Groundwater Age Research Articles

Groundwater arsenic content in quaternary aquifers of the Red River delta, Vietnam, controlled by the hydrogeological processes

The relation between arsenic groundwater concentrations and hydrogeological processes was investigated in the proximal part of the Red River delta, Vietnam, west of the depression cone formed by the exploitation of groundwater in Hanoi. Flow paths in the Quaternary aquifers were modeled based on previously interpreted geological structure and hydrogeological data gathered during field work in 2014–2017. Sedimentary structures and simulated flow patterns were compared with the spatial distribution of the groundwater arsenic concentration.The regression of the sea in the area started 4 ka BP in the Holocene. The low tectonic subsidence rate of the Red River delta led to intensive erosion and replacement of fine grained sediments of the sea level high stand by sandy channel belts, resulting in hydraulic connections between the Pleistocene and Holocene aquifers. The Pleistocene aquifer is recharged by both regional flow paths and naturally occurring vertical recharge through Holocene sand and clay layers. Young groundwater (<40 a) in the shallow Holocene aquifer generally discharges to surface water bodies. The shallow flow system is also seasonally recharged with surface water, as indicated by δ18O enrichment of groundwater and oscillating groundwater ages in wells in the vicinity of water channels. The deeper flow system discharges into the Red River and Day River or flows parallel to the rivers, toward the sea.The overall pattern of arsenic groundwater concentrations (decreasing with increasing sediment age) is modified by groundwater flow. The arsenic contamination of the Pleistocene aquifer of the Red River delta is not only caused by the intensive groundwater abstraction in Hanoi, as reported previously, but also by the natural flow of high arsenic groundwater from Holocene to Pleistocene aquifers in areas located outside of the depression cone. Groundwater with < 50 µg L-1 arsenic is found in the Pleistocene aquifer close to the recharge zone in the mountains bordering the Red River delta and in the Holocene and Pleistocene aquifers where clay deposits were eroded. Close to the recent Red River channel, recharge of older Holocene and Pleistocene sediments occurs partially by arsenic-contaminated groundwater from the youngest Holocene aquifers, and here arsenic concentrations exceed 50 µg L-1. A high arsenic concentration is also present in the early Holocene-Pleistocene aquifer, beneath thick clay layers, indicating a limited extent of flushing and the inflow of fresh organic matter.

Groundwater age persistence in topography-driven groundwater flow over paleohydrogeologic time scales

AbstractThe Pleistocene-Holocene climate transition resulted in a dramatic reduction in groundwater recharge in many aquifers in arid and semiarid regions throughout the world. This study conducted numerical experiments to compare the evolution of groundwater hydraulics and age patterns in arid and semiarid aquifers in response to transient conditions associated with recharge decline from the Pleistocene to the Holocene. Our results show that after a rapid reduction in recharge, the amplitude of water-table undulations and regional groundwater slope both reduced. This resulted in a general, and relatively rapid, contraction of local flow systems and an increase in the extent of intermediate and regional systems. The previous hierarchy of local, intermediate, and regional flow systems was completely replaced by largely horizontal and regional flow patterns after ~10,000 yr. However, in stark contrast, we observed that the original Pleistocene age patterns have remained almost unchanged throughout the 10,000 yr Holocene period. Thus, groundwater age is more likely to be indicative of past rather than current flow systems. Consequently, due to this age persistence, the use of modern groundwater age data to calibrate models or compute recharge with methods that do not account for this potentially significant spatial and temporal mismatch between age and hydraulics will be misleading and erroneous. This has significant implications for hydrogeologic analyses. The findings of this study may also apply to areas that have undergone dramatic changes in land cover or land use that strongly influence transient groundwater recharge processes.

