خصوصيات شيميايي آب زيرزميني دشت شهركرد

Groundwater Quality Mapping in Shahrekord Aquifer

It is necessary to have a working knowledge of the quantity and quality of the water resources especially in arid and semi-arid regions. Shahrekord plain has a 551 Km2 total area where groundwater is discharging almost at 230 MCM annually for drinking and agriculture. The main objective of this study was mapping of Shahrekord aquifer groundwater quality and comparison with the drinking water standard limits. Some of the chemical properties such as Cations, Anions, Electrical Conductivity (EC), Total Hardness (TH) and Total Dissolved Solids (TDS) were measured four times (spring and autumn 2004 and 2005). The results show that the chemical concentration in north-western part of the aquifer was lower than southern part. Bicarbonate and Calcium have the maximum value however they are lower than the standard limits. The TH in the wells varies from 150 to 300 (mg/lit/CaCo3). The water is classified as hard water based on TH. The results show that there was not considerable difference in TH and TDS values between spring and autumn seasons. Increasing population increases more water needs and wastewater production. Surface waters and groundwater are polluted where there is no wastewater treatment plant. The manmade activity increases Electrical Conductivity (EC), Na, CI, S04 and N03 in groundwater. The maximum concentration level (MCL) of CI and S04ʹs ion in surface water and groundwater is 250 ppm. The main objective of this study was mapping of Shahrekord aquifer groundwater quality and comparison with the water standard limits. Shahrekord plain with 551 km2 area is located in the CHB province, Iran at 32° 07" to 32° 25" latitude and 50° 38" to 51° 10" longitude. Shahrekord aquifer includes 417, 59 and 159 agricultural, drinking water and industrial usage wells, respectively. It also has 79 Qanats and 40 springs where they are discharging almost 230 MCM water from the aquifer annually. Eleven wells were selected for groundwater sampling and measurement of chemical indexes. The positions of the wells were determined using global positioning system (GPS) and drawn using Maplnfo software (Fig. 1). Fig. 1 also shows the groundwater flow direction in the aquifer. The overall flow direction is from north to the south. Water sample was collected from the 11 wells in spring and autumn 2004 and spring and autumn 2005. The water sample analyzed for total hardness (TH), total dissolve solid (TDS), electrical conductivity (EC), cations and anions (Carbonate, Bicarbonate, Sulfate, Chloride, Calcium, Potassium, Magnesium and Sodium) based on the standard method. The water quality was classified using Wilcox diagram and the anion and cation compared by Aquachem software. The nitrate concentration was measured in 17 wells, in summer 2004. Base on the maps and the field observation the research results were concluded. As the results for the spring and autumn were similar, therefore, only the map for the spring is presented here. According to Fig. 1 and 2 there are two regions in the aquifer based on TDS, southern and northern part. The northern part (well no 1 to 6) has less TDS where the southern (well no 7 to 11) has grater TDS value. Maximum of TDS concentration happened at well no. 9 where concentration is close to 500 ppm. It is almost equal to the international standard limits for drinking water. Therefore, it may be considered as point source of TDS pollution. Wells no. 9 was located in a village where the wastewater discharges to the aquifer directly. 600 ----------j 550 r~l 500 ] 450 | ^ 400 I I 350 [-1 | •2= 300 - ] p 250 i— p-i r~| [ 200 | 150 | 100 | 50 | 0 lLJ LJ L L. I-I r 1, ʹ 1 23456789 10 11 Well no Fig. 2: The TDS value in spring 2005 From the groundwater flow direction it is expectable that TDS increase gradually to the southern part of the aquifer. This process happens in four seasons where is analyzed (spring and autumn 2004, 2005). Other part of the aquifer doesnʹt have a pollution risk. Fig. 3 shows the mapping of TH in spring 2004. Fig. 3 shows that most part of the aquifer has TH values from 150 to 300 mg/lit/CaCo3 categorized as hard water. This is expected because of the geology of the plain. However the TH values of total wells are lower than the standard limits. Based on the data, the Shahrekord groundwater quality is classified hard water. Groundwater Quality Mapping in Shahrekord Aquifer Fig. 3: Total hardness distribution in spring 2004 EC has the same results as TDS because it has a linear relation with TDS. Fig. 4 shows Wilcox diagram. It shows that ten of the wells classified as C2Si (i.e. average EC and low Na). Only one well is classified (well no 9) in C3Si class (i.e. saline water). CI 33d 1330 « Salinity Hazard (Cond) Fig. 4: Water quality classification using Wilcox diagram Nitrate mapping is presented in Fig. 5. The maximum nitrate concentration value was 37 mg/lit in the northern parts of the aquifer where it has the minimum EC and TDS. It seems that the most important factor on nitrate contamination is agricultural deep percolation and N-based fertilizer consumption in agriculture. The northern part of the aquifer has agricultural land use and the vadose zone is too thin. Thus, the water table is close to the soil surface and runoff and nitrate is leached to the groundwater. Nevertheless the total concentration is lower than the standard limits (50 mg/1) but it is increasing and should be monitored. 18 Fig. 5: The Nitrate Concentration in the Shahrekord Aquifer (mg/1)