Groundwater Recharge Processes Research Articles

Estimation of models for cumulative infiltration of soil using machine learning methods

ABSTRACT Knowledge of cumulative infiltration of soil is necessary for irrigation, surface flow, groundwater recharge and many other hydrological processes. In the present study, the Support Vector Machine (SVM), artificial neural network (ANN) and adaptive Neuro-fuzzy inference system (ANFIS) were employed to estimate the cumulative infiltration of soil. For this study, a data set containing 106 experimental observations were analyzed. Out of 106, 70 % of data was selected for preparing different algorithms whereas rest 30% data was selected to test the models. The models accuracy was depended upon the two performance evaluation parameter which is correlation coefficient (CC) and root mean square error (RMSE). Results of performance evaluation parameters suggest that triangular membership function-based ANFIS model works well than SVM and ANN models. While SVM and ANN models also give a good estimation performance. Sensitivity analysis concludes that the parameter, time (t) is the most influencing parameter for the modeling of cumulative infiltration of soil for this data set.

Rainfall isotope variations over the Australian continent – Implications for hydrology and isoscape applications

This paper presents a continental scale interpretation of δ2H and δ18O in Australian precipitation, incorporating historical GNIP data at seven sites (1962–2002) and 8–12 years of new monthly data from 15 sites from 2003 to 2014. The more than doubling of stations and the significant time series duration allow for an improved analysis of Australian precipitation isotopes. Local meteoric water lines were developed for each site, and for the Australian continent. When the annual precipitation weighted values were used, the Australian meteoric water line was δ2H = 8.3 δ18O + 14.1‰.Precipitation amount was found to be a stronger driver of precipitation isotopes than temperature at most sites, particularly those affected by tropical cyclones and the monsoon. Latitude, elevation and distance from the coast were found to be stronger drivers of spatial variability than temperature or rainfall amount.Annual isoscapes of δ2H, δ18O and deuterium excess were developed, providing an improved tool to estimate precipitation isotope inputs to hydrological systems. Because of the complex climate, weather and oceanic moisture sources affecting Australia, regional groupings were used instead of the climate zone approach and additional data was included to improve the coverage in data poor regions. Regression equations for the isoscape were derived using latitude, altitude and distance from the coast as predictor variables.We demonstrate how this isoscape can be used as a tool for interpreting groundwater recharge processes using examples from across Queensland and New South Wales, including the Murray Darling Basin. Groundwater isotopes at sites where direct local recharge occurs are similar to rainfall, but for inland sites, which are often arid or semi-arid, a disconnect between shallow groundwater and local rainfall is observed; the departure in deuterium excess for these sites increases with aridity and distance from the headwaters where flooding originates.

Assessment of groundwater recharge processes through karst vadose zone by cave percolation monitoring

AbstractRecharge processes of karst aquifers are difficult to assess given their strong heterogeneity and the poorly known effect of vadose zone on infiltration. However, recharge assessment is crucial for the evaluation of groundwater resources. Moreover, the vulnerability of karst aquifers depends on vadose zone behaviour because it is the place where most contamination takes place. In this work, an in situ experimental approach was performed to identify and quantify flow and storage processes occurring in karst vadose zone. Cave percolation monitoring and dye tracing were used to investigate unsaturated zone hydrological processes. Two flow components (diffuse and quick) were identified and, respectively, account for 66% and 34% of the recharge. Quickflow was found to be the result of bypass phenomenon in vadose zone related to water saturation. We identify the role of epikarst as a shunting area, most of the storage in the vadose zone occurring via the diffuse flow component in low permeability zones. Relationship between rainfall intensity and transit velocity was demonstrated, with 5 times higher velocities for the quick recharge mode than the diffuse mode. Modelling approach with KarstMod software allowed to simulate the hybrid recharge through vadose zone and shows promising chances to properly assess the recharge processes in karst aquifer based on simple physical models.