Estimation of groundwater recharge rates using soil-water isotope profiles: a case study of two contrasting dune types on Langeoog Island, Germany

The creation of artificial dunes for coastal protection may have important consequences for freshwater lenses in coastal aquifers. The objective of this study was to compare the recharge processes below such a young dune with scant vegetation to an older dune covered by grass and herbaceous vegetation. To this aim, soil and water samples were collected from the unsaturated zone at two sites on Langeoog Island in northern Germany, and the soil water was analysed for stable water isotopes and chloride. Recharge rates were calculated by using a new version of HYDRUS-1D, which was modified to simulate isotope fractionation during evaporation. Both the model outcomes and the data highlight the importance of fractionation, which is slightly more pronounced at the older, more vegetated dune. At the newly constructed dune, vegetation dieback seemingly reduces the importance of transpiration during summer. Recharge occurs year-round, albeit predominantly during the winter months. Calculated recharge rates are consistent with lysimeter measurements, but are significantly higher than previously reported rates based on groundwater age data, which is primarily attributed to the absence of dune shrub at the sites investigated here. More data are needed to establish the importance of soil-water repellency and overland flow. Based on the results, it is proposed that repeated isotope sampling can yield important insights into the dynamics of recharge processes, including their response to climate change.

Inference of young groundwater ages and modern groundwater proportions using chlorofluorocarbon and tritium/helium-3 tracers from West Hawai‘i Island

The geologically complex western aquifers of Hawai‘i Island serve as the primary reservoir of fresh potable water for residents, yet the area’s hydrogeologic characteristics are still not well understood. Groundwater age is a fundamental parameter used to better understand groundwater flow and storage, yet few studies have attempted to estimate apparent groundwater ages in Hawai‘i. This study implements a multi-tracer approach using chlorofluorocarbons, sulfur hexafluoride, and tritium/helium-3 to determine the apparent ages of young water masses from the West Hawai‘i aquifers. Chlorofluorocarbon and tritium/helium-3 tracer measurements suggest that the aquifers contain young groundwater with a median age of 53 years, but that a significant volume of older groundwater exists, which was recharged prior to the release of these anthropogenic tracers. Mixing models suggest that the groundwater consists of mixtures with an average 28% modern water (<70 years), with the remainder being recharged prior to 1953. Further, helium measurements demonstrate that the area’s groundwater is influenced by hydrothermal and magmatic processes from Hualālai volcano, which bias estimated groundwater ages if not properly considered. It is therefore beneficial to utilize a multi-tracer approach in an oceanic volcanic island setting, where groundwater ages are impacted by various external processes. The analysis of young apparent ages and proportions can aid in coastal aquifer characterization and subsequent groundwater quality assessment and is crucial for sustainable groundwater management.

Groundwater recharge timing based on 14C and 2H within Indian Wells Valley, California, USA

A large carbon-14 dataset for groundwater in Indian Wells Valley, California, is used to identify groundwater recharged during the pluvial climate of the previous glacial period, more than 12,000 years before present. Additional data collected in 2019, including measurements of 13C and 14C in the recharge areas and dissolved organic carbon isotopes, substantiated the unusually heavy-isotope-enriched 13C and relatively low 14C compositions in the Sierra Nevada canyons on the northwestern margin of the basin. The carbon isotopic characteristics in this area may be due to aeolian deposits of sodium carbonate minerals from dried lake beds along the ancient Owens River, or dissolution of metasediments within the Sierra Nevada batholith, or geothermally sourced CO2 gas, or a combination of these. Recognizing these isotope characteristics is essential for correcting apparent 14C groundwater ages. Additional complications arise from sulfate reduction and related conversion of 14C-dead sedimentary organic material to dissolved inorganic carbon with a light 13C composition. The lacustrine deposits of pluvial China Lake and organic-rich spring deposits along the Sierra Nevada range front offer reducing environments across much of the valley. A similarly large 2H dataset provides an independent marker for recharge in a previous climate, facilitated by exhaustive regional studies of 2H content of precipitation, springs, and groundwater (Smith et al., 2002). The 14C data support an association of δ2H values less than −100‰ with pluvial-climate groundwater and δ2H values greater than −96‰ with groundwater recharged under current climatic conditions. Groundwaters with δ2H between these values cannot be assigned with certainty to either group. Evaporation is also evident for a small set of groundwater samples, rendering 2H unreliable as a climate indicator for them. Using 14C and 2H in combination, much of Indian Wells Valley is found to contain groundwater consistent with recharge during current climatic conditions, with ages generally less than 6000 years. Groundwater with pluvial characteristics occurs primarily in the far end of the flow system, in the eastern part of the valley near the evapotranspiration discharge area. The footprint of this older water is more extensive in the deeper parts of the flow system and the vertical distribution of ages indicates it moves upward from the deeper parts of the flow system to the shallow zone to discharge. Estimates of groundwater travel time from a numerical flow model of the basin are generally consistent with the isotopic interpretations.

