude and latitudinal hydrogeochemical zonality, respectively. The total salinity of the groundwater increases with the lowering altitude conditions and changing landscape conditions in the Altai–Sayan region, and with the latitudinal changes of landscape in the adjacent structures of the West Siberian plate. In the same sequence, the contents of most mobile chemical elements and the role of complex forms of element migration increases, and the variety of minerals in equilibrium secondary formations expands. The main parageneses (geochemical water types and the associations of secondary newly-formed minerals) in the Region are oxide-hydroxide-forming, aluminosilicate forming, carbonate forming, and sulphate-forming. The parageneses are formed in genetic unity.

Natural and human-induced factors controlling the phreatic groundwater geochemistry of the Longgang River basin, South China

Abstract Groundwater chemical evolution is the key to ensuring the sustainability of local society and economy development. In this study, four river sections and 59 groundwater wells are investigated in the Longgang River (L.R.) basin in South China. Comprehensive hydrochemical analysis methods are adopted to determine the dominant factors controlling the chemical evolution of the local phreatic groundwater and the potential impact of human activities on groundwater quality. The results indicate that the ionic composition of the local phreatic groundwater is dominated by Ca2+ (0.9–93.8 mg/L), HCO3 − (4.4–280.0 mg/L), and SO4 2− (1.0–91.0 mg/L). Ca–Mg–HCO3, Ca–Na–HCO3, and Na–Ca–HCO3 are the major groundwater hydrochemical facies. Water–rock interactions, such as the dissolution of calcite and dolomite, are the primary source of the major ions in the local groundwater. Cation-exchange reaction has its effects on the contents of Ca2+, Mg2+, and Na+. Ammonia concentration of the sampling sections in the L.R. increases from 0.03 to 2.01 mg/L along the flow direction. Groundwater nitrate in the regions of the farmland is attributed to the lowest level of the groundwater quality standards of China, while the same test results are obtained for heavy metals in the industrial park and landfill, suggesting a negative impact of the anthropogenic activities on the local phreatic groundwater quality.

Identifying groundwater discharge zones in the Central Mackenzie Valley using remotely sensed optical and thermal imagery

Landsat 4–5 Thematic Mapper, Landsat 8 Operational Land Imager, and RapidEye-3 data sets were used to identify potential groundwater discharge zones, via icings, in the Central Mackenzie Valley (CMV) of the Northwest Territories. Given that this area is undergoing active shale oil exploration and climatic changes, identification of groundwater discharge zones is of great importance both for pinpointing potential contaminant transport pathways and for characterizing the hydrologic system. Following the work of Morse and Wolfe (2015), a series of image algorithms were applied to imagery for the entire CMV and for the Bogg Creek watershed (a sub watershed of the CMV) for selected years between 2004 and 2017. Icings were statistically examined for all of the selected years to determine whether a significant difference in their spatial occurrence existed. It was concluded that there was a significant difference in the spatial distribution of icings from year to year (α = 0.05), but that there were several places where icings were recurring. During the summer of 2018, these recurrent icings, which are expected to be spring sourced, were verified using a thermal camera aboard a helicopter, as well as in situ measurements of hydraulic gradient, groundwater geochemistry, and electroconductivity. Strong agreement was found between the mapped icings and summer field data, making them ideal field monitoring locations. Furthermore, identifying these discharge points remotely is expected to have drastically reduced the field efforts that would have been required to find them in situ. This work demonstrates the value of remote sensing methods for hydrogeological applications, particularly in remote northern locations.

Aquifer distribution and groundwater geochemistry in Bojonegoro Sub-district, Bojonegoro District, East Java Province, Indonesia

Bojonegoro Sub-district is the capital of Bojonegoro District, East Java Province, Indonesia. This area has quite high economic growth supported by large oil and gas reserves in the district. An increasing number of population and improving economy will cause more water needs. People in this area use groundwater as their main source for daily purposes. However, information on the potential of groundwater resources is not yet well available. Therefore, the study aims to determine groundwater flow patterns, the distribution of aquifers, and groundwater geochemistry. The study was conducted by measuring groundwater level as many as 42 points; both dug wells and deep wells, 18 points geo-electrical surveys, and chemical analysis of eight groundwater samples. The results showed groundwater flow direction into the Bengawan Solo River with the primary aquifer of the sandstone lens. All groundwater samples indicate that the type of groundwater is calcium magnesium bicarbonate (Ca 2+ - Mg 2+ - HCO 3 - ) and come from one system. High nitrate content was found in some areas, probably due to contamination from agricultural or urban wastewater. Therefore, it needs some actions for groundwater protection and conservation in this area to support the sustainable use of groundwater.

