1 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 1735 GROUND WATER QUALITY FOR IRRIGATION IN BENGHAZI REGION Hasan G. Mohamed Department of Civil Engineering, University of Garyounis, Libya ABSTRACT Groundwater quality is an important as its quantity. In general, the Benghazi plain suffers from bad quality water. It is not clearly known whether the source of contamination is the sea water or there are possible sources of contamination by contact of water with evaporitic sediments in the area. Investigation based on large amount of data collected from the study area revealed that the water quality varies with depth of wells, time of pumping and during pumping of the wells. The concentration and composition of dissolved constituents in the groundwater determines it quality for irrigation use. From the values of electrical conductivity and sodium adsorption ratio for the water samples obtained in the region and using the salinity diagram, it is concluded that the majority of the water samples fall in the C 3 -S 1, C 4 -S 1 and C 4 -S 3 classes. A few samples were in the C 2 -S 1, C 3 -S 2 and C 4 -S 1 classes. The C 3 -S 1 class is well suited for irrigation. The water samples in the eastern part of the study area are better suited for irrigation than that in the western and central parts. Keywords: Groundwater quality, groundwater contamination, Irrigation water 1- INTRODUCTION Groundwater is a precious and the most commonly distributed resource of the earth. At present nearly one fifth of all the water used in the world is obtained from groundwater resources. Agricultural is the main user of water accounting for 80% of all consumption. For example, it takes about 1000 tons of water to grow one ton of grain and 2000 tons to grow one ton of rice. Many countries in the Arab world are dependent entirely on ground water resources or desalinated water or a suitable combination of both for domestic, agricultural and industrial requirements. It is now generally recognized that the quality of groundwater is as important as its quantity. All groundwater contains salts in solution that are derived from the location and past movement of the water. The quality required of all groundwater supply depends on its purpose; thus, needs for drinking water, industrial water, and irrigation water vary widely. In order to establish quality criteria, measures of chemical, physical, biological, and radiological constituents must be specified, as well as standard methods for reporting and comparing results of water analysis. Dissolved gases in groundwater can also pose hazards if their presence goes unrecognized.
2 1736 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 2- QUALITY OF IRRIGATION WATER AND THE STANDARDS TO BE APPLIED Not all water is suitable for irrigation use. Just as every water is not suitable for human beings, in the same way; all types of waters are not suitable for plant life. Water containing impurities which are injurious to plant growth is not satisfactory for irrigation and is called unsatisfactory water. Unsatisfactory water contains: i- Chemicals toxic to plants or to persons using the plant as food. ii- iii- Chemicals which react with the soil to produce unsatisfactory moisture characteristics. Bacteria injurious to persons or animals eating plants irrigated with the water. The quality of suitable irrigation water is very much influenced by the constituents of the soil which is to be irrigated. A particular water may be harmful for irrigation on particular soil, but the same water may be acceptable or even useful for irrigation in some other soil. The various types of impurities, which make the water useful for irrigation, are classified as: i- Sediment concentration: the effect of sediments present in the irrigation water depends upon the types of irrigated land. The fine sediment from the water may improve the fertility of the sandy soil, but if the sediment has been derived from the eroded area, it may reduce fertility or decrease soil permeability. In general, groundwater does not have sufficient sediment to cause any serious problem in irrigation. ii- Total concentration of soluble salts in water: salts of calcium, magnesium, sodium and potassium present in the irrigation water prove injurious to plants. Then injurious effects of salts on soils, causing changes in soil structure. Permeability and aeration, indirectly affect the plant growth and it depends upon the concentration of salts left in the soil. At the beginning of irrigation with undesirable water, no harm may be evident, but with the passage of time, the slat concentration in soil may increase to a harmful level, as the soil solution gets concentrated by evaporation. The salt concentration is generally expressed in ppm (particle per million) or mg/liter (milligram per liter). The critical salt concentration in the irrigation water depends upon many factors. However, an amount in excess of 700 ppm is harmful to some plants, and more than 2000 ppm is injurious to all crops. Salt concentration is generally measured by determining the electrical conductivity of water which is expressed in micromohs per centimeter, (see Tables 1 and 2).
