[1] We present a coupled groundwater-stream model of catchment-scale solute transport, using a Lagrangian stochastic advective-sorptive travel time approach. We consider distributed solute input over an entire catchment and investigate the resulting stream solute breakthrough, subject to the possible solute spreading mechanisms: (1) variable groundwater advection and solute mass transfer between mobile and immobile groundwater zones and (2) in-stream advection, mixing, and solute mass transfer between stream water and hyporheic zone. Among these mechanisms we show that fractal solute spreading over a wide timescale range is, for realistic parameter values, obtained in the stream only for the condition combination of both variable solute advection and solute mass transfer in the groundwater, with mean groundwater advection to mass transfer rate ratio that falls within a certain value range and with only a small fraction of solute input mass following fast overland and/or storm soil water flow to the stream.

[1] We analyze and compare simulations and controlling processes of the past 60 years and possible future short-and long-term development of phosphorus and nitrogen loading from the Swedish Norrström drainage basin to the Baltic Sea under different inland source management scenarios. Results indicate that both point and agricultural source inputs may need to be decreased by at least 40% in order to reach a long-term sustainable 30% reduction of anthropogenic coastal nitrogen loading, as required by national environmental goals. A corresponding 20% anthropogenic phosphorus load reduction goal may be reached in the short term by analogous combined 40% source input reduction, but appears impossible to maintain as a long-term achievement by inland source abatement only. In general, realistic quantification of the slow subsurface nutrient transport and accumulation-release dynamics may be essential for accurately predicting and managing nutrient loading to surface and coastal waters.Citation: Darracq, A., G. Lindgren, and G. Destouni (2008), Long-term development of phosphorus and nitrogen loads through the subsurface and surface water systems of drainage basins, Global Biogeochem. Cycles, 22, GB3022,

We simulate and analyze long-term dynamics of coastal nitrogen (N) loading and the inland source changes and processes that may have determined its development over the past 60-year period and may govern its possible future responses to various N source management scenarios. With regard to processes, the results show that average basin-scale N delivery fractions to the coast may not be representative of the coastal impacts of either diffuse or point inland sources. The effects of inland source changes may be greatly redistributed in space-time and delayed by slow N transport and mass transfer processes in the subsurface water system of coastal catchments. Extrapolation of current N transport-attenuation conditions for quantification of future abatement effects may therefore be misleading if the extrapolation models do not realistically represent delayed long-term influences of slow subsurface processes. With regard to policy, the results show that and why national Swedish and international Baltic Sea region policies for coastal N load abatement may be difficult or impossible to achieve by inland source abatement only. Large mitigation of both point and diffuse sources may be necessary to achieve targeted coastal N load reductions fast and maintain them also in the long term.

[1] We show that the large spatial aggregation of model parameters in common catchment scale nitrogen budget modeling leads to artifacts that may, for instance, be a factor in explaining reported decreases of calibrated instream nitrogen loss rates, l s *, with increasing stream size. In general, the common assumption of a single representative solute travel time for an entire stream reach may lead to considerable underestimation of the actual underlying local biogeochemical loss rate l s by l s *, which increases with actual l s value and/or increasing mean solute travel time and travel time variability in the stream. We propose an up-scaling methodology to overcome such model artifacts, in form of closed-form expressions of catchment-scale, in-stream nitrogen delivery factors for diffuse and point sources, as functions of localscale nitrogen loss rates, l s .