Release of geogenic uranium and arsenic results in water-quality impacts in a subarctic permafrost region of granitic and metamorphic geology

This study investigates geogenic U and As mobilization in relation to bedrock geology, groundwater geochemistry, and the presence of mineral deposits in the Dawson Range, Yukon, Canada, a remote subarctic region that has drawn recent interest from mining industry. For this study we compiled and interpreted a geochemical dataset from the region that includes 1075 rock samples, 365 sediment samples, 3189 surface water samples, and 384 groundwater samples. Elevated concentrations of U and As occur through natural weathering processes. Median U concentrations exceed the Canadian guideline for the protection of aquatic life of 15 μg/L (long-term exposure) at 8% of 547 surface-water monitoring locations and the maximum observed concentration is 337 μg/L. 39% of monitoring wells have median groundwater U concentrations above this guideline and the maximum observed concentration is 589 μg/L. Uranium mobilization is driven by weathering of granitic and metamorphic bedrock that has contents near or slightly above the average upper crustal abundance of 2.7 μg/g and by formation of soluble calcium-carbonato-uranyl complexes in groundwater. Arsenic is more heterogeneously distributed in rock than U, with localized enrichment occurring near sulfide-mineral bearing ore deposits. While >2000 μg/L As is observed present in chemically reduced groundwater, As attenuation is also observed through sorption onto Fe-(oxyhydr)oxides. Despite the occurrence of As-rich groundwater, surface-water As concentrations are consistently low (<1 μg/L), likely due to As attenuation in groundwater discharge zones. This work provides understanding of processes controlling U and As at a regional scale and presents baseline against which possible water-quality changes induced by climate change or industrial activity can be evaluated.

Assessment of groundwater geochemistry and human health risk of an intensively cropped alluvial plain, NW Italy

Groundwater chemistry data play an essential role to identify the water quality status and assess the exposure to human health. Therefore, in the present research, combined approaches used to assess the geochemistry, sources of ions and human health risk in the groundwater of the Tronzano Vercellese. Moreover, a geographic information system (GIS) technique was applied to identify locations have the most risk to human health. The results of the present study show that the groundwater samples had sequence of Ca2+>Mg2+>Na+>K+ in cationic, HCO3 −>SO4 2−>NO3 −>Cl−>F− in anionic and Zn > Fe > Ba > Ni > Mn > Cu > Cr > As in metals abundance. The hydrogeochemical approaches indicate that the major ions chemistry was mainly controlled by the weathering of carbonate and silicate minerals, and ion-exchange reactions. Furthermore, statistical analysis reveals that the non-lithogenic origin was the primary sources of some elements (Mn, Zn, Ba, NO3 − and K+) in the water samples. The hazard quotient (HQ) and hazard index (HI) were estimated to assess the risk to human health using the United States Environmental Protection Agency (USEPA) methods. Estimated HQ and HI values were higher for the child as compared to the adult and suggesting that the elements posed little hazard individually in the case of the child.

Geochemistry of groundwater and saltwater intrusion in a coastal region of an island in Malacca Strait, Indonesia

Most people in Bengkalis island use groundwater as their primary resource, but brackish water discovered in the wells located near the shore as well as in the distance. A hydrogeology study through geochemistry and isotope aspect conducted for the first time to identify the effect of saltwater intrusion (SWI) on groundwater. Na &gt; Mg &gt; K &gt; Ca were the most dominant cation sequence in groundwater, while Cl &gt; HCO3 &gt; SO4 was the most dominant for anion. NaCl is the main type of groundwater in shallow aquifer and NaCl &amp; NaHCO3 both became the most dominant type of groundwater in the deeper aquifer system. Most of the groundwater samples from dug wells indicating the weak acid exceeding strong acid. Meanwhile, the groundwater samples from bore wells dominated by alkalis exceeding alkaline earth. SWI analysis shows 36.4% of groundwater samples from dug wells and 40% of bore water samples experiencing the salinization process. From isotope analysis, the groundwater samples found to be originated from the higher elevation or distance source. Meanwhile, there are lighter 2H values in the samples that are indicating the groundwater flown in the deeper part of the aquifer.

