A Comparative Study of Ion Chemistry of Groundwater Samples of Typical Highland and Midland Sub-watersheds of the Manimala River Basin, Kerala, South India

Results of the detailed study of the chemistry of ions present in the groundwater samples of Peruvanthanam and Valiyathodu sub-watersheds of the Manimala river basin are analysed using the AQUACHEM 4.0 software to understand general chemical characteristics and geochemical processes involved. The study reveals that cations, such as sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg) and anions, such as bicarbonate (HCO3 ), sulphate (SO4 ), chloride (Cl), nitrate (NO3 ) are present. Recognition methods such as (a) Box and Whisker diagram (b) Piper diagram (c) Durov diagram (d) Radial plot and (e) Stiff diagram were prepared to delineate the seasonal variations in chemical constituents. The major ionic concentration of the groundwater samples of Peruvanthanam sub-watershed is Mg2+>Ca2+>HCO3 ->Cl->SO4 2->NO3 >Na>K and that of Valiyathodu is Mg2+>Ca2+>HCO3 ->Cl->NO3 -


Introduction
Millions of people rely on groundwater that contains an excess of cations and anions (Ramesh et al. 2012). The character of groundwater in different aquifers over space and time proved to be an important technique in solving different geochemical problems (Chebotarev 1955, Hem 1959, Back and Hanshaw 1965, Gibbs 1970, Srinivasamoorthy 2005. Due to interactions with the atmosphere, surface environment, nature of soil and bedrock a wide range of different elements can dissolve in groundwater (Subramanian 2000). The solute load of water determines the total dissolved solids which in turn affect the quality of water. It is a very useful indicator of geological evolution, mode of origin within hydrological cycles, soil or rock mass influences, the extent of pollution and contamination etc. Usually groundwater has much higher concentrations of most constituents as it has been in contact with rock for longer time than shallow or young water. The present study is carried out to understand the concentration and distribution of both cations and anions of Peruvanthanam and Valiyathodu sub-watersheds during different seasons.

Methods
In this study, 17 and 15 groundwater samples were collected from Peruvanthanam and Valiyathodu sub-watersheds respectively for pre monsoon, monsoon and post monsoon seasons, and chemical analysis of both cations and anions were carried out as per the methodology prescribed by APHA (1989). It is convenient to divide the dissolved constituents into major components i.e. the predominant cations and anions. Sodium (Na + ), potassium (K + ), calcium (Ca 2+ ) and magnesium (Mg 2+ ) are dominant cations and bicarbonate (HCO 3 -), sulphate (SO 4 2-), chloride (Cl -) and nitrate (NO 3 -) are dominant anions; these ions were added as attribute data to the AQUACHEM 4.0 software. The study illustrates the use of recognition methods such as (a) Box and Whisker diagram, (b) Piper diagram, (c) Durov diagram, (d) Radial plot and (e) Stiff diagram to delineate seasonal variations in chemical constituents, and thereby to point out the source contributions of vital major ions in the groundwater resources under various geomorphic conditions of both sub-watersheds. Here the chemistry of ions is depicted in different ways using the AQUACHEM 4.0 software to understand general chemical characteristics of groundwater and geochemical processes that are involved in chemical evolution of water that flows through Peruvanthanam and Valiyathodu sub-watersheds.

Box and Whisker diagram or Box plot
Box and Whisker diagram or Box plot shows the distribution of major ions in the ground water samples during the study period. It represents an inter quartile range and either end of the line indicates the minimum and maximum values. Box plot like other visual methods is more than a substitute for a table. It is a tool that can improve our reasoning about quantitative information (Williamson 1989). In descriptive statistics, Box plot is a convenient way of graphical depicting of the groups of numerical data through their five number summaries: the smallest observation (sample minimum), lower quartile (Q1), median (Q2), upper quartile (Q3) and largest observation (sample maximum). Box plot displays differences between populations without making any assumptions of the underlying statistical distribution. The spacing between the different parts of the box helps indicate the degree of dispersion and skewed data. It can be drawn either horizontally or vertically.

