PROCEEDINGS, TOUGH Symposium 2003 Lawrence Berkeley National Laboratory, Berkeley, California, May 12–14, 2003 - 1 - MULTI-CONSTITUENT MODELLING OF A GASOLINE SPILL USING THE T2VOC NUMERICAL SIMULATOR Fritjof Fagerlund and Auli Niemi Department of Earth Sciences, Uppsala University Villavägen 16 Uppsala, 75236, Sweden e-mail: Fritjof.Fagerlund@hyd.uu.se , Auli.Niemi@geo.uu.se ABSTRACT Gasoline is a mixture of many constituents which have different physical and chemical properties. Because of differences in solubility, volatility as well as adsorption and biodegradation properties the different constituents partition differently between the phases. Taking multi-constituent behaviour of gasoline into account in numerical models is complicated by its large number of constituents. To address this problem constituents with similar properties have been grouped together into eight fractions for which the fate and transport properties have been estimated. Using the T2VOC numerical simulator the migration of each fraction can be studied separately. Since the composition of the free- phase gasoline changes with time the simulations are conducted stepwise and chemical parameters are changed accordingly after each simulation step. The approach has been used to simulate a gasoline spill in connection with a tanker accident in a geological setting typical of Sweden. The migration with time of the free-phase gasoline is presented together with the concentrations of one of the fractions in the aqueous and gaseous phases. INTRODUCTION Due to its widespread use throughout society, gasoline is one of the most commonly occurring hazardous chemicals of today. It is present as a soil and groundwater contaminant at an enormous number of contaminated sites and is together with diesel fuel the most commonly occurring chemical in spills in connection with accidents in Sweden (Björklund et al., 2001). In order to assess risks and plan remedial actions at the sites of spills and leaks, it is necessary to understand the spreading behaviour of gasoline. Since it is immiscible with water but still partly soluble and highly volatile, gasoline may spread as a separate phase, as dissolved in soil- and groundwater and as fumes in the soil air. The migration of gasoline in the sub-surface is hence a matter of multi-phase flow. Various numerical simulators have been developed to study multi-phase flow, e.g. Kaluarachchi and Parker (1989), Abriola and Pinder (1985), Falta et al. (1995). Modelling gasoline is, however, complicated by the fact that it is a mixture of more than a hundred different constituents which have different chemical and physical properties. Because of differences in solubility and volatility, the different constituents partition differently between the phases and consequently also the composition and properties of the free-phase gasoline change with time. To correctly model migration behaviour in the subsurface and keep track of concentrations of especially interesting constituents (such as the BETX fraction) it is necessary to take into account the multi- constituent nature of gasoline. The work of Adenekan et al. (1993) and Pruess and Battistelli (2002) resulted in the TMVOC a numerical simulator which is capable of handling several chemical constituents at a time. Here a different approach is taken using a single-constituent multi-phase flow simulator, the T2VOC by Falta et al. (1995), simulating one constituent at a time. Ideally, both using the TMVOC and when the approach described below is taken, every constituent should be treated separately. However, because of the large number of constituents, such approach would require a huge computational effort and is as of to date not feasible. It is therefore necessary to reduce the number of modelled constituents by assigning them to a smaller number of representative groups or fractions. MODELLING APPROCH Grouping of constituents The approach taken here is to group constituents with similar physical and chemical properties into a smaller number of fractions, and is similar to that of the Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) (Gustafson et al., 1997). Based on the equivalent carbon number (EC number), which describes the equivalent length of the carbon chain based on the boiling point, the hydrocarbon constituents are classified as belonging