Incidence of the pollution source on the selective dissolution and retention of hydrocarbons in saturated porous medium
In the case of groundwater contamination hy petroleum products, the constituents of the immiscible pollutant source will be selectively solubilized and transporled over large distances by the water movement. Then, long-term pollution of aquifers is essentially due to the transfer and transport of dissolved hydrocarbon traces. The behaviour of soluble hydrocarbons and the extent of pollution depend on hydrodynamical conditions of the infiltration area and phase partitioning between water, oil, gas and solid in the aquifer. This article deals with some aspects linked to the transfer mechanisms which take place during the selective dissolution of hydrocarbons and those relating to physico-chemicai exchanges accompanying the transport of soluble substances of the pollution source through saturated porous media. The experiments have been conducted with physical models constituted of glass columns : The glass column source of pollution (L = 75 cm, Ф = 10.4 cm) contained both natural quartz land and a motionless oil phase uniformly distributed in quantifies below residual saturation; it allows to study the selective dissolution of the constituents of the impregnation body. The exchange phenomena between phases were examined in a second glass column (L = 70 cm, Ф = 9.3 cm) which received the water charged with dissolved hydrocarbons coming from the glass column source, and filled with solid matrixes which were not contaminated at the beginning. The solid phase in the second glass column consisted of quartz sand or a mixture of agricultural soil and sand. The soil contained an average of 1.3 % of organic materials, 6.7 % of clay minerals and 27 % of limestone. The influence of the adsorbing properties of the solid material and the role of residual air on the transport and retention of soluble hydrocarbons were investigated. The variation of residual air content was obtained according to the filling method of the second glass column : “dry filling” favours the presence of the gaseous phase (water introduced by imbibition); “lifting by sedimentation” can reduce or partially avoid it (the second column is filled with the porous medium under water). For a pollutant source made of a mixture of hydrocarbons at equal fractions and belonging to a same homologous series (alkanes and aromatics), the experimental results allowed to establish the importance of solubility parameter as well on the mode of selective leaching of the impregnation body, as on the selective retention of soluble elements of the pollutant source. The behaviour of the constituents of the pollution source made of several hydrocarbon species appears differently and then, the solubility dues not constitute, for such a source, a sufficient criterion to apprehend the observed transfer and exchange mechanisms. - Selective dissolution : the modifications of behaviour especially result to the fact that some components of the pollution source are represented at very low proportions then, they can’t respect, during the partition phenomenon, the gradation of solubility values as well at the beginning of the leaching of the impregnation body, as at the decreasing stages of concentrations. This situation must be regarded for studying the transfer of soluble hydrocarbons after accidental spill of petroleum products because it will affect the transport and the contamination prediction. - Exchange phenomena : we observed that the liquid/gas exchange is the major factor of retention of soluble alkanes masking the effects of adsorbing materials. The influence of adsorbing character of the solid material appears clearly, only when the air content of the porous matrix is low. On the other band, the residual air content has no influence on the transport of soluble monoaromatics but the main phenomenon observed is in that case the liquid/solid exchange. Moreover, the impact of the gaseous phase on the alkanes involves a modification of sorption process of hydrocarbons by the solid phase : the selective retention of various constituents of a pollution source made of several hydrocarbon species should be interpreted by the knowledge at their respective distribution coefficient Kd as it was admitted by several authors, when the air content of the porous matrix is low. The sorption process should be more complex when the residual air content is great because the gaseous phase plays a preponderant role for the alkanes; in that case, the simultaneous effects of the solubility, the Henry's constant and the distribution coefficient must be taken into account together for getting a best understanding of each component behaviour of the pollution source and assessing their fate in the case of alluvial aquifer contamination.