Displays

Transit time distributions estimated from stable water isotopes (deuterium and oxygen-18) are frequently used to assess transport and storage of water in catchments. We analyzed 2.25 years of 7‑hourly and 4.5 years of weekly measurements of stable water isotopes in precipitation and streamwater at the Plynlimon catchments in Wales, UK using the ensemble hydrograph separation technique. We thereby quantified new water fractions – the average contribution of recent precipitation to streamflow – in the different subcatchments, and determined transit time distributions as the contribution of precipitation to streamflow over a range of lag times.

We found that on average only 3 % of streamwater was made up of precipitation that fell within the last 7 hours, and 13-15 % of streamwater was made up of precipitation that fell within the previous week. However, these new water fractions increased with discharge, indicating that more recent precipitation reached the stream when the catchment was wet, and the contributions of recent precipitation to streamflow were highest during large events. This dependence of new water fractions on water fluxes was also reflected in their seasonal variations, with lower new water fractions and more damped catchment transit time distributions in the drier spring and summer compared to fall and winter.

A comparison between changes in solute concentrations and new water fractions with discharge provides additional insight into the storage and release of water and solutes from the catchments. Our analysis demonstrates that changes in solute concentrations primarily reflect changes in flowpaths between dry and wet conditions, rather than changes in the fraction of recent precipitation in streamflow.

How to cite: Knapp, J. and Kirchner, J.: New water fractions, transit time distributions, and solute concentrations at Plynlimon, Wales. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5993, https://doi.org/10.5194/egusphere-egu2020-5993, 2020.

The distribution of groundwater ages with depth provides information about subsurface structures and flow dynamics. Upslope measured groundwater age stratifications are commonly used to estimate groundwater recharge rates, whereas downslope measured age stratifications are influenced by recharge conditions, the aquifer structure and interactions between groundwater and surface water. In our study we address the question whether downslope measured groundwater ages from different locations can provide spatial and temporal information about catchment-scale groundwater dynamics and the relationship between groundwater recharge and discharge.
We derived an overall groundwater age stratification, representative for the Svartberget subcatchment (0.47 km²) located within the Krycklan study site, by measuring CFCs from 9 different sampling locations with depths of 2 m to 18 m. All sampling locations were downslope and located in basal till which is overlain by ablation till. 
The CFC-based groundwater age stratification reveals an unexpected pattern, with groundwater ages of already 30 years immediately below the water table. Groundwater ages increase then with depth. We could reproduce the observed groundwater age stratification by using a groundwater flow model and show that the lag of rejuvenation, noticeable in groundwater ages of 30 years at the water table, derives from return flow of groundwater at a subsurface discharge zone that evolves at the interface between the two soil types (basal and ablation till). Furthermore, we demonstrate by varying the infiltration rate how the extent of the discharge zone and the partitioning of the infiltration amount to the two layers change, i.e. young runoff in the upper layer (ablation till) and old groundwater circulation through the deeper layer (basal till).
By providing a simple analytical approximations of the observed groundwater age stratification, we show that the extent of the subsurface discharge zone is a powerful indicator of the relation between the recharge and discharge zone, while the vertical gradient of the age-depth relationship provides information about the overall aquifer recharge.

How to cite: Kolbe, T., Marçais, J., de Dreuzy, J.-R., Labasque, T., and Bishop, K.: Catchment-scale groundwater age stratification reveals groundwater recharge and discharge processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17164, https://doi.org/10.5194/egusphere-egu2020-17164, 2020.

The quality and quantity of alluvial groundwater in mountainous areas are particularly susceptible to the effects of climate change, as well as increasing pollution from agriculture and urbanization. Understanding mixing between surface water and groundwater as well as groundwater travel times in such systems is thus crucial to sustain a safe and sufficient water supply. We used a novel combination of real-time, in-situ noble gas analysis to quantify groundwater mixing of recently infiltrated river water (F_rw) and regional groundwater, as well as travel times of F_rw during a two-month groundwater pumping test carried out at a drinking water wellfield in a prealpine valley in Switzerland. Transient groundwater mixing ratios were calculated using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of F_rw consequently allowed us to infer the travel times from the stream to the wellfield, estimated based on radon-222 activities of F_rw. Additionally, we compared and validated our tracer-based estimates of F_rw using a calibrated surface water-groundwater model. Our findings show that (i) mean travel times of F_rw are in the order of two weeks, (ii) during most of the experiment, F_rw is substantially high (~70\%), and (iii) increased groundwater pumping only has a marginal effect on groundwater mixing ratios and travel times. The high fraction of F_rw in the abstracted groundwater and its short travel times emphasize the vulnerability of mountainous regions to present and predicted environmental changes.

