Adjustment of DRASTIC vulnerability index to assess groundwater vulnerability for nitrate pollution using the advection diffusion cell

Field: Environmental Science (Physical Sciences)

This study aims to estimate aquifer vulnerability by applying the standard DRASTIC index as well as utilizing the Emission-Transmission-Immission (ETI) concept to assess the groundwater vulnerability to groundwater contamination by nitrate in a part of the Venlo Block (German name: Venloer Scholle) in the Northwestern of North Rhine-Westphalia in Germany. Data for land use, soil, and hydrogeological settings in the study area were collected from different sources such as: The Geological Survey (GD), Ministry for the Environment, Conservation, Agriculture and Consumer Protection in North Rhine-Westphalia (MUNLV), and the German Working Group on Water Issues of the Federal States (LAWA). These data have been used to build the seven parameters of the standard DRASTIC vulnerability index. The final groundwater vulnerability map was constructed. It is clear that more than 65% of the study area can be classified to have high to very high groundwater vulnerability. Geostatistical and sensitivity analysis of the standard DRASTIC index showed that the groundwater recharge and the impact of the vadose zone have the greatest influence on the groundwater vulnerability. Analysis of the effective weights of the DRASTIC parameters indicated that the groundwater recharge and the protective layer represented with the impact of the vadose zone are also the most effective parameter in the final DRASTIC vulnerability index which coincides with the results of the map removal sensitivity analysis. An additional objective is to demonstrate the combined use of the DRASTIC index and the ETI model as an effective method for groundwater pollution risk assessment. The ETI concept was utilized to assess nitrate retardation in different soils using the Advection-Diffusion (AD) cell. The convective-dispersive solute transport equation has been solved analytically to calibrate the results of the Advection-Diffusion cell, and high agreement has been shown. The amount of nitrate sorbed in soil was depending on the grain size istribution and on the organic matter content and was ranging from about 10% in sandy soils to about 60% in clayey soils. The retardation parameter of nitrate in soil was also estimated based on the Advection-Diffusion results which were validated by the analytical solution of the convective-dispersive solute transport equation. The highest retardation parameter occurred in the soil sample with the highest silt and clay portions and the highest organic matter content. The nitrate retardation parameter was integrated in the DRASTIC index and adjusted vulnerability maps (DRAS’TIC, DRASTIC-ETI, DRASTIC-Rt indices) were produced considering nitrate retardation in soils. It was clear that integrating the retardation factor in DRASTIC index has reduced vulnerability values in the study area, these findings were enhanced by the positive strong correlation between the new modified DRASTIC indices values and nitrate concentrations in groundwater, land use classes and the direction of groundwater flow. The study concluded that (1) the standard DRASTIC index has overestimated the groundwater vulnerability to nitrate pollution, and (2) using the Advection-Diffusion cell based on the Emission-Transmission-Immission concept produced more reliable vulnerability maps after considering nitrate sorption and retardation in soil.