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Nitrates leaching from agricultural land in Hamadan, western Iran
Agriculture, Ecosystems and Environment 110 (2005) 210–218 www.elsevier.com/locate/agee
Nitrates leaching from agricultural land in Hamadan, western Iran Mohsen Jalali * Bu-Ali Sina University, College of Agriculture, Department of Soil Science, Hamadan, Iran Received 5 May 2004; received in revised form 19 March 2005; accepted 5 April 2005 Available online 24 June 2005
Abstract Nitrogen (N) is vital for plant and microbial growth and rather large amounts are required by most arable and horticulture plants. High nitrate (NO3) levels of water supplies have been attributed to leaching from the soil and into water systems. In the arid and semi-arid regions, irrigation water carries NO3 into groundwater. This study was conducted to investigate NO3 pollution of groundwater in Hamadan, western Iran. The water samples were mostly taken from domestic and community wells. In this area, the drinking water supply comes mainly from groundwater sources. Nitrate concentrations in the well samples varied from 3 to 252 with the average of 49 mg l1. Results showed that of 311 wells, 196 (63%) had levels less than 50 mg l1 and 115 (37%) had levels in excess of the 50 mg l1 NO3. Agriculture is the dominant land use in the area and application of N fertilizers clearly has an impact on groundwater. If agricultural losses remain stable, it could be expected that the concentration of NO3 in groundwater will reach or exceed the international recommendations for drinking water (50 mg l1) in the future. Irrigation with high NO3 groundwater can minimise the requirement for N fertilizers. To maintain yield increase and minimise NO3 pollution of the groundwater, best management practices, for N fertilizer use should be applied and excessive fertilizer application prevented. # 2005 Elsevier B.V. All rights reserved. Keywords: Nitrate; Leaching; Groundwater; Pollution; Iran
1. Introduction Nitrogen is a constituent of chlorophyll, proteins and many other molecules vital for plant growth. Plant tissues usually contain more N than potassium and phosphorus. Nitrogen influences the yields and quality of arable and horticulture plants and is a widely used plant nutrient. Nitrogen is most often the limiting nutrient in plant growth and to overcome this * Tel.: +98 811 4227090; fax: +98 811 4227012. E-mail address: [email protected].
limitation, N fertilizers are used to increase crop yields. Nitrogen fertilizer is used annually and plant uptake and microbial immobilization can not remove the entire NO3 from the solution. While N provides large responses in crop yield and is an extremely valuable nutrient, it is the major nutrient of concern in water pollution (Groffman, 2000; Davies, 2000). Contamination can render groundwater unsuitable for human consumption. Nitrate frequently pollutes groundwater supplies (Spalding and Exner, 1993). The leaching of NO3 from farmland has become an important environmental issue because high
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NO3 in water supplies can cause ecological damage and health hazards. Several studies document adverse effects of high NO3 levels, most notably methemoglobinemia, stomach cancer and non-Hodgkin’s Lymphoma (Johnson et al., 1987; Knobeloch et al., 1992; Ward et al., 1994; Addiscott et al., 1992). Marked increase in fertilizer N rates applied to agricultural soils has caused N to be leached below the rooting zone (Pratt, 1985; Hallberg, 1989). The groundwater NO3 problem is international in scope. Strebel et al. (1989) and Fried (1991) reviewed NO3 pollution of groundwater in Europe. Their data showed a rising trend in NO3 concentrations over time. In Europe, NO3 concentrations exceeding the international (WHO, 1993) recommendations for drinking water (50 mg l1) have been found in groundwater under 22% of cultivated land (Laegreid et al., 1999). Rivers et al. (1996), identified NO3 and ammonium in a sandstone aquifer beneath Nottingham, England. Similar high concentrations have been found in USA and China (Laegreid et al., 1999). In Australia, high groundwater NO3 concentrations have been identified in all states across differing landuses (Keating et al., 1996). Zhang et al. (1996) studied NO3 pollution of groundwater in northern China. Over 50% of 69 locations investigated in the study area contained NO3 concentration above 50 mg l1. Kacaroglu and Gunay (1997), identified NO3 pollution in an alluvial aquifer beneath the urban complex of Eskisehir, Turkey. High NO3 levels in groundwater have also been found in southern Australia (Dillon et al., 1991). Nitrate is added directly to soils in some fertilizer types and is also supplied by nitrification of other fertilizers and organic materials (Groffman, 2000). Important nonagricultural sources of NO3 in studied areas include municipal and industrial discharges containing N bearing effluent and atmospheric deposition. The major increases in N use by agriculture during the last decades in developed and some part of Asia have been associated with large rises in N losses both as NO3 in drainage water and gaseous emissions. In Iran, agricultural land may be considered to be the main source of NO3, where intensification in the last 30 years has increased NO3 leaching from soils into both surface and groundwaters. Iran, Egypt and Turkey account for 75% of the fertilizer-N consumption in the Near East (Bijay-Singh et al., 1995).
