Geology Faculty Research and Publications
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Item Open Access Direction of linear oceanic features subducting in the Pacific basin, their projection under the subducting plate, and arc volcano density number and volume density along the Pacific ring of fire(2022-11-04) Adam, Claudia; Vidal, Valérie; Grosse, Pablo; Ichihara, MieItem Open Access Swell and magmatic fluxes along the Louisville hotspot chain. Data associated with Adam et al. (2022) published in Geochemistry, Geophysics, Geosystems(2022-08-16) Adam, Claudia; Smith, Madison N.; Kempton, Pamela D.; Brueseke, MatthewColumn 1: longitude Column 2: latitude Column 3: distance from active volcanism (in meters) Column 4: volcanism age (in Ma) Column 5: magmatism flux (in m3s-1) Column 6: swell flux (in m3s-1)Item Open Access Cu-isotope evidence for subduction modification of lithospheric mantle(2022-07-01) Kempton, Pamela; Mathur, Ryan; Harmon, Russell S.; Bell, Aaron; Hoefs, Jochen; Shaulis, BarryA. Filename: 2021GC010134_Table_S1_probe_data_silicates.xlsx Short description: Excel file containing major element mineral compositions for silicate minerals (olivine, clinopyroxene, orthopyroxene, phlogopite and amphibole) in GVF ultramafic xenoliths determined by electron microprobe analysis B. Filename: 2021GC010134_Table_S1_probe_data_silicates.pdf Short description: pdf version of the Excel file described in (A) C. Filename: 2021GC010134_Table_S2_probe_data_nonsilicates.xlsx Short description: Excel file containing major element mineral compositions for nonsilicate minerals (spinel, sulfides, FeTi oxides) in GVF ultramafic xenoliths determined by electron microprobe analysis D. Filename: 2021GC010134_Table_S2_probe_data_nonsilicates.pdf Short description: pdf version of the Excel file described in (C) E. Filename: 2021GC010134_Table_S3_mineral_trace_elements.xlsx Short description: Excel file containing mineral trace element analyses (clinopyroxene, orthopyroxene, olivine, amphibole, and glass) determined on hand-picked mineral separates by Instrumental Neutron Activation Analysis (INAA) and in situ Laser-Assisted Inductively Coupled Mass Spectrometry on thin sections (LA-ICPMS) F. Filename: 2021GC010134_Table_S3_mineral_trace_elements.pdf Short description: pdf version of the Excel file described in (E) G. Filename: 2021GC010134_Table_S4_Cu_isotope_rock_standard_data.xlsx Short description: Excel file containing the Cu isotope compositions of rock standard BHVO-2 run during the project. H. Filename: 2021GC010134_Table_S4_Cu_isotope_rock_standard_data.pdf Short description: pdf version of the Excel file described in (G) I. Filename: 2021GC010134_Table_S5_WR_major_and_trace_elements.xlsx Short description: Excel file containing whole rock major and trace element analyses determined by XRF and LA-ICPMS for GVF ultramafic peridotites J. Filename: 2021GC010134_Table_S5_WR_major_and_trace_elements.pdf Short description: pdf version of the Excel file described in (I)Item Open Access Testing Luminescence Dating Methods for Small Samples from Very Young Fluvial DepositsSpencer, Joel Q. G.; Huot, Sebastien; Archer, Allen W.; Caldas, Marcellus M.; joelspenThe impetus behind this study is to understand the sedimentological dynamics of very young fluvial systems in the Amazon River catchment and relate these to land use change and modern analogue studies of tidal rhythmites in the geologic record. Initial quartz optically stimulated luminescence (OSL) dating feasibility studies have concentrated on spit and bar deposits in the Rio Tapajós. Many of these features have an appearance of freshly deposited pristine sand, and these observations and information from anecdotal evidence and LandSat imagery suggest an apparent decadal stability. The characteristics of OSL from small (~5 cm) sub-samples from ~65 cm by ~2 cm diameter vertical cores are quite remarkable. Signals from medium-sized aliquots (5 mm diameter) exhibit very high specific luminescence sensitivity, have excellent dose recovery and recycling, essentially independent of preheat, and show minimal heat transfer even at the highest preheats. These characteristics enable measurement of very small signals with reasonable precision and, using modified single-aliquot regenerative-dose (SAR) approaches, equivalent doses as low as ~4 mGy can be obtained. Significant recuperation is observed for samples from two of the study sites and, in these instances, either the acceptance threshold was increased or growth curves were forced through the origin; recuperation is considered most likely to be a measurement artefact given the very small size of natural signals. Dose rates calculated from combined inductively coupled plasma mass spectrometry/inductively coupled plasma optical emission