Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial aquifers of southwest Punjab

Assessment of hydraulic connectivity among shallow and deep aquifers and surface water of Central Gujarat: An isotopic approach

The primary objective of this study is to identify the surface water - groundwater interaction and interaction among groundwater at different depths using stable isotopes (δ18O and δ2H). The Fence diagram of the study area describes the subsurface geology where the main aquifer system includes both confined and unconfined in the VRB area and is complex multi-layered with different sources of recharge. The flow pattern of groundwater is generally from the highland of north-east to low-lying south-west of VRB and is indicated by the piezometric surface map of pre-monsoon, and post monsoon, 2016. The δ2H (‰ VSMOW) and δ18O (‰ VSMOW) values do not show any symmetric distribution among them. The values for shallow groundwater ranges between -23.0‰ and 1.9‰, and -3.2‰-2.3‰, respectively with a mean value for δ18O and δ2H are -1.26‰ and -11.194‰ respectively. The depth wise interpretation revealed that the locations Virpur and Nana Pura show connectivity. Therefore, deep and shallow aquifers are contaminated due to this connectivity. The plots of d-excess vs. electrical conductivity, d-excess vs δ18O and electrical conductivity vs. δ18O indicate that most of the groundwaters are primarily influenced by evaporation process, high mineral dissolution and enrichment of δ18O. The plot of δ18O vs. nitrate indicate the anthropogenic influences like agricultural activities in this region. The SI values of calcite and dolomite indicates oversaturated condition along with dissolution of these minerals in the aquifer matrix.

Integrated application of a Bayesian mixing model, numerical model, and environmental tracers to characterize groundwater recharge sources in a mountainous area

In this study, the combined use of a Bayesian mixing model (BMM), numerical model (random walk particle tracking-RWPT), and environmental tracers (δ¹⁸O-δD, ³H, and CFC) was applied to elucidate the probabilistic contribution of the recharge sources, flow path, and residence time of groundwater across the mountainous area of Jeju Island, South Korea. Especially, the BMM ability to estimate the variable recharge contributions to the aquifer by different elevations and seasons was investigated. The δ¹⁸O-δD isotopes showed that groundwater in the study area was primarily fed by precipitation during the wet season, and the BMM estimated that wet season recharge contributed to approximately 64% of the total. The BMM-based probabilistic estimation of recharge sources revealed a mixed contribution of source waters from different elevations. A notable difference in recharge flow path was observed between highland (>450 masl) and lowland (<400 masl) wells, where the inflow of source water from the regional flow was dominant in the former and both regional and local recharges served as significant groundwater sources in the latter. Evidence from age tracers (³H and CFC-12) also supported different recharge mechanisms between highland and lowland wells. A reasonable match between the BMM- and RWPT-derived recharge contributions (RMSE 0.02-0.06) was achieved within the uncertainty ranges, with RWPT being particularly useful for capturing different flow paths between highland and lowland wells. The dynamics revealed here provide important information for establishing an improved and informed groundwater management plan for the mountainous area of Jeju Island. Ultimately, this study highlights the advantageous integrated analysis of BMM, RWPT, and environmental tracer analyses to enhance the reliability of recharge area estimation and increase the collective understanding of complex hydrogeological systems in mountainous areas.

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

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

Appraising the factors favouring uranium mobilization and associated health risk assessment in groundwaters of north-western India

An attempt has been made in this study to evaluate the factors favoring the uranium mobilization into the groundwater of Northwest India using uranium isotope activity ratio (²³⁴U/²³⁸U), radon (²²²Rn) and environmental isotopes of water (²H, ¹⁸O and ³H). The values range from 23 - 597 µg/L for total uranium and 634-3210 Bq/m³ for radon and the corresponding annual effective dose is estimated to be 18.9-490 µSv/a and 6.2-31.5 μSv/a respectively. Uranium activity ratio (UAR) varies from 0.68 - 1.17 and maximum samples indicate secular equilibrium. Environmental isotopic data indicates that the source to groundwater is vertical percolation of rainwater in the case of shallow zone while regional flows from outcrop areas recharge the deep groundwater. A wide scatter is noticed in environmental ³H content (0.23-6.62 TU) indicating both fast and sluggish water flows. The UAR phase diagram suggests that leaching process controls the uranium mobilization into the groundwater. The correlations among UAR, uranium and U_excess further indicate oxidative nature of leaching process. Statistical treatment of the obtained data along with available geochemical and isotope evidences suggest that source of uranium is common but the driving processes are different for shallow and deep zone. Influences of root zone CO₂, oxic species from irrigation return flows and water level fluctuations are also evaluated. Low uranium, low UAR, low ³H and high ²²²Rn activity in deep zone suggest uranium being released from the roll front as well as transported from outcrop regions. This study highlights the application of uranium isotope ratio, radon and environmental isotopes in assessing vulnerability of alluvial aquifers towards uranium contamination.

