Multiple Environmental Tracers Research Articles

Mechanism of groundwater recharge in the thick loess deposits by multiple environmental tracers

The loess−covered region accounts for ∼10 % of global land surface. Because of dry climate and thick vadose zones, the subsurface water flux is low but the water storage is relatively large . As a result, the groundwater recharge mechanism is complicated and currently controversial (e.g., piston flow or dual mode with piston and preferential flow). Taking typical tablelands in China's Loess Plateau as example study area, this study aims to qualitatively and quantitively evaluate the forms/rates and controls of groundwater recharge considering space and time. We collected 498 precipitation, soil water and groundwater samples in 2014–2021 for hydrochemical and isotopic analysis (Cl−, NO3−, δ18O, δ2H, 3H and 14C). A graphical method was employed to determine appropriate model to correct 14C age. Dual model exhibited in the recharge: regional−scale piston flow and local−scale preferential flow. Piston flow dominated groundwater recharge with a proportion of 77 %–89 %. Preferential flow gradually declined with increasing water table depths, and the upper depth limit may be <40 m. The dynamics of tracers proved that mixing and dispersion effects of aquifers limited the ability of tracers to capture preferential flow at short−time scales. Long−term average potential recharge (79 ± 49 mm/year) was close to actual recharge (85 ± 41 mm/year) at the regional scale, indicating the hydraulic equilibrium between unsaturated and saturated zones. The thickness of vadose zone controlled recharge forms, and precipitation dominated the potential and actual recharge rates. Land−use change can also affect the potential recharge rates at point and field scales but maintain the dominance of piston flow. The revealed spatially−varied recharge mechanism is useful for groundwater modeling and the method can be referred for studying recharge mechanism in thick aquifers.

Challenges in studying water fluxes within the soil-plant-atmosphere continuum: A tracer-based perspective on pathways to progress

Tracing and quantifying water fluxes in the hydrological cycle is crucial for understanding the current state of ecohydrological systems and their vulnerability to environmental change. Especially the interface between ecosystems and the atmosphere that is strongly mediated by plants is important to meaningfully describe ecohydrological system functioning. Many of the dynamic interactions generated by water fluxes between soil, plant and the atmosphere are not well understood, which is partly due to a lack of interdisciplinary research. This opinion paper reflects the outcome of a discussion among hydrologists, plant ecophysiologists and soil scientists on open questions and new opportunities for collaborative research on the topic “water fluxes in the soil-plant-atmosphere continuum” especially focusing on environmental and artificial tracers. We emphasize the need for a multi-scale experimental approach, where a hypothesis is tested at multiple spatial scales and under diverse environmental conditions to better describe the small-scale processes (i.e., causes) that lead to large-scale patterns of ecosystem functioning (i.e., consequences). Novel in-situ, high-frequency measurement techniques offer the opportunity to sample data at a high spatial and temporal resolution needed to understand the underlying processes. We advocate for a combination of long-term natural abundance measurements and event-based approaches. Multiple environmental and artificial tracers, such as stable isotopes, and a suite of experimental and analytical approaches should be combined to complement information gained by different methods. Virtual experiments using process-based models should be used to inform sampling campaigns and field experiments, e.g., to improve experimental designs and to simulate experimental outcomes. On the other hand, experimental data are a pre-requisite to improve our currently incomplete models. Interdisciplinary collaboration will help to overcome research gaps that overlap across different earth system science fields and help to generate a more holistic view of water fluxes between soil, plant and atmosphere in diverse ecosystems.

