The aim of this research is to evaluate arsenic distribution and associated hydrogeochemical parameters in 27 randomly selected boreholes representing aquifers in the Al-Kharj geothermal fields of Saudi Arabia. Arsenic was detected at all sites, with 92.5% of boreholes yielding concentrations above the WHO permissible limit of 10
Arsenic is a toxic and carcinogenic metalloid which is geogenic in origin and has been shown to be detrimental to human health when present in the environment [
Arsenic mobilization in groundwater is linked to geologic setting and sedimentary components, which control the geochemistry and release of As into groundwater from bedrocks. The geochemistry of groundwater is dominated by redox processes occurring at the sediment-water interface. Adsorption capacity of the mineral surfaces also depends on geochemical parameters, such as pH, electrical conductivity (EC), ionic composition, and mineral type. Thus hydrogeochemical characteristics, such as the oxidation state of the mineral phases, and the associated cofactors affecting arsenic-containing solid phases, are responsible for arsenic mobilization. Aquifers with high arsenic content are characterized by high concentrations of bicarbonate, high pH and dissolved iron under reducing conditions and by sulfate, low pH, and iron precipitate under oxic conditions [
Studies of arsenic hydrogeochemistry and mobilization often involve evaluation of numerous factors, and these factors can be analyzed with multivariate statistics to elucidate underlying processes. Processes like carbonate dissolution, silicate weathering, and ion exchange were found to control major-ion chemistry by using the geochemical modeling and principal component analysis [
The aim of this work is to (i) report the detection of As above the WHO permissible limit in Al-Kharj geothermal fields, which has been done for the first time from this region and (ii) quantitatively evaluate the Al-Kharj area for arsenic contamination
The studied region is Al-Kharj agricultural area, located southeast of Riyadh in Saudi Arabia (23°59′N 47°09′E-24°22′N 47°06′′′E) (Figure
Geography of studied region of Al-Kharj Governorate in Riyadh Province East of KSA (a). The geographical layout of sedimentary rock formation in Saudi Arabia (adapted from: Saudi Geological Survey, 2008). (b) The GPS location indicating the sampling location in the Al-Kharj region in central Saudi Arabia.
The groundwater sampling was collected from 27 boreholes in and around the centrally habitated Al-Kharj agricultural area (Figure
The correlation between the arsenic concentrations and physiochemical properties of groundwater was obtained in the form of Pearson correlation coefficients from the raw data. Principal component analysis (PCA) was used to reduce large number of variables to representative factors called “Principal Components”. The aim was to delineate underling processes defined by parameters/variables like the physiochemical properties of groundwater, organic content, and trace element data. The components accounting for the maximum variance in the PCA output were chosen as significantly relating to the arsenic hydrogeochemistry. Proc factor procedure in the Statistical Analysis Software (SAS) was used to compute the eigenvalues and the components with eigenvalues greater than one were considered. Varimax rotation of component matrix was performed to maximize the variance between the components and simultaneously reduce the number of variables having high loading/score in each component to facilitate easy interpretation of the components. Hierarchical cluster analysis (HCA) was used as classifying tool aiming to club different sampling locations in the Al-Kharj region into few clusters with common underlying structures and to possibly explain the components obtained from PCA as detailed [
The geochemical parameters for groundwater in Al-Kharj are given in Table
Hydrogeochemical parameters of Al-Kharj groundwater forming the characteristic of aquifers spread in an area of 50 sq.Km.
