Tracing Copper from society to the aquatic environment : Model development and case studies in Stockholm

Abstract: Copper remains at elevated levels in the aquatic environment of Stockholm due to diffuse urban sources. Management of these diffuse sources requires their quantification but they cannot be measured directly by field observations. The working hypothesis of this thesis was that Copper levels in the sediments of urban lakes would reflect diffuse emissions within their catchment areas. In order to test this hypothesis, a source – transport – storage conceptual model was developed for tracing the urban diffuse sources of Copper to the sediment in the urbanised catchment. A substance flow analysis (SFA) approach was taken in the source module and a fate, mass-balance model was applied in the lake module. Five separate urban lakes (Judarn, Laduviken, Långsjön, Råcksta Träsk and Trekanten) within the Stockholm area and a main water flow pathway from Lake Mälaren to the inner archipelago of the Baltic Sea, through Stockholm, were selected as case studies. In comparison to actual source strength data in the literature for the five case study lakes, the SFA approach gave similar results to previous models, but with reduced uncertainty. The SFA approach was also able to indicate the actual sources of urban copper, which was not accomplished by the other approaches and which is a great advantage in managing the sources. For the five lakes in Stockholm, traffic and copper roofs were found to be major contributors of Copper. For the three more polluted lakes, good agreement was obtained between simulated sediment copper contents and independent field observations, thereby supporting the applicability of the model in such cases. Furthermore, simulation results showed sediment copper content to be linearly dependent on the urban load. While this suggests that the urban copper sediment level reflects the urban load, considerable integration of this load over time (decade(s)) was suggested by the simulation results, so time must be allowed in order to detect a change in the urban load by field monitoring of the sediments. Published data on the main water flow pathway from Lake Mälaren to the archipelago showed a peak in sediment copper content close to the city centre, confirming a considerable urban influence. An approach to quantitatively follow Cu from its urban source through such a complex, aquatic system was developed and applied to Stockholm. The compliance of future quantitative model results with monitoring data may help test the choices made in this conceptual model and the applicability of the model. Data availability proved to be a major obstacle to achieving a quantitative model, particularly as several municipalities with different levels of data availability surround the main water flow pathway studied. Finally, the applicability of the quantitative, coupled source – transport – storage was demonstrated in a simplified scenario analysis. The ability of the model to estimate the copper load to air and soil and to the urban aquatic environment was also demonstrated.