spectrometry (ICP-MS/ICP-OES) and high-resolution gamma spectrometry range from ~0.3 to 0.5 mGya−1 , and OSL ages for features so far investigated range from 13 to 34 years to several 100 years. Sampled sands are rich in quartz and yields of 212–250 µm or 250–310 µm grains indicate high-resolution sampling at 1–2 cm intervals is possible. Despite the use of medium-sized aliquots to ensure the recovery of very dim natural OSL signals, these results demonstrate the potential of OSL for studying very young active fluvial processes in these settings.Item Open Access Geochemical paleoredox indicators in organic-rich shales of the Irati Formation, Permian of the Parana Basin, southern BrazilGoldberg, Karin; Humayun, M.; kgoldberg; Goldberg, KarinPaleoredox indicators were used to assess the depositional environment of the Permian Irati Formation, and discuss the controls on the accumulation of organic-rich sediments. Geochemical data were measured for 43 drill-core samples in southern Parana Basin. We infer that redox boundary was located at the sediment-water interface during the accumulation of these sediments. Trace-element data demonstrate that Irati shales were similar to average shales, except during deposition of two organic-rich beds in which U and Mo are enriched, suggesting fully anoxic conditions. The variable degree of oxygenation of bottom waters is also attested by DOPT, Fe-T/Al, and sedimentological indicators of episodic ventilation. DOPT and Fe and S correlation points to an iron-limited environment, where the main sulfur sink is organic matter rather than pyrite. Cu and Cu/Mo ratio suggests bottom-water anoxia driven by increased organic flux. The Irati Formation was probably deposited in a salinity-stratified body of water, with positive water balance. The main control on the accumulation of the organic-rich sediments was the high organic carbon flux resultant from high primary productivity in surface waters. Anoxia arose as a consequence of increased productivity, not as the primary driver in the formation of these organic-rich rocks.Item Open Access Dissolved fulvic acids from a high arsenic aquifer shuttle electrons to enhance microbial iron reductionKulkarni, Harshad V.; Mladenov, Natalie; McKnight, Diane M.; Zheng, Yan; Kirk, Matthew F.; Nemergut, Diana R.; harshad; mfkirk; Kulkarni, Harshad V.; Kirk, Matthew F.It was demonstrated more than two decades ago that microorganisms use humic substances, including fulvic acid (FA), as electron shuttles during iron (Fe) reduction in anaerobic soils and sediments. The relevance of this mechanism for the acceleration of Fe(III) reduction in arsenic-laden groundwater environments is gaining wider attention. Here we provide new evidence that dissolved FAs isolated from sediment-influenced surface water and groundwater in the Bengal Basin were capable of electron shuttling between Geobacter metallireducens and Fe(III). Moreover, all four Bangladesh sediment-derived dissolved FAs investigated in this study had higher electron accepting capacity (176 to 245 μmol/g) compared to aquatic FAs, such as Suwanee River Fulvic Acid (67 μmol/g). Our direct evidence that Bangladesh FAs are capable of intermediate electron transfer to Fe(III) supports other studies that implicate electron shuttling by sediment-derived aqueous humics to enhance Fe reduction and, in turn, As mobility. Overall, the finding of greater electron accepting capacity by dissolved FAs from groundwater and other sediment-influenced environments advances our understanding of mechanisms that control Fe reduction under conditions where electron transfer is the rate limiting step.Item Open Access Interplay between Microorganisms and Geochemistry in Geological Carbon StorageKirk, Matthew F.; Altman, Susan J.; Santillan, Eugenio-Felipe U.; Bennett, Philip C.; mfkirkResearchers at the Center for Frontiers of Subsurface Energy Security (CFSES) have conducted laboratory and modeling studies to better understand the interplay between microorganisms and geochemistry for geological carbon storage (GCS). We provide evidence of microorganisms adapting to high pressure CO2 conditions and identify factors that may influence survival of cells to CO2 stress. Factors that influenced the ability of cells to survive exposure to high-pressure CO2 in our experiments include mineralogy, the permeability of cell walls and/or membranes, intracellular buffering capacity, and whether cells live planktonically or within biofilm. Column experiments show that, following exposure to acidic water, biomass can remain intact in porous media and continue to alter hydraulic conductivity. Our research also shows that geochemical changes triggered by CO2 injection can alter energy available to populations of subsurface anaerobes and that microbial