Exponential-piston Flow Model Research Articles

Transit time estimation of drying springs in Uttarakhand region using environmental tritium concentration

Many parts of the Uttarakhand state, situated in the hilly terrain of the Himalayan region of India are facing acute water crisis due to the drying up of the perennial springs which are the only source of potable water in those regions. Tritium (3H), the radioactive isotope of hydrogen (half-life of 12.32 years) and also a part of water molecule (in the form of HTO) acts as a very useful tracer in estimating the transit time of the hydrological systems. Tritium concentrations of three springs (S-1, S-2 and S-3) were monitored consecutively for three years (2017–2019) to better constrain the transit time estimation. The tritium concentrations of the springs are found to vary between 3.66 and 4.15 TU. All the springs show gradual decrease in tritium concentration with the passage of time indicating the diminishing percentage of freshly recharged modern water component. Among various lumped parameter models, the piston-flow model (PFM), exponential mixing model (EMM), exponential piston-flow model (EPM) and partial exponential mixing model (PEM) have been employed in this study. The historical record of weighted mean concentration of tritium in precipitation available for the Uttarakhand region is taken as input function in the modelling procedure. The application of various LPMs (PFM, EMM, EPM and PEM) indicates that the transit time of the S-1 spring ranges from 1.26 to 1.46 years whereas for S-2 spring, the transit time is found to vary from 5 months to 1.1 years. The MTT of S-3 spring ranges from 5 months to 11 months. The relatively short residence time of these springs indicates the actively recharged system. The estimation of accurate transit time is thus very crucial for understanding the renewability of the spring water systems.

Long-Term Variations in Spring Water Mean Transit Time in a Forested Headwater Catchment in Japan

In this study, we applied a new methodology that utilized stable isotopic concentrations of water and SF6 concentrations of the air to investigate temporal variations in the spring water mean transit time (MTT) of a forested headwater catchment in Japan. We monitored the area from June 2010 to November 2021, including the forest thinning that occurred in October 2011. The SF6 concentrations were used to estimate the apparent age of the spring water, which was used to initiate a parameter calibration that determined the best possible MTT for each study period. The MTT was evaluated by estimating the d-excess variations of the spring water stable isotopes using the exponential piston flow model. The MTT ranged from 40 to 55 months and tended to increase soon after the forest was thinned. In contrast, the MTT was shorter by approximately 6 years after thinning occurred. The results indicate that the characteristics of the hydrological processes in the catchment underwent long-term changes after the forest was thinned. This study demonstrates that using combined tracer methods to investigate the hydrological response to forest management practices improved the results and can be used for better forest and subsurface water resource management.

Constraining the mean transit time (MTT) of relatively modern thermal waters in Deccan volcanic geothermal area, India using tritium tracer

Constraining the mean transit time (MTT) of thermal waters is essential to understand the sustainability and renewal rate of geothermal systems. In this study, tritium concentrations of samples from five thermal water manifestations, three shallow groundwater wells (dug wells) and one deep groundwater (borehole) are analysed at regular time intervals for three years to estimate the MTT of the thermal and non-thermal waters in the Deccan Volcanic geothermal province by using lumped parameter models. The tritium concentrations of the thermal waters are found to vary between 0.77 and 2.77 TU (±0.0.23 TU at 1σ) which are lower than those of the shallow non-thermal groundwater (5.4–7.18 TU) (±0.40 TU at 1σ) whereas the tritium value for the deep groundwater ranges between 2.30 and 2.78 TU (±0.27 TU at 1σ). Among different lumped parameter models, PFM (piston flow model), EMM (exponential mixing model) and EPM (exponential piston flow model) are used to estimate the MTT of the groundwater and thermal waters in the study area. The historical records of weighted mean concentration of tritium in precipitation at the nearby Tural-Rajwadi geothermal area is taken as input function in the modelling procedure. The PFM and EMM models represent the lower and upper case of the EPM model mixing, respectively whereas the values obtained from the EPM models provide the actual range of mean transit times. The overall uncertainty in the MTT estimation has also been computed. The MTT of the younger fraction of the thermal waters in Ganeshpuri thermal water (HS-1) and Akloli thermal water (HS-7) are found to be 70 ± 5 and 90 ± 14 years old respectively. The MTT of both the shallow groundwater (DW-1 and DW-3) samples is estimated to be ~1.5 years whereas the MTT of the deep groundwater (BH-1) is 39 ± 6 years. Thermal springs in this area undergo mixing with non-thermal water during its ascent towards the surface as a result binary mixing model has been applied to find the MTT of the mixed thermal spring. The aggregate MTT of HS-1 turns out to be 7860 ± 870 years. This study of MTT estimation can be used for accessing the sustainability and renewability of these thermal waters compared to other thermal areas in Deccan volcanic region.