How Fluvial Deposits Distort Aquifer Recharge Estimated From Groundwater Age: Insights From a Landscape Evolution Model

AbstractSustainable management of groundwater relies on robust estimates of aquifer fluxes. As such, recharge can be estimated from groundwater age using analytical models assuming a homogeneous aquifer. We propose to assess how this assumption affects recharge estimates in fluvial deposits. First, we use CHILD, a landscape evolution model, to generate plausible deposits after tens of thousands of years of river evolution. Then, we use the fractional packing model to compute porosity and permeability of unconsolidated sediments assuming a mixture of a coarse and a fine fraction. Finally, we use PFLOTRAN to simulate groundwater flow and mean age over two thousand years of spatially uniform recharge. This reproduces the boundary conditions underlying Vogel's analytical model to estimate recharge rates. Our results are based on 20,000 realizations from varying inputs: grain sizes, aggradation rate of the river, porosity, recharge, etc. For most models, the mean estimate of the recharge rate remains below an absolute error of 25%. But fluvial heterogeneities can lead to local errors ranging from close to zero to several thousand percent, which means that estimates may vary drastically depending on sampling locations. A sensitivity analysis using the δ‐importance measure and CUSUNORO curves exposes how fluvial processes influence the range and distribution of the error. For instance, a high bank erodibility and small coarse grain size under low aggradation rates favor the reworking of deposits, which increases heterogeneity and leads to high errors. This work paves the way for a more reliable estimation of recharge by considering the sedimentary context.

Deciphering complex groundwater age distributions and recharge processes in a tropical and fractured volcanic multi‐aquifer system

AbstractGroundwater recharge in highly fractured volcanic aquifers in the humid tropics remains poorly understood. In this region, rapid demographic growth and unregulated land use change are resulting in extensive surface water pollution and a large dependency on groundwater extraction. Here we present a multi‐tracer approach including δ18Oδ2H, 3H/3He dating, and noble gases (NG) within the most prominent multi‐aquifer system of central Costa Rica, with the objective to assess dominant groundwater recharge mechanisms and age distributions. We sampled wells and large springs across an elevation gradient from 868 to 2421 m asl. Our results indicate relatively young apparent ages ranging from 0.0 ± 3.2 up to 43.5 ± 7.6 years within the unconfined aquifer system. Helium isotopes (R/Ra up to 5.4) indicate a dominant signal from the upper mantle and preclude 3H/3He dating in 50% of the samples. Potential recharge elevations (based on NG and δ18O) ranged from ~1350 to 2670 m asl. NG‐derived recharge temperatures ranged from 11.0°C to 19.4°C. Recharge estimates varied from 129 ± 78 to 1605 ± 196 mm/yr with a mean value of 642 ± 117 mm/yr, representing 20.1 ± 4.0% of the total mean annual rainfall as effective recharge. The shallow unconfined aquifer is characterized by young and rapidly infiltrating water, whereas the deeper aquifer units have relatively older water (&gt;60 years). These results are intended to guide the delineation and mapping of critical recharge areas in mountain headwaters to enhance water security and sustainability in the most important headwater dependent systems of Costa Rica.

Paleoclimatic interpretation in southern Laizhou Bay since the late Pleistocene: Evidence from groundwater and sedimentary strata

Understanding past climatic change and the associated responses of sea level may contribute to the ability to evaluate the relationship between these factors in the future under the background of sea-level changed. Here, we aimed to reveal the climate change and sea-level characteristics of southern Laizhou Bay (SLB) since the late Pleistocene by combining groundwater stable isotopes (2H, 18O and deuterium excess (dexcess)), age dating (14C) and the structural characteristics of sediments from several boreholes. The isotopic proxies for climates demonstrated the depletion of 2H, 18O and dexcess of groundwater recharged in the last glacial period of the late Pleistocene, indicating that the weather was cooler and wetter in the last glacial maximum, while the stable isotopes and dexcess of groundwater were enriched in the Holocene. Several transgressions were revealed through the structural characteristics of boreholes, which also indirectly reflected the large-scale changes of in global temperature. Finally, a conceptual model that describes the evolution of coastal saline groundwater based on the transgression process was proposed based on the sedimentary chronology of strata and groundwater age. These results not only contribute to the study of climate-related sea-level change since the late Pleistocene but also provides a theoretical explanation for the mechanism by which coastal saline groundwater forms.