Global Isotopic Hydrograph Separation Research History and Trends: A Text Mining and Bibliometric Analysis Study

Scientific research into isotope hydrograph separation (IHS) has rapidly increased in recent years. However, there is a lack of systematic and quantitative research to explore how this field has evolved over time. In this study, the methods of text mining and bibliometric analysis were combined to address this shortcoming. The results showed that there were clear periodical characteristics in IHS studies between 1986 and 2019. High-frequency words, e.g., catchment, stable isotope, runoff, groundwater, precipitation, runoff generation, and soil, were the basic topics in IHS studies. Forest and glacier/snow were the main landscapes in this research field. ‘Variation’, ‘spatial’, and ‘uncertainty’ are hot issues for future research. Today, studies involving the geographical source, flow path, and transit/residence time of streamflow components have enhanced our understanding of the hydrological processes by using hydrometeorological measurements, water chemistry, and stable isotope approaches. In the future, new methods, such as path analysis and ensemble hydrograph separation, should be verified and used in more regions, especially in remote and mountainous areas. Additionally, the understanding of the role of surface water in streamflow components remains limited and should be deeply studied in the future.

Spatial Analysis of Groundwater Hydrochemistry through Integrated Multivariate Analysis: A Case Study in the Urbanized Langat Basin, Malaysia

Rapid urbanization and industrial development in the Langat Basin has disturbed the groundwater quality. The populations' reliance on groundwater sources may induce possible risks to human health such as cancer and endocrine dysfunction. This study aims to determine the groundwater quality of an urbanized basin through 24 studied hydrochemical parameters from 45 groundwater samples obtained from 15 different sampling stations by employing integrated multivariate analysis. The abundance of the major ions was in the following order: bicarbonate (HCO₃^-) > chloride (Cl^-) > sodium (Na⁺) > sulphate (SO₄^2-) > calcium (Ca²⁺) > potassium (K⁺) > magnesium (Mg²⁺). Heavy metal dominance was in the following order: Fe > Mn > Zn > As > Hg > Pb > Ni > Cu > Cd > Se > Sr. Classification of the groundwater facies indicated that the studied groundwater belongs to the Na-Cl with saline water type and Na-HCO₃ with mix water type characteristics. The saline water type characteristics are derived from agricultural activities, while the mixed water types occur from water-rock interaction. Multivariate analysis performance suggests that industrial, agricultural, and weathering activities have contributed to groundwater contamination. The study will help in the understanding of the groundwater quality issue and serve as a reference for other basins with similar characteristics.

Use of multiple isotopes to evaluate nitrate dynamics in groundwater under the barrier effect of underground cutoff walls

Underground cutoff walls are useful in conserving groundwater and preventing seawater intrusion in coastal regions. However, the environmental effects of human activities on groundwater quality in the upstream area of the underground cutoff wall over the long term are not clear. In this study, combined analysis of multiple isotopes (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ²H, and δ¹⁸O-H₂O) and nitrate concentrations was used to assess the effect of underground cutoff walls on nitrogen dynamics in groundwater in an agricultural area in China. We sampled groundwater wells in the upstream and downstream areas of the underground walls in April, July, and September. The results indicated that the underground cutoff walls hampered the horizontal groundwater flow, making the upstream groundwater a closed system, which led to an increase in the nitrate concentration and accelerated nitrification processes. Manure was the main nitrate source in the upstream groundwater, and its levels in the groundwater were similar during the three seasons, indicating that there was no difference in the nitrate sources in the upstream groundwater among the three seasons. Hence, further management measures for manure application may be critical for groundwater protection in the upstream area of underground cutoff walls.

Distribution and correlation of radon and uranium and associated hydrogeochemical processes in alluvial aquifers of northwest India