Understanding water circulation with tritium tracer in the Tural-Rajwadi geothermal area, India

This study investigates the water circulation using tritium measurements in local precipitation, shallow dug wells and thermal springs of the Tural-Rajwadi area, which is one of the potentially productive geothermal areas in India. In the study area, the estimation of tritium-tracer mean transit times (MTTs) in the mixed thermal waters is lacking despite available tritium measurements in 1977 and 2013 and carbon-14 measurements in 2008. As a direct tracer of water movement, tritium radioisotope with a half-life of 12.32 years has been applied to estimate transit times of young waters, especially in the mixture of young and old water components, and tritium sampling was conducted in 2017 to reduce the ambiguity involved with tritium interpretation. For the study area, two scaling factors of latitude effect are established for anthropogenic and ambient tritium levels in precipitation to obtain continuous time-series of tritium in monthly precipitation from the Tokyo area. Annual weighted tritium concentrations in recharge are estimated as a difference between monthly precipitation and evaporation data and are utilized as the tritium input in the convolution integral to obtain tritium-tracer MTTs. Using the exponential piston-flow model (EPM), the representative MTT of shallow groundwater is estimated to be 12.5 ± 4.5 years at TDW-1 well and 15.5 ± 8.5 years at RDW-1 well and uncertainties in MTT due to the EPM ratio and scaling factor are found to be low. For two thermal springs, simulated tritium concentrations of the young groundwater in mixed thermal waters produce unique MTT solutions resulting in MTT of 28.0 ± 12.0 years for THS-1 spring and of 29.5 ± 12.5 years for RHS-1 spring and scaling factor uncertainties are lower than EPM ratio uncertainties, which could be further reduced by an additional tritium sampling. Combining carbon-14 concentrations of 2008 samples with tritium-tracer, the old water MTTs of THS-1 and RHS-1 thermal springs equal to 13500 ± 1500 years and 6900 ± 900 years, respectively, and aggregate MTTs of mixed water are 9189 ± 1023 years for THS-1 spring and 4701 ± 616 years for RHS-1 spring. Using these MTTs with the effective porosity values of 0.01 and 0.03 result in the geothermal reservoir volume of the Tural-Rajwadi area to vary between 347.3 ± 23.3 km3 and 115.8 ± 15.4 km3, respectively, and these volumes could be used to estimate the recharge area with geothermal system depth obtained from deep drilling. In conclusions, we highlight that the environmental tritium provides important information on water circulation in the Tural-Rajwadi area and similar tritium-tracer investigations of young and mixed waters could be conducted in other geothermal areas in Asia.

Using tritium and radiocarbon to determine groundwater age and delineate the flow regime in the Taiyuan Basin, China

Tritium (3H) and radiocarbon (14C) were used to determine the age, to delineate a flow regime, and to estimate flow velocity of medium-depth groundwater (50–200 m) from a Middle and Upper Pleistocene confined aquifer. Young groundwater age was determined using 3H and an exponential piston-flow model (EPM). Three correction models (Tamers, Pearson, and Fontes and Garnier) were chosen to date the 14C age of old groundwater. Groundwater ages of the medium-depth groundwater ranged from 3.5–25,790 a. Groundwater is relatively young in most of the piedmont area, although two high-age zones (with ages > 20,000 a) exist in the center of the plain. Groundwater age distribution characteristics indicated that groundwater flows from the mountain area to the basin area, and that the oldest groundwater in each high-age zone flows toward the depression cones. Two large local flow systems were formed because of overexploitation, and the regional flow system from north to south no longer exists. This study delineated the age of medium-depth groundwater in the entire Taiyuan Basin for the first time. The results can help promote the sustainable use of groundwater in the Taiyuan Basin.

