N. Ganesh

A study on the impact of weathering in groundwater chemistry of a hard rock aquifer

Hydrogeochemistry of groundwater in hard rock terrain are mainly governed by lithology and land use practices. A study area near Madurai region of central Tamil Nadu was selected with various litho-units and a hard rock sedimentary contact with an unconformity. Land use practices in these regions are also varied with lithology. The study was conducted by collecting 54 groundwater samples spatially covering the major litho-units. Collected samples were analyzed for electrical conductivity, pH, total dissolved solids (TDS), temperature, Ca, Mg, Na, K, Cl, HCO3, NO3, H4SiO4, PO4, and SO4. The results of the samples analyzed found to vary spatially. Dominance of ion shows that the alkalies are predominant and HCO3 is the dominant anion. Piper facies show that the samples are alkali-carbonate type indicating the predominance of weathering. Most of the parameters exceed the drinking water permissible limit. Standard plots and statistical analysis also indicate weathering as the major process governing the hydrogeochemistry of the groundwater in the region. Relative mobility of cations indicates that the rate of liberation of alkalies from the lithology is more prominent.

An attempt to understand the subsurface variation along the mountain front and riparian region through geophysics technique in South India

Geophysical resistivity survey was conducted in 35 locations along the foothills of Courtallam, Tamilnadu, in order to depict the groundwater potential zone. The layer parameters like apparent resistivity (ρa) and thickness (h) of different layers were arrived. A maximum of four layers were identified in few regions, but major part of the study area is dominated mainly by three layers. Three major zones has been delineated as weathered, fractured and massive. Generally, A, H, KH type curve are dominant in this region and minor representation of HK is also observed. Resistivity and thickness of both the layers show decreased value in riparian zone. More resistivity and thickness along mountain front (MF) indicates good groundwater potential. The observed high conductivity in Southern part is symptomatic of the contaminated groundwater leached and drained into the subsurface from the agricultural regions. VES 6 and 13 along MF and VES 1, 3 and 12 along riparian zone are the probable locations for productive groundwater abstraction. Resistivity values and their corresponding depth for some selected VES locations were used in producing two resistivity cross sections (A–B, C–D), which show the geoelectric distribution of the subsurface near and away from the MF.

Geo-electrical approach to determine the lithological contact and groundwater quality along the KT boundary of Tamilnadu, India

A geophysical survey involving electrical resistivity method has been carried out at Ariyalur region to delineate the geo-electric characteristics of the subsurface formation and evaluate the groundwater quality variation along the lithological contact. A total of 15 vertical electrical sounding (VES) stations were established along the various geological formation such as 3 in Archean, 7 in Cretaceous, 2 in Tertiary and 3 in Quaternary formation. The data acquisition was carried out by adopting Schlumberger configuration. The layer parameters like apparent resistivity (ρa) and thickness (h) of different layers were arrived. The results of interpretation revealed three distinct geoelectric layers, which is comprised of weathered, fractured and massive layer in hard-rock region and topsoil, sandy layer and clayey layer in sedimentary region. Sounding curves obtained in the area are mostly of A, H, K type. The 1st layer thickness shows a decreasing trend in the following order Tertiary > Alluvium > Achaean. However, for the 2nd layer, the trend is Achaean > Tertiary > Alluvium. The higher 1st layer thickness along the southeastern part of the study area is due to the overexploitation of the groundwater represented by alluvium. Resistivity values and their corresponding depth for some selected VES locations were used in producing three resistivity cross sections (A–A′, B–B′ and C–C′), which shows the geoelectric distribution of the subsurface. The observed high resistivity in a shallow depth in the western part is due to the variations in lithology. The relationship between the resistivity and conductivity are also taken into consideration to identify the shallow contaminated zones of the study area.

A study on mountain front recharge by using integrated techniques in the hard rock aquifers of southern India

Mountain front recharge (MFR) is the contribution of mountains to recharge the aquifers in the adjacent basins. The estimation of MFR is essential to obtain a detailed investigation of recharge of the groundwater at the mountain front. This study summarises the current understanding of recharge processes by comparing daily groundwater fluctuation to daily rainfall and identifies the recharge rates. The recharge rates vary with time due to difference in water table depth and travel time. Thus to understand the MFR along the foothills of Courtallam, a total of 14 surface water, rainwater and groundwater samples were collected and measured for stable isotopes. The isotopic data were used to investigate the recharge process and to identify the elevations to recharge. The study findings also suggest that predominantly rainfall along the foothills contributes recharge to the riparian zone (basin block region), whereas foothill regions receive recharge from rainfall over mountain block.