Multidisciplinary study for the assessment of the geometry, boundaries and preferential recharge zones of an overexploited aquifer in the Atacama Desert (Pampa del Tamarugal, Northern Chile)

Overexploited aquifers in drylands require a thorough assessment in order to get the best insight possible of their hydrogeological behavior and groundwater resource. With this aim we develop an integrated methodology, using the northern part of the Pampa del Tamarugal Aquifer (PTA), at the Atacama Desert, Northern Chile, as testing site. This multidisciplinary assessment was completed including 34 direct piezometric measurements in boreholes, 92 geophysical soundings (TDEM) and 9 stream gauging. Main results show strong correlation between a well-defined contrast of resistivity and the water-table of the unconfined aquifer (dry unsaturated zone ≥ 100 Ω.m vs saline groundwater in the saturated zone ≈ 10 Ω.m). TDEM results helped to map the piezometric levels and reassess the limits of the PTA. The eastern limit of the aquifer, delimited up to now by the basin floor, is repositioned from 10 to 50 km eastward in the Andean Piedmont, indicating larger groundwater reserves than those described previously. A high-resistivity anomaly (∼200 Ω.m) in the apex of alluvial fans reveals that frequent flood events cause rapid flows through the coarse permeable deposits leaching the vadose zone and contributing to recharge the aquifer (temporary recharge process). However, the updated piezometric map indicates the piedmont as the area where most of the aquifer recharge takes place. In the piedmont, at the downstream of tectonic structures or substratum outcrops, stream discharge changes reveal perennial river losses contributing to a located and permanent recharge of the PTA. This groundwater recharge process would be the main contribution to the aquifer recharge. By taking into account the regional geology, the results are summarized in a new conceptual model redefining the geometry, the spatial distribution of groundwater levels, the boundary conditions and the complex recharge processes (different flow magnitudes) in the northern part of the PTA. Finally, this study indicates an integrated and efficient methodology, providing original results that are useful for the characterization of other aquifers located in arid piedmonts and hillslopes.

Spatio-temporal variations in hydro-geochemistry of groundwater at rural, urban and industrial areas of Kanpur, India

Physicochemical characteristics of groundwater provide vital information regarding prevailing geochemical, geomorphological and predominant pollution conditions in an area. In the present work, hydro-geochemical properties of ground water were examined at rural, urban and industrial areas of Kanpur to determine its aptness for drinking and irrigation. Groundwater samples were randomly collected from deep borewell pumps across the city. The samples were examined for pH, electrical conductivity (EC), nitrate, sulphate, chloride, hardness, alkalinity, total dissolved solids, sodium, potassium, calcium, which ranged between 7.10 and 8.60, 140–3990 mS cm−1, 0.02–13.67, 4.67–285.37, 4–747.77, 108–1555.2, 32–532, 89.60–2550, 18–720, 2–19 and 5–290 mg L−1 in pre-monsoon season and 7.1–8.2, 400–5450 mS cm−1, 0.02–24.09, 7.62–258.21, 4–538.84, 122–750, 240–988, 256–3490, 29–730, 3–53 and 10–185 mg L−1 in post-monsoon season respectively. Water Quality Index of groundwater during post-monsoon were recorded to be better than the pre-monsoon season, reason being percolation of water during rainfall in monsoon. The pre and post-monsoon data provided valuable information regarding the suitability of groundwater for various purposes such as drinking and irrigation. Results showed ‘fairly’ and ‘good’ categories of water quality in the pre-monsoon season which further improved in post monsoon season. Results clearly specify that the natural process of groundwater recharge is an important mechanism to maintain the quality of groundwater.

Assessing groundwater storage in the Kairouan plain aquifer using a 3D lithology model (Central Tunisia)

The aquifer of the semi-arid Kairouan plain has been exploited for decades to supply the growing irrigated agriculture and the need of drinking water. In parallel, the major hydraulic works drastically changed the natural groundwater recharge processes. The continuous groundwater level drop observed since the 1970s naturally raises the question of groundwater storage sustainability. To date, hydrogeological studies focused on groundwater fluxes, but the total amount of groundwater stored in the aquifer system has never been fully estimated. This is the purpose of the present paper. A complete database of all available geological, hydrogeological and geophysical data was created to build a 3D lithology model. Then, the lithological units were combined with the hydraulic properties to estimate the groundwater storage. Over the 700 km2 of the modelled area, the estimated storage in 2013 was around 18 × 109 m3 (equivalent to 80 times the annual consumption of 2010) with a highly variable spatial distribution. In 45 years (1968–2013), 12% of the amount of groundwater stored in the aquifer has been depleted. According to these results, individual farms will face strong regional disparities for their access to groundwater in the near future.

