Background Monitored Natural Attenuation (MNA) is a remediation technique for cleaning contaminated sites by natural processes. Several European-level projects in the UK and Germany have researched the method and application guidelines have been developed. Before the method can be applied to North European countries, however, feasibility studies need to be carried under appropriate (hydro) geological and climatic conditions. Excavation is the most commonly used means of removing a contaminant source. This method, however, is not possible for large volumes of contaminated soil of relatively low contamination concentration. If the in situ techniques are applied, the contaminated substances are treated on site and not removed. The most suitable remediation technique are selected with due consideration of risk exposures to human health and possible environmental impacts. Shortage of available funding often results in neglecting the necessary assessments, therefore a prior assessment of similar procedures and the demonstration of their feasibility would prove highly valuable. The requirements of environmental law and the Water Framework Directive will also be taken into consideration when evaluating the feasibility of the MNA method. Objectives The biodegradation potential of petroleum hydro carbonates was studied at the Finnish Environmental Institute (SYKE) as part of a research project that ran from 1999 to 2002. The results of this project were used in a LIFE project to demonstrate the feasibility of the MNA method as a remediation technique for oil contaminated sites in Finland. The main objectives of the project were to establish the required timeframe for the remediation to take place and evaluate the acceptable risk exposure during and after the MNA-based remediation. The method was demonstrated on a full scale, and in order to evaluate all possible aspects of the remediation technology, a multidisciplinary assessment of associated risks from microbiological, chemical, hydro-geological, eco-toxicological perspectives was performed in close co-operation with the site owner (the municipality of Hanko) and the environmental authorities. Results The results obtained from the project were used to issue guidelines for application of the method in North European and Baltic Countries. The project's findings were widely disseminated at national and international forums and delivered to authorities and practitioners working in the field. The project also intensively researched and monitored a hydrocarbon-contaminated test site. It concluded that the effectiveness of the MNA method could be proven by geochemical indicators in the ground water and by the results of laboratory tests, which showed efficient biodegradation of the contaminants. The main recommendations were: • The MNA method is suitable for organic biodegradable contaminants such as petroleum hydrocarbons. • The MNA is suitable for sites without any pressing development needs. • The contamination source should be carefully considered if known. • The MNA can be used both in the saturated and unsaturated zones, but mostly recommended for areas with non-heterogeneous geological structure. • Proper risk assessment is essential Following the project duration, the test site will be continuously sampled and monitored by the local authority. The site could be used for demonstration activities. To ensure a long-term impact, the beneficiary is also continuing to offer consultation on alternatives to ex situ methods. The beneficiary will also deliver additional three scientific papers on the method and applications, and take part in the EU's soil strategy development and in the preparatory work for forthcoming regulations.