Chapter 5 RESULTS AND DISCUSSION
5.3 Hydrogeochemical Characteristics
5.3.1 Descriptive Statistics and Correlation Matrices of hydrochemical data
The descriptive statistics of the most important hydrochemical parameters analysed in the study area is given in Table 5.4. The corresponding Box plot that provides a useful and concise graphical display for summarising the distribution of the data is shown in Figure 5-4, while groundwater-surface water comparison statistics for the concentration of physicochemical parameters in the study area is shown in Table 5.5. The various physicochemical parameter values and measured groundwater and surface water trace elements concentration within the study area are presented in Appendix B and C, respectively.
The pH of groundwater in the study area ranges from 6 to 8, which is the rage commonly encountered in natural groundwaters (Hiscock, 2005). However, two outliers of an acidic pH of 5.21 and an alkaline pH of 9.59 were measured at a spring and borehole, respectively (Appendix B). The EC values range from 3.27 to 67.20 mS/m indicating very fresh to fresh groundwater with low salinity. Both groundwater and surface water samples have low variations in dissolved oxygen (DO) and it ranges from 4.07 to 5.87 mg/L in groundwater and from 2.79 to 6.30 mg/L in surface waters. The turbidity varies from 0.31 to 275 NTU with a mean of 43.56 NTU. The highest value of groundwater turbidity measured is 275 NTU from borehole MP14-003 and the minimum value is 0.31 from Spring04. Poor borehole development during drilling could account for the high turbidity from the borehole and/or unprotected boreholes with shallow groundwater indicates high turbidity as well due to impact from the surface. For the surface water samples, the highest and lowest turbidity values are 1.99 and 89.40 NTU obtained from sample locations SW18 and NCSW01 respectively. Sample point NCSW01 is collected from a surface runoff sediment dam hence the high turbidity while the sample location for sample point SW18 is along the Klipfonteinspruit which gets filtered by bamboo upstream.
The Ca2+ concentrations of groundwater and surface water samples vary from 0.60 to 56.90 and 4.66 to 94.70 mg/L, with mean concentrations of 8.17 and 30.67 mg/L respectively.
Concentrations of Mg2+, Na+, HCO3, and Cl- in groundwater vary from 0.53-26.30 mg/L, 1.53- 62.60 mg/L, 5.49-248.90 mg/L, and 1.25-24.50 mg/L, respectively. In surface water samples concentrations of Mg2+, Na+, HCO3, and Cl- vary from 3.00-19.10 mg/L, 4.6-68.7 mg/L, 43.92- 119.56 mg/L, and 3.620-14.90 mg/L, respectively. Surface water samples have elevated SO4
concentrations with a mean value of 97.22 mg/L while for groundwater samples is 52.76 mg/L.
For all the water samples, SO4 has thehighest standard deviation (81.25 mg/L), compared with
other parameters. The sources of SO4 may be attributed to the oxidation and dissolution of pyrite contained in the coal mined and used in the study area. It is clear that mining activities, predominantly coal, have a greater impact on the surface water chemistry within the study area.
Trace metals that are typically associated with coal-fired power stations are As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, and Zn (Dalton et al., 2018). Based on the results of trace element analysis undertaken in the study area as presented in Appendix C, the majority of samples have concentrations of As, Cd, Co, Cr, Hg, Pb and Sb that are below the laboratory detection limits.
Iron concentrations in groundwater samples is relatively high, with a mean concentration of 7.57 mg/L compared to a mean concentration of 1.19 mg/L in surface water samples. The mean concentrations of Mn in both groundwater and surface water are 0.16 and 0.06 mg/L, respectively. Borehole 10490-10 (37.2 mg-1 L) and NCSW01 (3.84 2 mg-1 L) has the highest concentration of iron for groundwater and surface water, respectively. These elevated iron and manganese concentrations could be related to the oxidation of pyrite which in turn leads to the precipitation of iron.
Table 5.4: Descriptive statistics of the hydrochemical variables measured within the study area (all values are reported in mg/L unless and otherwise stated).
Variables No. of
samples Min Max Mean (µ) Median Variance (σ2)
Standard deviation (σ)
Coefficient of variation (σ/µ)
Standard error of the
mean
pH 50 5.21 9.59 7.23 7.16 0.48 0.69 0.10 0.10
EC (mS/m) 50 3.27 67.20 13.65 22.72 353.70 18.81 0.83 2.69
Turbidity as N.T.U. 50 0.31 275.00 15.30 43.56 3872.95 62.23 1.43 9.38
Ca2+ 50 0.60 94.70 6.04 16.72 499.61 22.35 1.34 3.19
Mg2+ 50 0.53 26.30 3.67 6.33 33.20 5.76 0.91 0.82
Na+ 50 1.53 68.70 8.20 14.84 290.55 17.05 1.15 2.46
HCO3- 50 5.49 248.88 54.90 62.12 1847.30 42.98 0.69 6.14
Cl- 50 1.25 24.51 5.08 6.60 26.37 5.14 0.78 0.73
SO42- 50 0.09 271.00 5.88 48.84 6601.51 81.25 1.66 11.85
DO 50 2.79 6.30 5.55 5.27 0.48 0.69 0.13 0.10
Al 50 0.01 5.55 0.11 0.29 0.65 0.81 2.80 0.12
Fe 50 0.02 37.20 1.40 5.15 71.32 8.45 1.64 1.21
Mn 50 0.01 1.26 0.06 0.12 0.04 0.21 1.70 0.03
Table 5.5: Comparison of a statistical summary of physicochemical data of groundwater and surface water in the study area.