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In irrigated soils of arid and semi arid region, where water is applied in excess of evapotranspiration or to leach soluble salts, NO3 leaching may occur (Jalali and Rowell, 2003). During the last three decades, NO3 concentration of the groundwater has gradually increased and is reaching 50 mg l1 NO3 in some parts of Iran. The extent and intensity of agriculture in Hamadan, the west of Iran leading to much high annual rates of fertilizer application which has a strong influence over water quality. Recharge from precipitation and irrigation may carry N compounds from the soil into the aquifer, often resulting in elevated NO3 concentrations in wells. Nitrogen leaching in irrigated agriculture should receive considerable attention, because of possible pollution of groundwater in Iran. Nitrate pollution of groundwater in Hamadan, Iran, is of particular concern because of the proximity of the region to environmentally sensitive areas and the large number of people in city and rural areas relying on groundwaters for drinking. Large amounts of N-fertilizer and poorly managed irrigated systems may lead to NO3 leaching and pollution of groundwater. Thus, these effects were investigated in Hamadan, western Iran.
2. Materials and methods This study was conducted in Hamadan, about 400 km from Tehran, western Iran. The study area lies between longitudes 488200 and 498270 E and latitudes 348360 and 358150 N. The climate of the region is semiarid with a mean annual precipitation of 300 mm and mean annual temperature is 10 8C. Agriculture is a major industry and principal land use in Hamadan. It has 4118 km2 area, of which 32.5% is cultivated. Major crops grown in Hamadan are winter wheat (Triticum aestivum L.), potato (Solanum tubersum) and garlic (Allium sativum). Hamadan also yields considerable quantities of hay, barley (Hordeum vulgare L.), corn (Zea mays L.), soybean (Glysin max (L.) Merr.), sugar beet (Beta vulgaris), grape (Vitis vinifera L.), various nuts and vegetables. Fertilizers are applied throughout agricultural regions of Hamadan to enhance crop production. Most of the agricultural land in Hamadan is cropland and pasture. However, livestock also substantially contributes to the states agricultural industry. The preceding land uses threaten groundwater quality in Hamadan. Groundwater supplies approxi-
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Fig. 1. Study area location.
mately 60–70% of the water consumed in Hamadan, with the remainder coming from numerous surfacewater reservoirs. Most of the drinking and irrigation water are supplied from the groundwater. The soil of the area is mostly classified as Aridisol. The parent rocks are mainly limestone, calcareous shale and granitic material. Water samples for NO3 analysis were obtained during summer 2000 from the 311 wells (Fig. 1). Therefore, there was no seasonal variation in NO3 content in samples. In order to avoid contamination, clean plastic containers were used in drawing the water samples from wells. The samples were stored in polyethylene containers, adequately labeled and preserved in the refrigerator until they were taken to the laboratory for NO3 measurement. Sampling sites were selected to cover different agricultural activities and soil types. Samples were analyzed in the laboratory for the NO3 using a colorimetric method with an UV-vis spectrophotometer (Rowell, 1994). On the basis of NO3 concentrations the samples were grouped into one of three classes, low (<20 mg l1), medium (20–50 mg l1) and high (>50 mg l1). Nitrate concentrations in the high class exceed the international (WHO, 1993) recommendations for drinking water, so all groundwaters in this class was for wells with NO3 concentrations, high enough to indicate the influence of human activities (Spalding and Exner, 1993). The low class represented wells with a low risk to human health or the environment.
3. Results and discussion The data regarding NO3 concentration were statistically analysed. In Fig. 2 the result of water analysis for NO3 concentration are shown as frequency distribution. Nitrate concentration varied from 3 to 252 mg l1 with an average of 49 mg l1. The distribution of NO3 concentration is positively skewed. The maximum safe NO3 concentration in drinking water was considered to be 50 mg l1 (or 11.3 mg l1 NO3-N) according to the World Health
Fig. 2. Frequency distribution for nitrate concentration in water samples from the Hamadan area.
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Fig. 3. Frequency distribution of water samples in which nitrate concentration is less than 50 mg l1.
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Organisation (WHO, 1993). In 16% of samples (50) NO3 concentration was low (<20 mg l1) and NO3 concentration in 47% of samples (146) were in the range of 20–50 mg l1. Of 311 samples 115 (37%) had levels in excess of the 50 mg l1 NO3. Concentration of NO3 in 8% of samples was above 100 mg l1. In Fig. 3 frequency distributions of water samples in which NO3 concentration were less than 50 mg l1 is shown. Concentrations of NO3 in 23% of samples (46) are in the range of 40–50 mg l1 and approaching the limit of the WHO. As further nitrate moves through the soil profile with percolating water, it can be expected that the number of wells that have concentration above recommended guidelines be increased in future. High NO3 concentrations mainly occurred in three areas, northeast, centre and some parts of the south (Fig. 4). The high percentage of the wells occurred in the south east of the study area. The result revealed that average NO3 content in deep (>70 m depth), shallow (40–70 m depth) and
Fig. 4. Distribution of nitrate (mg l1) in water samples in the studied area.
