Approaches to Quantifying the Relevance of Sources and Pathways

Obviously quantifying the loads introduced to the waterbody from the major sources via major pathways will provide the best basis for identifying the most effective measures to control nutrient loads. However, key parameters necessary for quantitative models are often unknown and not readily measured. Qualitative assessments then are a valuable beginning. Rickert et al. (2016) describe a qualitative approach to assessing loads, roughly categorising their relevance from negligible to extreme and highlighting uncertainties necessitating more in-depth assessment.

In some settings, a qualitative assessment of potential sources of nutrient loading can provide a sufficiently clear basis for planning control measures even without quantifying the relative relevance of different sources and pathways of loads. This is possible if conditions are quite clear. For example, in a densely settled, largely urbanised catchment, much of the nutrient load will originate from sewerage, and focusing on measures that reduce sewage nutrient emission loads will directly impact concentrations in the waterbody. For such a setting, it will be clear that without control of such substantial point sources, further load reduction measures cannot achieve sufficiently low loads to the waterbody to reach target nutrient concentrations. Similarly, for a largely agricultural catchment, identifying the steepest slopes for implementation of the most stringent management of fertilisation and tillage may be a sufficiently effective basis for achieving a substantial load reduction, even if the reduction achieved can hardly be quantitatively predicted and only assessed by subsequent monitoring of the change in concentrations in the waterbody. A merely qualitative approach - based on “getting the job done” for obvious measures - can provide substantial load reductions, particularly in situations in which resources for more elaborate approaches are lacking. Qualitative or semiquantitative approaches may suffice for estimating loads that are either self-evidently major or likely to be low.

Quantification becomes important where the load that a source contributes may be relevant but uncertainties are too significant to make decisions on investments or regulations to reduce it. Approaches to quantification cover a wide range, requiring different levels of information, staff capacity and resources. The European Union summarises some in its Guidance Document No. 28 (EC, 2012) in the context of its Common Implementation Strategy for the EU Water Framework Directive. This document has the advantage not only of being harmonised as outcome of discussions between a number of countries, but also of giving guidance at 4-5 different levels of complexity and data requirements. Thus, the technical guidance on the development of an inventory of emissions, discharges and losses of substances described in this document can be used for a range of situations with different resources. While this document focuses on priority hazardous substances, it outlines general principles which can be applied for nutrients as well. It therefore uses some key considerations from the Guidance Document No. 28 (EC, 2012), giving specific attention to situations where data availability is lower than can be expected in EU countries.

In face of the complexity of systems and the challenges associated with data collection, three broad quantitative approaches in the establishment of inventories can be distinguished, which are shown in Figure 7.2 with their scope indicated by the dashed boxes in diagram and their complexity increasing from right to left:

• • the riverine load-oriented approach, which estimates the observed total load that a river carries into a lake or reservoir. This information can be used together with a quantification of point source inputs to calculate an estimate of the diffuse inputs (green dashed line in Figure 7.2);
• • the pathway-oriented approach (POA), also called “regionalised pathway analysis” (RPA), which models the different transport phenomena for the final input routes to the river system starting from the “interface media” as soil, groundwater or wastewater treatment plants. This approach calculates regionalised emissions for small catchments (termed “analytical units”), which can be subsequently aggregated to river basins or subunits (yellow dashed line in Figure 7.2);
• • the source-oriented approach, which addresses the whole system starting from the principal sources of substance release. Such an approach includes substance flow analysis (SFA; red dashed line in Figure 7.2).

As situations differ strongly in the range of information and data sources available, the following introduces a tiered (or level) approach whereby the complexity increases with each progressive tier, beginning with purely qualitative (tier 0) or semiquantitative (tier 1) assessment. Quantitative tiers (2-4) require further data as well as more in-depth understanding of sources and pathways, resolution and detail. On this basis, they allow a better discrimination of the relevance of sources, for example, the relative contribution of those emitting nutrients to sewers and wastewater treatment plants rather than the (lower tier) lumped treated effluent discharge which does not allow for discrimination of the original source. Thus, the different tiers support a progressively improved understanding of the emission situation and, therefore, the ability to effectively allocate financial resources and evaluate (cost-)effective measures for emission reduction.

The approach to select for a given catchment will depend on its size, the availability of data and resources as well as the relevance of the problem. Five levels or “tiers” (one qualitative, one semiquantitative and the three quantitative approaches outlined in Figure 7.2) of emission estimation methods are summarised in Table 7.1 and explained in the following.

Table 7.1 Five tiers for the elaboration of emission inventories in catchments - overview

Table 7. / (Continued) Five tiers for the elaboration of emission inventories in catchments - overview

Source: Adapted from European Commission (EC) (2012).