Geochemical Assessment of Potential Sources for Nitrate in the Wasia Aquifer, Al Kharj Area, Central Saudi Arabia

Nitrate (NO3−) represents one of the major groundwater constituents with increasing distribution and concentration in the Kingdom of Saudi Arabia. This study aimed to determine potential sources of nitrate in the Early to Late Cretaceous Wasia aquifer system at the Al Kharj area (Central Saudi Arabia) by an integrative approach using groundwater geochemistry, nitrate isotopes (15N–NO3 and 18O–NO3), and tritium (3H) measurements. The lowest saline groundwater samples (TDS = 1400–2000 mg/L) from the peripheral zone were representative for pristine groundwater from the Wasia aquifer with nitrate concentrations below 20 mg/L and low 18O–NO3 ratios (8.7–20.6‰) but enriched 15N–NO3 values (up to 10.8‰). In contrast, 11 out of 34 analyzed water samples from irrigation wells and cattle watering wells exceeded the World Health Organization (WHO) drinking water guideline value for nitrate of 50 mg/L with maximum concentrations of up to 395 mg/L. Nitrate fertilizers and atmospheric deposition are the main sources of nitrate in groundwater in the eastern and northern sections of the study area. The combination of elevated salinities (4940–7330 mg/L), NO3 (111–395 mg/L), boron (516–1430 μg/L), and enriched 18O–NO3 (21.7–25.8‰) ratios with depleted 15N–NO3 (5.7–7.6‰) confirm the local influx of evaporated irrigation water with remnants of dissolved fertilizer into the Wasia groundwater system. There was no evidence for the influx of animal or human wastes from adjacent dairy, poultry, and housing infrastructures. Tritium concentrations below the detection limit of 0.8 TU for most borehole samples implied the absence of recent natural recharge. The estimated annual average N influx of 3.34 to 6.67 kg/ha to the Wasia aquifer requires a combination of atmospheric deposition and anthropogenic sources (mainly nitrate fertilizers) to increase the nitrogen content of the Wasia aquifer.

Hydrotalcite stability during long-term exposure to natural environmental conditions.

Hydrotalcite-like compounds are a group of layered double hydroxides widely studied as sorbents to remove organic and inorganic contaminants under laboratory conditions. This study is a proof-of-concept of the long-term fate of hydrotalcite compounds under natural environmental conditions, to bridge the gap between laboratory studies and their field application as sorbents. Hydrotalcite (HT) with intercalated carbonate species (HT-CO3) and dodecyl sulphate (HT-DS) were synthesised and placed in two groundwater monitoring wells in Denmark, one contaminated with chlorinated hydrocarbons and another with uncontaminated groundwater. To assess the structural and surface compositional changes of hydrotalcite compounds upon prolonged exposure to groundwater, the material was analysed with powder X-ray diffraction (PXRD), Fourier-transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The results showed that the stability and dissolution behaviour of hydrotalcite compounds under groundwater conditions depended on the intercalated anion (CO32- > DS) and groundwater dynamics (static flow > dynamic flow), while the hydrotalcite aggregate size only had a minor effect. Groundwater geochemistry influenced the precipitation of insoluble species (CaCO3, and adsorbed sulphate) on the hydrotalcite surface. The instability of hydrotalcite compounds, especially in the case of HT-DS, may constitute a significant limiting factor on their future application as sorbents under dynamic flow conditions.

