The Use of Soil-Gas Sampling in the Study of Groundwater Pollution by Volatile Solvents (VOC): The Example of the Porto Feliz (São Paulo, Brazil) Case

Abstract

In 1983, a serious accident occurred in Porto Feliz, São Paulo, Brazil when the rupture of a storage tank released 400,000 litres of volatile organic solvents. A large portion of the underlying aquifer was contaminated including some deep wells and shallow hand-dug wells. Soil-gas sampling was the technique used to evaluate the degree and extent of contamination. The method is based on the analysis of small quantities of soil vapors extracted at shallow depths. The origin of these vapors is the underlying contaminated groundwater and the vapor concentration distribution reflects the degree and extent of aquifer contamination. Other techniques were rejected since the physico-chemical characteristics of the pollutants did not allow their detection by traditional geophysical methods and the thick unsaturated zone and difficult geology would have resulted in exorbitant drilling costs for monitoring wells. The use of soil-gas sampling allowed mapping of the pollution plume, defining its two-dimensional shape and location. Vapor analyses at different depths permitted the establishment of concentration gradients and an estimation of vertical vapor flow behavior in the unsaturated zone as well as surface losses by volatilization. Water levels in existing wells made it possible to construct potentiometric surface maps, defining the groundwater flow pattern. Periodic groundwater quality monitoring furnished data for the evaluation of the plume migration velocity and the influence of rainfall on the increase of pollutant concentrations in the aquifer. The groundwater pollutant concentrations obtained gave a good correlation between the two means.