| | | 4.3 Sources and retention |
Since, loads in rivers normally are well documented, accurate and precise estimations of emissions from various sources (i.e. source apportionment) are a prerequiste for our understanding and our ability to control nutrient fluxes. Source apportionment has normally been performed through inventories of point and diffuse sources. Point source emissions are normally easy to obtain directly from monitoring data from municipal wastewater treatment plants (WWTP) or indirectly from per-capita emission coefficients. For example, by assumptions that humans excrete 4.4 kg N yr-1 per person combined with information of the purification in WWTP, e.g. by assuming a per capita nitrogen load in wastewater of 3.3 kg N yr-1 per person. Areal nutrient leakage from diffuse sources is normally more problematic but can be estimated from emission coefficients for different land use categories obtained from plot or field experiments or through modelling of the upper soil layer. Geographical Information Systems (GIS) are often used to perform regional assessments. Alternatively, source apportionment may be based on statistical analysis of observed river nutrient loads (i.e., transport) and explanatory variables (i.e., factors explaining variability in loads between sites or in time). This methodology can be divided into two categories: regression analysis between observed concentration and water discharge, and regression analysis between observed load and watershed characteristics (see e.g Alexander et al., [3]). Recently another alternative of source apportionment has become available because dynamic process based models have been successfully applied in large watersheds. Dynamic models, which conceptually describe all physical, chemical and biological processes, aim to simulate time series of nutrient transport at the root-zone or in first-order or second-order streams. Source apportionment is a parallel result even if the main purpose of these models is usually prediction or better understanding of processes. For large-scale water management, all the above mentioned methods are potential tools that give results of different temporal and spatial scales. The different methods, however, also have a large variety of input demands in the form of data and work. For decision makers and scientists faced with a specific water management problem, it is therefore essential to choose methods that meet the often limited available input data, gives the wanted results, and is economically feasible. Unfortunately, comparable studies of different methods and models for nitrogen source apportionment are scarce. Model applications are often performed and described for specific watersheds, where other models have not been applied. Continuous changes in land use and agricultural practices also prevent fair comparison, even if models are applied in the same watersheds but on different occasions.
Monitoring of nutrients is important for the identification of emissions sources, e.g. from various land use categories, municipal wastewater and septic systems. Therefore, planning of actions for water protection measures require an understanding of the forces that drive different nutrient transformation processes in soils, ground waters, streams and rivers. However, the efficiency of source control measures in rivers depends not only on the quantification of the load, sources and loss of nutrients in the soils, groundwater and river network. This is due to the fact that the total sum of emissions of nutrients to surface waters in a river basin is normally larger than the nutrient load at the river mouth. Retention is a collective and lumped expression for a large number of biogeochemical and hydrological processes that temporarily decrease, decay, degrade, transform, or permanently retard or remove the substance from a river basin. Nutrient retention capacity in the subbasin is dependent from various factors: trophic status, depth of the waterbody, water residence time, ratio of lake area to the catchment area, nitrogen loading, loading of organic matter, denitrification activity (primarily dependent on sufficient amounts of reducable organic substrates, low oxygen concentrations and high temperature). Nutrient retention processes in rivers and in their drainage basins could therefore be of significance against increased nutrient loads to the maritime waters. Consequently retention processes/potentials and buffering capacities in river basins should also be considered and be an integrated part when planning water protection measures and strategies. In practise, policy implementation and remediation have been hampered by a substantial uncertainty involved in assessment of the contribution different sources made to the riverine transport of N and P. This has for example been observed in the Baltic Sea region. The importance of taking retention into account when estimating the contribution of nutrients from nonpoint and point sources have been emphasises in numerous studies. However, many studies on a global, regional and national scale are marred by defectives about the relative importance of retention when estimating the quantity reaching the sea from different sources: see e.g. the classical work by Howarth [235]. Nutrient retention is normally defined as the amount of nutrients that is biogeochemically transformed or retarded (temporarily or permanently). It is traditionally divided into:
Retention in river basins, and in particular retention in streams, rivers and lakes, are of great importance when using a mass balance approach to assess nutrient sources and retention on regional or national level. Estimates of nutrient delivery from various sources will obviously be affected by permanent or temporary retention. The retention potential at a river and drainage basin scale has in literature been given considerable attention. Nutrient and phosphorus in particular is usually subject to substantial retention. However, N and P-retention are highly variable in time and space and could range between 0% and 100% depending on the flow paths and water course character (e.g. presence of lakes in a drainage basin). In Sweden, the N-retention from the root-zone to the coastal sea has been estimated to 48%. In Vistula and Oder, nitrogen retention has been estimated to ~ 50% and 30%, respectively, while the corresponding estimates for phosphorus were in the range 25-30%. These examples clearly illustrate the great range in various retention estimates.
Besides retention in the main stream of a river, in smaller channels and streams, in lakes and reservoirs there can be retention in the terrestrial part of the river basin: in the topsoil, in the root-zone, and in deeper soil layers and groundwater. For instance, may N be lost from the soils via denitrification in the case of anaerobic conditions, an organic carbon source and high temperature. In addition, wet soils in combination with high pH may cause significant gaseous losses of N through ammonia volatilisation (and thereby reduced leaching losses). Even though the biogeochemical and physical processes have been extensively studied on an experimental scale and even on the scale of small catchments, there are yet very few studies that have attempted to examine the significance of these processes on a large river-basin scale [46]. Retention processes could also be one reason for the rather weak water quality response to the dramatic decline in industrial and agricultural production in the Baltic States after the independence [201]. Another aspect of retention is related to the balance between various substances. For example, may reduced emissions of phosphorus and organic matter (e.g. due to installation of sewage treatment plants), under certain conditions, reduce the denitrification capacity in rivers and thus increase the load of nitrogen on maritime areas.
To conclude: even though detailed knowledge about processes and mechanisms related to self-purification and retention of nutrients are relatively well-known, still a lot of research on the governing factors for retention of nutrients at catchment/river basin scale are needed. The quantification of retention is regarded of particular challenge.
| | | 4.3 Sources and retention |