Hydrodynamic conceptual model of groundwater in the headwater of the Rio de Oro, Santander (Colombia) by geochemical and isotope tools

Abstract Metamorphic, igneous and sedimentary rocks, with low to no primary porosity, outcrop in the La Moza micro-basin stream (headwater of the Rio de Oro). In this high mountain system, water isotopic composition of rainwater, water isotopes and geochemistry of groundwater (springs) and surface water were determined. Groundwater flows are associated to phreatic aquifers in relationship with secondary porosity generated by fracturing, which is increased by dissolution processes in case of carbonate formation producing karstic systems and by the weathering phenomenon mainly affecting granodioritic rocks. Water geochemistry shows low to medium electrical conductivity (EC) depending on the geological formation, but a unique calcium bicarbonate facies. Spring water EC shows limited temporal variations. The isotopic composition of spring indicates a meteoric origin, local infiltration and groundwater flows with low residence time. A conceptual model of the recharge zone is proposed that crosses the surface watershed and covers part of the adjacent Rio Jordán basin, where the Berlin Paramo is located.

Isotopic behavior and self-organizing maps for identifying groundwater salinization processes in Jerba Island, Tunisia

The purpose of this study is to assess the groundwater evolution in Jerba Island. This paper also focuses on the relationship between mineralization and anthropogenic activities and the identification of processes controlling groundwater geochemistry. An approach combining the use of water dissolved chemical species, isotopic fingerprint, and the self-organizing maps has been used to understand the behavior of a unconfined aquifer system and to determine the origin of its different contamination sources. In the present work, a geochemical characterization of the coastal aquifer from the unconfined Jerba aquifer, Southeast Tunisia, was carried out, using hydrogeochemical and isotopic (2H and 18O) data, correlation matrix, and self-organizing maps. For this aim, 79 wells were sampled and analyzed. Geochemical modeling and isotopic data demonstrated that seawater intrusion, exchange reactions occurring within the aquifer matrix, and anthropogenic activities were the major processes controlling the groundwater contamination in the island. The isotopic tracers indicate that the increasing salinity in the shallow coastal aquifer is caused by seawater intrusion. The methodology appears as a valid tool applicable in other coastal areas to determine the main factors governing the groundwater mineralization.

Removal of Trichloroethylene by Sulfide-Modified Nanoscale Zerovalent Iron Coated with Different Stabilizers in Aqueous Solution

In this study, the dispersion stability and reactivity of sulfide-modified nanoscale zerovalent iron (SNZVI) coated with different surface stabilizers (i.e., starch, sodium dodecylbenzene sulfonate (SDBS), or carboxymethyl cellulose (CMC)) were investigated. All the three types of surface stabilizers could enhance the dispersion stability of SNZVI but exerted differing influences on the reactivity toward trichloroethylene (TCE) removal. The coating of starch on SNZVI markedly improved TCE removal, which was positively correlated with the enhanced dispersion stability (i.e., more active surface sites). However, although the SDBS and CMC could enhance the dispersion stability of SNZVI, they resulted in an inhibition in TCE removal, especially for CMC. It was presumed that the coated SDBS/CMC on the surface of SNZVI occupied the active surface sites for TCE removal. Besides, the effect of groundwater geochemistry (i.e., pH, Ca2+, and humic acid (HA)) was examined. The increasing pH from 5 to 9 led to a slight decrease for all stabilized SNZVI particles. The presence of Ca2+ or HA exerted distinct influence on the TCE removal by the three stabilized SNZVI. The Ca2+/HA exerted an insignificant effect on the reactivity of CMC-SNZVI but markedly decreased the reactivity of starch-SNZVI and SDBS-SNZVI. The varying effects of Ca2+/HA should be due to their distinct interactions with different types of stabilizers on the surface of SNZVI.

Detailed geochemical assessment & indexing of shallow groundwater resources in metropolitan city of Nagpur (western Maharashtra, India) with potential health risk assessment of nitrate enriched groundwater for sustainable development