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hand pumped (<40 m depth) wells were 41, 65 and 51 mg l1, respectively. Therefore, NO3 concentrations in hand pumped and shallow wells are higher than deep wells. Of 75 and 26 well samples collected from fields under potato and vegetable crops, 35 and 13 had NO3 contents over the allowable limit, respectively. However, of 88 well samples collected from fields under gardens and alfalfa (Medicago sativa) only 18 had a NO3 content over the allowable limit. Nitrogen in the irrigation water has much the same effect as soilapplied fertilizer N and excess will cause problems, just as too much fertilizer would. If excessive quantities are present or applied, production of several commonly grown crops may be upset because of overstimulation of growth, delayed maturity or poor quality (Ayres and Westcot, 1985). Sensitive crops may be affected by N concentrations above 22 mg NO3 l1. Most other crops are relatively unaffected until NO3 exceeds 133 mg l1 (Ayres and Westcot, 1985). According to guidelines for evaluation of water
quality for irrigation (Ayres and Westcot, 1985), 3% of samples had concentrations of NO3 more than 133 mg l1 and NO3 in 79% of samples were in the range of 22–133 mg l1 and 18% less than 22 mg l1 NO3. Distribution of NO3 in water samples based on guidelines for evaluation of water quality for irrigation is given in Fig. 5. The water from wells located in some parts of the studied area has no restrictions for irrigation. The degree of restriction on use of most of the studied area is moderate according to the FAO guidelines (Ayres and Westcot, 1985), and should not be used for sensitive crops. The situation in the north east area is regarded to be severe, as the NO3 concentration is above 133 mg l1 and most crops in this area may be affected by high N levels in irrigation wells (Fig. 5). Therefore, in the north–east area sensitive crops such as sugar beet (Beta vulgaris) and grapes (Vitis vinifera L.) may be affected by excessive N. The soils in this area may be predominately light in texture and possibly at several farms fertilizers are poorly managed. However, high N water can be used as a fertilizer early in the
Fig. 5. Distribution of nitrate (mg l1) in water samples in studied area based on guidelines for evaluation of water quality for irrigation.
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season. Another option is to plant a less sensitive crop, which can utilise the N from the irrigation water more effectively. Potential sources of NO3 in agricultural regions include fertilizer, animal waste, and mineralization of soil organic N (in plant residues, bacterial biomass, and soil constituents). In the studied area various anthropogenic activities such as intensive agriculture, includes vegetable production, poultry production have been going on for about 35 years. The highest NO3 concentration levels occur in areas where due to intensive arable production, large amounts of N fertilizers (commonly urea, nitrate or ammonium compounds) are used. Nitrate pollution in groundwater originating from fertilizers has been reported from many parts of the world. Pacheco and Cabrera (1997) reported NO3 contamination in a karstic limestone aquifer beneath the Yucata Peninsula of Mexico. They attributed the contamination to human and agricultural wastewater. Strebel et al. (1989) and Fried (1991) found that fertilization and livestock manure were principal sources of the NO3 in groundwater. Rivers et al. (1996) indicated that nitrate pollution of groundwater originated from fertilizer and soil organic N. Oenema et al. (1998) attributed NO3 pollution of groundwater in the Netherlands to agricultural activity, mainly fertilizer and livestock manure.
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The average N fertilizer application in Iran is currently over 200 kg N h1. In high yielding crop regions of Iran, N fertilizer application is usually over 300 kg N ha1, with accompanying decreases in utilisation efficiency. Table 1 compared the amounts of N added to the soil with the crop N uptake in several crops in Hamadan area. It is apparent that the N input is higher than the N requirement. Thus, fertilizer use efficiency is low, even when the farmers inputs of N fertilizer were made according to soil and water research institute. These figures do not include the N input from manure, so the total N input could be substantially higher. In vegetable and field crops in the studied areas, the use of organic manure (in potato fields about 10 t ha1 is being used) is common (poultry manure is the type most frequently used). Organic manure (human excrement) is also applied for vegetable production. Some parts of the studied area have been irrigated with sewage effluent since 1970. The waste water receives no prior treatment and the irrigated area with waste water has been growing as sewage volume increases. Presently, farmers do not use any system at all, and fertilize according to experience. In this region, excessive amounts of N fertilizers are frequently applied by farmers for vegetable production in order to achieve high yields and profits. Approximately, all of vegetable growers in these areas apply N fertilizer more than recommended
Table 1 Average crop yield, applied N-fertilizer and crop N-uptake in Hamadan province Crop
Average crop yield (t ha1)a
Applied N-fertilizer (kg N ha1)b
Crop N-uptake (kg N ha1)c
Percentage of N-fertilizer applied taken by crop
Wheat Barley Corn Potato Sugar beet Tomato (Lycopersicon esculentum) Onion (Allium cepa L.) Other vegetables
1.68 2.5 8.8 29 33 34 6.14 7.7
138 138 184 184 207 207 184 230
29.4d 37d 104e 130f 140g 98h 92i 116i
21 27 57 71 68 47 50 50
a b c d e f g h i
Ministry of Agriculture (2003). Recommended by soil and water research institute (Malakouti and Gheibi, 2000). Calculated based on average crop yield and utilization of nitrogen by crops (Tisdale et al., 1985). Grain and straw. Grain and stover. Tubers and vines. Roots and tops. Fruit and vines. Calculated base on 1.5% N uptake.