Land use as a major factor of riverine nitrate in a semi-arid farming-pastoral ecotone: New insights from multiple environmental tracers and molecular signatures of DOM

The nitrogen contamination in rivers has become significant concern in arid and semiarid areas due to water resource shortage and extensive anthropogenic activities in relation to land-use changes in China. As a major nitrogen species, identifying driving factors, transformation and sources of nitrate is crucial for managing nitrogen pollution in rivers. In this study, nitrate sources and transformations were deciphered using physicochemical variables, molecular signature of dissolved organic matter and coupled isotopes of nitrate under different land use types in the Yang River, a typical farming-pastoral ecotone in the semi-arid area of North China. The results of river water showed a significant positive correlation between NO3− concentrations, δ15N-NO3− values and percentage of urban land and cropland, which confirmed the critical role of land use in the variations of riverine nitrate. The correlation between dissolved organic matter composition (aliphatic and lignin-like compounds) and NO3−/Cl− ratios as well as Cl− concentrations verified the effect of agricultural activities on nitrate source and transport. The variation in water chemical variables and dual isotopes of nitrate in river and soil extracts (δ15N-NO3− and δ18O-NO3−) was indicative of the concurrence of in-soil nitrification process and assimilation, whereas denitrification was inhibited under aerobic conditions in the semiarid area. The Bayesian model revealed that about 60% of nitrate was derived from non-point sources (manure, soil organic nitrogen and chemical fertilizer) and 36% from sewage. Although urban is not the major land-use type in the farming-pastoral ecotone, sewage contributed to about 36% of nitrate. The source identification of nitrate stresses the importance of the management of non-point pollution and demand for sewage treatment facilities in the farming-pastoral ecotone. This multiple-tracer approach will help gain deeper insights into nitrogen management in semi-arid areas with extensive human disturbance.

Quantitative estimation of groundwater recharge in the thick loess deposits using multiple environmental tracers and methods

Accurate estimation of groundwater recharge in regions with limited water and thick unsaturated zones is challenging because of the limited water flux. This study aims to employ multiple tracers and methods to investigate the mechanism of groundwater recharge in the thick loess deposits of China. We collected soil samples under different vegetation, unconfined groundwater, and precipitation to determine the water isotopes (δ2H, δ18O, and 3H), chloride (Cl-), nitrate (NO3–). Then, these tracers were employed to estimate recharge rates with methods considering the differences in shallow- and deep-rooted vegetation, as well as those in unsaturated and saturated zones. Results indicated that local precipitation predominantly recharges groundwater through matrix flow at a velocity of 14.5 cm year−1 with the residence time of > 200 years. The recharge rates range from 0 to 37 mm year−1, equivalent to 0–7% of annual average precipitation. However, the deep-rooted vegetation greatly reduced deep drainage compared with shallow-rooted vegetation, which will perturb the current hydraulic connectivity between unsaturated and saturated zones. The multiple tracers and methods can complement each other to enhance the understanding of groundwater recharge mechanisms. The indicated relationship between recharge form and land use suggests the urgency in developing strategies to balance the agroforestry system and water resource sustainability.

Assessing the groundwater dependence of valley bottom wetlands in coal-mining environment using multiple environmental tracers, Mpumalanga, South Africa

Assessing the groundwater dependence of valley bottom wetlands in coal-mining environment using multiple environmental tracers, Mpumalanga, South Africa

Combined effects of recharge and hydrogeochemical conditions on nitrate in groundwater of a highland agricultural basin based on multiple environmental tracers

δ18O, δD, 3H, chlorofluorocarbon (CFC), and hydrogeochemical parameters were investigated to evaluate factors controlling nitrate levels in groundwater in an agricultural basin with steep topographic gradients. Groundwater is predominantly recharged from mountainous terrain toward the bottom of the basin. Cultivation of Chinese cabbage and radish as highland summer crops is widespread in the vegetable fields, supported by heavy application of chemical fertilizers. The nitrate concentration was low (0.02–6.8 mg/L as NO3-N) in rice paddies of the northern region and high (2.0–15.2 mg/L as NO3-N) in vegetable fields of the southeastern region. The δD and δ18O values of water samples ranged from −79.9 to −60.5‰ and from −10.6 to −7.7‰, respectively in the study area. Water stable isotopes showed that recharge occurs mainly from local precipitation, and evaporation signatures were observed in the rice paddies. The apparent groundwater ages determined from CFCs ranged from <20 to 54 years. Older and more mineralized groundwater was observed in the discharge zone of the downgradient area, while young groundwater was found in the recharge zone of the northern region, indicating greater vulnerability to contamination. Elevated nitrate concentrations (6.8–8.5 mg/L as NO3-N) in some old groundwater in the vegetable fields of the southeastern region may be due to binary mixing of old stagnant water and newly recharged water, as identified by comparing 3H and CFC-113 with CFC-12 levels. Groundwater in the rice paddies had lower nitrate concentration and higher concentrations of HCO3, Fe, and Mn, indicating denitrification under reducing conditions. Principal component analysis revealed four major processes controlling groundwater flow and chemistry: agricultural contamination under reducing conditions in the vegetable fields, occurrence of old groundwater under reducing conditions, and denitrification in the rice paddies. These results suggest that management of nitrate sources is needed to alleviate nitrate contamination, particularly in the vegetable fields of both upgradient and downgradient areas with fast infiltration, high fertilizer usage, and low natural attenuation of nitrate.