Isotopic discrimination of natural and anthropogenic perchlorate sources in groundwater in a semi-arid region of northeastern Oregon (USA)

Perchlorate (ClO4−) has synthetic and natural sources. Synthetic ClO4− is released to the environment from its use as an oxidant in military and aerospace applications, and from its presence in a variety of common commercial products, such as safety flares, chlorate herbicides, and fireworks. Natural sources of ClO4− in the environment include imported nitrate fertilizers derived from salt deposits in the Atacama Desert of Chile and indigenous natural ClO4− that accumulates in unsaturated soils and groundwaters in other arid and semi-arid environments, largely from atmospheric deposition. The stable isotope ratios of chlorine (37Cl/35Cl) and oxygen (18O/16O, 17O/16O) and the isotopic abundance of radioactive 36Cl in ClO4− can be used to discriminate these different sources. Perchlorate was previously detected at relatively high concentrations (3.8–34.7 μg/L) in groundwater from many wells in the Boardman-Umatilla area near the Columbia River in northeastern Oregon, which is a semi-arid, highly agricultural, heavily irrigated area that includes several past and current military installations. Eight representative groundwater wells were sampled throughout this region and isotopic characteristics of ClO4− collected from each well were measured along with other chemical and isotopic parameters including tritium and other groundwater age indicators. Isotopic data indicate that indigenous natural ClO4− was present in groundwater from all sampled wells and was the predominant source in five of the wells. Synthetic ClO4− was present in the three remaining wells with natural ClO4−, and a minor fraction of Atacama-fertilizer-derived ClO4− was indicated in one of the wells. Data from this study expand the geographic area of the USA in which indigenous natural ClO4− has been detected to include the semi-arid northwest. This study also illustrates the role of irrigation recharge as a mechanism for producing relatively high concentrations of indigenous natural ClO4− in groundwater by flushing accumulated salts from the unsaturated zone.

Isotopic and Chemical Tracing for Residence Time and Recharge Mechanisms of Groundwater under Semi-Arid Climate: Case from Rif Mountains (Northern Morocco)

Karstic aquifers play an important role for drinking and irrigation supply in Morocco. However, in some areas, a deeper understanding is needed in order to improve their sustainable management under global changes. Our study, based on chemical and isotopic investigation of 67 groundwater samples from the karst aquifer in the Rif Mountains, provides crucial information about the principal factors and processes influencing groundwater recharge and residence time. The δ18O and δ2H isotopic values indicate that the recharge is derived from meteoric water at high, intermediate, and low elevations for Lakraa Mountain, North of Lao River, and Haouz and Dersa Mountain aquifers, respectively. All samples show an isotopic signature from Atlantic Ocean except for those from the Lakraa Mountain aquifer, which shows Mediterranean Sea influence. Groundwater age determined by radiocarbon dating using the IAEA model indicates that the ages range from modern to 1460 years. This short residence time is consistent with the detectable tritium values (&gt;2.7 TU) measured in groundwater. These values are similar to those of precipitation at the nearest GNIP stations of Gibraltar and Fez-Saiss, situated around 100 km north and 250 km south of the study area, respectively. This evidence indicates that groundwater in the Rif Mountains contains modern recharge (&lt;60 years), testifying to significant renewability and the vulnerability of the hydrological system to climate variability and human activities. The results also indicate the efficiency of isotopic tracing in mountainous springs and would be helpful to decision makers for water in this karstic zone.