Solution to Green–Ampt infiltration model using a two-step curve-fitting approach

Infiltration is an important hydrological process affecting the runoff, groundwater recharge and solute transport process. The Green–Ampt (GA) model, describing one-dimensional infiltration process, yields an implicit equation for estimation of cumulative infiltration with time. In this paper, an explicit approximation to the GA model is derived using two-step curve-fitting technique. The Marquardt’s algorithm is employed for least-squares estimation of nonlinear parameters. Performance of the proposed model is compared with the implicit GA model using numerical and published field experimental data. The quantitative statistical indicators namely, percent relative error (RE), maximum absolute percent relative error (MARE), percent bias (PB), and Nash–Sutcliffe modeling efficiency (E), are used to assess the performance of the proposed model. The solution of the proposed model matched very well with that of the implicit GA model with MARE ≤ 0.146%, PB ≤ 0.070%, and E value approximately equal to 1 for both the dimensionless numerical and dimensional field experimental infiltration estimates. Simulation results of the proposed model demonstrated the capability of the derived model in estimating infiltration rate and cumulative infiltration accurately and can be applied to solve variety of real-life hydrological problems.

Recharge processes and vertical transfer investigated through long-term monitoring of dissolved gases in shallow groundwater

We investigated temporal variations and vertical evolution of dissolved gaseous tracers (CFC-11, CFC-12, SF6, and noble gases), as well as 3H/3He ratio to determine groundwater recharge processes of a shallow unconfined, hard-rock aquifer in an agricultural catchment. We sampled dissolved gas concentration at 4 locations along the hillslope of a small experimental watershed, over 6 hydrological years, between 2 and 6 times per years, for a total of 20 field campaigns. We collected groundwater samples in the fluctuation zone and the permanently saturated zone using piezometers from 5 to 20 m deep. The purpose of this work is i) to assess the benefits of using gaseous tracers like CFCs and SF6 to study very young groundwater with flows suspected to be heterogeneous and variable in time, ii) to characterize the processes that control dissolved gas concentrations in groundwater during the recharge of the aquifer, and iii) to understand the evolution of recharge flow processes by repeated measurement campaigns, taking advantage of a long monitoring in a site devoted to recharge processes investigation.Gas tracer profiles are compared at different location of the catchment and for different hydrologic conditions. In addition, we compare results from CFCs and 3H/3He analysis to define the flow model that best explains tracer concentrations. Then we discuss the influence of recharge events on tracer concentrations and residence time and propose a temporal evolution of residence times for the unsaturated zone and the permanently saturated zone. These results are used to gain a better understanding of the conceptual model of the catchment and flow processes especially during recharge events.

Corroborating stable isotopic data with pumping test data to investigate recharge and groundwater flow processes in a fractured rock aquifer, Rivirivi Catchment, Malawi

Fractured rock aquifers continue to support millions of people in arid and semiarid regions. However, due to heterogeneous nature of their hydrogeology and comparative low yields, research in these formations is regarded expensive, and thus, very little is known about their dominant hydrological processes. In this study, we corroboratively used pumping test and stable isotopic data to understand groundwater recharge and flow processes in fractured hornblende-biotite-gneiss. It was established that rain is the main source of groundwater recharge. The groundwater system seems to retain the stable isotopic signatures of precipitation which has undergone evaporation before infiltration. Geochemical data also show that the groundwater system has not undergone pronounced geochemical evolution as evidenced by low total dissolved solids in both seasons. A combination of stable isotopic data and derivative plots of drawdown data helped identification of apparent recharge zones, no-flow boundaries and dominant flow types in the pumped wells with bilinear flow being the dominant type of flow in BP1 and BP2, while linear flow was dominant in BP3 and BP4. Geochemical, stable isotopic and hydraulic data further revealed hydraulic connection between two wells juxtaposed across Ntcheu Fault, indicating potential for groundwater flow across the fault and highlighting that the fault may be acting as a groundwater conduit across it. The established phenomena are important when considering groundwater development and sustainable management of the resource.

Impact of urbanization on groundwater recharge and urban water balance for the city of Hyderabad, India

Groundwater recharge processes in an urban area are different than in non-urban areas. There are various new components that must be considered in the case of urban groundwater recharge in addition to the natural recharge from precipitation. These various components are estimated in this study based on the water balance equation for the Hyderabad city of India. Initially, urban recharge components including leakages from water supply network and sewage networks were calculated. To estimate the natural recharge from precipitation, actual evapotranspiration and surface runoff were estimated using remote sensing and GIS techniques. Results indicated that the urban recharge component of groundwater was more than ten times greater than the natural recharge. The net urban recharge component of groundwater was estimated to be approximately 568mm yr-1 and the natural recharge component was observed to be 53mm yr-1. Water inflow and outflow components were also estimated to provide the complete scenario of the total urban water balance of Hyderabad. This analysis has provided the information regarding the extent and intensity of percolation of urban contaminants into the aquifer.