S/N | pH | EC | TDS | Na+ | K+ | Ca+ | Mg+ |
|
Cl− |
|
|
|
TOC |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
mS/cm |
|
(mg/L) | (mg/L) | (mg/L) | (mg/L) | (mg/L) | (mg/L) | (mg/L) | (mg/L) | (mg/L) | (mg/L) | ||
AGW01 | 7.11 | 1.650 | 1056 | 214.8 | 1.08 | 136.5 | 56.90 | 403.5 | 94.6 | 1.35 | 24.15 | 525.8 | 14.47 |
AGW02 | 7.38 | 1.664 | 1064.96 | 286.4 | 9.24 | 150.8 | 51.78 | 333.6 | 119.85 | 4.21 | 39.5 | 676.95 | 7.69 |
AGW03 | 7.27 | 2.309 | 1477.76 | 273.6 | 12.32 | 207.2 | 85.81 | 338.5 | 188.75 | 2.76 | 24.5 | 870.25 | 9.50 |
AGW04 | 7.45 | 2.251 | 1440.64 | 291.5 | 13.43 | 194.7 | 84.50 | 348.6 | 146.65 | 1.35 | 8.05 | 1327.5 | 14.04 |
AGW05 | 7.07 | 5.113 | 3272.32 | 461.6 | 24.37 | 477.2 | 156.5 | 427.3 | 454.1 | 4.89 | 111.4 | 2726.5 | 13.00 |
AGW06 | 7.24 | 3.451 | 2208.64 | 349.1 | 12.94 | 246.2 | 94.62 | 403.1 | 297.55 | 3.78 | 57.2 | 1765.15 | 11.14 |
AGW07 | 7.51 | 2.663 | 1704.32 | 314.5 | 11.79 | 203.9 | 82.43 | 351.5 | 189.65 | 3.87 | 46.9 | 1190.2 | 9.24 |
AGW08 | 7.73 | 4.184 | 2677.76 | 361.8 | 21.89 | 304.1 | 116.1 | 327.1 | 243.3 | 4.88 | 26.5 | 3053.55 | 8.75 |
AGW09 | 6.65 | 6.030 | 3859.2 | 452.6 | 40.99 | 334.7 | 171.0 | 742 | 481.2 | 5.79 | 53.45 | 3535.4 | 21.42 |
AGW10 | 7.52 | 5.905 | 3779.2 | 466.7 | 48.25 | 339.9 | 176.3 | 429.2 | 458.35 | 13.19 | 99.6 | 3540.15 | 13.78 |
AGW11 | 7.31 | 7.236 | 4631.04 | 520.4 | 31.68 | 459.7 | 163.6 | 400.6 | 1815.27 | 1.17 | 116.64 | 3128.76 | 11.66 |
AGW12 | 7.70 | 5.308 | 3397.12 | 259.5 | 20.39 | 461.6 | 141.9 | 244.7 | 1229.53 | 1.71 | 221.52 | 1767.37 | 9.02 |
AGW13 | 7.71 | 1.393 | 891.52 | 187.9 | 10.20 | 102.0 | 40.76 | 326.7 | 291.68 | 8.74 | 65.31 | 492.17 | 7.31 |
AGW14 | 7.84 | 2.996 | 1917.44 | 323.5 | 21.80 | 330.8 | 58.31 | 231.1 | 360 | 10.05 | 137.70 | 1493.37 | 5.54 |
AGW15 | 7.30 | 2.912 | 1863.68 | 276.2 | 23.65 | 360.1 | 85.84 | 449.4 | 265.54 | 4.85 | 81.42 | 1657.25 | 11.96 |
AGW16 | 7.29 | 1.671 | 1069.44 | 214.8 | 12.11 | 128.4 | 52.01 | 339.3 | 204.07 | 2.33 | 27.40 | 745.17 | 7.89 |
AGW17 | 7.03 | 3.016 | 1930.24 | 265.9 | 13.97 | 211.8 | 82.25 | 449.4 | 261.47 | 1.97 | 22.06 | 1399.13 | 9.64 |
AGW18 | 7.56 | 3.277 | 2097.28 | 290.2 | 15.55 | 304.6 | 76.77 | 297.9 | 364.92 | 5.64 | 63.53 | 1853.74 | 7.89 |
AGW19 | 7.39 | 3.226 | 2064.64 | 319.6 | 26.16 | 249.8 | 65.05 | 262.7 | 299.77 | 6.48 | 38.07 | 1418.04 | 6.82 |
AGW20 | 8.24 | 290.2 | 185728 | 104.8 | 1.95 | 16.31 | 3.75 | 24.8 | 59.28 | 0.63 | 11.54 | 101.93 | 0.574 |
AGW21 | 7.35 | 4.187 | 2679.68 | 341.4 | 24.90 | 445.6 | 85.45 | 171.9 | 361.3 | 0.30 | 179.85 | 1955 | 4.07 |
AGW22 | 7.67 | 4.644 | 2972.16 | 359.3 | 16.91 | 359.7 | 87.40 | 301.5 | 464.7 | 5.38 | 248.2 | 1834.55 | 6.76 |
AGW23 | 7.17 | 4.094 | 2620.16 | 313.2 | 21.52 | 455.2 | 93.74 | 328.9 | 377.3 | 2.79 | 136.2 | 1965.95 | 8.27 |
AGW24 | 7.40 | 2.124 | 1359.36 | 182.8 | 10.35 | 217.6 | 58.52 | 338.5 | 125.9 | 0.32 | 21.9 | 1213.3 | 7.21 |
AGW25 | 8.28 | 2.717 | 1738.88 | 290.2 | 23.86 | 363.7 | 86.17 | 578.4 | 190.85 | 3.45 | 98.55 | 1747.5 | 15.55 |
AGW26 | 7.51 | 2.090 | 1337.6 | 213.5 | 12.12 | 192.3 | 53.92 | 338.5 | 124.7 | 0.71 | 40.75 | 1139.2 | 8.08 |
AGW27 | 7.93 | 50.73 | 32467.2 | 1589.5 | 36.33 | 249.3 | 210.2 | 226.9 | 275.32 | 1.09 | 26.22 | 158.74 | 7.52 |
Trace element data for Al-Kharj groundwater forming the characteristic of aquifers spread in an area of 50 sq. Km.