Upwelling of Mantle‐derived Material in Southeast China: Evidence from Noble Gas Isotopes

AbstractShallow groundwater collected in Chaozhou, Huizhou, and Guangzhou allowed testing of concentrations and the isotope ratios of noble gases. Based on the calculated noble gas temperature (NGT) and the ratio of noble gas isotopes, the recharge temperature, recharge source, and residence time of groundwater can be calculated. In addition, the contribution of noble gas components from different sources to the sample components can be assessed. In the Huizhou area, according to the 1/Xe vs. Ne/Xe and NGT data, the shallow sandstone‐confined water samples in the Shiba area and the unconfined water samples of the Huangshadong are in different temperature ranges, indicating that they have different recharge sources, both in time or space. The He components in the samples are calculated to obtain the content of radiogenic 4He in the crust and to simulate the groundwater ages. The noble gas isotope ratios show the addition of mantle components into the basalt aquifers and sandstone aquifers in Chaozhou and Huizhou. Except for atmospheric and crustal sources, there is a certain proportion of mantle‐derived components in the shallow underground cold water in Huizhou and Chaozhou. The noble gases in the Chaozhou groundwater have an obvious mantle signature, allowing speculation that there is a deep fluid carrying mantle characteristics. This upwelling of mantle‐derived material might be caused by the India‐Eurasia collision or that between the Philippine Sea Plate and the Eurasian Plate.

Isotope-Geochemical Studies of Groundwater in the Zapolyarny Region of the Nenets Autonomous Okrug

The aquifer of sandy deposits in the talik zone of the Pechora River valley was studied. The use of information on the distribution of tritium isotopes made it possible to clarify the recharge conditions of groundwater and their genetic structure in different parts of the aquifer. The tritium age of groundwater in coastal wells was 20–25 years, on the basis of which it was concluded that the groundwater that entered the aquifer before 1995–2000 was almost completely replaced. In wells, remote from the river bank, the age of the water is in the range from 20 to 50 years. However, unlike onshore wells, these groundwaters contain “bomb” water and old water that entered the aquifer be- fore 1952. The age of the old water reaches 12.9±2.5 thousand years. It has been established that the ratio between stable 13C and radioactive 14C isotopes of carbon changes with age, indicating an increase in the carbon exchange between dissolved inorganic carbon and soil carbon dioxide caused by permafrost thawing.

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking 234U-235U-238U, Groundwater Ages, and Hydrogeochemistry

Uranium (U) contamination in groundwater from geogenic sources affects water quality globally. Here, we use a multifaceted isotopic and geochemical approach to elucidate U sources and controls on g...

Unravelling 30 ka recharge history of an intensely exploited multi-tier aquifer system in North West India through isotopic tracers – Implications on deep groundwater sustainability

Northwest part of India is an agriculturally active region experiencing rapid rise in food production and steep decline in groundwater levels. The freshwater requirement is mostly met by regional aquifers which are inherently heterogeneous and undergoing extensive human inducted perturbations. These factors pose great challenge in planning sustainable groundwater management. In this study, environmental isotopes (2H, 18O, 13C, 3H and 14C) were applied to understand the regional recharge mechanism during the last 30 ka and hydrogeological controls impacting the aquifer dynamics and inter-aquifer connectivity of the Ghaggar River basin. Rayleigh distillation modeling indicates that major groundwater recharge is through monsoonal rains while rainfall during other seasons is lost either through evaporation or surface runoff. The evaporation loss is estimated to be 1.5 to 10% and more pronounced in the southern part of the study area. Regional recharge from Siwalik foothills contributes to groundwater up to a depth of 250 m below ground level (bgl). The lumped parameter modeling (LPM) using 3H data estimated groundwater ages 34.7 ± 12.1 and 95.8 ± 11.3 years for shallow and deep aquifers respectively. Radiocarbon dating indicates presence of paleogroundwater (0.4 to 28.6 ka before present, BP) in the deeper aquifer of central part of the study area. Interpretation of the paleowater and paleoprecipitation isotope data in conjunction with available paleogeomorphologic information suggests two different recharge phases. Phase I extending from ~28.6 to 10.1 ka, showed ~48–61% contribution from isotopically depleted perennial river system. Phase II spanning from ~12.5 to 0.4 ka BP showed insignificant contribution from river recharge, which can be attributed to the decreased strength of the perennial river flows. The research methodology proposed in this study will be beneficial in improving the understanding of groundwater storage and its variability with changes in regional climatic conditions.

