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Hydrchemical evaluation of groundwater in Khartoum state, Sudan
3. Geology and Hydrogeology
The geologic setting is composed of Basement Complex, Omurman formation, and Gezira formation [3]. The basement complex consists of acid grey gneisses and granite. Omdurman formation is composed of a sequence of sandstone, conglomerate and mudstone, of more than 400 m thick, rests uncomformably on the Basement complex [4]. Gezira formation covers the area between the Blue and White Nile, and small strip east of the Blue Nile. It consists of a sequence of unconsolidated interbeded clay, silt, sand and gravel layers. Calcrete, evaporite and salt rocks are Characteristic features of the upper part of Gezira and Omdurman formation. The thickness of Gezira formation ranges from few meters to more than 80 m [5]. Omdurman and Gezira formation encompass the main aquifer systems.
Hydrogeologic system is composed of an upper aquifer developed in the upper part of Gezira and Omdurman formations and a lower aquifer at the deeper parts of these formations. The upper aquifer is semi-confined and up to 120 m thick, whereas, the lower aquifer is dominantly confined with thickness variation of 150-500m [4]. Depth to water level varies from few meters to more than 85 m. The aquifer system is exclusively recharges from Blue Nile, White Nile, River Nile, direct precipitation and seasonal streams. The hydraulic conductivity of the lower aquifer (21.6 m/d) is ten orders higher than the upper one [5]. Aquifer systems are partially hydraulically connected. The groundwater flow direction is generally outwards of river courses with hydraulic gradient varies from 0.01 close the rivers to 0.0006at far distance.
4. Sampling and Analytical Procedure
Eighty- three samples from boreholes at depths ranges from 30 to 85 m were collected for hydrochemistry (Figure 1). They were analyzed for the major ion chemistry, using the standard methods. Electric conductivity (EC) and hydrogen ion concentration (pH) were measured immediately at the field site, using portable Orion EC- and pH-meter. Further analyses for major ions were performed in the Non-Nile Sudan water research central Laboratories: total dissolved solids (TDS) were measured by sample evaporation techniques. Bicarbonate (HCO-3) and total alkalinity (T Alk.) were estimated by titration with HCl. Total hardness (TH) and calcium (Ca2+) were analyzed titrimetrically.. Sodium (Na+) was analyzed by flame photometry. Chloride (Cl-) was estimated by titration with AgNO3. The analytical precision for the measurements of cation-anion is indicated by ionic balance error, which is observed to be within the stipulated limit of 5%. All values were in milligram per liter (mg\l) unless otherwise indicated. The hydrochemical characteristics of groundwater in the study area were summarized as statistical overview in table 1.
Table 1: Statistic overview of hydrochemical characteristics of groundwater
| Parameters |
Eastern area |
Central area |
Omdurman area |
| Min. |
Max. |
Mean |
Min. |
Max. |
Mean |
Min. |
Max. |
Mean |
| PH |
6.5 |
8.4 |
7.6 |
7.2 |
8.4 |
7.8 |
6.8 |
9.6 |
7.7 |
| EC (µs/cm) |
281 |
4070 |
918 |
263 |
3180 |
718 |
116.1 |
4690 |
867 |
| TDS |
150 |
2890 |
626 |
184 |
2204 |
492 |
180 |
3283 |
751 |
| Hardness |
52 |
618 |
208 |
42 |
920 |
174 |
70 |
950 |
194 |
| Alkalinity |
124 |
372 |
232 |
75 |
573 |
239 |
2 |
402 |
30.3 |
| HCO-3 |
122 |
372 |
237 |
98 |
573 |
233 |
110 |
402 |
239 |
| Cl- |
12 |
817 |
105 |
10 |
585 |
78 |
12 |
1007 |
137 |
| Na+ |
25 |
980 |
116 |
8 |
304 |
96 |
16 |
504 |
127 |
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