Groundwater quality and vertical electrical sounding data of the Valliyar River Basin, South West Coast of Tamil Nadu, India

The present study was carried out to assess the drinking water quality and aquifer characteristics of the Valliyar river basin in Kanyakumari district. 71 groundwater samples were collected and analyzed to understand the quality of water based on the index parameters. 23 vertical electrical soundings (VES) were conducted to understand the subsurface characteristics and their impact on the quality of water. The VES data suggested that the subsurface lithology consist of three distinct layers. The water quality index (WQI) showed that 25% of the samples falling under excellent water and 51% of the samples were classiﬁed as good water and another 24% belong in the poor water category.


Data
This dataset contains six figures and five tables that represent the suitability of the groundwater for drinking and irrigation purposes of Valliyar River basin, Kanyakumari district, India. Fig. 1 shows the geochemical and geophysical data location map of Valliyar River basin. Fig. 2 shows spatial distribution of water quality index. Hydrogeochemical facies for groundwater shows in Fig. 3. Table 1 indicates data of various physio-chemical parameters such as pH, electrical conductivity (EC), total dissolved solids Specifications  [1].

Value of the data
The geochemical data set was used to identify the drinking water quality The irrigation water quality (Na%, SAR, and PI) was suggesting the suitability of water. The vertical electrical sounding data is helpful to understand the subsurface lithology, aquifer resistivity and thickness. (TDS), Ca 2þ , Mg 2þ , K þ , Na þ , Cl À , HCO 3À and SO 4 2À and their comparison with existing firm standards.
This comparison disclose the quality of water and its suitability for drinking and irrigation needs. pH is the molar concentration of hydrogen (H) ions which express the alkaline or acidic condition of water. pH value ranges from 6.5 to 8.5 is suitable for drinking water based on WHO. EC value used to get an idea about salt enrichment in groundwater [1]. Total dissolved solids (TDS) can be taken as a measurement of dissolved inorganic salts and some organic matter in water. In this study area TDS value ranges from 36 to 1718 mg/l. Table 2 shows the water quality indices of Canadian water quality index (CWQI), Sodium percentage, Sodium Absorption Ratio and Permeability index. Irrigation parameters such as Total hardness (TH), Na%, SAR, PI, and drinking water quality index (WQI) with its classification for individual sample are given in Table 3. Presence of calcium (Ca 2þ ) and magnesium (Mg 2þ ) ion content determines the hardness of water. The quality criteria for determining the viability of groundwater for agricultural purposes include salinity indices, comprising Na%, SAR and PI [2]. The water quality index (WQI), was calculated to enumerate the impact of natural and anthropogenic activities. According to CWQI, the water can be classified into five types namely poor (0e44), marginal (45e64), fair (65e79), good (82e94) and excellent (94e100). Table 4 shows interpreted subsurface layer parameters (r, h), aquifer resistivity and aquifer thickness. It provides vertical variation of lithology as well as depth-to-aquifer and aquifer condition [3]. The aquifer thickness of the study area ranges from 2.7 to 46 m, and the average is 12 m.

Groundwater sample collection
Groundwater samples were randomly collected from 71 open and bore wells during January 2015. For the sample collection, high density polyethylene bottles were used. The bottles are immediately sealed after the sample collection to avoid the reaction with the atmosphere. The sample bottles were labeled systematically. The collected samples were analyzed in the laboratory for various physicochemical parameters. During sample collection, handling, preservation and analysis standard procedure recommended by the American public health organization [9] were followed to ensure data quality and consistency.

Field analysis (physical parameters)
The physical parameters such as pH, Electrical Conductivity (EC) were measured in-situ using Hanna water quality meter (HI-9828, USA). The Total Dissolved Solids (TDS) were calculated by multiplying the electrical conductivity by a factor of 0.64 [5].

Laboratory analysis (chemical parameters)
The major ions(Ca 2þ , Mg 2þ , Na þ , K þ , HCO 3 À , SO 4 2À , Cl À ) were analyzed in the laboratory using the standard methods suggested by the American Public Health Association (APHA, 1995). Among the analyzed ions, sodium (Na þ ) and potassium (K þ ) were determined by using flame photometer. Calcium (Ca 2þ ), magnesium (Mg 2þ ), bicarbonate (HCO 3 À ) and chloride (Cl À ) were analyzed by volumetric  ) ions in the groundwater samples were determined by using acid titration method, in which the sulphuric acid with 0.01 N is used. Chloride (Cl À ) ion concentration is calculated using argentometric (AgNO 3 ) titration. Sodium and potassium content were sorted out using flame photometer instrument (DEEP VISION, Model-381). The amount of sulphate ions was found using the UVeVisible photometer.
To measure WQI a set of 11 physical and chemical parameters such as pH, EC, TDS, TH, Ca 2þ , Mg 2þ , Na þ , K þ , HCO 3 À , Cl À , and SO 4 2À were resolved. The analysis for water quality index has been done with the help of Canadian Water Quality index (CWQI) programmed excel software [10]. ArcGIS10.1 software was used for spatial analysis of various physico-chemical factors. An Inverse Distance Weighted (IDW) technique was used to interpolate the data spatially and enumerated the value for each grid node by inspecting the encompassing data points that lie within a user defined search area.

Hydrogeochemical facies
The geochemical histories and flow pattern of groundwater can be determined by hydrogeochemical facies interpretation. The changes in groundwater quality within an aquifer can be understood by plotting the concentrations of dominant ions in the piper tri-linear diagram [11]. This diagram mainly consist of two triangle shaped fields each represents the composition of cations and anions, and a diamond shaped field represents composition of both cations and anions present in the groundwater. The classification of hydrogeochemical facies for groundwater plotted by piper trilinear diagram is shown in (Fig. 3). Most of the water samples fall in the NaCl segment followed by mixed CaMgCl > CaNaHCO 3 > CaHCO 3 .

Vertical electrical sounding
To understand the subsurface lithology and to study the groundwater potentiality within Valliyar river basin, 23 vertical electrical sounding (VES) surveys were carried out. That data were processed (unit for all parameters are mg/l except pH (on scale) and EC (ms/cm)).

Table 2
Summary of water quality indices for drinking and irrigation [4e6].

Indices Acronym Formula
Canadian Water Quality index CWQI and interpreted manually using IPI2WIN software. The interpreted VES data reveals that the study area consist of three to four geoelectrical layers with different curve types.

Transparency document
Transparency document associated with this article can be found in the online version at https:// doi.org/10.1016/j.dib.2019.103919.  Where, r (U,m) and h (m) means resistivity and thickness of the subsurface layers.