Big data and environmental sustainability based integrated framework for isotope hydrology applications in India

Water contains environmental isotopes, which can act as tracers to study the source, origin, and distribution of water in the hydrological cycle. Recently, the environmental isotopes have been widely applied in hydrological investigations and it is established that the spatio-temporal variations in isotopic abundance provide valuable information to enhance the qualitative and quantitative understanding of hydrological processes such as mixing of different flow components and recharge rate, identify origin of water, delineation of groundwater recharge zones, geothermal dynamics, paleo-recharge and many others. Here, a review of case studies is presented to highlight the impact of isotope applications in water resource management in India. Along with a review of studies across India, a study is carried out for the assessment of groundwater interactions using isotopes of water (18O, 2H and 3H) in dry/semi-arid region (Jaipur) and wet/humid region (Guwahati). The major outcome of the study is the formulation of groundwater baseline for Guwahati (post-monsoon in 2017 and 2018: δD= 4.98 δ18O + 7.50) and Jaipur (post-monsoon and pre-monsoon in 2018: δD= 4.85 δ18O – 9.49). The results indicate that tritium has strong negative correlation (R2 = 0.59) with depth in Jaipur, indicating increasing groundwater age with depth. In Guwahati, the tritium shows weak positive (R2= 0.25) relationship which might be because of the presence of sandyclay layers in the shallow zone limiting vertical recharge to aquifer. Moreover, based upon the review and isotope results of the two study sites, a novel integrated framework for isotope hydrology application for India is proposed, incorporating the principles of environmental sustainability, economic efficiency and social equity. The isotopic techniques in hydrological investigations not only complements the traditional methods used in water resource management but also provides new insights and an enhanced understanding of the hydrological systems.

Analytical solutions for flow and advective solute transport in unconfined watershed aquifers with depth-dependent hydraulic conductivity

We present analytical and semi-analytical flow and transport models for 2D (x–z vertical plane) flow and particle transport in stratified rock of sedimentary type or vertically non-uniform bedrock aquifers, which are essentially phreatic. The main focus is on the flow induced by the areal discharge in watershed aquifers in hard-rock formations (e.g. granite, gneiss), the weathering profiles of which comprise two or more stratiform zones differing in hydraulic conductivity, k, which thus can be represented by a decay function of the vertical coordinate, i.e., the depth, k(z). Several models of depth-dependent hydraulic conductivity (stepwise, power-law, exponential) have been tested to show the sensitivity of groundwater flow paths and ages (transit or residence time distributions) to various geological and hydraulic simplifications related to rock heterogeneity. The mathematical setup handles the Dupuit–Forchheimer approximation the validity of which, as well as the suggested solutions, were verified via comparison with numerical results obtained using MODFLOW–PMPATH packages.

Mineral, thermal and deep groundwater of Hesse, Germany

Abstract. The composition of mineral, thermal or deep groundwaters is of interest for several geotechnical applications, such as drinking-water supply, spas or geothermal energy. Verified and reliable knowledge of temperature, pH, hydrochemical composition and other parameters is crucial to extract fluids with as few technical problems as possible and exploit groundwater reservoirs economically and environmentally sustainably. However, at sites where empirical data are lacking, the correct prediction of fluid properties is often difficult, resulting in considerable economic risks. Here we present the first comprehensive and publicly available database of mineral, thermal and deep groundwaters of Hesse compiled from published and our own data. Presently, it contains 1035 datasets from 560 different springs or wells sampled since 1810. A dataset consists of metadata like location; altitude; depth; rock type or stratigraphic unit; information on the water type; references; physical–chemical parameters; concentrations of major, minor and trace elements; and content of dissolved and free gases as well as isotope data. The dataset allows the evaluation of time series and distribution of groundwater properties both laterally and vertically. We show a simple statistical evaluation based on the five major hydrogeological regions of Hesse. Our database can be used to re-evaluate genesis and circulation of deep groundwaters, to estimate reservoir temperatures with a solution geothermometer, or to assess groundwater ages by means of isotope data. It can also be useful for a first conception of deep geothermal utilization. In the future, an update and extension of the database is intended. The database of mineral, thermal and deep groundwaters of Hesse has been made available by Schäffer et al. (2020) in xlsx and csv file format at TUdatalib, https://doi.org/10.25534/tudatalib-340. The second updated version of the database can be accessed directly at https://tudatalib.ulb.tu-darmstadt.de/handle/tudatalib/2508.2 (last access: 20 October 2021).