feedbacks on this effect can influence carbon storage. Our research documents the impact of CO2 on microorganisms and in turn, how subsurface microorganisms can influence GCS. We conclude that microbial presence and activities can have important implications for carbon storage and that microorganisms should not be overlooked in further GCS research.Item Open Access Thermodynamic and Kinetic Response of Microbial Reactions to High CO2Jin, Qusheng; Kirk, Matthew F.; mfkirkGeological carbon sequestration captures CO2 from industrial sources and stores the CO2 in subsurface reservoirs, a viable strategy for mitigating global climate change. In assessing the environmental impact of the strategy, a key question is how microbial reactions respond to the elevated CO2 concentration. This study uses biogeochemical modeling to explore the influence of CO2 on the thermodynamics and kinetics of common microbial reactions in subsurface environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results show that increasing CO2 levels decreases groundwater pH and modulates chemical speciation of weak acids in groundwater, which in turn affect microbial reactions in different ways and to different extents. Specifically, a thermodynamic analysis shows that increasing CO2 partial pressure lowers the energy available from syntrophic oxidation and acetoclastic methanogenesis, but raises the available energy of microbial iron reduction, hydrogenotrophic sulfate reduction and methanogenesis. Kinetic modeling suggests that high CO2 has the potential of inhibiting microbial sulfate reduction while promoting iron reduction. These results are consistent with the observations of previous laboratory and field studies, and highlight the complexity in microbiological responses to elevated CO2 abundance, and the potential power of biogeochemical modeling in evaluating and quantifying these responses.Item Open Access Geology of the Kansas Flint Hills: Ancient Ice Ages, Sea Levels, and Climate ChangeMiller, Keith B.; kbmillDuring the Wolfcampian the mid-continent of North American lay within the low relief interior of the supercontinent Pangea in near equatorial latitudes. Throughout the Permian and into the Triassic this landmass drifted slowly to the north into higher latitudes (Rowley et al., 1985; Scotese, 1986; Witzke, 1990). In the Wolfcampian, the study area would have been relatively far from areas of active tectonism. The highlands of the ancestral Rockies lay ~500 km to the west and the Ouachita and Wichita uplifts were an approximately equal distance to the south. A broad low lying cratonic area probably lay to the north and east. The carbonate and fine clastic facies of the Council Grove and Chase Groups in northeastern Kansas suggest a vast shallow marine to marginal marine shelf periodically exposed during relative sealevel lowstands.Item Open Access Landscape response to late Pleistocene climate change in NW Argentina: Sediment flux modulated by basin geometry and connectivitySchildgen, T. F.; Robinson, R. A. J.; Savi, S.; Phillips, W. M.; Spencer, Joel Q. G.; Bookhagen, B.; Scherler, D.; Tofelde, S.; Alonso, R. N.; Kubik, P. W.; Binnie, S. A.; Strecker, M. R.; joelspenFluvial fill terraces preserve sedimentary archives of landscape responses to climate change, typically over millennial timescales. In the Humahuaca Basin of NW Argentina (Eastern Cordillera, southern Central Andes), our 29 new optically stimulated luminescence ages of late Pleistocene fill terrace sediments demonstrate that the timing of past river aggradation occurred over different intervals on the western and eastern sides of the valley, despite their similar bedrock lithology, mean slopes, and precipitation. In the west, aggradation coincided with periods of increasing precipitation, while in the east, aggradation coincided with decreasing precipitation or more variable conditions. Erosion rates and grain size dependencies in our cosmogenic Be-10 analyses of modern and fill terrace sediments reveal an increased importance of landsliding compared to today on the west side during aggradation, but of similar importance during aggradation on the east side. Differences in the timing of aggradation and the Be-10 data likely result from differences in valley geometry, which causes sediment to be temporarily stored in perched basins on the east side. It appears as if periods of increasing precipitation triggered landslides throughout the region, which induced aggradation in the west, but blockage of the narrow bedrock gorges downstream from the perched basins in the east. As such, basin geometry and fluvial connectivity appear to strongly influence the timing of sediment movement through the system. For larger basins that integrate subbasins with differing geometries or degrees