Methods for the assessment of diffuse nutrient pollution in rural catchments


The Aquisafe project aims at mitigation of diffuse pollution from agricultural sources to protect surface water resources. The first project phase (2007-2009) focused on the review of available information and preliminary tests regarding (i) most relevant contaminants, (ii) system-analytical tools to assess sources and pathways of diffuse agricultural pollution, (iii) the potential of mitigation zones, such as wetlands or riparian buffers, to reduce diffuse agricultural pollution of surface waters and (iv) experimental setups to simulate mitigation zones under controlled conditions. The present report deals with (ii), presenting existing diagnostic methods for agricultural diffuse pollution on a river basin scale. The report focuses on methods with low to moderate data requirements and analytical effort. Generally no numerical models but mostly GIS based approaches have been considered. The described methods were distinguished along two questions: 1. Does diffuse agricultural pollution play an important role in a given catchment? 2. Which areas within the catchment contribute highly to diffuse pollution of the receiving river, i.e. which areas are critical source areas (CSAs)? Question 1 can be answered by using nutrient measurements, mass balance approaches or land use based methods. For most catchments some nutrient measurements and land use data are available, which allow a first assessment whether diffuse pollution could play a role. For question 2, the identification of CSAs, a number of GIS-based methods was found in scientific literature. Since most available methods focus on nutrients and since spatial data on other contaminants, such as pesticides, are typically not available, the report outlines methods for the two critical nutrients nitrogen and phosphorus. Each method can be looked up separately, as they are summarized in a similar structure. Moreover Table 8 in Appendix G provides a quick overview of all the presented methods. All the described approaches focus on nutrients, as they are a major concern and often in the focus of research projects. In general the presented methods consider three aspects to assess the risk of pollution from an area within a river basin: 1. The source of nutrients on agricultural land is included through fertilizer application, livestock numbers or indirectly via land use. 2. Transport to the river is mainly assessed via soil type, land cover, elevation and distance to the river 3. In addition several methods take retention processes into account during transport to or within the river It is important that different contaminants show different behaviour. For instance, phosphorus is pre-dominantly particle-bound, enters rivers via soil erosion and can be retained by adsorption or plant export. Nitrate, the dominant form of nitrogen, is very well soluble, is lost mostly through leaching and most efficiently retained by denitrification. Consequently, methodologies for the assessment of CSAs for phosphorus and nitrogen were looked at separately. While many promising methods with limited data requirements and analytical efforts were identified in the report, few concepts (such as the Universal Soil Loss Equation for phosphorus) seem to be well established. Most literature concerns specific local or regional case studies. As a result, transferability to other catchments is questionable. The highest potential is seen in qualitative, multi-criteria methods (such as the scoring approach by Trepel and Palmeri, 2002), which can be adapted by the user depending on the diagnostic aim as well as local data availability. In summary, it is recommended to test several of the presented GIS methods on one or two catchments to gain experience in their handling and their transferability.

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