General Quantification of Catchment-Scale Nutrient and Pollutant Transport through the Subsurface to Surface and Coastal Waters GEORGIA DESTOUNI,* KLAS PERSSON, CARMEN PRIETO, AND JERKER JARSJ ¨ O Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91, Stockholm, Sweden Received August 1, 2009. Revised manuscript received January 4, 2010. Accepted January 25, 2010. This study develops a general quantification framework for consistent intermodel and intercatchment comparison of the nutrient and pollutant mass loading from multiple sources in a catchment area to downstream surface and coastal waters. The framework accounts for the wide spectrum of different transport pathways and travel times through the subsurface (soil, groundwater, sediment) and the linked surface (streams, lakes, wetlands) water systems of a catchment. The account is based on key flow partitioning and mass delivery fractions, which can be quantified differently by different flow and transport and reaction models. The framework application is exemplified for two Swedish catchment cases with regard to the transport of phosphorus and of a generic attenuating solute. The results show essential differences in model quantifications of transport pathways and temporal spreading, with important implications for our understanding of cause and effect in the catchment-scale nutrient and pollutant loading to downstream waters. 1. Introduction Much of the nutrient and pollutant loading that threatens the quality and ecosystems of inland and coastal waters is transported through the subsurface (soil, groundwater, sediment) part of the terrestrial water cycle. The nutrient and pollutant loads originate from a variety of sources at and below the land surface such as heavy metal loading from mining wastes and abandoned mine voids (1-3), organic pollutant loading from contaminated land areas (4, 5), and nutrient loading from agricultural land, private sewage systems, and pools that remain in the subsurface from earlier anthropogenic inputs (6-8). Potential chemical accidents on land (9) and failure of subsurface nuclear waste reposi- tories (10), for instance, also involve subsurface transport of chemicals and radionuclides to surface and coastal waters. However, models of catchment-scale solute transport, which have so far mainly focused on nutrients, often neglect the spatio-temporally diffuse and variable nature of sub- surface mass transport and loading into surface waters, as found in the model reviews and classifications by Darracq and Destouni (11) and Destouni et al. (12). In order to clarify different model assumptions that are often adopted, and their effects on the quantification of catchment-scale nutrient and pollutant transport, we have developed a general, model- independent conceptualization and mathematical repre- sentation framework. The framework can be used for intermodel and intercatchment comparison of the mass transport from different sources in a catchment area through its different subsurface and surface transport pathways to the resulting total load into downstream surface or coastal waters. The application of the framework is exemplified by comparative analysis of different model results for the total phosphorus (P) loading to the Baltic Sea from the Swedish Water Management District (WMD) Northern Baltic Proper (13) and the main Norrstro ¨ m Drainage Basin (NDB) (8) within it. This application example identifies and quantifies essential differences in different model accounts of the transport pathways, processes and time scales that determine the catchment-scale mass transport, and loading to downstream waters. An additional application example for the coastal catchment area of Forsmark (14-21) further clarifies the whole spectrum of physical transport (advection) pathways and travel times through a catchment and analyzes how different model handling of this spectrum affects the quantification of total mass loading to downstream waters. 2. General Problem Formulation and Quantification Framework This section outlines the development of the general framework for consistent intermodel and intercatchment comparison of catchment-scale nutrient and pollutant transport. The need for such comparison has been empha- sized by previous studies (11, 12, 22, 23), showing that different process assumptions and parametrizations used in different models have quite different implications for our understanding of catchment-scale transport. Consistent intermodel comparison across various catchment conditions is needed to discriminate among different models and understand the limits of their applicability. Consistent intercatchment comparison can clarify the general versus the site-specific aspects of the catchment-scale nutrient and pollutant transport. The proposed quantification framework for such com- parisons considers the releases of nutrient or pollutant mass into the subsurface water system of a catchment area, which discharges its water and waterborne mass loading into a recipient surface water system (Figure 1). The recipient may be a downstream river stretch, lake, wetland, or coastal zone, and the catchment area may include several adjacent stream and groundwater (sub)catchments. There is no need to assume that surface water catchments coincide with ground- water catchments. On the basis of the experiences and insights from previous conceptualizations and classifications of catchment-scale mass transport (11, 12, 22, 24, 25), we express the total resulting mass load (S r tot ) into the recipient (r) from the different source inputs within the catchment area as the sum where S in i is the mass input from source i, and S in i γ gw-s i R gw-s i and S in i (1 - γ gw-s i )R gw i are the corresponding mass load components delivered to the recipient through the linked soil-to-groundwater-to-stream network pathways (orange and green pathway lines in Figure 1) and the direct * Corresponding author phone: +46 8 16 47 85; fax: +46 8 16 47 94; e-mail: georgia.destouni@natgeo.su.se. S r tot ) ∑ i S in i [γ gw-s i R gw-s i + (1 - γ gw-s i )R gw i ] (1) Environ. Sci. Technol. 2010, 44, 2048–2055 2048 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 6, 2010 10.1021/es902338y  2010 American Chemical Society Published on Web 02/16/2010