Identification of complex processes controlling the groundwater chemistry and the indexing of urban groundwater for drinking were carried out with special emphasis on health risk associated with nitrate contamination in the urban areas of Nagpur City in western India. The hydrochemical facies of forty-seven samples collected from shallow aquifers were of Ca–HCO3 and mixed Ca–Na–HCO3 type indicating a transition from pure recharge water to mixed type water. 75% of the samples were very hard water. Geochemical signatures evaluated from the Na+-Cl− plot, Na+:Na+ + Cl−, Ca2+ + Mg2+ vs. HCO3− + SO42− plot, (Na++ K+ - Cl−)/(Na++ K+ - Cl− + Ca2+), Ca2+/Na+ vs. Mg2+/Na+, SO42−/Cl− signposted the collective contribution of calcite, dolomite, silicate, and gypsum weathering on groundwater geochemistry. The dominancy of cation exchange followed by reverse ion exchange, anthropogenic inputs, and mineral dissolution are resolved based on bivariate plots. The supersaturated thermodynamic states of calcite and dolomite minerals played a major factor for low Ca2+ in some samples. Nitrate higher than the drinking-water thresholds of 50 mg/L (as per WHO) in around 51.1% of the samples reflected the consequences of urbanization on shallow aquifers. The EWQI method ranked 74.5% of the urban groundwater as moderately suitable and remaining samples are not suitable for drinking. The non-carcinogenic risk (HI Total) were computed as 1.82, 2.16, and 2.47 for an adult male, adult female, and children of Nagpur urban region. The non-carcinogenic risk was higher in children as compared to the adult residents. The noteworthy finding of the study may help in building effective policies and enhancing individual cooperation for the sustainable development of groundwater reservoirs in this region.

Groundwater mixing in a heterogeneous multilayer aquifer driven by geogenic CO2 fluxes: Evidence from chemical and isotopic composition of Ferrarelle waters (Riardo Plain, southern Italy)

The successful management of carbon in the Earth's crust is critical for mitigating the increase of anthropogenic CO2 in the atmosphere. Carbon Capture and Storage (CCS) requires an understanding of the behavior of carbon in the crust and the development of robust monitoring techniques to constrain the movement, mechanisms, and pathways for any potential CO2 leakage. Here, we examine an aquifer from the Riardo Plain (Campania Region, southern Italy), which serves as a suitable natural analogue for CO2 migration to the critical zone (i.e., shallow crust and aquifers) and as a case study to evaluate the geochemical processes that occur when CO2-saturated fluids mix with freshwater in shallow aquifers. We investigate the behavior of various geochemical constituents (major and trace elements, δ18O–H2O, δ13C-DIC, and Rn content). Water from this area has a high degree of mineralization (EC 2500–3000 μS/cm), high HCO3- (~2.5 g/L), is saturated with respect to CaCO3, and is enriched in alkali ions (e.g., Na+ + K+). The high degree of mineralization occurs in groundwater that discharges from the basal aquifer of the Roccamonfina volcanic edifice (~6 km NW), with vast CO2 inputs that promote host rock leaching. Superficial volcanic aquifers are recharged by fresh meteoric precipitation when groundwater flows from carbonates at the edge of the plain to aquifers hosted in the southeastern slope of the Roccamonfina volcano. The presence of normal faults in this area permits natural upwelling of CO2-rich groundwater, which locally mixes with shallow freshwater present within the upper volcanic succession. Significant (R > 0.8) linear correlations between conservative elements suggest that groundwater geochemistry is dominated by a mixture of two main endmembers: (i) deep/mineralized waters and (ii) shallow/diluted waters. The intrusion of freshwater to volcanic aquifers induces oxidation, leading to adsorption of select elements (e.g., As and Ba) onto Fe-oxyhydroxide precipitates within these aquifers. Geochemical modeling suggests that CO2 saturation approaches 3 g/L, which agrees with direct measurements of CO2 flux. We conclude that our conceptual geochemical model helps to constrain mixing of CO2 with freshwater and to diagnose the secondary geochemical processes that influence aqueous geochemistry within CO2-influenced groundwater.

Chemical weathering and atmospheric carbon dioxide (CO2) consumption in Shanmuganadhi, South India: evidences from groundwater geochemistry.

