Herbicides transport experimentation and modelling in the unsaturated zonefor groundwater risk assessment

A methodology of groundwater contamination risk assessment by non-point sou rce xenobiotics is presented for a test-site area in the Pistoia alluvial p lain (Northern Tuscany, Italy) with land use almost exclusively devoted to nurseries for ornamental horticulture. The assessment is based on the compa rison between the expected active ingredient concentration in groundwater a nd the maximum admissible concentration for drinkable use (either the resou rce actually exploited or potentially exploitable in the future). To determ ine the concentration of the pesticide in groundwater, a linkage is propose d between the cumulative mass flux arriving at the base of the unsaturated zone in a steady state condition over one year and the dilution capacity of the shallow groundwater, expressed by an Aquifer Transfer Function (ATF). The ATF is determined on the basis of hydraulic transmissivity and the patt ern of groundwater flow and recharge. The cumulative mass flux was predicte d on the basis of the leaching model CHEMFLO, according to 12 hydrogeologic scenarios of the unsaturated zone and 2 hydrologic scenarios of effective recharge (average and extreme events of precipitation). The risk assesment was validated against the results of a chemical monitoring survey conducted on 19 wells tapping the shallow aquifer. Model calibration was also based on the results of an experimental field-plot with tensiometers and suction lysimeters for the study of unsaturated zone flow and transport.