of connectivity (like Humahuaca), sedimentary responses to climate forcing are likely attenuated.Item Open Access High arsenic (As) concentrations in the shallow groundwaters of southern Louisiana: Evidence of microbial controls on As mobilization from sedimentsYang, N.; Shen, Z.; Datta, Saugata; Johannesson, K. H.; sdattaStudy region: The Mississippi Delta in southern Louisiana, United States. Study focus: The probable role that microbial respiration plays in As release from the shallow aquifer sediments. New hydrological insights for the region: Shallow groundwaters in southern Louisiana have been reported to contain elevated As concentrations, whereas mechanisms responsible for As release from sediments have rarely been studied in this region. Microbial respiration is generally considered the main mechanism controlling As release in reducing anoxic aquifers such as the shallow aquifers in southern Louisiana and those of the Bengal basin. This study investigates the role microbial respiration plays in As release from shallow aquifer sediments in southern Louisiana through sediment incubation experiments and porewater analysis. Arsenic concentrations were the lowest in the sterilized control experiments, slightly higher in the un-amended experiments, and the highest in the experiments amended with acetate, and especially those amended with both acetate and AQDS (9,10-anthraquinone-2,6-disulfonic acid). Although Fe and Mn generally decreased at the beginning of all the experiments, they did follow a similar trend to As after the decrease. Porewater analysis showed that As and Fe concentrations were generally positively correlated and were higher in the coarse-grained sediments than in the fine-grained sediments. Results of the investigation are consistent with microbial respiration playing a key role in As release from the shallow aquifers sediments in southern Louisiana. © 2015 The Authors.Item Open Access Broad-scale evidence that pH influences the balance between microbial iron and sulfate reductionKirk, Matthew F.; Jin, Qusheng; Haller, Ben R.; mfkirkItem Open Access Solute Concentrations Influence Microbial Methanogenesis in Coal-bearing Strata of the Cherokee Basin, USAKirk, Matthew F.; Wilson, Brien H.; Marquart, Kyle A.; Zeglin, Lydia H.; Vinson, David S.; Flynn, Theodore M.; mfkirk; lzeglinItem Open Access Precambrian nomenclature in Kansas(2014-05-27) Sawin, Robert S.; Franseen, Evan K.; Ludvigson, Greg A.; Watney, W. Lynn; West, Ronald R.; rrwestThe informal stratigraphic term “Precambrian” is replaced by formal nomenclature—Proterozoic and Archean Eonothems/Eons—and the informal term Hadean. The Phanerozoic Eonothem/Eon, representing all rocks younger than the Proterozoic, is added. The Proterozoic is further divided into Paleoproterozoic, Mesoproterozoic, and Neoproterozoic Erathems/Eras. The name Rice Formation (Scott, 1966) is abandoned, and the use of the informal term “Rice unit” is recommended. The proposed name Rice Series (Berendsen, 1994) is not accepted. These changes are adopted by the Kansas Geological Survey (KGS) and the stratigraphic nomenclature of Zeller (1968) has been revised accordingly.Item Open Access Cambrian Wichita igneous province in the Southern Oklahoma rift zone(2013-11-07) Hanson, Richard E.; Puckett, Robert E. Jr; Keller, G. Randy; Brueseke, Matthew E.; Bulen, Casey L.; Mertzman, Stanley A.; McCleery, David A.; brueseke; cabulenSouthern Oklahoma and adjacent parts of Texas contain an extensive igneous province emplaced during Early Cambrian rifting within the Southern Oklahoma rift zone. The rift zone was initiated in association with the opening of the southern Iapetus Ocean during Rodinia supercontinent breakup and later became the site of a series of linked uplifts and basins as a result of late Paleozoic inversion. Igneous rocks within the rift are referred to as the Wichita province and are present mostly in the subsurface, with critical exposures located in the Wichita and Arbuckle Mountains in southwestern and southern Oklahoma. Wells drilled into basement in the region provide a wealth of information on the distribution and relations of the major igneous units in the upper crust, and geophysical data provide important constraints on deeper levels of the rift zone. The upper parts of the igneous rift fill comprise the Carlton Rhyolite Group, which has an estimated subsurface areal extent of ~ 40,000 km², and the related Wichita sheet granites, which intrude the base of the rhyolite succession. These rocks have A-type characteristics and were emplaced after intrusion, tilting and uplift of a large tholeiitic layered mafic complex; smaller bodies of gabbro also intruded the layered complex after it was tilted. U−Pb zircon and [superscript 40]Ar/[superscript 39]Ar geochronology