A multi-environmental tracer study to determine groundwater residence times and recharge in a structurally complex multi-aquifer system

Abstract. Despite being the main drinking water resource for over 5 million people, the water balance of the Eastern Mountain Aquifer system on the western side of the Dead Sea is poorly understood. The regional aquifer consists of fractured and karstified limestone – aquifers of Cretaceous age, and it can be separated into a Cenomanian aquifer (upper aquifer) and Albian aquifer (lower aquifer). Both aquifers are exposed along the mountain ridge around Jerusalem, which is the main recharge area. From here, the recharged groundwater flows in a highly karstified aquifer system towards the east and discharges in springs in the lower Jordan Valley and Dead Sea region. We investigated the Eastern Mountain Aquifer system for groundwater flow, groundwater age and potential mixtures, and groundwater recharge. We combined 36Cl ∕ Cl, tritium, and the anthropogenic gases SF6, CFC-12 (chlorofluorocarbon) and CFC-11, while using CFC-113 as “dating” tracers to estimate the young water components inside the Eastern Mountain Aquifer system. By application of lumped parameter models, we verified young groundwater components from the last 10 to 30 years and an admixture of a groundwater component older than about 70 years. Concentrations of nitrate, simazine (pesticide), acesulfame K (ACE-K; artificial sweetener) and naproxen (NAP; drug) in the groundwater were further indications of infiltration during the last 30 years. The combination of multiple environmental tracers and lumped parameter modelling helped to understand the groundwater age distribution and to estimate recharge despite scarce data in this very complex hydrogeological setting. Our groundwater recharge rates support groundwater management of this politically difficult area and can be used to inform and calibrate ongoing groundwater flow models.

How does precipitation recharge groundwater in loess aquifers? Evidence from multiple environmental tracers

Characterization of the rainfall infiltration mechanism, whether piston or preferential flow, through an unsaturated zone is necessary for groundwater resource assessment and for analysis of environmental geological hazards. However, several limitations and difficulties remain in determining the infiltration mechanism in arid and semiarid areas. This study combines the evaluations of soil water infiltration characteristics, recharge rate, and determination of groundwater age at a water table in a semiarid loess tableland to examine the recharge processes. The case study is the Xifeng loess tableland in the Loess Plateau of China, which has an unsaturated zone depth of 40–75 m and an obvious 1963-3H peak at a depth of 7.5 m. According to the piston flow, the soil water infiltration velocity is 0.12–0.14 m/yr, corresponding to a recharge rate of 37–41 mm/yr, based on the 1963-3H peak and chloride mass balance method. All of the groundwater from the water table and wells is old (tritium-free), with ages ranging from hundreds of years to 20,000 years, which suggests that no rapid modern recharge has occurred since the 1950s. The chloride content of soil water in the deep unsaturated zone, with an average value of 10.8 mg/L in the unsaturated zone below 30 m, overlaps with that in groundwater, at 4.0–10.9 mg/L. The comprehensive isotopic and chemical compositions for soil water and groundwater showed that piston flow was dominant during soil water infiltration at the field scale, resulting in the delay of modern water and related solutes entering the groundwater. Combined evaluations of the unsaturated zone with those of the saturated zone and the use of multiple environmental tracers, such as Cl, Br, 2H, 18O, 13C, 3H and 14C, can be regarded as a generic framework for studying the recharge processes in arid and semiarid areas with thick unsaturated zones at macroscopic and field scales.

