Gibbs diagram

Gibbs diagram:
To establish the relationship of water composition and aquifer lithological characteristics, the data plotted on Gibbs diagram. The major distinct fields such as precipitation, rock and evaporation dominances types in the Gibbs plot (Gibbs, 1970).
Gibb’s ratios are calculated with the formulae given below and expressed in meq/L.
Gibb’s ratio I (for anion) = Cl- /(Cl- +HCO3-) (1)
Gibb’s ratio II (for cation) = (Na+ + K+) / (Na+ + K+ + Ca+2) (2)
Gibb’s ratio I of the study area values ranges from 0.15 – 0.85 meq/L with an average of 0.46 meq/L and 0.31 – 0.90 meq/L with an average of 0.57 meq/L in pre and post-monsoon season (Table 1). Gibb’s ratio II for the study area values ranges from 0.18 – 0.79 with an average of 0.46 meq/L and 0.16 – 0.86 meq/L with an average of 0.48 meq/L in pre and post-monsoon seasons (Table 1). The study area the groundwater samples are fall in rock dominance and evaporation dominance field in pre and post-monsoon seasons (Fig. 4a and b).
Drinking water quality:
Drinking water quality the analytical results of physical and chemical parameters of groundwater were compared with the standard guideline values as recommended by the World Health Organization for drinking and public health purposes (WHO 2011) (Table 1). The table shows the most desirable limits and maximum allowable limits of various parameters. The concentrations of cations such as Na+, Ca2+, and Mg2+, K+ and anions such as HCO3-, CO32-, Cl- and SO42- are within the maximum allowable limits for drinking except for a few samples.

Total dissolved solids and Total hardness:
To ascertain the suitability of groundwater for any purposes, it is essential to classify the groundwater depending upon their hydrochemical properties based on their TDS values (Carroll 1962, Todd 2001). The TDS of groundwater in the study area varying between 165 – 2510 mg/L with an average of 928.08 mg/L and 272 – 1862 mg/L with an average of 824.38 mg/L, during pre and post-monsoon seasons (Table 1). The highest desirable limit TDS up to 500 mg/L (WHO 2011). Based on this classification fresh water is 67% and brackish water 33% in pre-monsoon seasons and post-monsoon season 69% is fresh water and 31% is brackish water of the study area (Table 2). The high concentration of TDS is due to the influence of anthropogenic sources, such as domestic sewage, septic tanks, and agricultural activities.
The Total Hardness of groundwater in the study area varying between 117 – 725 mg/L with an average of 365.40 mg/L and 76 – 863 mg/L with an average of 304.39 mg/L, during pre and post-monsoon seasons (Table 1). According to WHO specification TH is 500 mg/L is the maximum allowable limit. Based on this classification of the total hardness about 64%, 26% and 10% of the groundwater samples are fall in the very hard, moderate to hard and hard water category in pre-monsoon season. In post-monsoon season, the groundwater samples are fall in very hard 33%, hard 30% and moderate to hard 07% (Table 2), which indicates the hardness of the water is due to the presence of alkaline earth such as calcium and magnesium.

IRRIGATION WATER QUALITY
Salinity and alkalinity hazard:
Electrical conductivity is a good measure of salinity hazard to crops as it reflects the TDS in groundwater. Eight and four out of 42 samples in pre and post-monsoon seasons exceeded (Table 2) the permissible limit for irrigation (Ragunath 1987). Based on the this classification excess salinity reduces the osmotic activity of plants and thus interferes with the absorption of water and nutrients from the soil (Saleh et al., 1999).
Sodium Adsorption Ratio (SAR):
Ca and Mg in the proper proportion with sodium maintain soil in the good state. In irrigation water, often represented by a parameter called “Sodium Adsorption Ratio” (SAR) which can be calculated by the formula taking individual values of Na+, K+, Ca2+ and Mg2+ in milliequivalent per liter.

In the study area, sodium adsorption ratio values range from 0.49 – 6.52 meq/L with an average of 2.24 mg/L and 1.30 – 8.64 meq/L with an average of 4.16 mg/L in pre and post-monsoon seasons (Table 1). All samples fall in the excellent class in pre and post-monsoon seasons (Table 2), implies that no alkali hazard is anticipated to the crops.