How to cite: Popp, A. L., Pardo-Álvarez, Á., Schilling, O. S., Musy, S., Scheidegger, A., Peel, M., Kipfer, R., and Brunner, P.: Untangling transient groundwater mixing and travel times with noble gas time series and numerical modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5022, https://doi.org/10.5194/egusphere-egu2020-5022, 2020.

Combined ¹⁴C and ³H measurements on groundwater from Christchurch, New Zealand, are proving vitally important for revealing water age distributions, which are key to understanding the diverse flows in the system (including younger potentially polluting water flows from near the surface and much older pristine water flows from depth). The deep gravel aquifers 350-500m thick under Christchurch have been studied since 1970 using ¹⁴C along with ³H, ¹⁸O and chemical concentrations to characterize the residence times, sources and flowpaths of the water. Of note in this long-term study is the successful use of ¹⁴C to determine mean ages of groundwater in the age range from 5 years to 2000 years, which is made possible by the absence of carbonate in the aquifer rocks and the presence of bomb ¹⁴C in some samples (Stewart, 2012). The ¹⁴C mean ages are showing that the groundwater system has changed markedly over time because of exploitation of the system, from young ages (60-70 years) across the system in 1976 to much older ages (i.e. 400 years on the west and 1600 years on the east) in 2017. Increasing amounts of deep stored water are being tapped by the wells, especially on the east (coastal) side.

Wells on the west side of the system have moderate ¹⁴C mean ages (400-600 years) and some of the samples have ³H showing that they also contain fractions of young water. Using binary mixing models allows the proportions and mean ages of the young fractions to be estimated. The mean ages of the young fractions have become younger over time, showing that nitrate contamination is becoming more likely. On the other hand, the fraction of older water is becoming larger and therefore more able to dilute the young fraction. Wells on the east side have much older ¹⁴C ages (1600 years) and are ³H-free showing that there are no such younger contributions. The results are providing valuable information for improved understanding and better management of the resource.

How to cite: Stewart, M. and van der Raaij, R.: Use of carbon-14 and tritium to investigate flow and storage of water in the Christchurch groundwater system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3182, https://doi.org/10.5194/egusphere-egu2020-3182, 2020.

⁴He as a dating groundwater tool in shallow crystalline aquifers

Labasque T., Chatton E. , Aquilina L., Vergnaud V.

OSUR-Géosciences Rennes, Université Rennes1 – CNRS; Rennes France

Groundwater dating methods have been widely used during the last decades for studying subsurface water hydrological and hydrochemical processes. Estimation of groundwater residence time is essential for resource preservation, contaminant studies or groundwater recharge rates and flow velocities assessments. Due to the complexity of groundwater flow, the joint use of several environmental tracers has been often promoted as it offers integrative information on the structure of complex aquifers.

Anthropogenic gas tracers as CFC, SF₆, ⁸⁵Kr, ³⁶Cl or ³H have been widely used to study shallow groundwater with residence time of less than 70 yrs. For longer groundwater residence time (100- x1000 yr), ³⁹Ar, ¹⁴C, ³⁶Cl and ⁴He have been used. Although it informs mainly on residence times from several thousands to hundreds of thousands years, ⁴He can also cover an age range of 10 to thousands years. The residence time is estimated by taking into account all ⁴He fluxes from atmosphere, crustal and mantellic, but also taking into account diffusion processes in fractured media. The main difficulty is to estimate the crustal production rate through U and Th decay and its homogenity in the aquifer and the others ⁴He fluxes: atmosphere, crust and mantellic, and diffusion processes in fracture media. In many cases U-Th production deduced from U and Th concentrations is not sufficient to explain the ⁴He concentrations observed in the aquifer. Other ⁴He fluxes can then be estimated through the use of other tracers such as ¹⁴C, ³⁶Cl or modeling. Fracturing may also enhance ⁴He concentrations in groundwater through diffusion processes from the matrix to the circulating water and should also be evaluated.