An insight on the speciation and genetical imprint of bicarbonate ion in the groundwater along K/T boundary, South India

Groundwater stored in sand, clay, and underground rocks that act as huge aquifers of water have been vastly explored by human civilization for drinking and irrigation purposes. This underlying sand, clay, and rocks greatly influence the chemistry of groundwater by fluctuation in various concentrations of ions. The geochemistry of Ariyalur region was studied by taking 142 samples in both Pre monsoon (PRM) and Post monsoon (POM) seasons. Analysis for crucial cations and anions like K+, Na+, F−, Cl−, HCO3−, Mg2+, Ca2+, SO42−, PO43−, NO3−, and H4SiO4 and the physico-chemical variables like pH, total dissolved solid (TDS), and electrical conductivity (EC) were carried out to understand the suitability of groundwater. From all these information, Piper-plot has been used to analyze the water type in both the seasons. The study revealed that in both the seasons, HCO3− covers the major factors followed by H2CO3 which are responsible for the change of chemistry of groundwater and proved its importance in the study area. Hence to determine the relationship of HCO3− with other ions, various charts were used. To obtain the origin of this HCO3− and reactions related to it, correlation of different species like HCO3, CaHCO3, MgHCO3, NaHCO3, CaCO3, MgCO3, NaCO3, and H2CO3 were analyzed along with major physico-chemical variables like pH and logpCO2. The factor analysis for these species revealed the processes and reactions taking place in the study area. The contour diagrams for all the species in both the season has revealed the origin of HCO3− and dissolution by comparing lithology and geology of the study area.

A hydrochemical approach to estimate mountain front recharge in an aquifer system in Tamilnadu, India

Mountain-front recharge (MFR) is a process of recharging an aquifer by infiltration of surface flow from streams and adjacent basins in a mountain block and along a mountain front (MF). This is the first attempt in India to estimate MFR along the foothills of Courtallam using hydrogeochemistry and geostatistical tools. The estimation of MFR has been carried out by collecting groundwater samples along the foothills of Courtallam. Collected water samples were analyzed for major cations and anions using standard procedures. Hydrogeochemical facies show the existence of four water types in this region. Calcium-rich water derived from gneissic rock terrain indicates significant recharge from higher elevation. Log pCO2 and ionic strength of the samples were also calculated to identify the geochemical process. Majority of the collected samples have sodium-rich water and weak ionic strength, which indicate foothill recharge and low residence time. Silicate and carbonate weathering have an equal interplay along the foothills with a relatively large fraction of Mg from the MF. The spatial diagrams of three factors show that the southern part of the study area is dominated by both weathering and anthropogenic processes, whereas the northern part is dominated by both leaching and weathering processes. Thus, the dominant weathering process represented by the second factor indicates the large recharge process along the foothills.

A Comparative Study on the Arsenic Levels in Groundwaters of Gangetic Alluvium and Coastal Aquifers in India

Arsenic contaminated groundwater causes danger to the health of millions of people in India. In this context, a comparative study has been made to evaluate the Arsenic (As) concentration in groundwater of Gangetic alluvium, West Bengal and south eastern coastal aquifer at Kalpakkam, India. The variations of Ar concentration with lithologies, depth of aquifers, sources and distribution of Ar and hydrochemical facies of groundwater has been clearly identified from the two localities. In Gangetic alluvium, As concentrations appear to be mostly low in groundwater from deep aquifers and it is interesting to note that in almost all the formations of coastal aquifer, the concentration increases with increase of depth. The shallow ground water of the southern coastal aquifer reveals the presence of oxidizing conditions and frequently flushing during the monsoonal period, which may also dilute or reduce the concentrations. The poor relationship between As and Fe indicates the As release into the groundwater depends on several processes such as mineral weathering, O2 consumption, and NO3 reduction and is de-coupled from Fe cycling.