Groundwater quality problems have emerged in many geographical areas due to natural environmental processes and human intervention in the geosystems. Hydrogeochemical appraisal of fluoride contaminated groundwater in Qinkenpao area, Daqing State, China is carried out by means of groundwater quality investigations. Results obtained from aqueous speciation modeling using AQUACHEM 5.1 reveal that the groundwater is undersaturated with fluorite and with calcite. The samples also contained high concentrations of nitrate, which is a serious water quality issue due to the impact of human activity. Factor analysis modeling demonstrated that NO3− in the system are produced by anthropogenic sources. Strong correlation observed between NO3− and Ca2+, Mg2+, Cl− and F−, suggesting that they have the same origin. The factor analysis indicates that sodium plus nitrate bicarbonate groundwater have a high loading factor for fluoride, whereas that for calcium chloride and magnesium chloride groundwater is low. The plausible geochemical reactions in the area of study are dissolution of calcite and dolomite, carbon dioxide and sulphate minerals with ion exchange.
Key words: Aqueous speciation modeling, fluoride, nitrate, groundwater, hydrogeochemical, factor analysis.
RESULTS AND DISCUSSION
Fluoride and Nitrate hydrogeochemistry and aqueous speciation modeling
Concentrations of various groundwater quality parameters obtained for samples collected in post-monsoon 2010 are shown in Table 2. The water type for each sample is also indicated. It is evident that groundwater consists mainly of types Na-HCO3, Ca-HCO3 and Ca-NO3. The calcium concentration ranges between 6.25 and 193.5 mg/L, fluoride between 0.58 and 3.4 mg/L and nitrate between 0 and 624.1 mg/L. The correlation coefficients among various groundwater quality parameters were obtained to investigate their interdependence. The correlation matrix for the hydrochemical data of September 2010 is shown in Table 3. This shows that fluoride and nitrate have a positive relationship with all species, but sulfate and pH for nitrate, and further pH for fluoride have a negative relationship.
The NO3– concentrations at some locations (60% of the wells) exceeded the 50 mg/L limit specified by the WHO drinking water standard.
Groundwater also contained high concentrations of NO3, which indicates a serious water quality issue. This NO3 originates from the soil surface and enters the groundwater via infiltration. However, due to the influence of land use, there was still a high level of horizontal variation in groundwater quality in the upper aquifer. NO3 and cation concentrations (Ca2+ and Mg2+) showed strong correlations indicating that they originated from the same sources (Figure 3).The upper limits of the point distribution for observed fluoride and calcium concentrations form a hyperbolic curve that suggests solubility of calcium- and fluoride- containing minerals controlling the fluoride concentration (Figure 4). High fluoride with very low calcium and magnesium in water may be due to prior precipitation of CaCO3 from water and only limited incorporation of fluoride in the calcite structure, so that there is always a net balance of fluoride in the solution.
Aqueous speciation modeling was carried out using PHREEQC (Parkhurst and Appelo, 1999) geochemical code via AquaChem 5.1 (Waterloo Hydrogeologic Inc., 2009).
The saturation indices of fluorite were computed for all three sets of temporal data, sampled as well as earlier hydrochemical data. Figure 5 shows the plot of these saturation indices versus calcium concentrations. Results show that the groundwater is undersaturated with calcite, having a saturation index of -0.0024 to -0.6174 with an equilibrium state. The groundwater is undersaturated with fluorite, having a saturation index of -0.0453 to -2.1392 (Table 4).
The factor analysis was carried out using the hydrochemical data of the area of study. These data were considered for multivariate analysis without combining sampled data to make the homogeneous data set with reference to dynamic changes in hydraulic stresses, land-use characteristics and pollutant sources. The chemical constituents considered for factor analysis for the unconfined aquifer were calcium, magnesium, sodium, potassium, bicarbonate, sulphate, chloride, fluoride, nitrate, pH and TDS. The principal component analysis was carried out using Statistical Package for Social Services (SPSS) software. The two-factor model results revealed that the first two eigenvalues extracted from the matrix account for more than 72% of total variance, which shows that the hydrochemical data is well posed. A varimax rotated component matrix with Kaiser (1958) normalization was used for principal component analysis.