We present here the evaluation of ⁴He in a crystalline fractured aquifer in the Northwest of France (OZCAR – H⁺ national hydrogeological network), in order to investigate the range of groundwater residence time in this complex shallow aquifer. Previous studies on this aquifer reveal mixing between young (<70yrs) and old waters (>1000yrs) based on ¹⁴C. The Helium radiogenic production rate is then evaluated through in situ production (U, Th) and calibration with CFC and ¹⁴C. Mixing processes are estimated through a lumped parameter model approach and diffusion processes are discussed through an estimation of fracture aperture and fracture interval. Apparent ages are compared and uncertainties discussed. Once ⁴He production calibrated and diffusion processes characterized, ⁴He gives access to groundwater ages from decades to several centuries, and thus completes the range of groudwater ages obtained by the other tracers.

How to cite: Labasque, T., Chatton, E., Aquilina, L., and Vergnaud, V.: 4He as a dating groundwater tool in shallow crystalline aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7558, https://doi.org/10.5194/egusphere-egu2020-7558, 2020.

Determining the time taken for water to pass through catchments from where it is recharged to where it discharges into streams or is sampled from within the soils or aquifers (the transit time) is vital for understanding catchment functioning. Near-surface environments are dynamic and transit times are likely to vary at different stages of the hydrological cycle. Because of the lower input of bomb-pulse tritium in the southern hemisphere it is possible to determine transit times from individual tritium measurements. Additionally, because tritium is radioactive, transit times can be estimated where the catchment is not stationary. While the transit times are subject to uncertainties, this approach allows transit times at different stages of the hydrological cycles in dynamic environments to be determined.

In several southeast Australian headwater catchments, the mean transit times of stream waters at low flows range from several years to decades. The tritium activities increase at higher flows, implying that there is an input of younger water at that time. However, the tritium activities generally remain below those of recent rainfall implying that simple dilution by recent rainfall is not occurring; that conclusion is consistent with the variation in the concentrations of other geochemical tracers at different streamflows. Rather, the variations in geochemistry are consistent with shallower younger stores of water from the soils and regolith being progressively mobilised as the catchments wet up during winter. These younger water stores typically have mean transit times of at least a few years. The generally long transit times imply that the southeast Australian headwater catchments have large storage capacities, probably due to the catchments being unglaciated and deeply weathered. The observation that the transit times at high flows are still relatively long suggest that, even though they may only be active for part of the year, the shallow water stores also have relatively large volumes.

Understanding the transit times improves our ability to predict the behaviour and management of these catchments. The large storage capacities result in the catchments being resilient to year-on-year variations in rainfall and many of the headwater streams in southeast Australia have continued to flow through recent droughts. Similarly, the streams are less susceptible to inputs of surface contamination but contaminants stored in the soil water or shallow groundwater may impact the streams over prolonged periods. As the bomb-pulse tritium decays over the next few decades, determining mean transit times from single tritium measurements will become possible in northern hemisphere catchments. This will enable a better global understanding of catchment functioning in a wider range of environments.

How to cite: Cartwright, I.: Determining water transit times in dynamic environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1874, https://doi.org/10.5194/egusphere-egu2020-1874, 2020.

Groundwater pollution threatens human and ecosystem health in many areas around the globe. Shortcuts to the groundwater through enlarged cracks and fissures, often referred to as concentrated recharge, are known to transmit short-lived pollutants into carbonate aquifers endangering water quality of around a quarter of the world population. However, the large-scale impact of concentrated recharge on water quality remains poorly understood. Here we apply a continental-scale model to quantify for the first time the danger of groundwater contamination by degradable pollutants through concentrated recharge in carbonate rock regions. We show that concentrated recharge is the primary reason for the rapid transport of contaminants to the groundwater, increasing the percentage of non-degraded pollutants from <1% in areas without concentrated recharge to around 20-50% in areas where concentrated recharge is present. Our findings are most pronounced in the Mediterranean region where agricultural pollutants in groundwater recharge like Glyphosate can exceed allowed concentrations by up to 19 times. Our results imply that in regions where shortcuts to the groundwater exist, continuing industrial agricultural productivity to optimize food production may result in a widespread reduction of available drinking water and harm ecosystem services more intense than presently available large-scale modelling concepts suggest.

