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

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

Fate and transport of strontium in groundwater from a layered sedimentary aquifer system.

In this article, strontium distribution in sedimentary coastal aquifers of Eastern India was studied and its association with groundwater particles has been ascertained using hydrochemical and morphological tools. Groundwater contains Sr2+ in the range of 0.08-4.0 mg/L with higher concentrations in Cretaceous formation. The particle number in groundwater varies from 4.5 × 105 to 3.3 × 106 per liter and follows the power law distribution with respect to the particle diameter. The calculated ß values (2.54 and 4.03) signify the abundance of smaller particles over larger ones. The particle concentration of size range 0.45-8 µm is found to be 0.64-2.6 mg/L. Elemental data of groundwater particles clearly suggest their origin from the host rock minerals. Zeta potential data indicates diverse nature of colloids suggesting prevalence of both positive and negative charged species in the groundwater. The hydrochemical interpretation along with speciation studies infers that high Sr2+ in groundwater is a result of incongruent dissolution of carbonate minerals and the dissolved Sr2+ partitions into both dissociated and un-dissociated forms. Based on the Sr2+/Ca2+ ratio and mineral saturation indices, it can be inferred that the Sr2+ is preferentially associated with colloids over large particles and the migration takes place through sorption of Sr2+ onto clay-bound (extrinsic) colloids in groundwater. This study describes the mechanism of strontium release into groundwater and provides insights into the role of groundwater particles in controlling the strontium migration to deep aquifers.

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.

Arsenic distribution in sediments of multi-tier sedimentary formation of coastal Pondicherry, India - Implications on groundwater quality.

In this work, sediments collected from a 150-m deep litho-section of a coastal region and encompassing Quaternary, Tertiary and Cretaceous sedimentary formations were studied for arsenic distribution and other trace and redox sensitive elements. Arsenic concentration in the sediments is found to vary from <0.5 to 30 mg·kg-1 and showed an increasing trend with the depth. The highest As content is observed at a depth of 129-131 m bgl belonging to Cretaceous formation. Though XRD studies do not indicate any arsenic mineral in sediments, positive correlations between As-Fe and S, as well as ESEM-EDS and TXRF studies confirm the presence of As sorbed to FeS mineral. The organic matter and As correlations suggest microbial mediated reduction process, which may pose future risk to water quality of this prolific coastal fresh water aquifer system. The elemental profiles infer that Cretaceous and Tertiary formations demonstrate inner shelf and marginal marine condition respectively.

Isotope and hydrochemical systematics of groundwater from a multi-tiered aquifer in the central parts of Indo-Gangetic Plains, India - Implications for groundwater sustainability and security.

The Indo-Gangetic multi-aquifer system provides water supplies to the most populous regions of the Indian subcontinent, however precise knowledge on the sources and dynamics of groundwater is still missing. Environmental isotopes (2H, 18O, 13C, 3H and 14C) and hydrochemical modeling tools were used in this study in the multi-tiered aquifers underlying the Middle Gangetic Plains (MGP) to investigate the source of recharge, aquifer dynamics and inter-connectivity among aquifers. Within a depth span of 300 m, three aquifers, with contrasting recharge sources and dynamics, were delineated in this Sone-Ganga-Punpun interfluve region, with limited cross-aquifer hydraulic interconnections. The chemistry evolves from Ca-HCO3 to Na-Ca-HCO3 in the shallow semiconfined Aquifer-I with a mean transit time of 20-23 years. The dominant recharge to Aquifer-I is from the river inflows and rainwater percolation through paleochannels. The semi-confined to confined Aquifer-II holds fresh quality groundwater with mixed water facies (Mg/Ca-Na-HCO3). The modeled age of Aquifer-II groundwater is found to be 205-520 years, which is supported by presence of negligible tritium and minor variations in stable isotopes. Outcrop regions of Aquifer-II sediments in the marginal alluvial areas and deep-seated paleochannels in the southwestern part are the potential zones for Aquifer-II recharge. A deep confined Aquifer-III with fresh quality of groundwater is identified below 220 m. This aquifer is characterized by old age (~3.5 to 4.7 ka BP) and enriched δ18O (-5.7‰). These results along with the existing paleoclimate records of this region infer that Aquifer-III is recharged during an arid climate. The marginal alluvial plains are the probable recharge zones for Aquifer-III. This study helped in conceptualizing the groundwater flow paths in multi-tiered aquifers of MGP. The knowledge and understanding would extend crucial inputs for the sustainable development of deep aquifers not only in the MGP but also in other regions of Indo-Gangetic Plains.