Chemical weathering in a groundwater basin is a key to understanding global climate change for a long-term scale due to its association with carbon sequestration. The present study aims to characterize and to quantify silicate weathering rate (SWR), carbon dioxide consumption rate and carbonate weathering rate (CWR) in hard rock terrain aided by major ion chemistry. The proposed study area Shanmuganadhi is marked with superior rainfall, oscillating temperature and runoff with litho-units encompassing charnockite and hornblende-biotite gneiss. Groundwater samples (n = 60) were collected from diverse locations and analysed for major chemical constituents. Groundwater geochemistry seems to be influenced by geochemical reactions combining dissolution and precipitation of solids, cation exchange and adsorption along with minor contribution from anthropogenic activities. The SWR calculated for charnockite and hornblende-biotite gneiss was 3.07tonskm-2year-1 and 5.12tonskm-2year-1, respectively. The calculated CWR of charnockite and hornblende-biotite gneiss was 0.079tonskm-2year-1 and 0.74tonskm-2year-1, respectively. The calculated CO2 consumption rates via silicate weathering were 1.4 × 103molkm-2year-1 for charnockite and 5.8 × 103molkm-2year-1 for hornblende-biotite gneiss. Lithology, climate and relief were the key factors isolated to control weathering and CO2 consumption rates.

Active sulfur cycling in the terrestrial deep subsurface.

The deep terrestrial subsurface remains an environment where there is limited understanding of the extant microbial metabolisms. At Olkiluoto, Finland, a deep geological repository is under construction for the final storage of spent nuclear fuel. It is therefore critical to evaluate the potential impact microbial metabolism, including sulfide generation, could have upon the safety of the repository. We investigated a deep groundwater where sulfate is present, but groundwater geochemistry suggests limited microbial sulfate-reducing activity. Examination of the microbial community at the genome-level revealed microorganisms with the metabolic capacity for both oxidative and reductive sulfur transformations. Deltaproteobacteria are shown to have the genetic capacity for sulfate reduction and possibly sulfur disproportionation, while Rhizobiaceae, Rhodocyclaceae, Sideroxydans, and Sulfurimonas oxidize reduced sulfur compounds. Further examination of the proteome confirmed an active sulfur cycle, serving for microbial energy generation and growth. Our results reveal that this sulfide-poor groundwater harbors an active microbial community of sulfate-reducing and sulfide-oxidizing bacteria, together mediating a sulfur cycle that remained undetected by geochemical monitoring alone. The ability of sulfide-oxidizing bacteria to limit the accumulation of sulfide was further demonstrated in groundwater incubations and highlights a potential sink for sulfide that could be beneficial for geological repository safety.

Geophysical investigation of resistivity and groundwater quality in Ogbe-Ijoh coastal area of the western Niger Delta of Nigeria

Groundwater quality assessment is essential for groundwater resource management, especially as it concerns the health of those that consume it. Consequently, we assembled resistivity and hydrogeochemical data to assess the adverse effect of anthropogenic sources on groundwater quality in Ogbe-Ijoh, a coastal area of the western Niger Delta. Geoelectrical data showed no evidence of quality degradation as reflected by high resistivity values that ranged from 29.3 to 349.9 Ωm. Aquiclude of adequate thickness capping aquifers was uncovered in most areas assessed, with the exception of areas near the bank of the river. The delineated clay sedimentary horizon probably enhanced groundwater quality sustainability by acting as a barrier to migrating contaminants, especially from leachate and septic tanks. The physical properties of groundwater sampled indicated moderate pH, TDS and EC magnitudes, with a majority having magnitudes lesser than the requisite standards. The geochemistry of groundwater revealed cations and anions with magnitudes that are distinctively lesser than the desired ranges. Piper plot revealed two different water types: NaCl− and Ca2+ + Mg2+ + Cl− + SO4, which implied that cation exchange has a weighty effect on quality of groundwater. Overall, the hydrogeochemical results are consistent with resistivity data and revealed the efficacy of the pairs in groundwater quality assessment in the absence of lithologic information. The groundwater condition is relatively safe for consumption; nevertheless, due to health implications we recommend that pH should be increased to the required standard before drinking.

Rapid groundwater recharge dynamics determined from hydrogeochemical and isotope data in a small permafrost watershed near Umiujaq (Nunavik, Canada)

