Human impacts on groundwater resources in Iran

Transcription

1 Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 230, Human impacts on groundwater resources in Iran ALI VALI-KHODJEINI Irrigation and Reclamation Engineering Department, University of Tehran, Karaj, Iran Abstract The groundwater which underlies many parts of Iran is exploited by ghanats, springs, artesian wells and deep and shallow wells. At present, their annual discharge is estimated to be 53.5 km 3. Until 1945, groundwater was utilized by means of the ghanats system, which provided about 75 % of water used for irrigation. There are some ghanats, of which almost half are still in operation. After the Second World War, abstraction of groundwater increased as a result of drilling of deep and shallow wells. As a consequence, water tables have been lowered and a large number of ghanats have become dry in areas with high population densities. The increase in groundwater consumption will create new problems, some of which, such as saline intrusion, are more difficult to solve. This paper reviews the quality and quantity of Iran's groundwater development and its related problems. INTRODUCTION Iran, with an area 1636 x 10 3 km 2, relies on surface water resources that derive from rainfall and snowmelt and the groundwaters that underlie many part of the country. Historically, water has presented a persistent problem. Many people living either on farms or in hamlets and villages in the interior are dependent upon groundwater resources. The use of groundwater increased sharply with the introduction of large-scale industrialization in recent decades. Cities outgrew their water supplies and the equilibrium of the groundwater supply has been most seriously affected. Since this resource is limited, conditions dictate that the groundwater resources must be effectively developed, conserved and managed to meet the nation's goals. GROUNDWATER RESOURCES DISTRIBUTION The mean annual precipitation of Iran averages about 250 mm. Most of this amount is used where it falls, percolates into the ground or is lost to évapotranspiration. The Iranian groundwater is distributed in two zones: 57% in the western zone (four regions of Azarbaijan, North coastal, centre and Khuzestan Border) and 43% in the eastern zone (four regions of Esfahan-Yazd, Fars-South Coastal, Khorassan and Kerman-Sistan- Baluchestan). The eastern zone includes the two great deserts of Iran, the Dasht-e-Lut and the Dasht-e-Kavir. Furthermore, in numerous regions the water resources are of a poor quality, either because of the effects of evaporation or due to the underlying geology.

2 142 Ali Vali-Khodjeini HISTORICAL UTILIZATION OF GROUNDWATER, THE GHANAT SYSTEM Up to 1945, groundwater in Iran was utilized in a special way. The Iranian system of irrigation, by means of ghanats, has been known for thousands of years. More than two thousand years ago, underground conduits were dug to introduce mountain groundwater to the arid plains. Flow conditions in a ghanat and the water profiles are schematically represented in Fig. 1. Almost all of the ghanats are associated with large alluvial fans in the piedmont zone between the high mountains and the Kavir or salt desert, or in a large alluvial valley on the desert margin. The existence of of the ghanats has been reported of which about half are still in operation. Many ghanats are being continually repaired or even extended and deepened to tap falling water tables. After the Second World War extraction of groundwater by pumps from wells started to increase and rapidly develop. With the rapid development of deep wells, the danger of lowering the water table by excessive pumping has been realized. In the future it seems likely that the yield from the ghanats will continue to decline with a considerable number of them drying up in areas of high population density. In remote areas of low population density and in areas of low or poor yielding aquifers, the ghanat will still have an important role in supplying irrigation water for many years to come. However, existing ghanats in the country comprise a tremendous investment created by many generations. Attending, understanding and protecting them will ensure the survival of this investment. PLAN OF GHANAT Out let Mother well Conveyance canal BEDROCK Fig. 1 Plan and cross section of a ghanat and flow condition.