The interpretation of factors was made in terms of the square of the coefficients of that factor. The rotated component matrix for the geochemical data is given in Table 5. There is almost identical loading for sodium, magnesium, nitrate and bicarbonate.
Therefore, the variance in the chemical composition of the hydrochemical system is controlled by sources of sodium and bicarbonate. For Factor 1, the sum of the squares of calcium and magnesium (1.45) is approximately that of nitrate and chloride bicarbonate, sulphate and chloride (1.56). Thus, the combined relationship suggests that there is more than one component or more than one solid phase that adds or removes calcium, magnesium, nitrate and chloride into the groundwater. For Factor 1, there is no replacement mechanism as there is a lack of mutually exclusive components. Nitrate is naturally present in groundwater at very low concentrations, and its source is human activity such as domestic or industrial waste or agricultural. Factor 1 shows calcium chloride and magnesium chloride waters. Factor 2 shows that the sum of the squares of sodium and potassium (1.15) is approximately that of bicarbonate, sulphate and chloride (1.37). The combined relationship suggests that there is more than one component or more than one solid phase that adds or removes sodium, potassium, bicarbonate, sulphate and chloride. The presence of negative correlations indicates that some components are controlled by equilibrium with the minerals in the aquifers. Thus, there is a reaction path by which one set of chemical products replaces another set. Moreover, it is observed that there is positive correlation between calcium and fluoride for both the factors.
For Factor 1, there is probably dissolution of fluorite, calcite and dolomite, whereas for Factor 2, the decrease in fluoride may be due to adsorption on clay surfaces. Thus, these results obtained from factor analysis help in understanding the possible grouping of chemical constituents in the groundwater.
The actual changes in concentrations of chemical species as a function of sulphate concentration help in obtaining the information on possible geochemical reactions that may be occurring in the area of study. To investigate this, analysis was carried out using hydrochemical data. The increase in calcium and magnesium and pH as sulphate increases are shown in Figure 6a-c. The groundwater in the area of study is undersaturated with calcite and dolomite (Table 4), and dissolution of calcite and dolomite adds calcium and magnesium to the groundwater. Calcite dissolution causes a pH increase due to consumption of H+ by carbonate during the dissolution process. The decrease in the CO2 in the solution leads to the dissolution of calcite and dolomite and CO2 dissolution, thereby increasing the magnesium and calcium concentration in the solution. The geochemical equations for possible reactions in the aquifer are as follows:
Where (g) in Equation 4 refers to the gaseous phase and (aq) to the aqueous phase.
Although the trends in calcium, magnesium and pH with sulphate are evident in the groundwater in the area of study, there is variation in the temperature and other reactions in addition to dedolomitization or dissolution of carbonates. Dedolomitization is a specific geochemical process and has been reported by Plummer et al. (1990) and Kloss and Goebelbecker (1992).
The plot of bicarbonate alkalinity as a function of dissolved sulphate concentration (Figure 6d) shows that bicarbonate alkalinity is decreasing as sulphate increases, and is possibly indicative of dedolomitization or carbonate dissolution. Cation exchange with calcium and magnesium cations could contribute to additional bicarbonate on the flow path with uptake of calcium and magnesium and release of sodium from exchange sites on clay minerals causing dissolution of carbonate minerals. The molar concentrations of sodium plus potassium are slightly more than the concentration of chloride. This is indicative of presence of evaporites. These waters have high bicarbonate concentration. Thus, there is a tendency to form sodium-bicarbonate waters. For the groundwater in the area of study of Qinkenpao area, the sodium bicarbonate water is derived from the dissolution of carbonate minerals.
CONFLICT OF INTEREST
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