Numerical Simulation of Chromium Changes Trend in Aquifer of Birjand plain

Numerical quality and quantity simulation of groundwater, is an important tool for managing groundwater resources. According to the measurements taken from the density of chromium in drinking water wells of urban water in distribution network of Birjand, the chromium content is higher than permissible limit (Density of more than 0.05 mg/lit). Therefore, the purpose of this research is quantitative and qualitative simulation of groundwater in unconfined aquifer of Birjand using mathematical model and change trend of chrome density. To do this, numerical codes of MODFLOW and MT3D which is in groundwater modeling software (GMS), were used. At first, needed statistics and information for preparing a conceptual model of plain was collected. Then the simulation was done in the software and the model was ran. Then the calibration of qualitative and quantitative mathematical model in Birjand aquifer was donein transient conditions. Hydraulic conductivity and specific yield values for the quantitative model and longitudinal diffusivity coefficient values and prosity was calibrated for qualitative model. Finally, the simulation was performed for a one-year period. The forecast was done for 6 years after the simulation with 30% reduction and increase of harvest. In the quantitative model, the highest water level was estimated in the eastern part of the aquifer is 1296 m and the lowest water level is 1263 m close to the groundwater drainage in the southwest aquifer region. Qualitative model output indicated the highest density of chromium in the middle of the aquifer. Also the gradual movement curve of the maximum chromium aquifer density was from the central zones toward the east and west of the aquifer. For evaluating the calibration, features of GMS software such as Mean Error, Mean Absolute Error and Root Mean Square Error was used that the values of these statistical parameters were 0.09, 0.944 and 1.071 m for quantitative model and 0.003, 0.004 and 0.008 mg/lit for quantitative model. The results of qualitative and quantitative modeling of Birjand aquifer shows that the density of chromium in the central areas of Birjand groundwater is more than 0.05 mg/lit, which is more than WHO standards for drinking water. this density increases during simulation and chromium Density and reaches 0.09 mg/lit. Also the density variation of this contaminant depends on fluctuations in groundwater levels and by increasing the water level, chromium levels can be reduced in the aquifer. As well as during recent years, chromium density in the aquifer always have increasing trend. By applying the reduction scenario of harvest from the aquifer, the area of the critical zone has decrease and increasing scenario of harvest, in comparison with harvest reduction, shows less changes in density and area of the critical zone.