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This case study tested an approach for large-scale monetary valuation of water purification services based on spatially explicit, biophysical mapping of nitrogen retention in river networks. The study area for the case study was the Mediterranean bio-geographical region. The monetary value of nitrogen retention was assessed using replacement costs (RCs), based on the construction, operation and maintenance of constructed wetlands (CWs). The value of nitrogen retention by rivers in 2005 was estimated at €2167 km–1, on average. If the capacity or the potential of rivers and lakes to retain nitrogen was considered, the monetary value was €35,000 km–1on average, much higher than the effective retention. The coupling between a biophysical model simulating the transport and retention of nitrogen in the aquatic environment and monetary estimates of potential and realized nitrogen removal resulted in detailed spatial information of one of the benefits that are provided by rivers and streams.