Multitracer determination of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman

CO2 sequestration in the form of carbonate minerals via alteration of oceanic crust and upper mantle is an important part of the global carbon cycle, but the annual rate of CO2 mineralization is not well quantified. This study aimed to constrain groundwater ages within the Samail ophiolite, Sultanate of Oman. Such ages could provide upper bounds on the time required for ongoing low temperature CO2 mineralization. While we were able to estimate apparent groundwater ages for modern waters, results from hyperalkaline boreholes and springs were disappointing. Waters from boreholes and hyperalkaline springs within the ophiolite were characterized using multiple environmental tracers including tritium (3H), noble gases (3He, 4He, Ne, Ar, Kr, Xe), stable isotopes (δ18O, δ2H), and chemical parameters (pH, Ca, Mg, DIC, etc.). Shallow peridotite groundwater and samples from boreholes near the mantle transition zone have a pH < 9.3, are 4-40 yrs old, have little to no non-atmospheric He accumulation, NGTs (noble gas temperatures) equivalent to the modern mean annual ground temperature, and stable isotopes within the range of current local precipitation. In contrast, hyperalkaline springs and deeper samples from peridotite boreholes have pH > 10, are pre-H-bomb (older than 1952), have significant non-atmospheric helium accumulation (30-70% of dissolved helium), often are isotopically heavier (enriched in δ18O), and can have NGTs 6-7°C lower than the modern ground temperature. These differences suggest that groundwater in deep (>50 m) peridotite aquifers is considerably older than shallow groundwater in peridotite and water in deeper aquifers near the mantle transition zone. Unfortunately, how much older remains an open question. The low NGT of groundwater from one deep (300 m) peridotite borehole indicates it is probably glacial in origin. If so, it must date back to at least the late Pleistocene, the most recent glacial period; He accumulation suggests it could be from 20-220 ka. The inefficacy of this suite of environmental tracers to quantitatively estimate apparent groundwater age for hyperalkaline fluids necessitates the use of different techniques. Future work to constrain groundwater ages should utilize a packer system to isolate discrete depth intervals within boreholes and less common environmental tracers such as 39Ar and 81Kr.

Hydrogeochemical and isotopic study of the origins of groundwater salinization in the deep confined aquifer of northern Yangtze River

The deep confined water of coastal plain of northern Yangtze River suffers salinization. That results from overexploitation. This work aims to investigate the geochemical processes that lead to the salinization for this aquifer. Multiple environmental tracers of major ions, minor ions and isotopes (18O, 87Sr, 13C) were used to yield reasonable conclusions. The TDS of the aquifer ranges from 387 to 2600 mg/L. The aquifer is mainly composed of fresh water. Brackish water is distributed in the eastern and southern coastal areas, and is scattered in some inland areas. The water chemical type evolves from HCO3-Na to Cl-Na as TDS increases. Groundwater salinization is caused by mixing with saline water of marine origin. The aquifer is also affected by other hydrochemical processes. Silicate weathering and carbonate dissolution add Na+, K+, Ca2+, Mg2+ and HCO3- to groundwater. Sulfate reduction makes sulfate be deficient and HCO3- be enriched. Carbonate minerals equilibrium limits the increase in the concentrations of Ca2+, Mg2+ and HCO3-. Cation exchange reduces the concentration of Ca2++Mg2+ and increases the concentration of Na++K+. The aforementioned processes have a combined influence on the formation of fresh water with HCO3-Na type.

