Preceding the construction of a future repository of spent nuclear fuel, a descriptive ecosystem model has to be developed in order to meet the demands set by a safety assessment. The descriptive ecosystem model should be able to describe stocks and flows of matter, and processes that may affect these on a broad landscape scale. Here we describe the planned strategy to develop such a model in three steps; the conceptual model, introduction of site-specific data and, transfer and accumulation of matter in the landscape. The suggested method is based on an ecosystem approach using site-specific data. A mass balanced ecosystem model with food webs provide a way of analysing how matter are linked to different ecosystem components through fluxes. It is suggested that estimated inflow and outflow of matter in mass balanced ecosystem units will reduce the potential variation of transport and accumulation of matter by setting the physical and biological limits to the system. In this paper we present a strategy to build a descriptive ecosystem model and shows examples from the ongoing descriptions at Forsmark and Simpevarp in the Swedish site investigation programme to illustrate the ideas.