How to cite: Hartmann, A. and research consortium, T. K. V.: Widespread underestimation of the danger of groundwater contamination by shortcuts into aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22641, https://doi.org/10.5194/egusphere-egu2020-22641, 2020.

Groundwater transit time refers to the travel time of a water molecule through an aquifer from recharge at the water table to discharge at a surface water body (e.g., river). Analysing transit times provides a primary way to understand the overall transport characteristics of a hydrological system and is of interest in many aspects of environmental management. For example, studying transit time distribution can facilitate the mitigation of pollutant transport risks and ecosystem restoration. Hydrogeological heterogeneity of an aquifer is a major controlling factor for groundwater flow paths and transit time distributions. In this study, we investigate the impacts of spatial variability of hydrogeological properties on transit times by combining measurements and a new semi-analytical numerical modelling scheme. Passive tracer transport data from several catchments in Europe are obtained from open databases. Groundwater transit time in these catchments are inferred from both tracer transport data and numerical modelling. Comparing the results in different catchments provides a comprehensive way of understanding the impact of hydrogeological heterogeneity on groundwater transit time.

How to cite: Rahman, M. and Hartmann, A.: Impacts of hydrogeological heterogeneity on groundwater transit time, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7211, https://doi.org/10.5194/egusphere-egu2020-7211, 2020.

Groundwater is an important source of water for the Wairau Plain, and there is concern about its sustainable yield because of declining water levels and spring flows in the Wairau Fan. The Wairau Fan is comprised of highly permeable fluvial gravels. The main source of groundwater is loss from the Wairau River channel. The underlying Pleistocene gravels form a significantly less permeable aquifer. Near the coast, estuarine sediments form an aquiclude over the Pleistocene gravels. The main groundwater flow from the gravel fan is forced back to the surface near the confinement boundary feeding highly valued streams with crystal-clear water but declining flow.

To understand the flow dynamics of the groundwater, we utilised tritium, SF6, and 14C. For the extremely young groundwaters in the unconfined Wairau Fan, <1 year, we developed a dating method that traces the seasonal river temperature variability through the aquifer. The lags of the temperature synodal signal were calibrated to true age via the ¹⁸O synodal signal.

All groundwaters within the Pleistocene gravels are very old, >100 years, and up to 39,000 years in the Deep Wairau Aquifer. In contrast, throughout the unconfined Wairau Fan we observed only very young groundwater, with mean residence time of 0–1 years, even in the deeper wells of >20 m.

Flow rates estimated from groundwater age gradients show that in its upper part the unconfined Wairau Fan is well connected to the Wairau River. Extremely high flow rates of up to > 30 km/y in this area indicate extremely high hydraulic conductivity in these Holocene deposits near the river. Towards the coast, the flow rates reduce considerably, to 13 km/y at around the boundary of the confinement, thereafter slowing further to 0.7 km/y near the coast. The reduction in flow rate near the coast, by a factor >20, is related to the flow loss from the aquifer, mainly to the spring belt and through abstraction.

Hydraulic conductivities, derived from the flow rates, are c. 12,000 m/day in the unconfined Wairau Fan near the river and in the central part of the unconfined Wairau Fan. Near the coast the estimated hydraulic conductivity is 800 m/day. Despite relatively uniform hydraulic conductivities, the Wairau Fan becomes less transmissive downstream due to decreasing piezometric gradients. This is likely to cause the restriction in the flow system. The ‘choking point’ in the flow system of the unconfined Wairau Fan appears to be not the recharge zone near the river but the lower Wairau Fan due to its lower transmissivity by a factor of two.

To understand the buffer of the entire system against prolonged drought, the mean transit time of the water through the Wairau River catchment was estimated from tritium time-series data to four years, and the active groundwater storage to approximately 6,200M m³. The Wairau catchment would be able to maintain baseflow in the river and the aquifer for several years.

How to cite: Morgenstern, U. and Stewart, M.: Groundwater age gradient to infer flow rates, hydraulic parameters, aquifer abstraction limits, and storage in the Wairau Plain, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7059, https://doi.org/10.5194/egusphere-egu2020-7059, 2020.