indicates the felsic rocks and at least some of the mafic units were emplaced in a relatively narrow time frame at ~ 539−530 Ma. Basalts and intermediate lavas are present only in the subsurface. Our new work shows these lavas to have tholeiitic to mildly alkaline compositions and to be more extensive than previously realized, forming thick sequences that both underlie and are intercalated with the rhyolites.A suite of late diabase dikes also cuts the rhyolites and granites, indicating that mafic magma was supplied to the rift throughout its magmatic history. Geophysical data show that an enormous mass of mafic rock occupies deeper parts of the rift, extending to depths of at least ~10 km beneath the surface. The total volume of igneous rock emplaced within the rift is estimated to be in excess of 250,000 km³. Detailed studies of Carlton Rhyolite exposures in the Wichita Mountains, complemented by studies of drill cuttings from basement wells, indicate that the rhyolites comprise a series of lava flows up to 400 m thick stacked on top of each other or separated by intervals of rhyolitic volcaniclastic deposits. The generally limited exposures of these rhyolite lavas in the Wichita Mountains are interpreted to be remnants of laterally extensive flow units similar to those documented from other A-type felsic provinces. Trace element contents define three distinct geochemical groups of rhyolite, suggesting derivation from three different sources or magma reservoirs. Flows belonging to the different geochemical groups are intercalated in the Wichita Mountains exposures, which may indicate that laterally extensive flows from separate magma chambers came to rest on top of one another during accumulation of the rhyolitic volcanic pile.Item Open Access CO2-induced shift in microbial activity affects carbon trapping and water quality in anoxic bioreactors(2013-10-10) Kirk, Matthew F.; Santillan, Eugenio F. U.; Sanford, Robert A.; Altman, Susan J.; mfkirkMicrobial activity is a potentially important yet poorly understood control on the fate and environmental impact of CO[subscript 2] that leaks into aquifers from deep storage reservoirs. In this study we examine how variation in CO[subscript 2] abundance affected competition between Fe(III) and SO[subscript 4]²ˉ-reducers in anoxic bioreactors inoculated with a mixed-microbial community from a freshwater aquifer. We performed two sets of experiments: one with low CO[subscript 2] partial pressure (~0.02 atm) in the headspace of the reactors and one with high CO[subscript 2] partial pressure (~1 atm). A fluid residence time of 35 days was maintained in the reactors by replacing one-fifth of the aqueous volume with fresh medium every seven days. The aqueous medium was composed of groundwater amended with small amounts of acetate (250 μM), phosphate (1 μM), and ammonium (50 μM) to stimulate microbial activity. Synthetic goethite (1 mmol) andSO[subscript 4]²ˉ (500 μM influent concentration) were also available in each reactor to serve as electron acceptors. Results of this study show that higher CO[subscript 2] abundance increased the ability of Fe(III) reducers to compete with SO[subscript 4]²ˉ reducers, leading to significant shifts in CO[subscript 2] trapping and water quality. Mass-balance calculations and pyrosequencing results demonstrate that SO[subscript 4]²ˉ reducers were dominant in reactors with low CO[subscript 2] content. They consumed 85% of the acetate after acetate consumption reached steady state while Fe(III) reducers consumed only 15% on average. In contrast, Fe(III) reducers were dominant during that same interval in reactors with high CO[subscript 2] content, consuming at least 90% of the acetate while SO[subscript 4]²ˉ reducers consumed a negligible amount (<1%). The higher rate of Fe(III) reduction in the high-CO[subscript 2] bioreactors enhanced CO[subscript 2] solubility trapping relative to the low-CO[subscript 2] bioreactors by increasing alkalinity generation (6X). Hence, the shift in microbial activity we observed was a positive feedback on CO[subscript 2] trapping. More rapid Fe(III) reduction degraded water quality, however, by leading to high Fe(II) concentration.Item Open Access Impact of commercial natural gas production on geochemistry and microbiology in a shale-gas reservoir(2013-04-18) Kirk, Matthew F.; Martini, Anna M.; Breecker, Daniel O.; Colman, Daniel R.; Takacs-Vesbach, Cristina; Petsch, Steven T.; mfkirkWe consider the effect that commercial gas production has had on microbiology and water and gas geochemistry in the northern producing trend of the Antrim Shale, an unconventional gas reservoir in the Michigan Basin, USA. We analyzed gas, water, and microbial biomass samples collected from seven wells in 2009 and compared our findings to the