## 4 Methodology  
The technical approach used to fulfill the objectives of the study can be split into six main phases. These are the preparation of the base map, field measurements, sampling, analysis, interpretation of results, and identification of the main hydrogeochemical processes in the subsurface. Polyethylene plastic containers with a capacity of 500 mL were used to collect groundwater samples from 40 wells (from dug and bore) in January 2020 (before the Covid19 pandemic lockdown) and January 2021. For each sampling, plastic containers were pre-rinsed with well water from the same well used for analysis. A plastic bucket was used to collect water samples from dug wells at a depth of 25 feet below the static water level. For bore wells, the water was pumped for a few minutes before groundwater samples were collected to avoid temporarily contaminating the water at the top to be sampled. The sampling sites were selected to cover the northern coastal strip of Ramanathapuram district and the landward side of about 15 km from the coast. However, due to the availability of wells in the region, the sampling sites were scattered throughout the study area. In situ measurements of the physico-chemical parameters, such as pH (hydrogen ion activity), TDS (Total Dissolved Solids), and EC (Electrical conductivity), were conducted in the field using a PCSTestr-35 portable meter [21]. The portable meter was calibrated with Merck standard buffer solution before being taken to the field. The samples were filtered with a 0.5 \(\upmu\)m (Whatman) filter before each analysis. A titrimetric procedure with EDTA standard was used to determine the calcium (Ca2+) and magnesium (Mg2+). The same procedure was used with AgNO3 standard for the determination of chloride (Cl-). Bicarbonate (HCO3-) was measured titrimetrically using HCL solution. For quality assurance and quality control (QA/QC), the samples were analyzed in duplicate. Flame spectrometric and spectrophotometric turbidimeters were used to determine Na+ (sodium), K+ (potassium) and SO42- (sulfate), and NO3- (nitrate), respectively. A selective ion electrode was used to measure the fluoride (F-) concentration in the samples [48]. The working standards were measured in between every five groundwater samples. The recovery rate of spiking was 93.2 \(\pm\) 5%, while the analytical precision of the tests was 2-4% RSD. The analytical values of the major and minor ions were used to calculate the ionic balance error, which was less than 10% and made the analytical results valid for further interpretation. The unit for all major and minor ion concentrations was milligrams per liter  
Figure 2: The hydrogeological profile and the information of groundwater flow. (mg/L), except for pH and EC. The unit for EC was micro-Siemens per centimeter (\(\mu\)S/cm). The inverse distance weighted interpolation (IDW) technique was applied to generate the spatial distribution maps for each physicochemical parameter using QGIS (Quantum GIS v3.10). IDW was chosen for interpolation, because the study area was a homogeneous formation and the samples were almost uniformly distributed. Considering this aspect, the bull's eye effect should not be a problem for deriving the spatial distribution pattern of each parameter. A Microsoft Excel spreadsheet was used to calculate the irrigation water quality indices and to generate the various hydrogeochemical diagrams. The modified equation (1 & 2) of the weighted arithmetic WQI (Water Quality Index) was used to estimate the overall groundwater quality for drinking and irrigation [49]. Various statistical analyses, such as the Pearson correlation matrix, principal component analysis (PCA), and hierarchical cluster analysis (HCA), were used to interpret the hydrogeochemical data [34; 50; 51; 52]. The use of HCA and PCA in hydrogeochemistry studies has increased because the results are easy to read and discuss.