A hydrogeochemistry and multi-isotope (Sr, O, H, and C) study of groundwater salinity origin and hydrogeochemcial processes in the shallow confined aquifer of northern Yangtze River downstream coastal plain, China

Economically developed coastal areas have a high water demand, and their groundwater resources can be threatened by salinization. Many methods and tracers have been used to discriminate the source of salinization because a single method does not yield reliable results. In this paper, the shallow confined coastal plain aquifer, north of the downstream Yangtze River in China, is used as a case study to investigate the origin of the salinity and the relevant geochemical processes for this aquifer. Multiple environmental tracers of major ions, minor ions (Br−, I−), and isotopes (18O, 2H, 13C, 87Sr, 3H, 14C) were used so as to provide reliable conclusions. The TDS distribution of the aquifer has an increasing trend, from below 500 mg/L in the inland areas to more than 20,000 mg/L around the southeast coastline. The water chemical type evolves from HCO3-Ca to Cl-Na as the TDS increases. The results suggest that the groundwater salinity is influenced by seawater intrusion. The seawater proportions in the groundwater samples range from 0.07% to 94.41% and show the same spatial distribution pattern as TDS. The 3H and 14C values show that the highest salinity was mainly caused by a seawater transgression around 6000a B.P. The aquifer is also affected by other hydrogeochemical processes: base exchange has enriched Ca2+ and depleted K+ and Na+, sulfate reduction has reduced the concentration of SO42− and enriched HCO3−, and iodine-rich organic matter decomposition has enriched the concentration of I−. The iodine enrichment also suggests paleo-seawater intrusion. In addition, the precipitation of carbonate minerals has decreased the concentration of Ca2+, Mg2+, and HCO3−, albeit to a limited extent.

Relationship between land-use and sources and fate of nitrate in groundwater in a typical recharge area of the North China Plain

Identification of different nitrate sources in groundwater is challenging in areas with diverse land use and multiple potential inputs. An area with mixed land-uses, typical of the piedmont-plain recharge area of the North China Plain, was selected to investigate different nitrate sources and the impact of land use on nitrate distribution in groundwater. Multiple environmental tracers were examined, including major ions, stable isotopes of water (δ2H-H2O, δ18O-H2O) and nitrate (δ15N-NO3− and δ18O-NO3−). Groundwater was sampled from four land-use types; natural vegetation (NV), farmland (FL), economic forestland (EF) and residential areas (RA). A mixing model using δ18O and Cl− concentrations showed that groundwater recharge predominantly comprises precipitation and lateral groundwater flow from areas of natural vegetation in the upper catchment, while irrigation return water and wastewater from septic tanks were major inputs in farmland and residential areas, respectively. Land use variation is the major contributing factor to different nitrate concentrations. In total, 80%, 49% and 86% of samples from RA, FL and EF, respectively exceeded the WHO standard (50mg/L NO3−), compared to 6.9% of samples from NV. Isotopes of δ15N-NO3− and δ18O-NO3− verified that nitrate in groundwater of the NV (with δ15N ranging from 1.7‰ to 4.7‰) was sourced from soil and precipitation. Examination of δ15N-NO3− vs δ18O-NO3− values along with multivariate statistical analysis (principle component and cluster analysis) helped identify sources with overlapping isotopic values in other land-use areas (where δ15N values range from 2.5‰ to 10.2‰). Manure and septic waste were dominant sources for most groundwater with high NO3− and Cl− concentrations in both farmland and residential areas. The lack of de-nitrification and fact that the area is a recharge zone for the North China Plain highlight the importance of controlling nitrate sources through careful application of manure and fertilizers, and control of septic leakage.

Detecting inter-aquifer leakage in areas with limited data using hydraulics and multiple environmental tracers, including 4He, 36Cl/Cl, 14C and 87Sr/86Sr

The investigation of regionally extensive groundwater systems in remote areas is hindered by a shortage of data due to a sparse observation network, which limits our understanding of the hydrogeological processes in arid regions. The study used a multidisciplinary approach to determine hydraulic connectivity between the Great Artesian Basin (GAB) and the underlying Arckaringa Basin in the desert region of Central Australia. In order to manage the impacts of groundwater abstraction from the Arckaringa Basin, it is vital to understand its connectivity with the GAB (upper aquifer), as the latter supports local pastoral stations and groundwater-dependent springs with unique endemic flora and fauna. The study is based on the collation of available geological information, a detailed analysis of hydraulic data, and data on environmental tracers. Enhanced inter-aquifer leakage in the centre of the study area was identified, as well as recharge to the GAB from ephemeral rivers and waterholes. Throughout the rest of the study area, inter-aquifer leakage is likely controlled by diffuse inter-aquifer leakage, but the coarse spatial resolution means that the presence of additional enhanced inter-aquifer leakage sites cannot be excluded. This study makes the case that a multi-tracer approach along with groundwater hydraulics and geology provides a tool-set to investigate enhanced inter-aquifer leakage even in a groundwater basin with a paucity of data. A particular problem encountered in this study was the ambiguous interpretation of different age tracers, which is attributed to diffusive transport across flow paths caused by low recharge rates.