Application of environmental tracers to study the drainage system of the unsaturated zone of the Ljubljansko polje aquifer

The groundwater recharge processes were investigated in the unsaturated zone of the Ljubljansko polje intergranular aquifer. The study based on application of environmental tracers. The monitoring of the drainage system was established at an urban lysimeter consisting of artificial and autochthonous sediment layers. Groundwater was sampled with the suction cups instaled at depths of 0.3, 0.6, 1.2, 1.8, 3.0 and 4.0 m. The results pointed out the most important factors that control the hydrodynamic processes and solute transport in the study site where the soil cover and Vegetation are missing. The physico-chemical and isotopic properties of sampled groundwater indicated that the clayey layers have an important role in the hydraulic behaviour of the unsaturated zone. As a consequence of a piston effect the event and pre-event water concentrate above these layers causing lateral flow. A vertical breakthrough of this water into lower layers of the unsaturated zone could occur through preferential paths during intensive precipitation events in dependence on pre-accumulated water volumes. Under such conditions groundwater residence time is about 2 months in the unsaturated zone 3 m below the surface.

Kinematic diffusion approach to describe recharge phenomena in unsaturated fractured chalk

Abstract When dealing with groundwater resources, a better knowledge of the hydrological processes governing flow in the unsaturated zone would improve the assessment of the natural aquifer recharge and its vulnerability to contamination. In North West Europe groundwater from unconfined chalk aquifers constitutes a major water resource, therefore the need for a good hydrological understanding of the chalk unsaturated zone is essential, as it is the main control for aquifer recharge. In the North Paris Basin, much of the recharge must pass through a regional chalk bed that is composed of a porous matrix with embedded fractures. The case study regards the role of the thick unsaturated zone of the Cretaceous chalk aquifer in Picardy (North of France) that controls the hydraulic response to rainfall. In order to describe the flow rate that reaches the water table, the kinematic diffusion theory has been applied that treats the unsaturated water flow equation as a wave equation composed of diffusive and gravitational components. The kinematic diffusion model has proved to be a convenient method to study groundwater recharge processes in that it was able to provide a satisfactory fitting both for rising and falling periods of water table fluctuation. It has also proved to give an answer to the question whether unsaturated flow can be described using the theory of kinematic waves. The answer to the question depends principally on the status of soil moisture. For higher values of hydraulic Peclet number (increasing saturation), the pressure wave velocities dominate and the preferential flow paths is provided by the shallow fractures in the vadose zone. With decreasing values of hydraulic Peclet number (increasing water tension), rapid wave velocities are mostly due to the diffusion of the flow wave. Diffusive phenomena are provided by matrix and fracture-matrix interaction. The use of a kinematic wave in this context constitutes a good simplified approach especially in cases when there is a lack of information concerning the hydraulic properties of the fractures/macropores close to saturation.

Stable isotopes of soil water: Implications for soil water and shallow groundwater recharge in hill and gully regions of the Loess Plateau, China

There is relatively abundant groundwater in some hill and gully regions of the Loess Plateau aquifer. The region has an arid to semiarid climate with low precipitation and high potential evaporation, and thus, groundwater recharge processes remain unclear. This study aimed to investigate recharge mechanisms and soil water movement in the Loess Plateau aquifer and to assess the recharge to shallow loess aquifers using soil profiles. In general, we found that the δ18O and δ2H values of soil water varied both with season and with soil profile depth. The seasonal stable isotopic values in the soil profiles also differed between wet and dry years. In particular, soil water and groundwater isotopic values responded to precipitation. This response suggests that seasonal precipitation is the main source of recharge to soil water and groundwater and that soil water is involved in groundwater recharge. The precipitation infiltration, soil water replenishment, and groundwater recharge are complicated processes. Two main modes of infiltration allow water to move downward through the unsaturated zone and recharge groundwater can be concluded. The primary mode is preferential infiltration through microtopographic features. This channel-like infiltration can rapidly move precipitation downward to deep soil layers, greatly reducing evaporation even under the arid to semiarid conditions of the study area. The secondary mode is a combination of matrix flow, which includes inside preferential infiltration though finger-like flow in the upper layer and piston-like flow in the deeper layer. The upper finger-like flow can cause rapid infiltration to below the root zone during less intense rain or snowmelt. The annual precipitation through preferential flow is the dominant recharge mode for the shallow groundwater in the loess aquifers. These two modes of recharge maintain perennial groundwater in the arid to semiarid hill and gully regions of the Loess Plateau.

