Pollution vulnerability assessment of groundwater in Sirjan Plain using DRASTIC and GIS

Groundwater is one of the most important water resources in many parts of Iran as well as in Sirjan plain. Although in comparison with surface waters, groundwaters are less vulnerable to pollution, their pollution abatement is much more difficult. It is thus necessary to protect them from being polluted. Additionally, in contrast with stream waters which are concentrated in linear areas across the world, groundwaters are present in vast areas. This means they are much more accessible and human beings are much more dependent on them. They are also present under terrains having different uses. That is, their protection against pollution and preparing their vulnerability maps is of utmost importance. There are different methods to determine the pollution potential of groundwater, the most widely used and the most comprehensive method is DRASTIC. DRASTIC is a model that considers the main hydrological and geological factors which potentially may impact aquifers. It considers seven relevant parameters which include depth to water table (D), recharge rate (R), aquifer material (A), soil characteristics (S), topographic slope (T), vadose zone impact (I), and aquifer,s hydraulic conductivity (C). The depth to the water table (D, in meters) is the distance from the land surface to the groundwater level, which means the distance the polluter must pass to reach groundwater. The net recharge (R, in mm per year) is the amount of infiltrated water that reaches the aquifer from the surface. The soil characteristics determine the ease with which a polluter can pass the soil layer toward groundwater. The probability of a surficial polluting agent reaching groundwater is inversely related to surface slope, which is expressed as T in DRASTIC. The material comprising the vadose zone plays an important role in blocking the movement of polluting agents to reach the water table. In DRASTIC, each parameter has devoted a rate of 1 (the least important) to 10 (the most important) which depends on its value. Subsequently, the DRASTIC index is determined using the weights considered for each of the seven parameters. Sirjan plain is one of the most developing plains in Iran which has many different land uses, including agriculture, urban areas, roads, and factories. The majority of these uses may pollute groundwater. It is, therefore, necessary to adapt the different land uses with groundwater vulnerability rates. So, the purpose of this article is to determine the groundwater pollution potential of this plain using the DRASTIC model.

In this study, the relevant seven parameters were first prepared. For preparing the D layer, the depths of the water table measured from monitoring wells across the plain were employed. The (R) layer was prepared using the data of rainfall infiltration, runoff infiltration, and infiltration from residential and agricultural areas. For preparing the (A) layer, 70 geological logs of the area were used. The (S) layer was prepared according to field studies and geological logs. The digital elevation model (DEM) was used to prepare the (T) layer. The (I) layer was prepared using the geological logs of the area. Finally, the (C) layer was prepared by dividing the transmissibility values of the aquifer by its thicknesses. All maps were rated according to and combined to calculate the DRASTIC index (DI). DI values were validated according to nitrate concentrations, and sensitivity analysis was undertaken by omitting the layers successively.

The relevant DRASRIC layers are given in Figures 2 to 8. In the depth rating map (Fig 2 ), the majority of the surface belongs to rate 1. This means that due to high water-level depths, this parameter has lowered pollution potential. In the recharge rating map (Fig 3), the major part of the plain is green (rate 1), meaning very low levels of recharge and a low role in pollution. The aquifer material map (Fig 4) is mainly covered by 5, 6, and 7 rates. The soil rating map (Fig 5 ) indicates that the 7, 8, and 9 rates cover the major parts. This means a rather high pollution potential share of this parameter. In the topographic slope rating map (Fig 6) the 9 and 10 rates cover vast surfaces at the west and south, and the 4-7 rate values of the vadose zone rating map (Fig 7) represent the average contribution from this parameter. The aquifer's hydraulic conductivity map (Fig 8) indicates a rather wide range contribution of this parameter. DI values of the study area range from 60 to 128, representing rather medium vulnerability. The higher vulnerability values are mostly observed in the western part, where recharge from agriculture is high, the topographic slope is gentle, and the water level is rather high. Low and medium values of DI are mostly observed at the north center and east. In a qualified vulnerability map comparatively low, medium, and high vulnerability values are separated. The high values mostly belong to residential and agricultural areas, which are at the risk of nitrate, pesticide, saltwater, detergent, and heavy metal risk factors.

The DRASTIC index values of the Sirjan alluvial aquifer range from 60 to 128, meaning low- to high vulnerability values. The most vulnerable locations mainly include residential and agricultural areas which may deliver considerable amounts of nitrate, detergents, pesticides, heavy metals, and dissolved salts to the aquifer. It is, therefore, necessary to reduce the pollution potential in these areas by such means as reducing pesticide use, using green agricultural practices, and collecting and treating sewage from residential areas. Considering the high water level dropdowns in this plain, which means decreasing groundwater resources, it is necessary to abate the pollution risk of these limited resources via such means as adapting land use with vulnerability at every location.