Hydrogeochemical data are used to better understand recharge dynamics and to support a hydrogeological conceptual model in a 2-km2 watershed in a discontinuous permafrost zone in Nunavik, Canada. The watershed contains an upper (surficial) and lower aquifer within Quaternary deposits, above and below a marine silt layer containing ice-rich permafrost mounds. The analysis is based on water samples from precipitation, groundwater monitoring wells, ground ice in permafrost mounds, thermokarst lakes and a perennial stream. Groundwater geochemistry in both aquifers reflects young, poorly evolved waters, with mainly Ca-HCO3 water types and low mineralisation ranging from 11 to 158 mg/L total dissolved solids (TDS), implying short pathways and rapid travel times of a year or less. While relatively low, TDS signatures in groundwater and surface water show increasing values downgradient. Groundwater isotope values (δ18OH2O and δ2HH2O) are often strongly influenced by snowmelt, while those of thermokarst lakes show evidence of evaporation. Recharge along the cuesta contributes to a transverse component of groundwater flow within the valley with higher TDS and δ13CDIC values influenced by open-system weathering. Even where permafrost-free, the marine silt unit has a strong confining effect and plays a more important role on recharge dynamics than the discontinuous permafrost. Nevertheless, the vulnerability of these types of hydrogeological aquifer systems is expected to increase due to rapid recharge dynamics associated with the gradual loss of the confining effect of permafrost. This hydrogeochemical data set will be useful as a baseline to document impacts of permafrost degradation on the hydrogeological system.

Using cluster analysis for understanding spatial and temporal patterns and controlling factors of groundwater geochemistry in a regional aquifer

Understanding spatial and temporal patterns of groundwater geochemistry at the regional scale over a long time period is challenging, due to the lack of data and effective statistical approaches to characterize complex natural processes and anthropogenic activities. We applied a recently developed cluster analysis method to investigate spatial and temporal patterns and controlling factors of groundwater geochemistry in the confined aquifer of the Jianghan Plain, China. The cluster analysis is applied to a dataset of 13,024 groundwater geochemical measurements for 11 geochemical parameters of 1,184 groundwater samples collected over 23 years from 29 monitoring wells distributed over the Jianghan Plain. The cluster analysis yielded a classification of seven clusters, and the classification was confirmed by using principal component analysis and Stiff and Piper diagrams. Based on the spatial distribution of the seven clusters, the Jianghan Plain is separated into four geochemical zones (i.e., recharge zone, transition zone, flow-through zone, and discharge-mixing zone) along the regional groundwater flow path, which has not been attempted in the past. The temporal changes of groundwater geochemistry are controlled by short- and long-term factors of water-rock interactions and anthropogenic activities. A particular finding of this study is that the Three Gorges Reservoir has a long-term impact on groundwater geochemistry, because the reservoir increased river discharge to groundwater after 2009. This study demonstrates that using the cluster analysis method together with hydrogeochemical analysis can identify spatiotemporal patterns and controlling factors of groundwater geochemistry at the regional scale over a long monitoring period.

Role of Quaternary glacial-outburst megaflood paleochannel deposits in a basalt-dominated aquifer system in the West Plains area of eastern Washington, USA

Approximately 2 km2 of sand- and gravel-rich Pleistocene glacial-outburst megaflood sediment has accumulated in a series of five subparallel paleochannels that are an important component of a basalt-dominated aquifer system on the semiarid, ~450-km2 West Plains area. Located near Spokane, Washington (USA), the West Plains paleochannel deposits have been used for artificial storage and recovery wells and for planned and passive stormwater disposal in this rapidly growing area. Recent recognition of perfluorooctanoic acid contamination and excessive drawdowns have led to the closure of multiple municipal wells on the West Plains, highlighting the challenges of developing a sustainable groundwater supply in this complex aquifer system. Geologic and hydrogeologic data, from more than 1,400 water-well reports, surface exposures, existing groundwater geochemistry reports, and modeling, reveal the interconnections between the paleochannel deposits and surrounding basalt flows in three dimensions (3D). Initially incised into the Miocene Columbia River Basalt and excavated by Pleistocene glacial-outburst megaflooding, the West Plains paleochannels are projected in unprecedented 3D. This study’s modeled interactions between the more highly hydraulically conductive (100×) paleochannel sedimentary deposits and surrounding basalt beds provide insight into 14C ages and tritium values that had been measured previously. Sediment-filled closed depressions at the bases of the paleochannels, which are features likely formed by turbulent bursts (‘kolks’) during peak megaflood flows, are potentially promising groundwater storage sites. However, because of the relatively higher permeability of the unconfined paleochannel deposits, the closed depressions also are potential conduits for infiltration of contamination to deeper parts of the aquifer system.