Fractured and karstified aquifers are often vulnerable to pollution by nitrate or other anthropogenic compounds. Therefore, a better understanding of the flow and transport processes in these aquifers is imperative for effective drinking water management. In this study, we used the analysis of tritium and tritiogenic helium-3 concentrations to estimate the residence and exposure time of nitrate transported in a fractured groundwater system of the Upper Muschelkalk in southwest Germany.

The recharge area is characterised by elevated nitrate concentrations of up to 60 mg/L which are in accordance with dominating agricultural landuse in the catchment. Further along the groundwater flow direction a significant decrease in dissolved oxygen as well as nitrate concentrations to values close to the detection limit is observed.

Tritium/³He ages were found to be in the range of zero to forty years. However, in the fractured aquifer the age tracers were most probably affected by mixing and exchange processes that might change the concentration as well as the ratio of tritium and helium-3 in addition to radioactive decay. Therefore, we investigated the impact of different transport processes such as mixing of water parcels at fracture joints or exchange between mobile water on fractures and the pore matrix using forward convolution approaches for both isotopes separately.

In combination with hydrochemical, multi-isotopic, petrographical, and molecular biological data, the groundwater residence time data was intended to gain crucial insight into the processes and limiting factors of autotrophic denitrification found within the Muschelkalk aquifer.

How to cite: Osenbrück, K., Fünfgeld, F., Sültenfuss, J., Blackwell, N., and Grathwohl, P.: Tritium-Helium Dating of Groundwater in a Fractured and Karstified Carbonate Rock Aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10148, https://doi.org/10.5194/egusphere-egu2020-10148, 2020.

Faults play an important role in flow and transport in regional groundwater systems. The inclusion of faults during the conceptualization of regional groundwater systems and their incorporation during the construction of groundwater models is crucial, particularly during performance assessments of radioactive waste repositories as well as risk assessment for other deep subsurface activities. Faults can act as: i) barriers slowing down groundwater flow, ii) conduits speeding up groundwater flow, or iii) a combination of both. Determining flow and transport behaviors across these structures is difficult since they are rarely exposed on the surface and their hydraulic behavior vary spatially. Environmental tracers may provide valuable information potentially useful to determine flow pathways, travel times, and groundwater age. If these latter are affected by the presence of fault zones, and they can yield important information for the parameterization of faults in groundwater models. For the Neogene aquifer in Flanders, groundwater flow and solute transport models have been developed in the framework of safety and feasibility studies for the underlying Boom Clay Formation as potential host rock for geological disposal of radioactive waste. However, the simulated fluxes and transport parameters of these models are still subject to large uncertainties, as they are typically constrained by hydraulic heads only and their current conceptualization does not differentiate the fault zones from the undisturbed aquifer materials. This study investigates how groundwater flow and solute transport in the sedimentary Neogene aquifer are disturbed by the Rauw fault – a 55 km long normal fault – across the Nete catchment, in Belgium. To this end, we use a combination of hydraulic head observations and several environmental tracers: hydrochemical analyses, stable isotopes, carbon-14 (¹⁴C), helium-tritium (³He-³H), helium-4 (⁴He) and temperature-depth (TD) profiles. This will allow us to: i) test our current understanding of the system as well as the corresponding model performance, and ii) decrease the uncertainties on forward model outcomes for future scenarios and inverse models by including an advanced conceptualization. The Rauw fault has a displacement of >7 meters which increases with depth. The observed hydraulic gradient across the fault zone appears significant, with head differences of 1.8-2.0 meters over an horizontal distance of 60 meters. Two sampling campaigns have taken place, in 2016 and 2019, for collection of ³He-³H, ⁴He, ¹⁴C, and TD data at a total of 38 selected wells across the Nete river catchment. These will be further used as observations points for the transport modelling. Here, we will present the first results and interpretations of the gathered temperature and environmental tracer data in complementation with hydraulic head levels to evaluate the effects of the Rauw Fault on the hydrogeological system and the implications future conceptualization and numerical modelling.

How to cite: Casillas-Trasvina, A., Rogiers, B., Beerten, K., Wouters, L., and Walraevens, K.: The influence of faults on groundwater flow and transport dynamics: the raw fault in the Neogene aquifer, Belgium., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8713, https://doi.org/10.5194/egusphere-egu2020-8713, 2020.