Using dual‐domain advective‐transport simulation to reconcile multiple‐tracer ages and estimate dual‐porosity transport parameters

AbstractDual‐domain transport is an alternative conceptual and mathematical paradigm to advection‐dispersion for describing the movement of dissolved constituents in groundwater. Here we test the use of a dual‐domain algorithm combined with advective pathline tracking to help reconcile environmental tracer concentrations measured in springs within the Shenandoah Valley, USA. The approach also allows for the estimation of the three dual‐domain parameters: mobile porosity, immobile porosity, and a domain exchange rate constant. Concentrations of CFC‐113, SF6, 3H, and 3He were measured at 28 springs emanating from carbonate rocks. The different tracers give three different mean composite piston‐flow ages for all the springs that vary from 5 to 18 years. Here we compare four algorithms that interpret the tracer concentrations in terms of groundwater age: piston flow, old‐fraction mixing, advective‐flow path modeling, and dual‐domain modeling. Whereas the second two algorithms made slight improvements over piston flow at reconciling the disparate piston‐flow age estimates, the dual‐domain algorithm gave a very marked improvement. Optimal values for the three transport parameters were also obtained, although the immobile porosity value was not well constrained. Parameter correlation and sensitivities were calculated to help quantify the uncertainty. Although some correlation exists between the three parameters being estimated, a watershed simulation of a pollutant breakthrough to a local stream illustrates that the estimated transport parameters can still substantially help to constrain and predict the nature and timing of solute transport. The combined use of multiple environmental tracers with this dual‐domain approach could be applicable in a wide variety of fractured‐rock settings.

Investigating the age distribution of fracture discharge using multiple environmental tracers, Bedrichov Tunnel, Czech Republic

The transit time distribution (TTD) of discharge collected from fractures in the Bedrichov Tunnel, Czech Republic, is investigated using lumped parameter models and multiple environmental tracers. We utilize time series of $$\delta ^{18}O$$ , $$\delta ^{2}$$ H and $$^3$$ H along with CFC measurements from individual fractures in the Bedrichov Tunnel of the Czech Republic to investigate the TTD, and the uncertainty in estimated mean travel time in several fracture networks of varying length and discharge. We compare several TTDs, including the dispersion distribution, the exponential distribution, and a developed TTD which includes the effects of matrix diffusion. The effect of seasonal recharge is explored by comparing several seasonal weighting functions to derive the historical recharge concentration. We identify best fit mean ages for each TTD by minimizing the error-weighted, multi-tracer $$\chi ^{2}$$ residual for each seasonal weighting function. We use this methodology to test the ability of each TTD and seasonal input function to fit the observed tracer concentrations, and the effect of choosing different TTD and seasonal recharge functions on the mean age estimation. We find that the estimated mean transit time is a function of both the assumed TTD and seasonal weighting function. Best fits as measured by the $$\chi ^2$$ value were achieved for the dispersion model using the seasonal input function developed here for two of the three modeled sites, while at the third site, equally good fits were achieved with the exponential model and the dispersion model and our seasonal input function. The average mean transit time for all TTDs and seasonal input functions converged to similar values at each location. The sensitivity of the estimated mean transit time to the seasonal weighting function was equal to that of the TTD. These results indicated that understanding seasonality of recharge is at least as important as the uncertainty in the flow path distribution in fracture networks and that unique identification of the TTD and mean transit time is difficult given the uncertainty in the recharge function. However, the mean transit time appears to be relatively robust to the structural model uncertainty. The results presented here should be applicable to other studies using environmental tracers to constrain flow and transport properties in fractured rock systems.

