Organic pollutants such as policyclic aromatic hydrocarbons (P AH) are found in groundwater of tar-oil contaminated former manufactured gas-plant or coking-plant sites and have been investigated in detail in the past. Tar-oil is often described as dense or light non aqueous phase liquid (DNAPL / LNAPL) and a main part of the water soluble fraction (i.e. solubilities of mg/L up to g/L) consists of polar PAH, N,S,O-heterocyclic compounds and their metabolites. The increased water solubility ofN,S,O-heterocyclic compounds compared to the parent non-polar P AH results in greater mobility and biological uptake with often no decrease in toxicity.