Groundwater in coastal area is a strategic but fragile resource since it undergoes high anthropogenic pressure that can lead to saltwater intrusion. Therefore the use of coastal groundwater needs a thorough understanding of the groundwater flow and mixing to assure a suitable management of the resource.

The coastal and thermal karstic hydrosystem of the Thau basin (South of France) shows a good example of the pressure that can undergoes coastal groundwater as it is a strategic resource for drinking water, spa activities as well as shellfish aquaculture. In this aquifer, groundwater originates from 3 different naturally sources with contrasted mean residence time (MRT): 1) cold and young karstic water, 2) warm and mineralized thermal waters with long MRT, and 3) marine waters (Thau lagoon and/or seawater). In this context, age dating tracers can be valuable tools for the characterization of the groundwater flow circulations, the estimation of their residence time but also of the mixing which can affect the thermal system.

We used dissolved gases (CFCs and SF6) and ³H age dating tracers to characterize the young end-member, as these tracers are particularly suitable for identifying and quantifying water mixing of different ages (Newman et al., 2010). Strategic locations representative of each component of the system (surficial and deep karst, springs and thermal boreholes) were sampled during different hydrogeological contexts (high flow/base flow).

The first results show that as expected, in general, the thermal component has a very low level of dissolved gas indicating long MRT and few mixing whereas karstic springs show high contents of dissolved gas. However, some thermal wells show important and variable gas content indicating mixing with the karstic component and rapid circulation in some parts of the system. These data will contribute to determine the groundwater transfer model(s) in the Thau system and to estimate the contribution of the current karst water to the Balaruc thermal system. These results will in turn, be used within the framework of the “Dem’Eaux Thau project” to develop tools for groundwater resources management allowing decision-makers to take on the challenges of this region.

How to cite: de Montety, V., Lemaitre, L., Ladouche, B., Bailly-Comte, V., Pérotin, L., Hery, M., Guilhe-Batiot, C., Pétré, M.-A., and Seidel, J.-L.: Groundwater circulation and mixing inferred from age dating with dissolved gas tracers in a complex Mediterranean karstic and thermal aquifer (Thau lagoon area, Montpellier, France). , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9021, https://doi.org/10.5194/egusphere-egu2020-9021, 2020.

The TMG aquifer is one of the largest aquifer systems in South Africa and is currently targeted as a potential source of potable water for the City of Cape Town (CoCT) which recently experienced a period of extreme water stress. Groundwater in the TMG aquifer typically has very low total dissolved salts, on the order of 50 mg/L of less, making it challenging to constrain the groundwater residence time. However, residence time is a key parameter to provide proper constraints on turnover time of groundwater in the aquifer system before large-scale abstraction takes place, in order to evaluate the sustainability of the resource. This study used the ³H/³He system to date modern water (<100 years) and ¹⁴C to date older groundwater (>500 years). Groundwater residence times were determined for the TMG aquifer and five associated aquifer systems in the Western Cape of South Africa, namely the alluvial, Witteberg, Bokkeveld, Cape Granite Suite (CGS) and Malmesbury aquifers. Good correlation between ³H/³He and ¹⁴C ages indicate relatively short residence times for the alluvial and TMG aquifers whereas groundwater from the Witteberg, Bokkeveld, CGS and Malmesbury aquifers indicate mixing of older water bodies with modern recharge resulting in distinctly different ages derived from the two dating systems. In an attempt to better constrain the mixing relationship with modern precipitation, ²²²Rn was used to assess the interaction between precipitation and groundwater after rainfall events. The basis for this approach comes from the assumption that precipitation has little ²²²Rn in it, with groundwater ²²²Rn derived from interaction with the groundwater host rocks. This should result in groundwater ²²²Rn activity being diluted through recharge with precipitation. However, since the half-life of ²²²Rn is only 3.82 days, ²²²Rn activities should respond rapidly to recharge, and should also recover rapidly from this recharge. Three behavioural characteristics were established; (1) groundwaters where the ¹⁴C activity was of ≥ 100 pMC (TMG and alluvial aquifers), and where an immediate dilution in radon’s activity was recorded due to direct recharge. (2) groundwaters where the ¹⁴C activity was 80% – 90% pMC (Malmesbury aquifer) where a delayed response in the dilution of radon’s activity was recorded; and (3) groundwaters where the ¹⁴C activity was ≤ 70% and radon activities were stable indicating little or no recharge. ²²²Rn thus proved an important mechanism for evaluating the validity of residence times derived from both ³H/³He and ¹⁴C.