Application of environmental tracers for investigation of groundwater mean residence time and aquifer recharge in fault-influenced hydraulic drop alluvium aquifers

Abstract. Investigating groundwater residence time and recharge sources is crucial for water resource management in the alluvium aquifers of arid basins. Environmental tracers (chlorofluorocarbons, 3H, 14C, δ2H, δ18O) and groundwater hydrochemical components are used for assessing groundwater mean residence times (MRTs) and aquifer recharge in fault-influenced hydraulic drop alluvium aquifers in the Manas River basin (China). Aquifers under the Manas River upstream (south of the fault) contains very high 3H activity (41.1–60 TU), implying water recharge affected by the nuclear bomb tests of the 1960s. Carbon-14 groundwater age correlates positively with distance from mountain area (3000–5000 years in the midstream to > 7000 years in the downstream) and groundwater depth, but correlates negatively to a decrease in 3H activity (1.1 TU) and more negative δ18O values. This phenomenon reveals that the source of the deeper groundwater in the semi-confined aquifer is paleo-meteoric recharge. Special attention has been paid to the estimation of MRTs using CFCs and 3H by an exponential piston flow model. The results show that MRTs vary from 19 to 101 years by CFCs and from 19 to 158 years by 3H. MRTs estimated from 3H are much longer than those from CFCs, probably due to the different time lag of liquid (3H) and gas-phase CFCs through the unsaturated zone. The MRTs estimated by CFCs show good correlations with pH and the concentrations of SiO2 and SO42-, which can provide a possible approach to estimate first-order proxies of MRTs for groundwater age. The young water fractions are investigated by the CFC binary mixing method in the south and north of the fault. Relatively modern recharge is found in the south of the fault with young (post-1940) water fractions of 87 %–100 %, whereas in the north of the fault the young water fractions vary from 12 % to 91 %. This study shows that the combination of CFCs and 3H residence time tracers can help in analysing the groundwater MRTs and the recharge sources for the different mixing end-members.

Assessment of hydrological pathways in East African montane catchments under different land use

Abstract. Conversion of natural forest (NF) to other land uses could lead to significant changes in catchment hydrology, but the nature of these changes has been insufficiently investigated in tropical montane catchments, especially in Africa. To address this knowledge gap, we aimed to identify stream water (RV) sources and flow paths in three tropical montane sub-catchments (27–36 km2) with different land use (natural forest, NF; smallholder agriculture, SHA; and commercial tea and tree plantations, TTP) within a 1021 km2 catchment in the Mau Forest complex, Kenya. Weekly samples were collected from stream water, precipitation (PC) and mobile soil water for 75 weeks and analysed for stable isotopes of water (δ2H and δ18O) for mean transit time (MTT) estimation with two lumped parameter models (gamma model, GM; and exponential piston flow model, EPM) and for the calculation of the young water fraction. Weekly samples from stream water and potential endmembers were collected over a period of 55 weeks and analysed for Li, Na, Mg, K, Rb, Sr and Ba for endmember mixing analysis (EMMA). Solute concentrations in precipitation were lower than in stream water in all catchments (p < 0.05), whereas concentrations in springs, shallow wells and wetlands were generally more similar to stream water. The stream water isotope signal was considerably damped compared to the isotope signal in precipitation. Mean transit time analysis suggested long transit times for stream water (up to 4 years) in the three sub-catchments, but model efficiencies were very low. The young water fraction ranged from 13 % in the smallholder agriculture sub-catchment to 15 % in the tea plantation sub-catchment. Mean transit times of mobile soil water ranged from 3.2–3.3 weeks in forest soils and 4.5–7.9 weeks in pasture soils at 15 cm depth to 10.4–10.8 weeks in pasture soils at 50 cm depth. The contribution of springs and wetlands to stream discharge increased from a median of 16.5 (95 % confidence interval: 11.3–22.9), 2.1 (−3.0–24.2) and 50.2 (30.5–65.5) % during low flow to 20.7 (15.2–34.7), 53.0 (23.0–91.3) and 69.4 (43.0–123.9) % during high flow in the natural forest, smallholder agriculture and tea plantation sub-catchments, respectively. Our results indicate that groundwater is an important component of stream water, irrespective of land use. The results further suggest that the selected transit time models and tracers might not be appropriate in tropical catchments with highly damped stream water isotope signatures. A more in-depth investigation of the discharge dependence of the young water fraction and transit time estimation using other tracers, such as tritium, could therefore shed more light on potential land use effects on the hydrological behaviour of tropical montane catchments.