Evaluation of groundwater residence time in a high mountain aquifer system (Sacramento Mountains, USA): insights gained from use of multiple environmental tracers

The New Mexico Bureau of Geology and Mineral Resources (USA) has conducted a regional investigation of groundwater residence time within the southern Sacramento Mountains aquifer system using multiple environmental tracers. Results of the tracer surveys indicate that groundwater in the southern Sacramento Mountains ranges in age from less than 1 year to greater than 50 years, although the calculated ages contain uncertainties and vary significantly depending on which tracer is used. A distinctive feature of the results is discordance among the methods used to date groundwater in the study area. This apparent ambiguity results from the effects of a thick unsaturated zone, which produces non-conservative behavior among the dissolved gas tracers, and the heterogeneous character and semi-karstic nature of the aquifer system, which may yield water from matrix porosity, fractures, solution-enlarged conduits, or a combination of the three. The data also indicate mixing of groundwater from two or more sources, including recent recharge originating from precipitation at high elevations, old groundwater stored in the matrix, and pre-modern groundwater upwelling along fault zones. The tracer data have also been influenced by surface-water/groundwater exchange via losing streams and lower elevation springs (groundwater recycling). This study highlights the importance of using multiple tracers when conducting large-scale investigations of a heterogeneous aquifer system, and sheds light on characteristics of groundwater flow systems that can produce discrepancies in calculations of groundwater age.

River infiltration to a subtropical alluvial aquifer inferred using multiple environmental tracers

AbstractChloride (Cl−), stable isotope ratios of water (δ18O and δ2H), sulfur hexafluoride (SF6), tritium (3H), carbon‐14 (14C), noble gases (4He, Ne, and Ar), and hydrometry were used to characterize groundwater‐surface water interactions, in particular infiltration rates, for the Lower Namoi River (New South Wales, Australia). The study period (four sampling campaigns between November 2009 and November 2011) represented the end of a decade‐long drought followed by several high‐flow events. The hydrometry showed that the river was generally losing to the alluvium, except when storm‐derived floodwaves in the river channel generated bank recharge—discharge cycles. Using 3H/14C‐derived estimates of groundwater mean residence time along the transect, infiltration rates ranged from 0.6 to 5 m yr−1. However, when using the peak transition age (a more realistic estimate of travel time in highly dispersive environments), the range in infiltration rate was larger (4–270 m yr−1). Both river water (highest δ2H, δ18O, SF6, 3H, and 14C) and an older groundwater source (lowest δ2H, δ18O, SF6, 3H, 14C, and highest 4He) were found in the riparian zone. This old groundwater end‐member may represent leakage from an underlying confined aquifer (Great Artesian Basin). Environmental tracers may be used to estimate infiltration rates in this riparian environment but the presence of multiple sources of water and a high dispersion induced by frequent variations in the water table complicates their interpretation.

A multi-tracer approach for assessing the origin, apparent age and recharge mechanism of shallow groundwater in the Lake Nyos catchment, Northwest, Cameroon

Summary The shallow aquifer in the vicinity of Lake Nyos (Northwest, Cameroon) is one of the main water supply sources to meet the water needs of the inhabitants to be resettled after 1986s tragedy. Unfortunately, there is a lack of knowledge on the groundwater recharge and flow systems in the area. Multiple environmental tracers (δ 18 O, δD, Cl − , CFCs and SF 6 ) and a yearly record of rainfall, surface waters and groundwater were employed to characterize the recharge mechanism of the shallow groundwater seeping in the fractured rock of the Lake Nyos catchment (LNC). The δ 18 O–δD relationship of the rainfall events gave the Nyos Meteoric Water Line: δD = 8.28 δ 18 O + 11.87. Inland moisture vapor may have impacted the isotopic composition of original vapor masses from Gulf of Guinea prior to precipitation. Shallow groundwater in the LNC shows a similar trend of enrichment in 18 O and D as surface waters indicating a well-mixed aquifer. The proportions of surface waters and rainfall in the groundwater reservoir were 87% and 13%, respectively. The high annual recharge rate (941 mm/yr) and the seasonal variability in the isotopic signatures of groundwater indicate a renewable aquifer system. CFCs apparent ages-based piston flow model revealed a young age (average of 24 a.) of the groundwater in the LNC. SF 6 -based ages were biased young as compared to relatively younger than CFCs-based ages, implying an additional terrigenic production of SF 6 . The conceptual model for groundwater flow suggests that three main flow regimes, mainly controlled by the physical properties of the rock heterogeneities govern the movement of water in the aquifer. The piston flow model appears, however, to be the better model to explain the flow regime in the highly faulted and fissured area where recharge occurs (∼1200–1600 masl). The rapid circulation and the low solubility lead to low mineralization. In the middle-lower area where waters circulate more in the weathered layer, exponential mixing model dominates, yielding more mineralized water. Downward to the valley, with prevailing gentle slope, interaction between surface and recently infiltrated groundwater better explain the observed similarity in the isotopic compositions and the mineralization of water.