How to cite: Miller, J., Harilall, Z., Agyare-Dwomoh, Y., Palcsu, L., and Botha, R.: Re-evaluation of groundwater residence time using a combined 3H/3He, 14C and 222Rn approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15244, https://doi.org/10.5194/egusphere-egu2020-15244, 2020.

In shallow aquifers, including weathered zones characteristic of crystalline geologic basements, subsurface flows strongly depend on the geomorphological evolution of landscapes as well as on the geological heterogeneity structures. Yet, it remains largely unknown how geomorphology and geology shape the residence times in the aquifers and the transit times  in the receiving stream water bodies.

We investigate this issue with 3D synthetic models of free aquifers. Aquifer models represent hillslopes from the river to the catchment divide with constant slopes, evolving widths and depths. They are submitted to uniform and constant recharge. All flows end up in the river either through the aquifer or through the surface as return flows and saturation excess overland flows. Steady-state flows and transit times to the river are simulated with Modflow and Modpath (Niswonger et al., 2011; Pollock, 2016). The mean and standard deviation of the transit time distribution are systematically determined as functions of the hillslope shapes (convergent or divergent to the river, thinning or thickening to the river) and the ratio of recharge to hydraulic conductivity.

We show that the mean transit time distribution is a function of the geology through the volume of the aquifer divided by the recharge rate even in the presence of seepage areas. The standard deviation of the transit time distribution is a function of the geomorphology through the bulk organization of the groundwater body from the river to the catchment divide. Without seepage, the organization of the groundwater body is efficiently characterized by its barycenter. When seepage occurs, the standard deviation becomes also sensitive to the extent of the seepage zone.

We conclude that mean of the transit time distribution is primarily determined by geology through the accessible aquifer volume while the ratio of the standard deviation to the mean (coefficient of variation) is rather determined by geomorphology through the profile of the aquifer from the river to the catchment divide. We discuss how geophysical data might help to determine the groundwater body and assess the transit time distribution. We illustrate these findings on natural aquifers in the crystalline basements of Brittany-Normandy (France).

References

Niswonger, R.G., Panday, S., Ibaraki, M., 2011. MODFLOW-NWT, A Newton formulation for MODFLOW-2005.

Pollock, D.W., 2016. User guide for MODPATH Version 7—A particle-tracking model for MODFLOW (Report No. 2016–1086), Open-File Report. Reston, VA. https://doi.org/10.3133/ofr20161086

How to cite: Gauvain, A., Leray, S., Marçais, J., Vautier, C., Aquilina, L., and de Dreuzy, J.-R.: Morphological controls on groundwater residence times of shallow aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14907, https://doi.org/10.5194/egusphere-egu2020-14907, 2020.

Groundwater transport of legacy contaminants (e.g., excess nutrients in agricultural watersheds) into streams and rivers is a likely contributor to the lag in surface water quality improvement. This lag being linked to the distribution of groundwater transit times, ample study of groundwater transit times becomes critical. Catchments are spatially complex and subsurface flow is invisible, so one can only infer the movement and mixing of waters from the chemical and isotopic tracer signatures that they carry.(JW Kirchner, X Feng, C Neal - Nature, 2000*). Thus, building a code that can explain the movement of inert tracers (like nitrogen) can reveal a lot of information.

To model TTDs in a catchment, conceptual lumped parameter models are most commonly used, which include long-established conventional models like piston flow, exponential, gamma & dispersion models, & recent ones like TRANSEP, ETNA etc—mostly claiming parsimony. But real-world catchments are not only heterogeneous, they are also nonstationary: their travel-time distributions shift with changes in their flow regimes, due to shifts in the relative water fluxes and flow speeds of different flow paths.

The distributed models (which mostly use Finite Element analysis to solve intricate PDEs) despite being more accurate are also quite complicated—with complex assembly processes, & often needless fineness in discretization, thereby rendering equifinality & overparameterization.