Transit times from rainfall to baseflow in headwater catchments estimated using tritium: the Ovens River, Australia

Abstract. Headwater streams contribute a significant proportion of the total flow to many river systems, especially during summer low-flow periods. However, despite their importance, the time taken for water to travel through headwater catchments and into the streams (the transit time) is poorly understood. Here, 3H activities of stream water are used to define transit times of water contributing to streams from the upper reaches of the Ovens River in south-east Australia at varying flow conditions. 3H activities of the stream water varied from 1.63 to 2.45 TU, which are below the average 3H activity of modern local rainfall (2.85 to 2.99 TU). The highest 3H activities were recorded following higher winter flows and the lowest 3H activities were recorded at summer low-flow conditions. Variations of major ion concentrations and 3H activities with streamflow imply that different stores of water from within the catchment (e.g. from the soil or regolith) are mobilised during rainfall events rather than there being simple dilution of an older groundwater component by event water. Mean transit times calculated using an exponential-piston flow model range from 4 to 30 years and are higher at summer low-flow conditions. Mean transit times calculated using other flow models (e.g. exponential flow or dispersion) are similar. There are broad correlations between 3H activities and the percentage of rainfall exported from each catchment and between 3H activities and Na and Cl concentrations that allow first-order estimates of mean transit times in adjacent catchments or at different times in these catchments to be made. Water from the upper Ovens River has similar mean transit times to the headwater streams implying there is no significant input of old water from the alluvial gravels. The observation that the water contributing to the headwater streams in the Ovens catchment has a mean transit time of years to decades implies that these streams are buffered against rainfall variations on timescales of a few years. However, impacts of any changes to land use in these catchments may take years to decades to manifest themselves in changes to streamflow or water quality.

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.

On the use of spring baseflow recession for a more accurate parameterization of aquifer transit time distribution functions

Abstract. Baseflow recession analysis and groundwater dating have up to now developed as two distinct branches of hydrogeology and have been used to solve entirely different problems. We show that by combining two classical models, namely the Boussinesq equation describing spring baseflow recession, and the exponential piston-flow model used in groundwater dating studies, the parameters describing the transit time distribution of an aquifer can be in some cases estimated to a far more accurate degree than with the latter alone. Under the assumption that the aquifer basis is sub-horizontal, the mean transit time of water in the saturated zone can be estimated from spring baseflow recession. This provides an independent estimate of groundwater transit time that can refine those obtained from tritium measurements. The approach is illustrated in a case study predicting atrazine concentration trend in a series of springs draining the fractured-rock aquifer known as the Luxembourg Sandstone. A transport model calibrated on tritium measurements alone predicted different times to trend reversal following the nationwide ban on atrazine in 2005 with different rates of decrease. For some of the springs, the actual time of trend reversal and the rate of change agreed extremely well with the model calibrated using both tritium measurements and the recession of spring discharge during the dry season. The agreement between predicted and observed values was however poorer for the springs displaying the most gentle recessions, possibly indicating a stronger influence of continuous groundwater recharge during the summer months.

Constraining groundwater recharge and the rate of geochemical processes using tritium and major ion geochemistry: Ovens catchment, southeast Australia

► Recharge rates in Ovens catchment constrained using high-precision 3H dating. ► Recharge estimates accord with those from hydrograph records. ► Geochemistry and Sr and C isotopes reflect water–rock interaction during recharge. ► Groundwater protection requires managing large areas of the catchment. Constraining recharge is essential for understanding and managing groundwater systems. This study uses high-precision Tritium ( 3 H) concentrations together with major ion and environmental isotope (Sr, C, O, H) geochemistry to constrain rates of recharge and geochemical processes in the Ovens catchment of the southeast Murray Basin, Australia. 3 H concentrations of groundwater in the Ovens catchment decline with depth but not with position in the catchment, implying that recharge occurs over broad areas. Recharge rates calculated using an exponential–piston flow model are 120–180 mm/yr in the coarse-grained alluvial gravels of the upper and middle Ovens and 30–45 mm/yr in the finer-grained sediments of lower Ovens. These recharge rates are similar to those calculated from the response of shallow bore hydrographs to rainfall. The geochemistry of groundwater with low (60–160 mg/L) total dissolved solids (TDS) from the upper and middle Ovens indicates that dissolution of both silicate minerals and calcite has occurred. Major ion geochemistry, 87 Sr/ 86 Sr ratios, and δ 13 C values in the upper and middle Ovens groundwater are correlated with groundwater residence times and constrain the progress of mineral reactions. Dissolution of silicates with high 87 Sr/ 86 Sr ratios (biotite and K-Feldspar) occurs prior to dissolution of silicates with low 87 Sr/ 86 Sr ratios (plagioclase) and calcite. Mineral dissolution mainly occurs in the upper 20 m of the aquifer system with relatively little change in geochemistry deeper in the flow system. Groundwater from the lower Ovens has higher TDS contents (600–9600 mg/L) due to higher degrees of evapotranspiration. Although a correlation between TDS and groundwater residence times exists, it is more likely to reflect differential recharge rates and not the rate of evapotranspiration. The conclusion that recharge occurs across the Ovens catchment indicates that protection of groundwater from surface contamination is dependent on the protection and management of large areas.