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rate. The situation is similar in other crops, such as potato and garlic grown in these areas. The framer surveys indicate that excessive N application often up to two to three times the recommended rate is very common in vegetables crops. Fertilizer application is done in autumn or spring. Most of the fields are flood irrigated, with poorly managed irrigated systems, resulting in low irrigation efficiency. Therefore, large amount of N fertilizer and an inadequate management of N fertilization coupled with a low irrigation efficiency are mainly responsible for the NO3 concentrations in wells. This has resulted in environmental pollution, economic and natural resource wastage as well as deterioration in the quality of crops, such as lower sugar content in water melons (Citrullus vulgaris) and very high NO3 contents in leafy vegetables. The ratio of N fertilizer used to that contained in harvested crops is estimated today as 1.5:1, with the most inefficient systems having ratios of as much as 2:1 (Pierzynski et al., 2000). This means that only about two thirds of the fertilizer N applied is harvested, the rest enters the soil N cycle where it can be conserved or lost to air or water (Stevenson and Cole, 1999). In the early 1950s, the Ministry of Agriculture started to import fertilizers and encouraged farmers to use them (Siadat et al., 1993). The weather dominates through the impact of rainfall and temperature on drainage, crop growth and N utilization (Addiscott and Powlson, 1992). Also rainfall in the weeks following N application is especially important (Powlson et al., 1992). Due to the adverse climate conditions (such as temperature and wind stress), consumption of NO3 by crops may be reduced, so NO3 is accumulated in the soil following harvest and finally is leached into water supplies. The availability of cheap N fertilizers encourages over applications. On fields where N fertilizer is poorly timed for plant use or coincident with heavy rainfall, leaching loss can be 50–80 kg ha1 of N annually (Miller and Gariner, 1998). McNeal and Pratt (1978) found that leaching losses accounted for 13–100% of applied N fertilizer in some irrigated California soils and commonly average 25–50% of the N applied in most cropping situations. Goulding (2000) on the basis of nutrient budgets on farms have shown that surpluses of N on well-managed arable land can be as much as 20 kg ha1 year1.
For decreasing ratios farmers should choose the highest-yielding variety appropriate to maximise the use of the available N (Goulding, 2000). Appropriate controls to minimise pest, disease and weed infestation are essential because a diseased crop is less able to use its N (Cussans, 1992). Agronomic practices such as cultivation, cropping and irrigation water management may decrease the average NO3 concentration in water draining from the soil zone (Johnson et al., 1997). Reducing N fertilizer application to crops (Davis and Sylvester-Bradley, 1995), the use of cover crops to remove NO3 present in the soil during the autumn (Johnson et al., 1997), and the incorporation of straw may help limit NO3 loss. Farmers are not able to reduce the loss of N to zero and some leaching of NO3 from farms is inevitable. Under winter wheat on Broadbalk experiment at Rothamsted experimental station, where no fertilizer N is applied, there is always some leaching loss, approximately 10 kg ha1 year1 (Goulding, 2000). For crops irrigated with water containing N, the rates of N fertilizer supplied to the crops can be reduced by an amount equal to that available from the water supply. The contaminated groundwater can supply some of N normally provided by N fertilizers. In a one hectare field, application of one irrigation (=10 cm) of the studied groundwaters having 3–252 mg l1 of NO3 ion can supply 3–252 kg of NO3. In 35% of groundwater samples supply of NO3 in this way would be above 50 kg ha1. Assuming that the quantity of fertilizer applied for cultivation of potato and wheat in the studied area is 300 and 200 kg N ha1, respectively, the operational cost of cultivation can be brought down substantially. This would further reduce the degradation of both soil and groundwater.
4. Conclusions Groundwater is immensely important for water supply to meet human needs in rural areas in Hamadan, western Iran. It has been found that well water in some localities of the Hamadan area in Iran contains NO3 in excess of 50 mg l1. In general, groundwater beneath areas of intensive agriculture in the Hamadan area can be classified mostly as medium to high in NO3 content. Only 16% of water samples were classified as having a low risk to human health or
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the environment. Regions of elevated NO3 occupying northeast, central and some part of the south, suggest the influence of regional land use patterns. Agricultural practices, especially cultivation and fertilisation, are principal causes of NO3 contamination on a regional scale. As it will take time for NO3 to reach to the groundwater, and N fertilizer is still being used, concentrations of NO3 in underground waters in the studied areas are expected to reach or exceed 50 mg l1 in future. Optimising management practices for the use of water and N fertilizers in agriculture is a possible means of avoiding, or at least minimising, environmental contamination by NO3. Fertilizer application should be done in the spring and summer, and the number of applications should be at least two, although a higher frequency is preferred. As the use of or such groundwaters for irrigation is likely to increase with time, the high levels of NO3 in groundwater should be taken into consideration when recommending N fertilizer application. The use of groundwaters with elevated NO3 would reduce the requirement of inorganic fertilizer applications.
Acknowledgements Two anonymous reviewers and editor made valuable and comprehensive comments on the manuscript. The author gratefully expressed his gratitude for their thoughtful and thorough reviews. The author also wish to thanks Dr. D.L. Rowell (Reading University, England) for helpful discussions and the revision of the first version of the manuscript.