Fluoride Geochemistry and Exposure Risk Through Groundwater Sources in Northeastern Parts of Rajasthan, India.

Exposure to fluoride concentrations above a threshold of 1.5 mg/L can cause joint pains, restricted mobility, skeletal and dental fluorosis. This study aims to determine the hydrochemical evolution of the fluoride-rich groundwater and estimate the risk of fluoride exposure to the residents of semi-arid northeastern part of Rajasthan, India. The methodology involves measurement of fluoride and other ionic concentrations in groundwater using ion chromatography, followed by an estimation of the cumulative density function and fluorosis risk. The fluoride concentration in water samples varied from 0.04 to 8.2 mg/L with 85% samples falling above the permissible limit. The empirical cumulative density function was used to estimate the percentage and degree of health risks associated with the consumption of F- contaminated water. It is found that 55% of the samples indicate risk of dental fluorosis, 42% indicate risk of deformities to knee and hip bones, and 18% indicate risk of crippling fluorosis. In addition, instances of high nitrate concentrations above the permissible limit of 45 mg/L are also found in 13% of samples. The fluoride rich groundwater is mainly associated with the Na-HCO3-Cl type water facies while low fluoride groundwater shows varied chemical facies. The saturation index values indicate a high probability of a further increase in F- concentration in groundwater of this region. The calculated fluoride exposure risk for the general public in the study area is 3-6 times higher than the allowed limit of 0.05 mg/kg/day. Based on the results of this study, a fluorosis index map was prepared for the study area. The northern and northeastern parts are less prone to fluorosis, whereas the south-central and southwestern parts are highly vulnerable to fluorosis. The inferences from this study help to prioritize the regions that need immediate attention for remediation.

Application of laser-induced breakdown spectroscopy (LIBS) coupled with PCA for rapid classification of soil samples in geothermal areas.

The Manuguru geothermal area, located in the Telangana state, is one of the least explored geothermal fields in India. In this study, characterization of the soil samples is carried out by laser-induced breakdown spectroscopy (LIBS) coupled with analytical spectral-dependent principal component analysis. A total of 20 soil samples were collected both from near the thermal discharges as well as away from the thermal manifestations. LIBS spectra were recorded for all the collected soil samples and principal component analysis (PCA) was applied to easily identify the emission lines majorly responsible for variety classification of the soil samples. In this submission, a modified PCA was developed which is based on the spectral truncation method to reduce the huge number of spectral data obtained from LIBS. The PCA bi-plot on the LIBS data reveals the presence of two different clusters. One cluster represents the soil samples collected from the close vicinity of the thermal manifestations whereas the other cluster contains the soil samples collected away from the thermal sprouts. PCA performed on the chemical dataset of the soil samples also reveals the same clustering of the soil samples. Both LIBS and chemical analysis data shows that soil samples near the thermal waters are found to be enriched in B, Sr, Cs, Rb, Fe, Co, Al, Si, Ti, Ru, Mn, Mg, Cu, and Eu concentrations compared to the soil samples located away from thermal manifestations. This study demonstrates the potential use of LIBS coupled with PCA as a tool for variety discrimination of soil samples in a geothermal area. LIBS is shown to be a viable real-time elemental characterization technology for these samples, avoiding the rigorous dissolution required by other analytical techniques.