Water Qualty in the Tazerbo Wellfield, Libya

WATMED3

Third International Conference on

WATER RESOURCES IN MEDITERREANBASIN

Tripoli – Lebanon 1 – 3 November 2006

WATER QUALITY OFTHEGREAT MAN-MADE RIVER PROJECT -LIBYA

(CaseStudy: TAZERBO WELLFIELD)

By

1- Prof.Dr. Farag .A. ElMabrouk

2- Dr. Naser. Im. Sahli

BENGHAZI - LIBYA

WATER QUALITY OF THEGREAT MAN-MADERIVER PROJECT -LIBYA

(CaseStudy:TAZERBO WELLFIELD)

*Prof. Farag .A. El-Mabrouk & **Dr. Naser Im Sahli

*Faculty of Engineering –Garyounis University – P.O Box9476

Benghazi- Libya Telefax: 218619080689 E-mail:

**Great Man-Made River Authority, Benghazi, Libya

E-mail:. Tel: 00218913796672

ABSTRACT:

This studyexplores the water quality of the Tazerbo Wellfield.Tazerbo Wellfield is one of Libya’s Great Man-Made River Project Wellfields, and it is designed to produce 1 million cubic meters per day of ground water, which will be subsequently conveyed to the northern coastal strip for towns water supply and agricultural utility.

This investigation explores the various criteria including the classification of water type, physical parameter, study of major and minor cations, anions and trace elements, and the determination of water aggressivityandwater quality for potable uses.

The water indicate has low sodium content and medium salinity, which means that it can be used with little danger on nearly all soils, and it can be used where a moderate amount of leaching in the soil occurs when it used for irrigation.

The Tazerbo water is classified as soft water and it is characterized by low mineral content.The lowest concentration of all anions in the Tazerbo Wellfield is nitrate which was found to be 0.0 mg/l for almost all wells.

The concentration of CO2, Mn+2 and CO2 in the Tazerbo Wellfield wateris high. Therefore, it is important to treat this water to reduce or remove irons, manganese and dissolved carbon dioxide to the admissible levels before it is supplied to cities.Tazerbo Well field water is characterized by reducing conditions with almost all wells having a negative oxidation-reduction potential (ORP). Several indexes were used to indicate the potential of Tazerbo Wellfield,and it has showed that the water is corrosive.

1 INTRODUCTION:

The Great Man Made River Project (GMRP) was launched in 1983, aiming at transporting 6.5 million cubic meters of water per day from desert wellfields to the coastal zone. The GMRP has established important water supply wellfields, including: Sarir-Sirt, Tazerbo-Benghazi System (SSBT); Kufra/Tazerbo System; Sirt/Assdada System; Jagboob/Tobruk System; Hasouna-Jeffara System; and Ghadamis/Zwara system, Figure 1.

The Tazerbo Wellfield is situated in south eastern Libya, Figure 1.

It is located in a flat desert area at the southern limit of the Sarir plain and covers an area of 1000 square kilometers.

The Tazerbo Wellfield lies along the south - west north – east alignment of the southern flank of the Hercynian uplifts which forms the boundary Separating the Kufra and Sirt basins.

2 DISTRIBUTIONS OF THE WELLS:

The Tazerbo Wellfield consists of 108 wells which will discharge their flow into a collector pipeline and then into the header tank at Tazerbo.

These wells are distributed into three parallel lines; each line is divided in two sections, and each line is 50 km long and consists of 36 wells. The well numbers in the first line are 101- 118 and 201- 218, the second line as 301- 318 and 401- 418, and the third line as 501- 518 and 601- 618. These lines are named line 100 to line 600.

The spacing between the wells is 1.3 km, and the distance between these three parallel lines is 10 km, (Figure1)

The total depth of the production wells (108 wells) in Tazerbo Wellfield ranges from 460 to 580 m.

Figure 1 Tazerbo Wellfield and the Layouts of the Great Man-Made River

Project and MajorSystems (GMRP, 2004).