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Nitrates leaching from agricultural land in Hamadan, western Iran Mohsen Jalali * Bu-Ali Sina University, College of Agriculture, Department of Soil Science, Hamadan, Iran Received 5 May 2004; received in revised form 19 March 2005; accepted 5 April 2005 Available online 24 June 2005
Abstract Nitrogen (N) is vital for plant and microbial growth and rather large amounts are required by most arable and horticulture plants. High nitrate (NO3) levels of water supplies have been attributed to leaching from the soil and into water systems. In the arid and semi-arid regions, irrigation water carries NO3 into groundwater. This study was conducted to investigate NO3 pollution of groundwater in Hamadan, western Iran. The water samples were mostly taken from domestic and community wells. In this area, the drinking water supply comes mainly from groundwater sources. Nitrate concentrations in the well samples varied from 3 to 252 with the average of 49 mg l1. Results showed that of 311 wells, 196 (63%) had levels less than 50 mg l1 and 115 (37%) had levels in excess of the 50 mg l1 NO3. Agriculture is the dominant land use in the area and application of N fertilizers clearly has an impact on groundwater. If agricultural losses remain stable, it could be expected that the concentration of NO3 in groundwater will reach or exceed the international recommendations for drinking water (50 mg l1) in the future. Irrigation with high NO3 groundwater can minimise the requirement for N fertilizers. To maintain yield increase and minimise NO3 pollution of the groundwater, best management practices, for N fertilizer use should be applied and excessive fertilizer application prevented. # 2005 Elsevier B.V. All rights reserved. Keywords: Nitrate; Leaching; Groundwater; Pollution; Iran
1. Introduction Nitrogen is a constituent of chlorophyll, proteins and many other molecules vital for plant growth. Plant tissues usually contain more N than potassium and phosphorus. Nitrogen influences the yields and quality of arable and horticulture plants and is a widely used plant nutrient. Nitrogen is most often the limiting nutrient in plant growth and to overcome this * Tel.: +98 811 4227090; fax: +98 811 4227012. E-mail address: [email protected].
limitation, N fertilizers are used to increase crop yields. Nitrogen fertilizer is used annually and plant uptake and microbial immobilization can not remove the entire NO3 from the solution. While N provides large responses in crop yield and is an extremely valuable nutrient, it is the major nutrient of concern in water pollution (Groffman, 2000; Davies, 2000). Contamination can render groundwater unsuitable for human consumption. Nitrate frequently pollutes groundwater supplies (Spalding and Exner, 1993). The leaching of NO3 from farmland has become an important environmental issue because high
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NO3 in water supplies can cause ecological damage and health hazards. Several studies document adverse effects of high NO3 levels, most notably methemoglobinemia, stomach cancer and non-Hodgkin’s Lymphoma (Johnson et al., 1987; Knobeloch et al., 1992; Ward et al., 1994; Addiscott et al., 1992). Marked increase in fertilizer N rates applied to agricultural soils has caused N to be leached below the rooting zone (Pratt, 1985; Hallberg, 1989). The groundwater NO3 problem is international in scope. Strebel et al. (1989) and Fried (1991) reviewed NO3 pollution of groundwater in Europe. Their data showed a rising trend in NO3 concentrations over time. In Europe, NO3 concentrations exceeding the international (WHO, 1993) recommendations for drinking water (50 mg l1) have been found in groundwater under 22% of cultivated land (Laegreid et al., 1999). Rivers et al. (1996), identified NO3 and ammonium in a sandstone aquifer beneath Nottingham, England. Similar high concentrations have been found in USA and China (Laegreid et al., 1999). In Australia, high groundwater NO3 concentrations have been identified in all states across differing landuses (Keating et al., 1996). Zhang et al. (1996) studied NO3 pollution of groundwater in northern China. Over 50% of 69 locations investigated in the study area contained NO3 concentration above 50 mg l1. Kacaroglu and Gunay (1997), identified NO3 pollution in an alluvial aquifer beneath the urban complex of Eskisehir, Turkey. High NO3 levels in groundwater have also been found in southern Australia (Dillon et al., 1991). Nitrate is added directly to soils in some fertilizer types and is also supplied by nitrification of other fertilizers and organic materials (Groffman, 2000). Important nonagricultural sources of NO3 in studied areas include municipal and industrial discharges containing N bearing effluent and atmospheric deposition. The major increases in N use by agriculture during the last decades in developed and some part of Asia have been associated with large rises in N losses both as NO3 in drainage water and gaseous emissions. In Iran, agricultural land may be considered to be the main source of NO3, where intensification in the last 30 years has increased NO3 leaching from soils into both surface and groundwaters. Iran, Egypt and Turkey account for 75% of the fertilizer-N consumption in the Near East (Bijay-Singh et al., 1995).