3 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 1737 Table 1: Types of water according to Electrical conductivity (E.C.) at 25 C Class No. Type of water Electrical conductivity (E.C.) in micromohs/cm C 1 Low conductivity water < 250 C 2 Medium conductivity water C 3 High conductivity water C 4 Very high conductivity water > 2250 Table2: Suitability of the different types of water No. Type of water Use in irrigation 1 C 1 : Low salinity water Can be used for almost all crops and for almost all kinds of soils. 2 C 2 :Medium salinity water Can be used if a moderate amount of leaching occurs; normal salt tolerant plants can be grown without much salinity control. 3 C 3 : High salinity water Can be used in soils with restricted drainage. Special precautions and measures are to be undertaken for salinity control. 4 C 4 : Very high salinity water Generally not suitable for irrigation. iii- Proportion of sodium ions to other cations: most of soils contain calcium and magnesium ions and a small quantity of sodium ions. High sodium soils are plastic, sticky when wet, prone to form clods and they crust on drying. Most normal soils of arid regions, as is the case at hand, have calcium and magnesium as the principal cations, with sodium representing generally less than 5% of the exchangeable cations. If the sodium percentage in the soil increases to about 10% or more, the aggregation of soil grains brake down and the soil becomes less permeable and of poorer tilt and crusts when dry. The sodium hazard is typically expressed as the sodium adsorption ratio (SAR). This index quantifies the proportion of sodium (Na + ) to calcium (Ca ++ ) and magnesium (Mg ++ ) ions in a sample. Calcium will flocculate (hold together), while sodium disperses (pushes apart) soil particles. This dispersed soil will readily crust and have water infiltration and permeability problems. General classifications of irrigation water based upon SAR values are presented in Table 3. By adding gypsum (CaSO 4 ) to the water or the soil, the SAR value can be reduced. The suitability of these four kinds of water for irrigation is illustrated in Table 4.
4 1738 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt SAR = + Na meq / L ( Ca meq / L ) + ( Mg meq / L ) (1) 2 where meq/l = mg/l divided by atomic weight of ion divided by ionic charge (Na + = 23.0 mg/meq, Ca ++ = 20.0 mg/meq, Mg ++ = mg/meq). Table 3. General classification of water sodium hazard based on SAR values SAR values Sodium hazard of water Comments 1-9 Low Use on sodium sensitive crops must be cautioned Medium Amendments (such as gypsum) and leaching needed High Generally unsuitable for continuous use. 26 Very High Generally unsuitable for use. Table 4. Uses of different types of water for irrigation versus salinity No. Type of water Use in irrigation 1 S 1 : Low salinity water 2 S 2 : Medium salinity water 3 S 3 : High salinity water Can be used for irrigation on all crops and for almost all crops except those which are highly sensitive to sodium. It may be used on coarse texture or organic soil with good permeabilities but in fine textured soil it is appreciably. May cause harmful effects on almost all the soils and it requires good drainage, high leaching, gypsum addition, etc.. 4 S 4 : Very high salinity water Generally not suitable for irrigation. iv- Concentration of potentially toxic elements present in water: A large number of elements may be toxic to plants. Traces of boron are essential to plant growth, but its concentration above 0.3 ppm may prove toxic to some plants. The concentration above 0.5 ppm is dangerous except to some crops; anyhow, the boron concentration should not exceed 4 ppm. Selenium even in low concentration is toxic and must be avoided. v- Bi-carbonate concentration as related to calcium plus magnesium: High concentration may result in precipitation of calcium and magnesium bicarbonate from the salt solution, increasing the relative proportion of sodium ions and causing sodium hazard.