S/N | Li | Sb | Mn |
|
Fe | B |
---|---|---|---|---|---|---|
(mg/L) | (mg/L) | ( |
( |
( |
(mg/L) | |
AGW01 | 0.049 | 0.012 | 6 | 31 | 129 | 0.0252 |
AGW02 | 0.062 | 0.272 | 1 | 2 | 139 | 0.445 |
AGW03 | 0.071 | 0.317 | 2 | 6 | 89 | 0.349 |
AGW04 | 0.074 | 0.556 | 3 | 17 | 249 | 0.388 |
AGW05 | 0.138 | 1.053 | 3 | 14 | 93 | 1.833 |
AGW06 | 0.081 | 1.051 | 1 | 25 | 49 | 0.548 |
AGW07 | 0.074 | 0.861 | 3 | 23 | 65 | 0.752 |
AGW08 | 0.125 | 0.539 | 13 | 20 | 173 | 1.013 |
AGW09 | 0.137 | 0.372 | 1 | 31 | 88 | 2.272 |
AGW10 | 0.203 | ND | 11 | 55 | 76 | 2.056 |
AGW11 | 0.151 | 0.661 | 4 | 88 | 75 | 1.350 |
AGW12 | 0.103 | 0.413 | 7 | 122 | 203 | 0.284 |
AGW13 | 0.052 | ND | 1 | 73 | 56 | 0.257 |
AGW14 | 0.123 | ND | 3 | 38 | 85 | 0.702 |
AGW15 | 0.123 | 0.776 | 6 | 30 | 560 | 0.601 |
AGW16 | 0.062 | 0.440 | 1 | 27 | 67 | 0.293 |
AGW17 | 0.093 | 0.599 | 7 | 25 | 145 | 0.567 |
AGW18 | 0.091 | 0.595 | 2 | 15 | 60 | 0.446 |
AGW19 | 0.121 | 1.085 | 14 | 17 | 236 | 0.515 |
AGW20 | 0.009 | 0.406 | 2 | 16 | 68 | 0.459 |
AGW21 | 0.107 | 0.115 | 1 | 16 | 95 | 0.537 |
AGW22 | 0.085 | 0.155 | 1 | 17 | 95 | 0.537 |
AGW23 | 0.130 | 1.158 | 1 | 20 | 59 | 0.663 |
AGW24 | 0.140 | 0.089 | ND | 18 | 42 | 0.290 |
AGW25 | 0.114 | 0.107 | 4 | 22 | 69 | 0.532 |
AGW26 | 0.063 | 0.003 | 1 | 26 | 256 | 0.327 |
AGW27 | 0.210 | 0.003 | 5 | 75 | 151 | 4.254 |
*ND: below the detection level.