Variables
Groundwater Surface water
No. of
samples Min Max Mean (µ)
Standard deviation
(σ)
No. of
samples Min Max Mean (µ)
Standard deviation
(σ)
pH 31 5.21 9.59 6.86 0.74 19 7.27 8.10 7.64 0.19
EC (mS/m) 31 3.27 67.20 16.20 13.64 19 10.30 62.90 33.34 21.91
Turbidity as N.T.U. 31 0.31 275.00 63.26 75.06 19 1.99 89.40 16.61 23.00
Ca2+ 31 0.60 56.90 8.17 13.50 19 4.66 94.70 30.67 27.44
Mg2+ 31 0.53 26.30 4.43 4.85 19 3.00 19.10 9.43 6.05
Na+ 31 1.53 62.60 11.04 14.04 19 4.60 68.70 20.84 20.28
HCO-3 31 5.49 248.88 54.79 50.15 19 43.92 119.56 74.10 26.36
Cl- 31 1.25 24.51 5.23 5.45 19 3.62 14.90 8.83 3.90
SO42- 31 0.09 250.00 17.14 52.76 19 5.36 271.00 97.22 95.83
DO 31 4.07 5.87 5.04 0.60 19 2.79 6.30 5.59 0.72
Al 31 0.01 5.55 0.32 1.05 19 0.05 0.54 0.25 0.17
Fe 31 0.02 37.20 7.57 10.11 19 0.32 3.84 1.19 1.02
Mn 31 0.01 1.26 0.16 0.26 19 0.02 0.24 0.06 0.05
Figure 5-4: Box plots of chemical parameters used in descriptive statistics of all the water samples. Plots of Al and Mn were excluded due to their low concentrations.
5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
pH
Box plot (pH)
0 10 20 30 40 50 60 70
EC (mS/m)
Box plot (EC (mS/m))
0 50 100 150 200 250 300
Turbidity as N.T.U.
Box plot (Turbidity as
N.T.U.)
0 10 20 30 40 50 60 70 80 90 100
Ca2+
Box plot (Ca2+)
0 5 10 15 20 25 30
Mg2+
Box plot (Mg2+)
0 10 20 30 40 50 60 70
Na+
Box plot (Na+)
0 50 100 150 200 250
HCO-3
Box plot (HCO-3)
0 5 10 15 20 25
Cl-
Box plot (Cl-)
0 50 100 150 200 250 300
SO-4
Box plot (SO4)
2.5 3 3.5 4 4.5 5 5.5 6 6.5
DO
Box plot (DO)
0 5 10 15 20 25 30 35 40
Fe
Box plot (Fe) Legend
Maximum
3rd Quatile
Median 1st Quartile Minimum Outlier
Pearson’s Corelation Matrix
Major hydrogeochemical processes that control chemical characteristics can be understood as correlations between the water quality variables. The relationship of the dissolved ions was assessed to identify the main hydrogeochemical processes responsible for the chemical composition of the water within the study area. The results of Pearson’s correlation matrix presented in Table 5.6 illustrate that the electrical conductivity has high correlations with Na (r
= 0.72), Ca (r = 0.89), Mg (r = 0.94), SO4 (r = 0.94) and moderate correlation with Cl (r = 0.67), indicating the contribution of major ions to the salinity of groundwater in the area. Sulphate has high correlation with calcium (r = 0.94) indicating possible buffering of acid mine affected groundwater by carbonate minerals. The moderate negative correlation of dissolved oxygen with iron (r = -0.51) suggests reducing conditions occurring in the system. Turbidity and iron are highly correlated (r = 0.80), indicating possible aeration of a reduced groundwater during pumping.
Table 5.6: Pearson's correlation matrix of hydrochemical variables analysed in the study area.
pH EC Turbidity Ca2+ Mg2+ Na+ HCO3- Cl- SO42- DO Fe
pH 1.00
EC 0.48 1.00
Turbidity -0.21 -0.20 1.00
Ca2+ 0.33 0.89 -0.20 1.00
Mg2+ 0.32 0.94 -0.10 0.82 1.00
Na+ 0.59 0.72 -0.15 0.33 0.64 1.00
HCO3- 0.66 0.54 0.04 0.29 0.48 0.73 1.00
Cl- 0.12 0.67 -0.16 0.48 0.71 0.57 0.23 1.00 SO42- 0.31 0.94 -0.24 0.94 0.90 0.51 0.26 0.63 1.00 DO 0.20 0.18 -0.46 0.24 0.12 0.06 -0.02 0.12 0.23 1.00 Fe -0.27 -0.03 0.80 -0.07 0.13 -0.03 0.04 0.04 -0.06 -0.51 1.00