Ecohydrological transformation in the Dry Chaco and the risk of dryland salinity: Following Australia's footsteps?

AbstractDuring the last century, the massive conversion of Australian dry forests to crops and pastures triggered the massive soil and groundwater degradation process known as dryland salinity. Currently, South American Chaco's dry forests are undergoing a similar transformation, leading global deforestation rates. The goal of this study was to review existing ecohydrological information about natural and cultivated systems in the Chaco to assess the dryland salinity risks. We review deep soil water, salt stocks, and groundwater recharge from agriculture or native dry forests stands located in a precipitation range of 450–1100 mm. We complement this with water table level records and geoelectric profiles together with personal observations. We use data from 15 Australian studies for comparison. Strong salt leaching, especially after 20 years of forest clearance, indicates the onset of deep drainage following forest conversion to agriculture in the Dry Chaco. Water stocks were more than double in the cleared stands compared to their dry forest pairs, and recharge rates were up to two order magnitude higher in agricultural areas. Although lower atmospheric salt deposition, younger sediments, and relatively high water‐consuming agricultural systems in the Dry Chaco attenuate salinization risks compared to Australia, the very flat topography and related shallow water table levels of the South American region could make groundwater recharge and salt mobilization processes more widespread and difficult to manage. The lack of awareness among the general public, farmers, and decision makers about this issue amplifies the problem, making land management plans for the Argentine dry forest territories essential.

Hydrogeochemical and isotopic features of the groundwater flow systems in the central-northern part of Jeju Island (Republic of Korea)

Groundwater from 10 production wells located in lowlands to highlands was investigated from July 2008 to February 2010 for chemical and isotopic compositions (δ18O, δ2H, and δ13CDIC) to understand groundwater recharge processes and to assess geochemical conditions of groundwater in the volcanic aquifers. The groundwater in lowlands had elevated concentrations of Cl and NO3 with greater seasonality when compared with those from higher areas, indicating that groundwater in the lowlands is affected by contamination. Based on δ18O values and altitudinal variations, groundwater was classified into two regional flow systems with different flow paths. The regional flow systems are likely connected from highland to lowland areas, considering consistent δ18O values and major ion contents throughout the seasons. Of the two regional flow systems, the wells clustered in one regional flow system showed low δ13CDIC values (average −14.3‰) and high PCO2 (average 2.04 in -logPCO2), reflecting relatively high contribution of soil CO2 to groundwater. The other regional flow system with low PCO2 (average 2.39 in -logPCO2), however, consisted of two highland wells with enriched δ13CDIC value, −9.3‰ and two lowland wells with relatively depleted δ13CDIC value, −13.7‰. These findings indicate that the two regional flow systems were mixed with local recharge from soil layers during recharge. The δ18O and δ2H values in groundwater from the regional flow systems were not clearly consistent with the local meteoric water line due to mixing of summer and dry season precipitation. The isotopic variations showed a phase lag and cyclic temporal patterns, suggesting that a considerable portion of groundwater was derived from fast recharge with short residence times on monthly time scales. This study suggests that groundwater in lowland areas in Jeju Island can be significantly contributed from highland areas, which was revealed by chemical and isotopic compositions, especially, altitude effect and seasonal variation of δ18O value. We expect that the knowledge on the volcanic aquifer system can be useful for management of groundwater resources including projects (e.g. artificial recharge technology) which has been carried out to mitigate water poverty over the world.