3 WATER QUALITY OF THE TAZERBO WELLFIELD:

Field measurement data have been analysis in the field, whichincludes temperature, pH value, dissolved oxygen, dissolved carbon dioxide and dissolved hydrogen sulphide were carried out.Table1 represents the statistical analysis of field measurements for all the Tazerbo Wellfields.

Table 1StatisticalAnalysis of Field Measurement,Tazerbo Wellfield

SD = Standard Deviation, %CV = Percentage Coefficient of Variation andN= number of wells

3.1 PHYSICAL AND GENERAL PARAMETERS:

Physical andgeneral parameters which havebeen analysis for the Tazerbo Wellfield are:TDS, EC, ORP, Eh, total hardness and total alkalinity. Table2 represents the statistical analysis of Physical andgeneral parametersfor all the Tazerbo Wellfield.

Table 2StatisticalAnalysis of Physical and General Parameter, Tazerbo Wellfield

SD = Standard Deviation, %CV = Percentage Coefficient of Variation and N= number of wells.

The major chemical constituents are considered to be the cations:Calcium, magnesium sodium, potassium, and the anions: bicarbonate, carbonate, chloride,sulphate, nitrate and silicate. Table 3 represents the statistical analysis of major cations and anions for all the Tazerbo Wellfields.

Table3 Statistical Analysis of the Cations and AnionsLines 100 - 600, Tazerbo Wellfield

SD = Standard Deviation, %CV = Percentage Coefficient of Variation and N= number of wells.

3.2 Chemical Indicators of Corrosion and Water Aggressivity

(Saturation Indices and Corrosion Indices)

Several indexes were used to indicate the potential of Tazerbo Wellfield water to cause corrosion or to precipitate incrustating deposits on wells pumps and pipelines.

The calculated indices are presented in Table 4 asstatistical analysis.

Table 4 Stastical Analysis of the Chemical Indicators of Corrosion and Water Aggressivity

SD = Standard Deviation, %CV = Percentage Coefficient of Variation and N= number of wells.

4 DISCUSSION:

The overall mean water temperature of Tazerbo Wellfield is 32.31oC, with a minimum value of 27.2 oC and a maximum value is 35.5 oC.

A noticeable variation was also observed in the pH values of Tazerbo Wellfield water. Thus the overall average pH value was 6.48, whereas a minimum value of 5.92 and a maximum value of 7.16.

Tazerbo Wellfield in general has very low concentrations of dissolved oxygen, with averages as 0.2 mg/l. The concentration of CO2is high and reported in most wells. The overall average is 32.7 mg/l.Their for it is necessaryto treatment of water before itis pumped into the GMRP pipe systems. In contrast to CO2, Tazerbo Wellfield has a very small concentration of dissolved hydrogen sulphide, where the overall average is 0.012 mg/l.

Tazerbo Wellfield water generally has moderate to low electrical conductivity (EC) values. Thus the wellfield overall EC mean value was found to be 319.4 µS/cm. Such moderate to low EC values for this wellfield certainly reflect the relatively low dissolved solids content of the water. The average value of the total dissolved solids concentration for Tazerbo Wellfield is 280.61 mg/l. The Wellfield water is characterized by reducing conditions with almost all wells having a negative oxidation-reduction potential ( ORP).

The Wellfield water is a soft water according to the measured total hardness as CaCO3.

The total alkalinity for the wellfield water is averaging value is 119.55 mg/l as CaCO3.

Calcium averaged 9.10 mg/l and. The median value was 9.05 mg/l, reflecting an approximatlynormal distribution of calcium concentrations.

Magnesium concentration in the wellfield was quite low. The overall average was 12.81 mg/l.

Sodium concentrations showed some variation throughout Tazerbo Wellfield. The overall average, however was 14.24mg/l.

The concentrations of potassium cations were found to be greater than that of sodium cations in Tazerbo wellfield. This case is seems to be in reverse to most natural waters, where sodium tends to be higher than potassium.