Piper diagram
Piper diagram is an excellent tool for hydro chemical analysis using a series of water quality data in a spatial context (Piper 1944). It has become a widespread method for understanding and describing chemical evolution of groundwater which depends on pattern recognition techniques and permits the classification of water. It can also define the patterns of spatial change in the water chemistry among geological units, along a line of the section or along a path line (Domenico and Schwartz 1998). It is therefore useful in understanding the water flow and water quality (Back 1960, Ophori and Toth 1989, Sikdar et.al 1993, Sikdar and Bhattacharya 1999, and the changes in water types and mixing relationships based on the relative proportions of major ions rather than the bulk concentrations. Hence, it is the most widely used method for delineation of hydrochemical evolution and identification of dominant processes that control water chemistry.

Durov diagram
The advantage of Durov diagram is that it displays some possible geochemical processes that could affect the water genesis (Durov 1948). Durov diagram of Peruvanthanm sub-watershed (Fig. 7) depicts that groundwater samples fall in fields 1, 2, 4 and 5. In field 1, HCO 3 and Ca 2+ are dominant which indicates recharging of water in limestone and sandstone aquifers. Pre monsoon groundwater samples are dominant in field 1. In field 2, the water type is dominated by Ca 2+ , Mg 2+ and HCO 3 ions. If Mg 2+ is significant, the association with dolomite is presumed, and if Na + is significant, an important ion exchange is presumed. Few samples come under this field. In field 4, SO 4 2is dominant. Few groundwater samples come under this field and they are post monsoon samples. In field 5 no dominant anion or cation are observed which indicates water exhibiting simple dissolution or mixing towards Cl -. Pre monsoon samples are dominant in this zone. In Valiyathodu sub-watershed (Fig. 8), in field 1 HCO 3 and Ca 2+ are dominant, indicating recharge of water in limestone and sandstone aquifers. In field 2 the water type is dominated by Ca 2+ , Mg 2+ and HCO 3 ions. If Mg 2+ is significant, association with dolomite is presumed, and if Na+ is significant, an important ion exchange is presumed. In field 4 SO 4 2is dominant. Few groundwater samples come under this field. In field 5 the absence of dominant anions or cations indicates water exhibiting simple dissolution or mixing towards Cl -.

Radial plots
A radial plot is a graphical display for comparing the estimates that have differing precisions. It is a scatter plot of standardised estimates against reciprocals of standard errors, possibly with respect to a transformed scale, designed so that the original estimates can be compared and interpreted. The estimates may be means, regression coefficients, proportions, rates, odd ratios, random effects, or indeed any parameter estimates that merit the comparison between individuals or groups. The water types of each well of both Peruvanthanam and Valiyathodu sub-watersheds are represented in Table  1 and Table 2 respectively.

Stiff diagram
Stiff diagram is a sophisticated method for demonstrating vertical changes in the chemical composition of water (Stiff 1951). The basic thing of Stiff diagram is a vertical line which has two functions. It is both depth scale of aquifers and vertical zero of axes from which the concentrations of ions are plotted on four parallel horizontal axes extending on each side. The depth of the pattern could be used as an approximate indication of the total ionic content. Cations are plotted in milli-equivalents per liter on the left side of the zero axis, and anions are plotted on the right side. Stiff diagrams can be used to visualise ionically related water from which the flow path can also be determined. If the flow path is known, it is possible to show how the ionic composition of a water body changes over space and time. It is a relatively distinctive method of showing differences or similarities in water and changes in the water composition with depth. It is useful especially for illustrating the chemical composition in hydro geologic cross sections. It can also be used for classification purposes and is useful as a symbol on a map. The stiff diagrams which represent the chemical composition of water types of Peruvanthanam subwatershed during the pre monsoon season are depicted in Figs 24, 25 and 26. Stiff diagrams reveal that Mg 2+ -Ca 2+ -Cl -, Mg 2+ -Ca 2+ -HCO 3 --Cl -, and Mg 2+ -Ca 2+ -HCO 3 are the dominant water types of the pre monsoon season. This also agrees with the radial plots during the pre monsoon season of Peruvanthanam sub-watershed.