result of analyses performed as early as 1991 on samples collected from the same wells. We also examined production records associated with six wells. Water production has decreased sharply over time and is currently at 0.2 to 14.6% of peak levels. While this has happened, the chemical and isotopic composition of gas and water produced from the wells has shifted. The proportion of CO[subscript 2] has increased by as much as 15 mole% while CH[subscript 4] content has correspondingly decreased. Isotopically, the δ[superscript 13]C and δD values of CH[subscript 4] decreased for most wells by averages of 1.3‰ and 9‰, respectively, while δ[superscript 13]C values of CO[subscript 2] increased for most wells by an average of 1.7‰. Alkalinity in the water from each well decreased by 10 mM on average and SO[subscript 4][superscript 2−] content increased from below 50 μM to over 200 μM on average in water from each well with initial values. Microorganisms most closely related to CO[subscript 2]-reducing methanogens were the most abundant group in archaeal clone libraries and SO[subscript 4][superscript 2−] reducers were the most abundant group in bacterial libraries. In contrast, no SO[subscript 4][superscript 2−] reducers were identified in a nucleic acid-based analysis of a sample collected in 2002 from one of the wells we sampled . Our results show that commercial gas production has not only caused chemical and isotopic changes in water and gas in the Antrim Shale but also an increase in the abundance of SO[subscript 4][superscript 2−]-reducing microorganisms, a change that can ultimately have a negative impact on biogenic CH[subscript 4] formation. Processes that can explain these changes include ongoing biogeochemical reactions, groundwater flow, gas desorption, and open-system degassing.Item Open Access Decline in firing technology or poorer fuel resources? High-temperature thermoluminescence (HTTL) archaeothermometry of Neolithic ceramics from Pool, Sanday, Orkney(2012-10-23) Spencer, Joel Q. G.; Sanderson, David C. W.; joelspenThe Neolithic ceramic assemblage from the multi-period coastal settlement at Pool on the island of Sanday, Orkney is unique because it stratigraphically spans both the earlier round-based (including possible Unstan bowls) and later flat-based (‘Grooved Ware’) traditions. High-temperature thermoluminescence (HTTL) analysis objectively demonstrates that ceramics from the earliest Neolithic layers have been consistently better fired compared to examples from later layers. We suggest two interpretations of these data: either firing technology declined with changing social structures and/or adoption of a different ceramic tradition or that there was greater pressure on fuel resource and management in the later Neolithic. Paleoenvironmental and chronological evidence indicate climatic deterioration in the later Neolithic, which adds further support to an interpretation of a poorer fuel resource at that time. In addition to studies of the HTTL signal, analysis of the ambient temperature modification of the TL signal has potential to support or evaluate dating evidence, and is readily applicable to optically stimulated luminescence (OSL) age data.Item Open Access Perennial ponds are not an important source of water or dissolved organic matter to groundwaters with high arsenic concentrations in West Bengal, India(2012-09-24) Datta, Saugata; Neal, Andrew W.; Mohajerin, T. Jade; Ocheltree, Troy; Rosenheim, Brad E.; White, Christopher D.; Johannesson, Karen H.; sdattaItem Open Access Hydrological control of As concentrations in Bangladesh groundwater(2012-06-01) Stute, M.; Zheng, Y.; Schlosser, P.; Horneman, A.; Dhar, R. K.; Datta, Saugata; Hoque, M. A.; Seddique, A. A.; Shamsudduha, M.; Ahmed, K. M.; Van Geen, A.; sdattaThe elevated arsenic (As) content of groundwater from wells across Bangladesh and several other South Asian countries is estimated to slowly poison at least 100 million people. The heterogeneous distribution of dissolved arsenic in the subsurface complicates understanding of its release from the sediment matrix into the groundwater, as well as the design of mitigation strategies. Using the tritium-helium (3H/3He) groundwater dating technique, we show that there is a linear correlation between groundwater age at depths <20 m and dissolved As concentration, with an average slope of 19 μg L−1 yr−1 (monitoring wells only). We propose that either the kinetics of As mobilization or the removal of As by groundwater flushing is the mechanism underlying this relationship. In either case, the spatial variability of As concentrations in the top 20 m of the shallow aquifers can to a large extent be attributed to groundwater age controlled by the hydrogeological heterogeneity in the local groundwater flow system.
