39.-Modelling groundwater pollution due to fluoride contaminated irrigation water

Abstract

Out of many natural geochemical substances contaminating ground water, fluoride is the major one. It invokes considerable interest due to its unique character as regards to its impact on physiological system of living beings. A very low dose (<0.6 ppm) of fluoride promotes decay of teeth, whereas when consumed in the ranges greater than 1.5 ppm it causes fluorosis and related diseases. Fluoride contaminated water when used for irrigation, percolates down and contaminates the soil and ground water. The movement of fluoride in the vadose zone is a complex phenomenon and is largely affected by water movement. In this study, an attempt has been made to study the water and fluoride movement in the vadose zone. An experimental soil column of height lm and diameter 20 cm was used. The movement of fluoride was studied by collecting the leachate from 90 cm depth at definite intervals of time. CHEMFLO model was used in the study for describing leachate movement. It is based on the convection-dispersion equation for water and chemical movement in vadose zone. Reasonably good correlation was noticed between observed and model predicted results. The results obtained from the study revealed that for shallow ground water table conditions, fluoride contaminated irrigation water of concentration 15 ppm produces leachate percolating the root zone, which exceeds the acceptable limit (1.5 mg/1) for drinking purpose and thereby will pollute the ground water. Under similar topsoil conditions, it was also found that the optimum number of irrigation cycles for ground water table depths of 2m, 5m and 10m are to be restricted to 2, 5 and 9 to prevent contamination of ground water.