A general reactive transport modeling framework for simulating and interpreting groundwater14C age and δ13C

A reactive transport modeling framework is presented that allows simultaneous assessment of groundwater flow, water quality evolution including δ13C, and 14C activity or “age”. Through application of this framework, simulated 14C activities can be directly compared with measured 14C activities. This bypasses the need for interpretation of a 14C age prior to flow simulation through factoring out processes other than radioactive decay, which typically involves simplifying assumptions regarding spatial and temporal variability in reactions, flow, and mixing. The utility of the approach is demonstrated for an aquifer system with spatially variable carbonate mineral distribution, multiple organic carbon sources, and transient boundary conditions for 14C activity in the recharge water. In this case, the simulated 14C age was shown to be relatively insensitive to isotopic fractionation during DOC oxidation and variations in assumed DOC degradation behavior. We demonstrate that the model allows quantitative testing of hypotheses regarding controls on groundwater age and water quality evolution for all three carbon isotopes. The approach also facilitates incorporation of multiple environmental tracers and combination with parameter optimization techniques.

Multiple tracer study in Horonobe, northern Hokkaido, Japan: 1. Residence time estimation based on multiple environmental tracers and lumped parameter models

A multiple tracer study was conducted in the coastal plain of Horonobe, northern Hokkaido, Japan. To accomplish reliable estimates of groundwater residence time, multiple tracer approach, employing both gaseous and nongaseous environmental tracers, and lumped parameter models (LPMs) were applied in combination. Environmental tracers including 3H, 3H/3He, 36Cl, and SF6 were analyzed and compared with values estimated from LPMs to select the most appropriate tracer and models to describe groundwater flow regime in the study area. Time series of 36Cl/Cl input in the study area were reconstructed and used in the LPMs. From comparison of the analyzed and modeled results, an index of 3H/3He was selected as the most appropriate for investigation of the study area. The relationship between the 3H/3He index and 36Cl/Cl indicated reliability of the 3H/3He index, but 36Cl/Cl values of the samples were lower than the models. This discrepancy was caused by underestimation of Cl− concentration of recharging water in the calculation of 36Cl/Cl input. SF6 mixing ratios of the samples were slightly higher than the models and it suggested involvement of terrigenic SF6. Three well groups, northern Shimonuma wells (NSW), southern Shimonuma wells (SSW), and Hamasato wells (HW), were respectively consistent with exponential piston flow model (EPM), binary mixing model (BMM), and exponential mixing model (EMM). Relational expressions between the 3H/3He index and mean residence time or mixing fraction of young groundwater (assumed to contain no tritiogenic 3He) with old groundwater (all 3H decayed to tritiogenic 3He) were obtained based on the models. The mean residence time determined from the 3H/3He index and the expressions were 19–78 years for NSW and 1–648 years for HW. The mixing fraction of young groundwater was less than 10% at all SSW. Nongaseous tracers, including 36Cl are not influenced by factors specific for gaseous tracers, such as excess air and degassing. Analyses of multiple tracers, including both gaseous and nongaseous tracers, and comparison using LPMs were effective to show plausibility of the estimated residence time, and they contributed to understanding of groundwater flow regime in the study area.