The 3H/3He Groundwater Age-Dating Method And Applications

Groundwater age-dating is an important tool for quantifying and managing water resources. Groundwater age is the elapsed time between recharge (at the land surface or water table) and the time when groundwater is sampled. If groundwater is sampled at the point of discharge from an aquifer, then the age represents the groundwater transit time. Groundwater that has recharged in recent decades is considered young groundwater. In many areas, the quality and quantity of young groundwater has been impacted by human activities and groundwater age-dating is useful for quantifying current and historical water and contaminant fluxes into and through aquifers. This review is focused on the tritium-helium (3H/3He) method, which is a robust and widely applied age-dating technique for young groundwater. We present the development of the 3H/3He method and practical considerations for sampling groundwater in shallow unconfined aquifers. Along the way, we highlight available resources: (1) educational software for building intuition around groundwater age-dating and selection of sampling sites and (2) software that can be used to calculate 3H/3He age from noble gas and 3H data. We also highlight strengths and potential uncertainties associated with the method. For example, while other age-dating techniques require a known historical record of tracer concentration in the atmosphere, the 3H/3He age-dating technique does not require such historical records. However, the 3H/3He method requires measurement of two tracers to produce a groundwater age estimate (“apparent age” or “tracer age”). Precise measurement of 3H and noble gases, plus careful analysis of noble gas data to calculate the tritiogenic 3He (i.e., the portion of 3He derived from decay of 3H in the aquifer) is required to calculate the groundwater apparent age. Sampling for noble gases is sometimes challenging and requires specialized sample containers and technique. We also introduce basic sampling methods in this review but highlight how practitioners should work closely with a noble gas laboratory to obtain the correct containers and assess field conditions and/or the overall feasibility of projects. Lastly, the review highlights recent applications of the 3H/3He method, including recharge rate estimation, characterization of contaminant input histories for aquifers, quantifying groundwater transit times by sampling at aquifer discharge points, and the use of isotope data to constrain and inform numerical and statistical models of groundwater and contaminant movement in the subsurface.

Helium-carbon systematics of groundwaters in the Lassen Peak Region

Carbon dioxide emissions from active subaerial volcanoes represent 20–50% of the annual global volcanic CO2 flux (Barry et al., 2014). Passive degassing of carbon from the flanks of volcanoes, and the associated accumulation of dissolved inorganic carbon (DIC) within nearby groundwater, also represents a potentially important, yet poorly constrained flux of carbon to the surface (Werner et al., 2019). Here we investigate sources and sinks of DIC in groundwaters in the Lassen Peak region of California. Specifically, we report and interpret the relative abundance and isotopic composition of helium (3He, 4He) and carbon (12C, 13C, 14C) in 37 groundwater samples, from 24 distinct wells, collected between 20 and 60 km from Lassen Peak. Measured groundwater samples have air-corrected 3He/4He values between 0.19 and 7.44 RA (where RA = air 3He/4He = 1.39 × 10−6), all in excess of the radiogenic production value (~0.05 RA), indicating pervasive mantle-derived helium additions to the groundwater system in the Lassen Peak region. Stable carbon isotope ratios of DIC (δ13C) vary between −12.6 and − 27.7‰ (vs. VPDB). Measured groundwater DIC/3He values fall in the range of 2.2 × 1010 to 1.1 × 1012. Using helium and carbon isotope data, we explore several conceptual models to estimate surface carbon contributions and to differentiate between DIC derived from soil CO2 versus DIC derived from external (slab and mantle) carbon sources. Specifically, if we use 14C to identify soil-derived DIC (assuming decadal-to-centennial groundwater ages and a soil CO214C activity equal to that of the atmosphere), we calculate that a hypothetical external carbon source would have an apparent δ13C signature between −10.3 and − 59.3‰ (vs. Vienna Pee Dee Belemnite (VPDB)) and an apparent C/3He between 7.0 × 109 and 1.0 × 1012. These apparent δ13C and C/3He values are substantially isotopically lighter than and greater than canonical MORB values, respectively. We suggest that >95% of any external (non-soil-derived) DIC in groundwater must thus be non-mantle in origin (i.e., slab derived or assimilated organic carbon). We further investigate possible sources of external DIC to groundwater using two idealized conceptual approaches: a pure (unfractionated) source mixing model (after Sano and Marty, 1995) and a scenario that invokes fractionation due to calcite precipitation. Because the former model requires carbon contributions from an organic source component with unrealistically low δ13C (~ − 60‰), we suggest that the second scenario is more plausible. Importantly, however, we caution that all conceptual models are dependent on assumptions about initial 14C activity. Thus, we cannot rule out the possibility that the true fraction of non-surface-derived DIC in these samples is lower or negligible, despite the pervasive mantle-derived He isotope signatures throughout the region. Following the 14C approach to deconvolving sources of DIC, we determine that the maximum passive carbon flux could be up to ~2.2 × 106 kg/yr, which is lower than previous magmatic carbon flux estimates from the Lassen region (Rose and Davisson, 1996). We find that the passive dissolved carbon flux could represent a maximum of ~4–18% of the total Lassen geothermal CO2 degassing flux (estimated to be ~3.5 × 107 kg/yr Rose and Davisson, 1996; Gerlach et al., 2008), which is still more than an order of magnitude smaller than soil gas CO2 flux estimates (7.3–11 × 107 kg/yr) for nearby volcanoes (Sorey et al., 1998; Gerlach et al., 1999; Evans et al., 2002; Werner et al., 2014). We conclude that passive dissolved carbon fluxes should be combined with geothermal fluxes and soil gas fluxes to obtain a complete picture of volcanic carbon emissions globally. Our approach highlights the utility of measuring helium isotopes in concert with the full suite of noble gas abundances, tritium, δ13C and 14C, which when interpreted together can be used to better elucidate the various sources of DIC in groundwater.