Predicting Pesticide Attenuation in a Fractured Aquifer Using Lumped‐Parameter Models

Neighboring springs draining fractured-rock aquifers can display large differences in water quality and flow regime, depending on local variations of the connectivity and the aperture size distribution of the fracture network. Consequently, because homogeneous equivalent parameters cannot be assumed a priori for the entire regional aquifer, the vulnerability to pollution of such springs has to be studied on a case by case basis. In this paper, a simple lumped-parameter model usually applied to estimate the mean transit time of water (or tracer) is presented. The original exponential piston-flow model was modified to take land-use distribution into account and applied to predict the evolution of atrazine concentration in a series of springs draining a fractured sandstone aquifer in Luxembourg, where despite a nationwide ban in 2005, atrazine concentrations still had not begun to decrease in 2009. This persistence could be explained by exponentially distributed residence times in the aquifer, demonstrating that in some real world cases, models based on the groundwater residence time distribution can be a powerful tool for trend reversal assessments as recommended for instance by current European Union guidelines.

Resolving the source and mixing proportions of modern leakage to the Memphis aquifer in a municipal well field using geochemical and 3H/3He data, Memphis, Tennessee, USA

The Memphis aquifer in southwestern Tennessee is a confined to semi-confined unconsolidated sand aquifer and is the primary municipal water source for the Memphis metropolitan area. Leakage of modern water from shallow unconfined aquifers through the upper Claiborne confining unit locally degrades water quality in the Memphis aquifer and makes the aquifer more vulnerable to contamination. Major solute chemistry, tritium, and 3H/3He data were used to investigate the source and mixing proportions of modern water in the Memphis aquifer in the Davis well field, Memphis, Tennessee. Water quality in several production wells in the Davis well field has declined over the past 30 years, mainly through increased total dissolved solids, iron, alkalinity, and hardness. Trends in chemical data, tritium, and other hydrogeologic data support a source for the leakage from the Mississippi River Valley Alluvial aquifer. Mixing proportions of alluvial water in production well water obtained by inverse chemical modeling with PHREEQCi range from 7 to 45%. For two of the production wells, MLGW 414 and 432, 3H/3He data yield mixing ratios similar to those obtained from PHREEQCi in three of four cases; the dissimilar ratio is likely due to a poor solution from the PHREEQCi inverse modeling. Modeling of the age distribution obtained from MLGW 414 and 432 using an exponential-piston flow model (EPM) with an inverse solution computer code yielded mixed results. The EPM solution obtained for MLGW 414 converged with difficulty only for a 5-year transit time in the shallow aquifer and is consistent with a source from the Mississippi River Valley Alluvial aquifer; however, the modeled age of the water is greater than that observed. In comparison, the EPM solution for MLGW 432 converged for the 5- and 7-year transit periods in the shallow aquifer and yielded model ages consistent with observed 3H/3He ages; however, the extent of the maximum radii for infiltration source is not consistent with a Mississippi River Valley Alluvial aquifer source. Other potential sources for leakage to MLGW 432 include water from the Fluvial-terrace aquifer migrating along a fault east of the well field or infiltration of water from a lake south of the well field.

Discriminant analysis for estimation of groundwater age from hydrochemistry and well construction: application to New Zealand aquifers

Concentrations of tritium, chlorofluorocarbons and sulfur hexafluoride have been measured at over 100 groundwater monitoring sites across New Zealand, followed by interpretation of groundwater age distribution using the exponential-piston flow model. Interpreted mean residence times ranged from less than 1 year to more than 100 years, with the 25th, 50th (median) and 75th percentiles being approximately 10, 40 and 100 years, respectively. Classification functions derived from discriminant analysis and based on nine input variables (well depth, electrical conductivity and the concentrations of the ions Na, K, Ca, Mg, HCO3, Cl and SO4) allowed assignment of 71% of the sites to the correct of four age categories (mean residence time 10 years or less, 11–40 years, 41–100 years, or more than 100 years). The discriminant analysis classification functions were more effective than regression methods for estimating groundwater age from hydrochemistry and well depth, and can thus be used to predict the groundwater age category for any monitoring site in New Zealand.