Arsenic was detected at all the sites, with 92.5% of the boreholes showing concentrations above the WHO permissible limit of 10
Piper diagram displaying the major ions present in the groundwater, indication of Ca+2-Mg+2-
The principal component analysis of standardized parameters resulted in five components with eigenvalues of 7.0518, 2.7257, 1.6086, 1.3456, 1.0368, accounting for 44.1%, 17.0%, 10.1%, 08.4%, and 06.5% of the total variance, respectively (Tables
Pearson correlation matrix for the hydrochemical properties indicating the extent of effect of parameters.
pH |
EC ( |
Na+ | K+ | Ca+ | Mg+ |
|
Cl− |
|
| |
---|---|---|---|---|---|---|---|---|---|---|
EC ( |
0.469 | |||||||||
Na+ | 0.113 | −0.026 | ||||||||
K+ | −0.116 | −0.242 | 0.561 | |||||||
Ca+ | −0.174 | −0.411 | 0.207 | 0.630 | ||||||
Mg+ | −0.210 | −0.272 | 0.725 | 0.826 | 0.617 | |||||
|
−0.519 | −0.520 | −0.013 | 0.374 | 0.241 | 0.381 | ||||
Cl− | −0.101 | −0.152 | 0.153 | 0.422 | 0.614 | 0.526 | 0.076 | |||
|
0.105 | −0.208 | −0.014 | 0.297 | 0.735 | 0.244 | −0.098 | 0.531 | ||
|
−0.347 | −0.344 | 0.014 | 0.706 | 0.720 | 0.607 | 0.521 | 0.502 | 0.370 | |
As (ug/L) | 0.183 | −0.055 | 0.343 | 0.349 | 0.270 | 0.485 | −0.049 | 0.709 | 0.347 | 0.108 |
Loading of physiochemical parameters in the principal components expressing the geochemical processes.
PC1 | PC2 | PC3 | PC4 | PC5 | |
---|---|---|---|---|---|
pH | −0.071 | −0.325 | 0.362 | 0.117 | 0.313 |
TDS | 0.361 | −0.116 | −0.098 | −0.013 | −0.077 |
Na+ | 0.340 | 0.041 | −0.012 | 0.175 | −0.148 |
K+ | 0.336 | 0.031 | 0.210 | 0.067 | −0.054 |
Ca+ | 0.282 | −0.253 | −0.117 | 0.003 | −0.307 |
Mg+ | 0.360 | 0.046 | −0.067 | −0.081 | 0.024 |
|
0.155 | 0.456 | −0.105 | −0.104 | 0.273 |
Cl− | 0.247 | −0.301 | −0.226 | −0.255 | 0.208 |
|
0.124 | 0.028 | 0.551 | 0.357 | 0.250 |
|
0.158 | −0.445 | −0.109 | 0.129 | −0.072 |
|
0.355 | 0.050 | 0.058 | 0.072 | −0.126 |
TOC | 0.185 | 0.416 | −0.087 | −0.219 | 0.278 |
Mn | 0.105 | −0.009 | 0.578 | −0.327 | −0.109 |
As | 0.145 | −0.304 | −0.023 | −0.381 | 0.568 |
Fe | −0.029 | 0.332 | 0.265 | −0.622 | −0.403 |
B | 0.331 | 0.213 | 0.057 | 0.181 | −0.020 |
The second component (PC2) indicates the reductive dissolution of iron oxides as a possible mechanism. This component had a maximum loading for TOC and
Multivariate statistical analysis of Al-Kharj groundwater data: (a) cluster analysis with respect to As and B (b) cluster analysis with respect to Fe, As, Mn, TOC,
While reductive dissolution of As from arsenopyrites/Fe oxihydroxides is a biotic process, the competitive effect of direct carbonate ions in ground water is proposed to be another major abiotic process of As release [
Bicarbonate and nitrification corresponding to correlation with the total arsenic
A direct comparison of the temperature from wellheads has in many cases shown low correlations comparing the arsenic levels with the temperature gradient [
Characterization of hydrogeochemistry and arsenic contamination in geothermal systems of Al-Kharj aquifers in Saudi Arabia has been done to understand the primary processes causing the arsenic mobilization into the groundwater. The main processes responsible are geothermal, and this has been established with different geothermal tracers and geostatistics. The reductive dissolution of arsenic bearing minerals could also be a process occurring, this has been observed and concluded as the aquifer systems in Al-Kharj region show significant amounts of TOC content and experience a slow water moment with low recharge rates, which is why the system has only two major water types classification. The processes like the mobilization due to competitive effects of carbonate ions to As and chemolithotropic dinitrification of arsenite to less mobile arsenate can be ruled out in this system which is characteristically anoxic and high in sulphate levels. A thorough investigation though is needed to comprehensively study this system which may include profiling mineralogical and morphological patterns of the sedimentary rocks and aquifer system along with hydrogeochemical studies.
This project was supported by Research Center, College of Food & Agricultural Sciences, Deanship of Scientific Research, King Saud University.