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In irrigated soils of arid and semi arid region, where water is applied in excess of evapotranspiration or to leach soluble salts, NO3 leaching may occur (Jalali and Rowell, 2003). During the last three decades, NO3 concentration of the groundwater has gradually increased and is reaching 50 mg l1 NO3 in some parts of Iran. The extent and intensity of agriculture in Hamadan, the west of Iran leading to much high annual rates of fertilizer application which has a strong influence over water quality. Recharge from precipitation and irrigation may carry N compounds from the soil into the aquifer, often resulting in elevated NO3 concentrations in wells. Nitrogen leaching in irrigated agriculture should receive considerable attention, because of possible pollution of groundwater in Iran. Nitrate pollution of groundwater in Hamadan, Iran, is of particular concern because of the proximity of the region to environmentally sensitive areas and the large number of people in city and rural areas relying on groundwaters for drinking. Large amounts of N-fertilizer and poorly managed irrigated systems may lead to NO3 leaching and pollution of groundwater. Thus, these effects were investigated in Hamadan, western Iran.
2. Materials and methods This study was conducted in Hamadan, about 400 km from Tehran, western Iran. The study area lies between longitudes 488200 and 498270 E and latitudes 348360 and 358150 N. The climate of the region is semiarid with a mean annual precipitation of 300 mm and mean annual temperature is 10 8C. Agriculture is a major industry and principal land use in Hamadan. It has 4118 km2 area, of which 32.5% is cultivated. Major crops grown in Hamadan are winter wheat (Triticum aestivum L.), potato (Solanum tubersum) and garlic (Allium sativum). Hamadan also yields considerable quantities of hay, barley (Hordeum vulgare L.), corn (Zea mays L.), soybean (Glysin max (L.) Merr.), sugar beet (Beta vulgaris), grape (Vitis vinifera L.), various nuts and vegetables. Fertilizers are applied throughout agricultural regions of Hamadan to enhance crop production. Most of the agricultural land in Hamadan is cropland and pasture. However, livestock also substantially contributes to the states agricultural industry. The preceding land uses threaten groundwater quality in Hamadan. Groundwater supplies approxi-
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Fig. 1. Study area location.
mately 60–70% of the water consumed in Hamadan, with the remainder coming from numerous surfacewater reservoirs. Most of the drinking and irrigation water are supplied from the groundwater. The soil of the area is mostly classified as Aridisol. The parent rocks are mainly limestone, calcareous shale and granitic material. Water samples for NO3 analysis were obtained during summer 2000 from the 311 wells (Fig. 1). Therefore, there was no seasonal variation in NO3 content in samples. In order to avoid contamination, clean plastic containers were used in drawing the water samples from wells. The samples were stored in polyethylene containers, adequately labeled and preserved in the refrigerator until they were taken to the laboratory for NO3 measurement. Sampling sites were selected to cover different agricultural activities and soil types. Samples were analyzed in the laboratory for the NO3 using a colorimetric method with an UV-vis spectrophotometer (Rowell, 1994). On the basis of NO3 concentrations the samples were grouped into one of three classes, low (<20 mg l1), medium (20–50 mg l1) and high (>50 mg l1). Nitrate concentrations in the high class exceed the international (WHO, 1993) recommendations for drinking water, so all groundwaters in this class was for wells with NO3 concentrations, high enough to indicate the influence of human activities (Spalding and Exner, 1993). The low class represented wells with a low risk to human health or the environment.
3. Results and discussion The data regarding NO3 concentration were statistically analysed. In Fig. 2 the result of water analysis for NO3 concentration are shown as frequency distribution. Nitrate concentration varied from 3 to 252 mg l1 with an average of 49 mg l1. The distribution of NO3 concentration is positively skewed. The maximum safe NO3 concentration in drinking water was considered to be 50 mg l1 (or 11.3 mg l1 NO3-N) according to the World Health
Fig. 2. Frequency distribution for nitrate concentration in water samples from the Hamadan area.
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Fig. 3. Frequency distribution of water samples in which nitrate concentration is less than 50 mg l1.
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Organisation (WHO, 1993). In 16% of samples (50) NO3 concentration was low (<20 mg l1) and NO3 concentration in 47% of samples (146) were in the range of 20–50 mg l1. Of 311 samples 115 (37%) had levels in excess of the 50 mg l1 NO3. Concentration of NO3 in 8% of samples was above 100 mg l1. In Fig. 3 frequency distributions of water samples in which NO3 concentration were less than 50 mg l1 is shown. Concentrations of NO3 in 23% of samples (46) are in the range of 40–50 mg l1 and approaching the limit of the WHO. As further nitrate moves through the soil profile with percolating water, it can be expected that the number of wells that have concentration above recommended guidelines be increased in future. High NO3 concentrations mainly occurred in three areas, northeast, centre and some parts of the south (Fig. 4). The high percentage of the wells occurred in the south east of the study area. The result revealed that average NO3 content in deep (>70 m depth), shallow (40–70 m depth) and
Fig. 4. Distribution of nitrate (mg l1) in water samples in the studied area.
