Waxy Crude Oils (WCO’s) are characterized by the presence of heavy paraffins insufficiently large concentrations. They exhibit quite complex thermodynamical andrheological behaviour and present the peculiar property of giving rise to the formation ofsegregated wax deposits, when temperature falls down the so called WAT, or Wax AppearanceTemperature. In extreme cases, segregated waxes may lead to pipeline occlusion due todeposition on cold walls. In this paper we review the mathematical models formulated todescribe: (i) wax cystallization or thawing in cooling/heating cycles; (ii) the mechanismsof mass transport in saturated non-isothermal solutions; (iii) the experimental deviceused to measure wax solubility and wax diffusivity; (iv) wax deposition in pipelinescarrying a warm, wax-saturated WCO through cold regions; (v) wax deposition accompanied bygelification during the cooling of a WCO under a thermal gradient.

The paper is devoted to mathematical modelling and numerical computations of anonstationaryfree boundary problem. The model is based on processes of molecular diffusion ofsomeproducts of chemical decomposition of a solid organic substance concentrated inbottom sediments.It takes into account non-stationary multi-component and multi-stage chemicaldecomposition oforganic substances and the processes of sorption desorption under aerobic andanaerobic conditions.Such a model allows one to obtain quantitative estimates of incoming soluteorganicsubstances of anthropogenic origin having different molecular weights from thebottom sedimentsinto water and to study the influence of seasonal variations of theconcentration of solute oxygenin the near-bottom water on the direction of exchange processes in the system“waterbottom”.
Identification of parameters of the mathematical model with the use ofexperimental data andwith the employment of a priori information of the model's structure is carriedout. Comparisonof the numerical simulations with experimental data is conducted to the end ofverification ofefficiency and plausibility of the proposed mathematical model of secondarypollution. It impliesassessment of water quality on account of the processes of exchange in thesystem “waterbottom”.The results of computations of non-stationary fluxes at the boundary“waterbottom” are analyzed.A model example is used to estimate the potentials of the biogenic load on thewater reservoir.