عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Water resources management comprises directives to protect both surface and groundwater. Agricultural activity is the most signi?cant factor and the main cause of the prominent increase in chemical pollutant (e.g. nitrates or pesticides) in many aquifers. The other source of pollution (organic) can be attributed to agriculture, generating a local increase in the concentration of nitrate levels from farming activities and waste water. A long-term intake of water with a high nitrate concentration is harmful to human health. Groundwater as a source of water supply, ecological environment factor and geological has become increasingly prominent. However, groundwater has confronted with gradually serious pollution. Nowadays, groundwater vulnerability assessment has become a useful tool for groundwater pollution prevention and control. Groundwater vulnerability mapping can indicate regions which are more vulnerable to contamination in the planning stage of socio-economic activities. In this study, intrinsic vulnerability mapping was evaluated. Nitrate has been often selected as an indicator for the assessment of groundwater vulnerability and pollution risks. Existing methods to assess groundwater speci?c vulnerability include: overlay index method, process simulation method and statistical models. Overlay index method, based on superimposing the selected sub-indexes to calculate a composite index re?ecting vulnerability extent, has four major factors which are indexes, weights, ranges and classi?cation.
The most widely used method of vulnerability evaluation is DRASTIC model, due to its ease to use, minimum data requirement, and clear explanation of groundwater vulnerability. In addition, DRASTIC model gives relatively accurate results for extensive regions with complex geological structure in absence of measured data of specialized indexes. DRASTIC model has been transformed by the modi?cation of its indexes and applied to assess the vulnerability of uncon?ned and con?ned groundwater Aquifers. However, its results may be inaccurate due to high obvious subjectivity. The most critical challenge of groundwater vulnerability is achieving the optimal balance among the complexity of method, cost and uncertainty of assessment results. While it is important to check the accuracy of the vulnerability mapping, previous researches have neglected to discuss the validation method for vulnerability mapping, integrally. The choice of an appropriate validation criterion is still disputed in the research. Plain Aleshtar in Lorestan province, Iran, is located in the longitudes of 48° 02' and 48° 31' E and the latitudes of 33° 43' and 34° 05'N. DRASTIC term is Based on seven parameters include: depth of water table, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity. One of the most important factors is the water table depth in DRASTIC Model. In order to provide the data layer of this factor, 21 observation wells in the plain Aleshtar were used. By interpolating these point data, DRASTIC model was evaluated according to the criteria. In this study, maps of slope, rainfall plains and net recharge criteria were classified and the overlapping layers were prepared. The top layer of soil environment and vadose zone is unsaturated area. To prepare this parameter layer, logging of 21 operating wells in plain Aleshtar was done. Then, the wells were ranked according to DRASTIC model criteria. The prepared maps of classification and valuation were based on the results of Aller and colleagues (1987). In this research, in order to validate the potential areas of vulnerability, results were compared with maps of the distribution of nitrate. Single-parametric sensitivity assesses the impact of each of the model parameters sensitivity on the vulnerability index. In this analysis, "effective" or "real" weight of each parameter is compared with the "theoretical" weight assigned through the analytical model.
In this study, seven layers of reference parameters required for the Standard Model Drastic were prepared according to Aller and colleagues (1987). Then, the effect of each of the aquifer parameters was investigated. After weighting DRASTIC parameters and overlaying the seven layers vulnerability index zoning maps were prepared. Based on this classification 3.4, 24.2, 41.4, 24 and 7 percent of the aquifer, respectively had the vulnerability of negligible, very low, low, medium and medium to high. In order to validate the model, its results compared with the measured nitrate pollution in groundwater aquifer and the Pearson correlation coefficient was 0.67. For the purpose of this study that was to try to increase the correlation coefficient based on existing methods and that model modified according to hydrogeological conditions, the model was developed and offered. Therefore, single parameter sensitivity analysis was done, based on the Pearson correlation coefficient. By the results, water table depth had the highest correlation while the net recharge parameters and soil environment had the lowest. Inconsistency due to the assigned weights of the parameters by the model and the results of sensitivity analysis method, AHP method was used. Results showed that the weight of topography parameter varied from 1 to 2 and weight of unsaturated zone was changed from 5 to 2.8. Also, net recharge parameters and soil environment were omitted due to negative correlation with the nitrate zoning map. By the Modified (DATIC) Model, the region was divided into three different classes of vulnerability, in which 67% was negligible, 24% too low and 9% in the classification of low vulnerability. Based on the results, the Pearson correlation coefficient was improved from 68 to 79 percent by the revised version.