Analysis of water movement through an unsaturated soil zone in Jeju Island, Korea using stable oxygen and hydrogen isotopes

Summary In order to analyze water movement through an unsaturated soil zone in Jeju Island, Korea, temporal variations in the oxygen-18 and deuterium isotopes of precipitation and soil waters at three different depths were monitored for about one year. The stable oxygen and hydrogen isotopic compositions of the soil waters are plotted between or near the two local meteoric water lines for summer and winter precipitation indicating that the soil waters were recharged from the year-around precipitation, and evaporation was negligible even during hot summer season in the study area. Mean residence times of the soil waters were then estimated from the δ18O, δD, and deuterium excess or d-values of the precipitation and soil waters using two well-mixed or exponential models (EM) and one exponential piston-flow model (EPM). The regression results show that the combination of the exponential piston-flow model and the d-values of the precipitation and soil waters gives the most reasonable estimation of the mean residence times of the soil waters in the study area. It suggests that the deuterium excess or d-values can be used as a better parameter than the δ18O or δD values alone for estimating mean residence times of subsurface waters not only in Korea but also in other Northeast Asian countries. The estimated low moisture contents and hydraulic diffusivities of the soils at the three different depths also indicate that the boundary between the upper fine-grained soil and the lower coarse-grained soil may act as a capillary barrier, and the soil waters probably flow slowly through micropores rather than rapidly through macropores in the unsaturated soil zone.

Groundwater mixing dynamics at a Canadian Shield mine

Temporal and spatial variations in geochemistry and isotopes in mine inflows at the Con Mine, Yellowknife, are studied to access the impact of underground openings on deep groundwater flow in the Canadian Shield. Periodic sampling of inflow at 20 sites from 700 to 1615 m depth showed that salinities range from 1.4 to 290 g/l, with tritium detected at all depths. Three mixing end-members are identified: (1) Ca(Na)–Cl Shield brine; (2) glacial meltwater recharged at the margin of the retreating Laurentide ice sheet at ∼10 ka; and (3) modern meteoric water. Mixing fractions, calculated for inflows on five mine levels, illustrate the infiltration of modern water along specific fault planes. Tritium data for the modern component are corrected for mixing with brine and glacial waters and interpreted with an exponential-piston flow model. Results indicate that the mean transit time from surface to 1300 m depth is about 23 years in the early period after drift construction in 1979, but decreases to about 17 years in the past decade. The persistence of glacial meltwater in the subsurface to the present time, and the rapid circulation of modern meteoric water since the start of mining activities underline the importance of gradient, in addition to permeability, as a control on deep groundwater flow in the Canadian Shield.

Determining the turnover time of groundwater systems with the aid of environmental tracers: 1. Models and their applicability

Three new lumped-parameter models have been developed for the interpretation of environmental radioisotope data in groundwater systems. Two of these models combine other simpler models, i.e. the piston flow model is combined either with the exponential model (exponential distribution of transit times) or with the linear model (linear distribution of transit times). The third model is based on a new solution to the dispersion equation which more adequately represents the real systems than the conventional solution generally applied so far. The applicability of models was tested by the reinterpretation of several known case studies (Modry Dul, Cheju Island, Rasche Spring and Grafendorf). It has been shown that two of these models, i.e. the exponential-piston flow model and the dispersive model give better fitting than other simpler models. Thus, the obtained values of turnover times are more reliable, whereas the additional fitting parameter gives some information about the structure of the system. In the examples considered, in spite of a lower number of fitting parameters, the new models gave practically the same fitting as the multiparameter finite state mixing-cell models. It has been shown that in the case of a constant tracer input a prior physical knowledge of the groundwater system is indispensable for determining the turnover time. The piston flow model commonly used for age determinations by the 14C method is an approximation applicable only in the cases of low dispersion. In some cases the stable-isotope method aids in the interpretation of systems containing mixed waters of different ages. However, when 14C method is used for mixed-water systems a serious mistake may arise by neglecting the different bicarbonate contents in particular water components.