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hand pumped (<40 m depth) wells were 41, 65 and 51 mg l1, respectively. Therefore, NO3 concentrations in hand pumped and shallow wells are higher than deep wells. Of 75 and 26 well samples collected from fields under potato and vegetable crops, 35 and 13 had NO3 contents over the allowable limit, respectively. However, of 88 well samples collected from fields under gardens and alfalfa (Medicago sativa) only 18 had a NO3 content over the allowable limit. Nitrogen in the irrigation water has much the same effect as soilapplied fertilizer N and excess will cause problems, just as too much fertilizer would. If excessive quantities are present or applied, production of several commonly grown crops may be upset because of overstimulation of growth, delayed maturity or poor quality (Ayres and Westcot, 1985). Sensitive crops may be affected by N concentrations above 22 mg NO3 l1. Most other crops are relatively unaffected until NO3 exceeds 133 mg l1 (Ayres and Westcot, 1985). According to guidelines for evaluation of water
quality for irrigation (Ayres and Westcot, 1985), 3% of samples had concentrations of NO3 more than 133 mg l1 and NO3 in 79% of samples were in the range of 22–133 mg l1 and 18% less than 22 mg l1 NO3. Distribution of NO3 in water samples based on guidelines for evaluation of water quality for irrigation is given in Fig. 5. The water from wells located in some parts of the studied area has no restrictions for irrigation. The degree of restriction on use of most of the studied area is moderate according to the FAO guidelines (Ayres and Westcot, 1985), and should not be used for sensitive crops. The situation in the north east area is regarded to be severe, as the NO3 concentration is above 133 mg l1 and most crops in this area may be affected by high N levels in irrigation wells (Fig. 5). Therefore, in the north–east area sensitive crops such as sugar beet (Beta vulgaris) and grapes (Vitis vinifera L.) may be affected by excessive N. The soils in this area may be predominately light in texture and possibly at several farms fertilizers are poorly managed. However, high N water can be used as a fertilizer early in the
Fig. 5. Distribution of nitrate (mg l1) in water samples in studied area based on guidelines for evaluation of water quality for irrigation.
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season. Another option is to plant a less sensitive crop, which can utilise the N from the irrigation water more effectively. Potential sources of NO3 in agricultural regions include fertilizer, animal waste, and mineralization of soil organic N (in plant residues, bacterial biomass, and soil constituents). In the studied area various anthropogenic activities such as intensive agriculture, includes vegetable production, poultry production have been going on for about 35 years. The highest NO3 concentration levels occur in areas where due to intensive arable production, large amounts of N fertilizers (commonly urea, nitrate or ammonium compounds) are used. Nitrate pollution in groundwater originating from fertilizers has been reported from many parts of the world. Pacheco and Cabrera (1997) reported NO3 contamination in a karstic limestone aquifer beneath the Yucata Peninsula of Mexico. They attributed the contamination to human and agricultural wastewater. Strebel et al. (1989) and Fried (1991) found that fertilization and livestock manure were principal sources of the NO3 in groundwater. Rivers et al. (1996) indicated that nitrate pollution of groundwater originated from fertilizer and soil organic N. Oenema et al. (1998) attributed NO3 pollution of groundwater in the Netherlands to agricultural activity, mainly fertilizer and livestock manure.
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The average N fertilizer application in Iran is currently over 200 kg N h1. In high yielding crop regions of Iran, N fertilizer application is usually over 300 kg N ha1, with accompanying decreases in utilisation efficiency. Table 1 compared the amounts of N added to the soil with the crop N uptake in several crops in Hamadan area. It is apparent that the N input is higher than the N requirement. Thus, fertilizer use efficiency is low, even when the farmers inputs of N fertilizer were made according to soil and water research institute. These figures do not include the N input from manure, so the total N input could be substantially higher. In vegetable and field crops in the studied areas, the use of organic manure (in potato fields about 10 t ha1 is being used) is common (poultry manure is the type most frequently used). Organic manure (human excrement) is also applied for vegetable production. Some parts of the studied area have been irrigated with sewage effluent since 1970. The waste water receives no prior treatment and the irrigated area with waste water has been growing as sewage volume increases. Presently, farmers do not use any system at all, and fertilize according to experience. In this region, excessive amounts of N fertilizers are frequently applied by farmers for vegetable production in order to achieve high yields and profits. Approximately, all of vegetable growers in these areas apply N fertilizer more than recommended
Table 1 Average crop yield, applied N-fertilizer and crop N-uptake in Hamadan province Crop
Average crop yield (t ha1)a
Applied N-fertilizer (kg N ha1)b
Crop N-uptake (kg N ha1)c
Percentage of N-fertilizer applied taken by crop
Wheat Barley Corn Potato Sugar beet Tomato (Lycopersicon esculentum) Onion (Allium cepa L.) Other vegetables
1.68 2.5 8.8 29 33 34 6.14 7.7
138 138 184 184 207 207 184 230
29.4d 37d 104e 130f 140g 98h 92i 116i
21 27 57 71 68 47 50 50
a b c d e f g h i
Ministry of Agriculture (2003). Recommended by soil and water research institute (Malakouti and Gheibi, 2000). Calculated based on average crop yield and utilization of nitrogen by crops (Tisdale et al., 1985). Grain and straw. Grain and stover. Tubers and vines. Roots and tops. Fruit and vines. Calculated base on 1.5% N uptake.