3 Human impacts on groundwater resources in Iran 143 GROUNDWATER RESOURCES AND THEIR DEVELOPMENT Data base Since the beginning of 1963, there has been a vast programme of groundwater survey including geophysical examinations, drilling test wells, test pumping and water analysis. The following has been accomplished: collection of data and preparation of hydrogeological maps; observation of water table fluctuations and preparation of related hydrographs; control of the variation in water table by means of the existing ghanats system; investigations of water quality; determination of hydrogeological coefficients by pumping tests. Groundwater availability The groundwater availability analysis was based on limited information, data and judgement. A summary of groundwater yield at the end of the hydrological year is given in Table 1. According to the data for 1991, the total groundwater potential is estimated at about 46 km 3. Total discharge was 37 km 3 in 1982, 46 km 3 in 1985 and 53.5 km 3 in Today about deep and shallow wells have been dug all over the country. Their development and total discharge from 1965 to 1992 is shown in Fig. 2. Of course, there is a wide range of groundwater level fluctuation and therefore change of storage. As abstraction approaches and exceeds recharge, the precise determination of the water balance becomes more important for future planning. In desert areas, due to the rapid evaporation salinization will be intensified. In the central plateau, water is saline. The patterns of dissolved chemicals in groundwater reflect the geology of areas over and through which these waters pass, and the relative solubilities of the materials. The dominant ions are Ca, Mg, Na, CI, S0 4 and HC0 3. At the present time many serious problems of water quality are created by modern developments e.g. use of detergents and pesticides, treatment of radioactive waste waters. EXAMPLES OF CRITICAL GROUNDWATER BASINS There are currently 59 basins (10% of all groundwater basins) that are in a state of critical or short groundwater supply. This number is expected to increase. Some examples of critical groundwater basins are given below (Fig. 3). Table 1 Summary of groundwater availability (1992). Source of water Number Discharge (km 3 year" 1 ) Ghanat Deep well Shallow well Spring Total

4 144 Ali Vali-Khodjeini B O U (S ft o ^ ^ r^^^ / O - T ~ \ \ i i i » i ' i ' -i i i ' i ' Year Fig. 2 Development of the total discharge by wells. (a) In the province of Khorassan about 88% of the water requirements (i.e. 8 km 3 ) is supplied by groundwater resources. Assessment of the hydrographs for observation wells in the twenty groundwater basins indicates that on the average, the reduction of the groundwater reservoir is equal to more than 10% of the annual rate of withdrawal. In recent years, drawdown has been about 70 cm and in some regions some parts of the aquifer have dried up. Fig. 4 shows the decline in groundwater level in the 3700 km 2 Torbat-Jam basin between 1987 and (b) The Zarand plain, which is located in the central basin with an area of 4116 km 2, had its first well drilled in 1950 and the number of wells is increasing. There were 786 wells in 1989 with a withdrawal rate 230 x 10 6 m 3. Groundwater table levels were lowered to about 25 m between 1965 and 1989 (Fig. 4). Today some parts of the aquifer have dried up. (c) The Ardestan plain has an area of about 6086 km 2, with an annual withdrawal of 520 x 10 6 m 3. Groundwater table levels were lowered to about 2.5 m between 1988 and 1990 (Fig. 4). (d) The 3340 km 2 Baghein plain of Kerman (Fig. 4) has a rate of over-withdrawal from Caspian Sea / ' 1-Ttorbat-jam 2-Zarand 3-Ardestan 4-Baghein-Kerman 5-Yazd & Ardekan 6-Kashan 200 Km Strait of '.-..^ Honruz ' A - Fig. 3 Location maps of critical groundwater basins.

5 Human impacts on groundwater resources in Iran (ajïbrbat-jam y ' 1990 (c) Ardestan Year Fig. 4 Declining groundwater levels in four critical groundwater basins. this aquifer of about 22.4 X 10 6 m 3 year" 1 for the period (e) The critical Basin of Yazd, the groundwater levels declined to 25 m over the past 30 years. Water samples from wells and ghanats show deterioration of groundwater quality reflecting the gypsum and salt formations in the margin of the plain. The maximum conductivity has increased from to micromhos cm" 1 and the maximum chloride content has changed from 4150 to 5720 mg l" 1 during the period (f) In the Kashan critical basin, with an area of 900 km 2 the groundwater level has lowered by about 28.5 m during the period Water samples from wells show deterioration of groundwater quality caused by saline intrusion; conductivity and chloride levels during this period increased considerably. The averages of EC and CI increased from 3935 to 9400 micromhos cm 4 and 960 to 2860 mg l" 1 respectively. CONCLUSIONS With rapid economic growth and industrial development, the excessive over-abstraction from groundwater is likely to have severe long-term costs: a national water plan for the country must be developed. This evaluation should include guidelines applicable to all levels of groundwater resources planning and incorporate the following elements: (a) long-term water resources planning; (b) detailed quantification of the spatial variation of water resources; (c) estimation of future water requirements; and

6 146 Ali Vali-Khodjeini (d) evaluation of existing and prospective water problems and formulation of recommendations to alleviate or solve problems and conflicts. Acknowledgements This study would not have possible without the help of the large number of people in the Groundwater Section of the Iranian Ministry of Energy who collected, edited and reported the data. This assistance is gratefully acknowledged.