The spatial and vertical distributions of radon and uranium are evaluated in relation to the hydrogeology, geomorphology, and hydrochemistry of southwest Punjab. Radon activity of the groundwater ranges from 580 to 3633 Bq/m³ (shallow groundwater 580 to 2438 Bq/m³ and deep groundwater 964 to 3633 Bq/m³), and uranium concentration varies from 24.4 to 253 μg/L (shallow groundwater 24.4 to 253 μg/L and deep groundwater 27.6 to 76.3 μg/L). Shallow groundwater shows higher U concentration compared with deeper ones, which can be attributed to the presence of dissolved oxygen (DO) and NO₃^- as oxidants and HCO₃^- as stabilizing agent in shallow zone. Unlike uranium, the radon activities were found to be similar in both shallow and deep groundwater. Rn_excess over secular equilibrium was used to confirm the possibility of additional sources of radon, such as secondary minerals present in the subsurface. Surface manifestations show significant influence on radon and uranium distributions in the shallow zone but not in deep zone due to limited hydraulic connectivity. Depth profiles and correlations of radon and uranium with trace elements and hydrochemical parameters indicate that groundwater exhibits different redox characteristics in shallow (younger and oxidizing) and deep zones (older and reducing). The present study provides critical information that can be helpful for planning sustainable groundwater development in this region and other similar regions without contaminating the relatively safer deep aquifers.

Use of multiple isotopes to evaluate the impact of mariculture on nutrient dynamics in coastal groundwater

Nitrogen and phosphate dynamics in groundwater and surface waters (aquaculture ponds and effluents and drainage channels) in the two seasons (April and September 2015) were investigated in a reclaimed coastal region in China. Multiple isotopes (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ²H-H₂O, and δ¹⁸O-H₂O) associated with the concentrations of dissolved inorganic nitrogen and phosphate were analyzed to assess the environmental impact of mariculture on coastal waters. Low phosphate concentration in the effluents was released from aquaculture ponds. Nitrate accounted for a larger proportion of dissolved inorganic nitrogen in September in aquaculture ponds and effluents and drainage channels. The groundwater in the mariculture zone was enriched with dissolved nitrogen components. In the offshore direction, seawater and precipitation were supplement of the groundwater, with nitrate deriving from mixing aquaculture effluents and wastewater. Denitrification occurred in groundwater in September 2015.

Integration of Isotopic (2H and 18O) and Geophysical Applications to Define a Groundwater Conceptual Model in Semiarid Regions

One-third of the global population depends on groundwater for drinking, which is an even larger proportion for arid regions. The integration of isotopic and geophysical applications has been very useful in understanding the process of groundwater recharge. The aim of this study is to define a conceptual model that describes groundwater functions within an aquifer located in a semi-arid region by identifying recharge patterns based on the isotopic characteristics of: Rainfall, surface water, shallow and deep groundwater, and incorporating regional geophysical data. We demonstrated that rainfall was affected by sub-cloud evaporation and altitude. Shallow and deep modern groundwater samples were clustered and exhibited similar evolution from rainfall. However, different groups recharged from different precipitation sources compared to the local one. In the current study, we analyzed the isotopic evolution of deep groundwater over a 10-year period, which was mainly affected by the incorporation of different flows with different isotopic signatures and the hydrodynamics of the area. We performed two geoelectrical sections in the study area to improve the understanding of the hydrogeological setting and water movement patterns. The new conceptual model should help stakeholders in the context of water management policies for the study area.

A study on the role of hydrogeology on the distribution of uranium in alluvial aquifers of northwest India

A study was undertaken to decipher the uranium distribution in relation to a number of hydrogeological factors in groundwater of southwest Punjab. Existing geological information for the region suggests that the shallow alluvial aquifer extends up to 50-70 m below ground level (bgl) and is in turn underlain by a deeper aquifer which extends to a depth of 250 m bgl. The presence of clayey units limits the vertical mixing of groundwater between the shallow and deep aquifers. Water level data (averaged over 5 years period) indicates that the south and southwestern regions of the study area have shallow water levels (3-5 m bgl) while the north and northeast regions have deep water levels (20-28 m bgl). This difference in water levels is found to be increasing with time. Higher concentrations of uranium occur in the central, southern, and southwestern parts of the study area where the water table occurs at shallow depth. Groundwater in the northern and northeastern parts of the study area shows U concentration within permissible levels for potable use (< 30 μg/L) while the highest concentration of U (341 μg/L) was found in the central part of the study area. Seasonal variation in dissolved U concentration is found to be statistically significant. The observed increases in U concentrations during the post-monsoon season are due to the addition of bicarbonate from the root zone as well as increased dissolved oxygen, nitrate, and sulphate concentration (oxic condition) in the groundwater while the decrease in U concentration is attributed to quick recharge by precipitation through sand dunes and contribution of surface water. Deeper groundwater does not show much seasonal variation in dissolved U concentration. Correlation between U and other hydrochemical parameters was evaluated. Cluster analysis of the data also indicates the oxidative mobilization of U from the sediments. Based on the lithological, hydrogeological, and dissolved U data, a schematic map is prepared depicting the various factors affecting the U distribution in alluvial aquifers, which can also be applied to other regions of similar hydrogeological setup. Graphical abstract ᅟ.