5 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 1739 vi- Bacterial contamination: Bacterial contamination of irrigation water is not a serious problem, except crops irrigated with highly contaminated water and are directly eaten without being cooked. 3- WATER QUALITY IN BENGHAZI REGION In general, the Benghazi plain suffers from bad quality water. It has not been definitely ascertained whether the source of saline water is the sea or there are other possible sources of contamination by contact of water with evaporitic sediments in the area. Based on the data collected and the different investigations conducted on water quality in this area, the results can be summarized in Table 5 and are as follows: Table 5: Chemical analysis of some selected water samples in the study area Well No. E.C. (micromohs/cm) Ca (ppm) Mg (ppm) Na (ppm) T.D.S (ppm) S.A.R (%) 3488III/A I/I I/I I/H III/A I/I I/H I/H III/I III/A NH34/II I/A II/G I/C I/E I/I I/E I/G I/I I/G I/G I/G I/H Variation of water quality with depth: In the eastern and central parts of Hawari area, no specific relation was observed between the quality of water and the depth of wells upto 140 m. the results of chemical
6 1740 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt analyses of the water samples collected at different depths to 240 m in an exploratory well in the Hawari area further correlates this view. On the other hand, studies in the Kuwayfiyah area shoed the deterioration in quality with depth. Upto a depth of 40 m, the electrical conductivity (E.C) was between to micro mohs/cm and chloride content ranged from 4400 to 5800 ppm. Below this depth, the E.C. increased from to micro mohs/cm and the chloride content also increased from to ppm. This clearly indicates the presence of salt-fresh water interface at shallow depths, at about 50 m below the land surface at this place. The results of the analysis of water samples in a number of wells at different locations and different depths indicates in general that: water quality is deteriorating with depth, and the salt-fresh water interface is at shallow depths in the western part (at about 50 m) and at deeper levels in the central part of the area (below 400 m) Variation of water quality during pumping: In order to study the variation of water quality with time during pumping, the total dissolved solids (TDS) and the chloride contents of water at the beginning, middle and end of pumping test period was measured in a number of wells in different parts of Benghazi plain. From the studies conducted, following points are observed: 1. During the pumping tests in one of the wells, the E.C. at the beginning was 2000 micro mohs/cm, after 5 hours of pumping it changed to 8000 micro mohs/cm and at the end, after 72 hours of pumping, the E.C. recorded was micro mohs/cm. This clearly shows that the quality of water deteriorates with time of pumping. 2. In another well, the recorded E.C. was micro mohs/cm during pumping where as in idle conditions the E.C. ranged from to micro mohs/cm indicating a clear deterioration in the quality of water with pumping. 3. Also, from the studies conducted it was noticed that though the quality of water is deteriorating with depth, at low pumping levels no appreciable change in quality with time during pumping could be observed Variation of water quality with time: During different years water samples were collected in a number of wells in different parts of the area. The quality was analyzed with respect to the TDS and chloride contents in the water samples. Based on the analysis, it is indicated that: 1. There is definite flushing of ground water in good rainfall years (decrease in TDS and chloride contents). 2. Heavy pumping in the wells in the area with overdraft can results in the rising of salt-fresh water interface and fast deterioration of water quality. 3. In general, the water quality in the Benghazi plain is deteriorating with time, especially in the heavily pumped well fields.
7 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt Suitability of groundwater for irrigation in Benghazi plain: In order to evaluate the suitability of groundwater in the Benghazi plain for irrigation needs, the values of E.C. and SAR were determined for a number of water samples collected from bore holes in the region. The U.S. Regional salinity laboratory has constructed a diagram for classification of irrigation water describing 16 classes (Fig. 1) with reference to SAR as an index for sodium hazard S and E.C. as an index of salinity hazard C. the values of E.C. and SAR for the samples where plotted in this diagram. From their positions in the diagram, the following observations can be made: - It has been found that a majority of the samples fall in the C 3 -S 1, C 4 -S 2 and C 4 -S 3 classes. - A few samples were in C 2 -S 1, C 3 -S 2 and C 4 -S 1 classes and some fall beyond the limits of the diagram. - The water samples of wells in the eastern parts of the area mostly were in the C 3 -S 1 classes, the quality of which is better when compared to C 4 - S 2 and C 4 -S 3 classes of water in which most of the water samples from the wells in the western and central parts fell. Fig. 1 Diagram for classification of irrigation waters (after Richards)
