Such understanding is required for developing regulations and guidance, even though facilities for the geological storage of CO2 will be designed to prevent leakage. Under Task 1 of RISCS, Quintessa led the development of some reference European receptor environments, together with associated high-level impact scenario descriptions. These scenarios provide the basis for mathematical modelling studies that will be undertaken later in the project, and input to experimental studies in both terrestrial and marine environments. For each receptor environment a baseline 'most likely' scenario has been defined in which the storage system evolves as designed, with no leaks occurring. For comparison, unexpected 'alternative evolution' scenarios have also been identified. For example, for terrestrial systems unexpected 'alternative evolution' scenarios include potential impacts on animals and plants caused by direct releases to the atmosphere following well seal failure, and localised releases of CO2 to aquifers and the near-surface environment.