Spatiotemporal variation of the surface water effect on the groundwater recharge in a low-precipitation region: Application of the multi-tracer approach to the Taihang Mountains, North China

Groundwater recharge variations in time and space are crucial for effective water management, especially in low-precipitation regions. To determine comprehensive groundwater recharge processes in a catchment with large seasonal hydrological variations, intensive field surveys were conducted in the Wangkuai Reservoir watershed located in the Taihang Mountains, North China, during three different times of the year: beginning of the rainy season (June 2011), mid-rainy season (August 2012), and dry season (November 2012). Oxygen and hydrogen isotope and chemical analyses were conducted on the groundwater, spring water, stream water, and reservoir water of the Wangkuai Reservoir watershed. The results were processed using endmember mixing analysis to determine the amount of contribution of the groundwater recharging processes. Similar isotopic and chemical signatures between the surface water and groundwater in the target area indicate that the surface water in the mountain–plain transitional area and the Wangkuai Reservoir are the principal groundwater recharge sources, which result from the highly permeable geological structure of the target area and perennial large-scale surface water, respectively. Additionally, the widespread and significant effect of the diffuse groundwater recharge on the Wangkuai Reservoir was confirmed with the deuterium (d) excess indicator and the high contribution throughout the year, calculated using endmember mixing analysis. Conversely, the contribution of the stream water to the groundwater recharge in the mountain–plain transitional area clearly decreases from the beginning of the rainy season to the mid-rainy season, whereas that of the precipitation increases. This suggests that the main groundwater recharge source shifts from stream water to episodic/continuous heavy precipitation in the mid-rainy season. In other words, the surface water and precipitation commonly affect the groundwater recharge in the rainy season, whereas the reservoir and stream water play important roles in the groundwater recharge in the low-precipitation period. The results should contribute not only to the understanding of the mountain hydrology but also to groundwater resource management in the North China Plain.

Effects of afforestation on groundwater recharge and water budgets in the western region of Uruguay

AbstractUruguay has stimulated the development of its forest sector since the promulgation of Forest Law N° 15 939 in December of 1987. Nevertheless, the substitution of natural grasslands with forest plantations for industrial use has raised concerns regarding hydrological processes of groundwater recharge and water consumption involving evapotranspiration. The purpose of this study is to assess the effects of this substitution approach on water resources. Input data were collected from two small experimental watersheds of roughly 100–200 hectares located in western Uruguay. The watersheds are characterized by Eucalyptus Globulus ssp. Maidenni and natural grasslands for cattle use. Total rainfall, stream discharge, rainfall redistribution, soil water content and groundwater level data were collected. Groundwater recharge was estimated from water table fluctuations and from groundwater contributions to base flows. Seasonal and annual water budgets were computed from October of 2006 to September of 2014 to evaluate changes in the hydrological processes. The data show a decrease in annual specific discharge of roughly 17% for mean hydrological years and no conclusive effects on annual groundwater recharge in the forested watershed relative to the reference pasture watershed. Reduced annual specific discharge is equivalent to the mean annual interception. The computed actual annual evapotranspiration is consistent with international catchment measurements. Reduction rates vary seasonally and according to accumulated rainfall and its temporary distribution. The degree of specific discharge decline is particularly high for drier autumns and winters (32 to 28%) when the corresponding rainfall varies from 275 to 400 mm. These results are of relevance for water resources management efforts, as water uses downstream can be affected. These findings, based on a study period dominated by anomalous wet springs and summers and by dry autumns and winters, oppose earlier results based on 34 years of rainfall and discharge data drawn from Uruguayan large basins. Copyright © 2016 John Wiley & Sons, Ltd.

Une approche multi-traceurs pour la détermination de la dynamique des eaux souterraines du bassin de la rivière Actopan, Etat de Veracruz, Mexique

Geochemistry and environmental tracers were used to understand groundwater resources, recharge processes, and potential sources of contamination in the Rio Actopan Basin, Veracruz State, Mexico. Total dissolved solids are lower in wells and springs located in the basin uplands compared with those closer to the coast, likely associated with rock/water interaction. Geochemical results also indicate some saltwater intrusion near the coast and increased nitrate near urban centers. Stable isotopes show that precipitation is the source of recharge to the groundwater system. Interestingly, some high-elevation springs are more isotopically enriched than average annual precipitation at higher elevations, indicating preferential recharge during the drier but cooler winter months when evapotranspiration is reduced. In contrast, groundwater below 1,200 m elevation is more isotopically depleted than average precipitation, indicating recharge occurring at much higher elevation than the sampling site. Relatively cool recharge temperatures, derived from noble gas measurements at four sites (11–20 °C), also suggest higher elevation recharge. Environmental tracers indicate that groundwater residence time in the basin ranges from 12,000 years to modern. While this large range shows varying groundwater flowpaths and travel times, ages using different tracer methods (14C, 3H/3He, CFCs) were generally consistent. Comparing multiple tracers such as CFC-12 with CFC-113 indicates piston-flow to some discharge points, yet binary mixing of young and older groundwater at other points. In summary, groundwater within the Rio Actopan Basin watershed is relatively young (Holocene) and the majority of recharge occurs in the basin uplands and moves towards the coast.