Geochemical processes in the groundwater of Soubré, Ivory Coast, West Africa: Evidence from 14C, 13C, 18O, 2H and major ions

Characteristics of the bedrock groundwater in the southwestern (SW) Ivory Coast (IC) (5–6°N, 4–7°W) are geochemically less known compared to those in the eastern part of the country. Several main cash crops, such as cocoa, coffee, rubber and palm oil, started being grown in this area twenty years ago because of the migration of agricultural activities from the East to the West of IC; this migration of agricultural activites was tied to climate variability. Within these large land use areas, food crops such as plantain bananas, yams, rice, maize, and vegetables are also grown. In this work, the mineralisation and groundwater pollution from East IC to West IC are studied within groundwater of the Archean and Paleoproterozoïce bedrock aquifers located in the SW IC. Stable isotopes ratios (δ18O, δ2H and δ13C), radiocarbon activity (A14C) and chemical contents (major ions) were measured on a set of 23 groundwater samples. In SW IC, the residence times calculated by an exponential mixing model (EMM) and a dispersion model (DM) with Dispersion Parameter values of 0.05 and 0.5 reveal that the oldest groundwater is located in the aquifer of migmatitic and granulitic gneiss (MiGn aquifer). Recent groundwater is located in the metagranitoid (MetGr) and the metasedimentary rocks (MetSe) aquifers. The most appropriate model to compute residence times is EMM because of the highest proportion of recharge in accordance with the nitrate content in groundwater after 1958, the beginning of Ivorian deforestation. However, this model lead to an overestimate of the residence time of groundwater particularly high in NO3. The oldest age of MiGn groundwater according to EMM calculation is ∼5 ka BP. The MiGn aquifer is less renewed (mean annual renewal rate of 0.03%) followed by those of MetSe (∼2.5 ka BP) and MetGr (∼2 ka BP), with mean annual renewal rates of 0.09% and 0.012%, respectively. Groundwater mineralisation is mainly governed by the hydrolysis of Al-silicates minerals and cultivation practice linked to K and N-fertilisers use, which leads to an increase of Ca, Mg and Na content. The values of TDS ranged from 49.4 to 720.4 mg L−1, which indicates the chemical characteristics of fresh water in SW IC. Most of the studied area groundwater are evaporated, likely due to both climate variability since the last 5 ka BP, delayed infiltration of rainwater which would be caused by impervious sediments covering the hard rock aquifers and agricultural practices affecting the most recently recharged groundwater. River water recharge from irrigation in cultivated areas may be possible. The groundwater with low 14C activities (64.33 pMC) has high residence times and is depleted in 13C content, while that with high 14C activities (99.58 pMC) is enriched in 13C content. This demonstrates that vegetation cover changed from C3 plants or forest (−23.9‰) to C4 plants, savannah or cultivation plants (−12.5‰) similar to those found in the southeastern IC.