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rate. The situation is similar in other crops, such as potato and garlic grown in these areas. The framer surveys indicate that excessive N application often up to two to three times the recommended rate is very common in vegetables crops. Fertilizer application is done in autumn or spring. Most of the fields are flood irrigated, with poorly managed irrigated systems, resulting in low irrigation efficiency. Therefore, large amount of N fertilizer and an inadequate management of N fertilization coupled with a low irrigation efficiency are mainly responsible for the NO3 concentrations in wells. This has resulted in environmental pollution, economic and natural resource wastage as well as deterioration in the quality of crops, such as lower sugar content in water melons (Citrullus vulgaris) and very high NO3 contents in leafy vegetables. The ratio of N fertilizer used to that contained in harvested crops is estimated today as 1.5:1, with the most inefficient systems having ratios of as much as 2:1 (Pierzynski et al., 2000). This means that only about two thirds of the fertilizer N applied is harvested, the rest enters the soil N cycle where it can be conserved or lost to air or water (Stevenson and Cole, 1999). In the early 1950s, the Ministry of Agriculture started to import fertilizers and encouraged farmers to use them (Siadat et al., 1993). The weather dominates through the impact of rainfall and temperature on drainage, crop growth and N utilization (Addiscott and Powlson, 1992). Also rainfall in the weeks following N application is especially important (Powlson et al., 1992). Due to the adverse climate conditions (such as temperature and wind stress), consumption of NO3 by crops may be reduced, so NO3 is accumulated in the soil following harvest and finally is leached into water supplies. The availability of cheap N fertilizers encourages over applications. On fields where N fertilizer is poorly timed for plant use or coincident with heavy rainfall, leaching loss can be 50–80 kg ha1 of N annually (Miller and Gariner, 1998). McNeal and Pratt (1978) found that leaching losses accounted for 13–100% of applied N fertilizer in some irrigated California soils and commonly average 25–50% of the N applied in most cropping situations. Goulding (2000) on the basis of nutrient budgets on farms have shown that surpluses of N on well-managed arable land can be as much as 20 kg ha1 year1.
For decreasing ratios farmers should choose the highest-yielding variety appropriate to maximise the use of the available N (Goulding, 2000). Appropriate controls to minimise pest, disease and weed infestation are essential because a diseased crop is less able to use its N (Cussans, 1992). Agronomic practices such as cultivation, cropping and irrigation water management may decrease the average NO3 concentration in water draining from the soil zone (Johnson et al., 1997). Reducing N fertilizer application to crops (Davis and Sylvester-Bradley, 1995), the use of cover crops to remove NO3 present in the soil during the autumn (Johnson et al., 1997), and the incorporation of straw may help limit NO3 loss. Farmers are not able to reduce the loss of N to zero and some leaching of NO3 from farms is inevitable. Under winter wheat on Broadbalk experiment at Rothamsted experimental station, where no fertilizer N is applied, there is always some leaching loss, approximately 10 kg ha1 year1 (Goulding, 2000). For crops irrigated with water containing N, the rates of N fertilizer supplied to the crops can be reduced by an amount equal to that available from the water supply. The contaminated groundwater can supply some of N normally provided by N fertilizers. In a one hectare field, application of one irrigation (=10 cm) of the studied groundwaters having 3–252 mg l1 of NO3 ion can supply 3–252 kg of NO3. In 35% of groundwater samples supply of NO3 in this way would be above 50 kg ha1. Assuming that the quantity of fertilizer applied for cultivation of potato and wheat in the studied area is 300 and 200 kg N ha1, respectively, the operational cost of cultivation can be brought down substantially. This would further reduce the degradation of both soil and groundwater.
4. Conclusions Groundwater is immensely important for water supply to meet human needs in rural areas in Hamadan, western Iran. It has been found that well water in some localities of the Hamadan area in Iran contains NO3 in excess of 50 mg l1. In general, groundwater beneath areas of intensive agriculture in the Hamadan area can be classified mostly as medium to high in NO3 content. Only 16% of water samples were classified as having a low risk to human health or
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the environment. Regions of elevated NO3 occupying northeast, central and some part of the south, suggest the influence of regional land use patterns. Agricultural practices, especially cultivation and fertilisation, are principal causes of NO3 contamination on a regional scale. As it will take time for NO3 to reach to the groundwater, and N fertilizer is still being used, concentrations of NO3 in underground waters in the studied areas are expected to reach or exceed 50 mg l1 in future. Optimising management practices for the use of water and N fertilizers in agriculture is a possible means of avoiding, or at least minimising, environmental contamination by NO3. Fertilizer application should be done in the spring and summer, and the number of applications should be at least two, although a higher frequency is preferred. As the use of or such groundwaters for irrigation is likely to increase with time, the high levels of NO3 in groundwater should be taken into consideration when recommending N fertilizer application. The use of groundwaters with elevated NO3 would reduce the requirement of inorganic fertilizer applications.
Acknowledgements Two anonymous reviewers and editor made valuable and comprehensive comments on the manuscript. The author gratefully expressed his gratitude for their thoughtful and thorough reviews. The author also wish to thanks Dr. D.L. Rowell (Reading University, England) for helpful discussions and the revision of the first version of the manuscript.
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