The U.S. salinity laboratory (USDA 1954) proposed a diagram for studying the suitability of groundwater for irrigation purposes based on SAR. The chemical parameters of the groundwater of the area are represented in the USSL diagram (Fig. 5 and 6). Based on this classification about 83% of the groundwater samples are fall under C3S1 class indicating high salinity – low sodium waters, 10% of the groundwater samples are fall under C2S1 class indicates medium salinity and low sodium waters, 5% of the groundwater samples fall under C4S1 class indicating very high salinity and low sodium water and 2% of the groundwater sample fall under C3S2 class indicating high salinity – medium sodium waters in pre and post-monsoon seasons. This implies that the groundwater quality can be used for irrigation on all types of soil.

Sodium percentage (Na %):
Irrigation water containing large amounts of sodium is of special concern due to sodium’s effects on soil and poses a sodium hazard. Excess sodium in water produces the undesirable effects of changing soil properties and reducing soil permeability (Subba Rao 2006). Hence, the assessment of sodium percentage is necessary while considering the suitability for irrigation, which is calculated using the formula and expressed in meq/L.
(4)
The sodium percentage values vary from 9.14 – 75.17 meq/L with an average of 36.76 meq/L in pre-monsoon season and 21.04 – 68.79 meq/L with an average of 44.78 mg/L in the post-monsoon season (Table 1). Based on this classification, about 14%, 17% of the groundwater samples are excellent to a good category, 64%, 40% of the groundwater samples are good to permissible, 10%, 26% of the samples are doubtful to unsuitable category in the pre and post-monsoon seasons respectively for irrigation purposes. About 5% of the groundwater samples fall in the field of unsuitable for irrigation in post-monsoon season.
Residual Sodium Carbonate (RSC):
Residual sodium carbonate has been used to determine the hazardous effect of carbonate and bicarbonate on the quality of water for agricultural purpose (Eaton 1950) and is determined by the formula and expressed in meq/L.
(5)
The RSC values range from -10.19 – 3.26 meq/L with an average of -2.64 meq/L in pre-monsoon season and -10.50 – 4.20 meq/L with an average of -1.27 meq/L in post-monsoon season respectively (Table 1), The classification of irrigation water according to RSC values containing greater than 2.5 meq/L of RSC are not suitable for irrigation. The study area which is classified on the basis of RSC values is presented in (Table 2) about 90%, 83% of groundwater samples are fall in the safe category for irrigation in pre and post-monsoon seasons, about 5%, 7% of groundwater sample are in marginal for irrigation in pre and post-monsoon seasons and about 5%, 10% of groundwater sample are in unsuitable for irrigation in pre and post-monsoon seasons.
Chloro Alkaline Indices (CAI):
It is essential to know the changes in the chemical composition of groundwater during its travel in the sub-surface (Aastri 1994). The Chloro-alkaline indices CAI 1, 2 are suggested by (Schoeller 1977) which indicate the ion exchange between the groundwater and its host environment. The Chloro-alkaline indices are calculated using the formula given below and expressed in meq/L).
1) Chloro Alkaline Indices (6)
2) Chloro Alkaline Indices (7)
If there is ion exchange of Na and K from water with magnesium and calcium in the rock, the exchange is known as direct when the indices are positive. If the exchange reversed, then the exchange is indirect, and the indices are found to be negative. The CAI-1 was calculated for the groundwater of the study area varying between -3.71 – 0.86 meq/L with an average of -0.21 meq/L and – 4.09 to 0.56 meq/L with an average of -0.39 meq/L in pre and post-monsoon seasons (Table 1). The CAI-2 are calculated for the waters of the study area varying from -0.67 -1.25 meq/L with an average of 0.10 meq/L and -0.58 – 1.90 meq/L with an average of 0.01 meq/L in pre and post-monsoon seasons (Table 1). In the pre-monsoon season, CAI-1 and 2 graph sample values are showing that 25 samples positive (60%) and 17 samples show negative (40%). The post-monsoon season CAI-1 and 2 graph sample values are shows that 17 positive and 25 negatives. The chloro-alkaline indices also support the fact that reverse ion exchange is the main hydrochemical process controlling the groundwater chemistry in the region (Fig. 8).