We follow a unique semi-distributed 1d modelling approach in determining TTDs -- our entire groundwater catchment is discretized into a bunch of interconnected Continuous Stirred Tank Reactors (CSTRs)—which best captures the geometry, heterogeneity, temporal assortation & varying flow conditions of the domain—in short, it performs as well as a distributed model, only with lesser parameters. The approach is often used by chemical engineers but is yet alien to hydrology. It’s a black-box with a simple GUI & a simple assembly process--just the solution of a bundle of 1^st order linear ODEs furnish us with a robust description of the processes going on within the system—for example, it can explain water balance issues in nested watersheds with layered heterogeneity.

We are now using the model to perform simple benchmark tests on the Kerbernez Site of South-Western French Brittany (which belongs to the observatory of research on environment AgrHys) to simulate the measured baseflow & stream nitrate concentration patterns at seasonal & inter-annual time scales.

Reference:

*Kirchner, James W., Xiahong Feng, and Colin Neal. "Fractal stream chemistry and its implications for contaminant transport in catchments." Nature 403.6769 (2000): 524.

How to cite: Bhaduri, B., Ruiz, L., Fovet, O., and Muddu, S.: Modelling the Water & Solute Transit Time Distributions in a Groundwater System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4008, https://doi.org/10.5194/egusphere-egu2020-4008, 2020.

The temporal variability of transit-time distributions (TTDs) and residence-time distributions (RTDs) in hydrological systems has received particular attention recently because of their ability to inform on elementary processes impacting geochemical signatures and water fluxes in ecosystems. To date, these distributions and their temporal variability have been mainly investigated through concentration measurements of conservative geochemical or isotopic tracers. Even though physically-based and distributed hydrological models can render interpretations of TTDs/RTDs in terms of processes and physical controls, the variability of TTDs and RTDs has barely been studied using distributed hydrological modeling. In this study, an integrated hydrological model has been coupled with particle tracking algorithms and applied to the Strengbach Catchment – a small mountainous catchment belonging to the French network of critical zone observatories – to investigate the eventual link between water storage in the catchment and the temporal variability of TTDs and RTDs. The model calibration is performed relying upon both classical streamflow measurements and magnetic resonance sounding, a geophysical measure sensible to the water content in the subsurface. The model is then run over a 10-year period for which time distributions are calculated at various deadlines. The results show that the response of the Strengbach catchment is uncommon with short mean transit times (approximately 150-200 days) and a weak variability of TTDs and RTDs with the water storage. This specific behavior is mainly linked to the small size of the system and specific climatic and topographic conditions. Because the hydrological model was calibrated on the basis of unusual data (local water contents inferred via MRS measurements), ongoing investigations target the evaluation of the sensitivity of transit time distributions with respect to uncertainties plaguing calibrating data.

How to cite: Weill, S., Lesparre, N., Jeannot, B., and Delay, F.: Combined integrated hydrological modeling and particle tracking algorithms to infer the temporal variability of transit and residence time distributions within the Strengbach catchment (Vosges mountains, France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6872, https://doi.org/10.5194/egusphere-egu2020-6872, 2020.

Numerical simulations of mean groundwater age are presented for a variety of complex flow systems including heterogeneous aquifers and discretely-fractured porous rock. We apply the finite element models FLONET/TR2 (in the 2D vertical plane) and SALTFLOW (in 3D systems), using the standard advection-dispersion equation with an age source term. The age simulations are applied in a variety of contexts including defining capture zones for pumping wells, characterizing fractured rock aquifers, and for improved understanding of flow systems and geochemical evolution. Applications include real field sites and hypothetical conceptual models. Comparisons are also made with advective particle-tracking derived ages which are much faster to compute but do not include dispersive age mixing. Control of numerical (age) dispersion is critical, especially within discrete fracture networks where high age gradients can develop between the fractures and matrix. The presentation will highlight the broad applications of mean groundwater age simulations and will show how they can be useful for providing insight into hydrogeological systems.

How to cite: Molson, J. and Frind, E.: Numerically Simulated Groundwater Age Distributions within Complex Flow Systems and Discrete Fracture Networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21254, https://doi.org/10.5194/egusphere-egu2020-21254, 2020.