Evolution of chemical and isotopic composition of inorganic carbon in a complex semi-arid zone environment: Consequences for groundwater dating using radiocarbon

Estimating groundwater age is important for any groundwater resource assessment and radiocarbon (14C) dating of dissolved inorganic carbon (DIC) can provide this information. In semi-arid zone (i.e. water-limited environments), there are a multitude of reasons why 14C dating of groundwater and traditional correction models may not be directly transferable. Some include; (1) the complex hydrological responses of these systems that lead to a mixture of different ages in the aquifer(s), (2) the varied sources, origins and ages of organic matter in the unsaturated zone and (3) high evaporation rates. These all influence the evolution of DIC and are not easily accounted for in traditional correction models. In this study, we determined carbon isotope data for; DIC in water, carbonate minerals in the sediments, sediment organic matter, soil gas CO2 from the unsaturated zone, and vegetation samples. The samples were collected after an extended drought, and again after a flood event, to capture the evolution of DIC after varying hydrological regimes. A graphical method (Han et al., 2012) was applied for interpretation of the carbon geochemical and isotopic data. Simple forward mass-balance modelling was carried out on key geochemical processes involving carbon and agreed well with observed data. High values of DIC and δ13CDIC, and low 14CDIC could not be explained by a simple carbonate mineral–CO2 gas dissolution process. Instead it is suggested that during extended drought, water–sediment interaction leads to ion exchange processes within the top ∼10–20m of the aquifer which promotes greater calcite dissolution in saline groundwater. This process was found to contribute more than half of the DIC, which is from a mostly ‘dead’ carbon source. DIC is also influenced by carbon exchange between DIC in water and carbonate minerals found in the top 2m of the unsaturated zone. This process occurs because of repeated dissolution/precipitation of carbonate that is dependent on the water salinity driven by drought and periodic flooding conditions. This study shows that although 14C cannot be directly applied as a dating tool in some circumstances, carbon geochemical/isotopic data can be useful in hydrological investigations related to identifying groundwater sources, mixing relations, recharge processes, geochemical evolution, and interaction with surface water.

The ecohydrological imprint of deforestation in the semiarid Chaco: insights from the last forest remnants of a highly cultivated landscape

AbstractThe semiarid Chaco plains present one of the highest rates of forest clearing and agricultural expansion of the world. In other semiarid plains, such massive vegetation replacements initiated a groundwater recharge and salt mobilization process that, after decades, raised regional water tables and salts to the surface, degrading agricultural and natural ecosystems. Indirect evidence suggests that this process (known as dryland salinity) began in the Chaco plains. Multiple approaches (deep soil profiles, geoelectric surveys and monitoring of groundwater salinity, level and isotopic composition) were combined to assess the dryland salinity status in one of the oldest and most active agricultural hotspots of the region, where isolated forest remnants occupy an extremely flat cultivated matrix. Full vadose moisture and chloride profiles from paired agriculture‐forest stands (17 profiles, six sites) revealed the following: a generalized onset of deep drainage with cultivation (32 to >87 mm year−1), full leaching of native chloride pools (13.7 ± 2.5 kg m−2) down to the water table after >40 years following clearing and differential groundwater table rises (0.7 to 2 m shallower water tables under agriculture than under neighbouring forests). Continuous level monitoring showed abrupt water table rises under annual crops (up to 2.6 m in 15 days) not seen under forests or pastures. Varying deep drainage rates and groundwater isotopic composition under agricultural plots suggest that these pulses are strongly modulated by crop choices and sequences. In contrast to other dryland salinity‐affected areas of the world, forest remnants in the study area (10–20% of the area) are not only surviving the observed hydrological shifts but also sustaining active salty groundwater transpirative discharge, as evidenced by continuous water table records. The overall impact of these forest remnants on lowering neighbouring water tables would be limited by the low hydraulic conductivity of the sediments. As highly cultivated areas of the Chaco evolve to new hydrological conditions of shallower saline water tables, innovative crop rotations that minimize recharge, enhance transpirative discharge and tolerate salinity will be needed. Copyright © 2016 John Wiley & Sons, Ltd.