Bicarbonate was found to be the dominant ion and anion species found in Tazerbo Wellfield.

The concentration of carbonate ions in Tazerbo Wellfield water is extremely low and averages concentration across the wellfield is 0.04 mg/l.

The chloride concentrations in The Wellfield water are moderately low and averaging at only 10.2 mg/l for the entire wellfield.

The overall averageconcentration of sulphate in the Wellfield water is 13.7 mg/l.

The lowest concentration of all major chemical constituents in the Wellfield is nitrate which was found to be 0.0 mg/l for all wells except two wells showed concentrations of 1.10 and 3.9 mg/l.

The Wellfield water is classified as a soft water; consequently it shows a low concentration of silica.

The Langelier index for Tazerbo Wellfield water was found as a negative (-ve) value, meaning that the water is under- saturated with calcium and will dissolve existing carbonate coating. In addition, the Ryznar saturation index for The Wellfield water has an average of 10.07, and as this is greater than 7, this implies that the water is under-saturated with respect to CaCO3 and would therefore tend to dissolve any existing solid calcium carbonate.

The calcium carbonate precipitation potential (CCPP) average CCPP value for the Wellfield water is -101.90 mg/l as CaCO3 which confirms the aggressive nature of this water.

Corrosivity index (CI) for the Wellfield water. According to this method, the CI value for The Wellfield water average of 0.215, which is corrosive?

The chloride corrosion index (CCI)for the Wellfield water average has classified the water in the uncertain zone of corrosion, which meansthe water is moderate in corrosion.

5 CONCLUSION:

Water type for the Tazerbo Wellfield is sodium bicarbonate.The dominance of these ions could not be explained by ion exchange, as their concentration was not high enough to allow ion exchange to take place. These differences can be explained by a mixing effect of the groundwater as it moves within the aquifer.

Tazerbo Wellfield water is a soft water according to the measured total hardness as CaCO3, Table 3.

manganese exists as Mn++, since the pH of the water is too low to form manganese oxide complexes and iron compounds (as Fe ++) solubility is low.

Tazerbo Wellfield water is characterized by reducing conditions with almost all wells having a negative oxidation-reduction potential (ORP), Variation in the sulphate concentration is likely due to reducing conditions.

The determination of several indexes used to indicate the potential of Tazerbo Wellfield water to cause corrosion or to precipitate incrustating deposits on wells pumps and pipelines have showed the aggressivity of water to the cementation materials of the GMRP pipeline.

Tazerbo Wellfield water meets the WHO Guidelines for drinking water standards, with the exception of iron, manganese, hydrogen sulphide and potassium. Thus the iron concentration is seven times higher than the admissible levels of the WHO, and the manganese concentration is double the admissible level for WHO guideline. The average concentration of potassium is 28.95 mg/l which is considered to be high, although of no great consequence for water quality. In conclusion, the water has been treated for remove iron, CO2 and manganese before delivery to the consumer.

Considering agricultural use for the Tazerbo Wellfields water, it is suitability for irrigation according to the international standers.

6 REFERENCES :

APHA 1997, Standard methods for the examination of water and wastewater, 18th edition, in A. E. Greenberg, L. S. Clesceri and A. D. Eaton (eds), American Public Health Association, 905 pp.

Appelo, C.A.J. and Postma, D. 1996, Geochemistry, groundwater and pollution, A.A. Balkema, Rotterdam, Brookfield.

Hem, J. D. 1985, Study and interpretation of the chemical characteristics of natural water, Water Supply Paper 2254, 3rd ed., USGS.

Langelier W.F. 1936, Analytical control of anti-corrosion Water Treatment, Journal Am. Wks. Assoc; vol. 28, p. 1500.

Lloyd JW. and Heathcote JA. 1985. Natural Inorganic Hydrochemistry in Relation to Groundwater Clarendon Press, Oxford, pp125.

Ryznar 1944, JWA. New index for Determining the amount of Calcium carbonate scale formed by water. J. Am. Wks. Assoc.; vol 36, pp272.

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