8 1742 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt The significance and interpretation of these quality ratings can be summarized as follow: 1- C 2 -S 1 class: this is low sodium and medium salinity water. Water of this type can be used for irrigation on almost all soils with little danger of the development of a sodium problem, if a moderate amount of leaching occurs. 2- C 3 -S 1 class: this is a low sodium and high salinity water which can be used for irrigation on almost all soils with good drainage but not good for sodium sensitive crops. Even with adequate drainage, special management for salinity control may be required and plants with good salt tolerance should be selected. 3- C 3 -S 2 class: this belongs to medium sodium and high salinity water group and may present a moderate sodium problem in fine-textured (clay) soils, unless there is gypsum in the soil. This water can be used on coarse textured (sandy) or organic soils that take water well. Special arrangements for salinity control are required. 4- C 4 -S 1 class: this is low sodium and very high salinity water. Water of this type is not suitable under ordinary conditions but because of the low sodium content, may be used on coarse textured and organic soils. The soils must be permeable, drainage must be adequate, irrigation water must be applied in excess to provide considerable leaching and very salt tolerant crops should be planted. 5- C 4 -S 2 class: this is a medium sodium and very high salinity water. It may present a moderate sodium problem in fine textured soils unless there is gypsum in the soil. Not ordinary suitable for irrigation but under special circumstances can be used on sandy or organic soils special circumstances can be used on sandy or organic soils. 6- C 4 -S 3 class: high sodium and very high salinity type. It produces sodium problem in most soils and not suitable for irrigation under ordinary conditions. It may be occasionally used under very special circumstances. Soils must be permeable, adequate drainage, good leaching should be ensured. Very salt tolerant crops should be selected. 7- outside the chart: generally not suitable for irrigation. In the classification of irrigation water, it is assumed that the water will be used under average conditions with respect to soil texture, infiltration rate, drainage, quantity of water used, climate, and salt tolerance of crop. Large deviations from the average for one or more of these variables may take it unsafe to use, what, under average conditions, would be good water or may make it safe to use hat under average conditions would be a water of doubtful quality. This relationship to average conditions must be kept in mind in connection with the use of any general method for classification of groundwater.
9 Twelfth International Water Technology Conference, IWTC , Alexandria, Egypt 1743 CONCLUSIONS The Benghazi Plain, in general, suffers from bad quality water. Based on the different investigation conducted on water quality in this area, the results can be summarized as follows: 1- The water quality is deteriorating with depth. 2- The salt-fresh water interface is at shallow depths in the western part (at about 50 m) and at deeper levels (below 400 m) in the central part of the area. 3- The quality of water deteriorates with time of pumping. 4- During heavy rainfall months there is flushing of groundwater leading to improvement in quality. 5- In general, the water quality in the Benghazi plain is deteriorating with time, especially in the heavily pumped well fields. The concentration and composition of dissolved constituents in the groundwater determine its quality for irrigation use. The characteristics of irrigation water that are most important in determining its quality are: i- Total concentration of salts. ii- Relative proportion of sodium to other cations. iii- Concentration of boron or other elements that may by toxic. iv- The bicarbonate concentration. From the values of electrical conductivity and sodium adsorption ratio for the water samples obtained from the different areas and using the salinity diagram, the irrigation water quality ratings can be summarized as follows: - The majority of the water samples fall in the C 3 -S 1, C 4 -S 1 and C 4 -S 3 classes. The C 3 -S 1 class is well suited for irrigation. However, C 4 -S 2 and C 4 -S 3 classes are not ordinarily suitable for irrigation. - A few samples were in the C 2 -S 1, C 3 -S 2 and C 4 -S 1 classes. - The water samples in the eastern part of the area are better suited for irrigation than the water in the western and central parts. REFERENCES 1- Todd, D. K., (1980): Ground Water Hydrology, John Wiley & Sons, New York. 2- Walton, W. C., (1970): Ground Water Resources Evaluation, McGraw-Hill, New York. 3- Khan, N. Y., Raju, T. S., and Ghosh, A. K., (1978): Water resources evaluation of the Benghazi Plain, Vol. 1, Benghazi. 4- Wilcox, L. V., (1948): The Quality of Water for Irrigation Use, U.S. Dept. Agri. Tech. Bull. 962, Washington; D. C., 40pp. 5- Wilcox, L. V., (1955): Classification and use of irrigation water, U.S. Dept. Agri. Circ. 969, Washington; D. C., 19p.