Identifying Key Nutrient Sources and Pathways Causing Loads

Once the critical load has been determined, this needs to be compared to the current load to the waterbody in order to assess by how much the load needs to be reduced in order to remain below the critical load. The current load then needs to be differentiated according to the locally relevant sources and pathways in order to identify measures for reducing or controlling loads from these sources. This is also useful in situations in which the critical load is not exceeded: this serves to identify situations and measures worth maintaining in order to ensure that a currently good situation does not deteriorate.

Background Information

Figure 7.2 shows principal sources, pathways and internal processes of nutrient loads to a waterbody. In this conceptual framework, all processes and activities that are likely to contribute to the input of nutrients are defined as sources. The most important point sources for nutrients are settlements which dispose wastewaters to surface waters via sewage without or after treatment and, depending on processes, also industrial facilities (the latter being typical point sources also for specific other pollutants). Relevant diffuse or nonpoint sources most frequently originate from agriculture, but they may include other fertilising activities, some urban emissions (including

Sources and pathways of nutrients and different levels (tiers) of their quantitative assessment in the context of emission inventories (Adapted from European Commission (EC), 2012.)

Figure 7.2 Sources and pathways of nutrients and different levels (tiers) of their quantitative assessment in the context of emission inventories (Adapted from European Commission (EC), 2012.)

into air and then precipitating on the water surface, contribution to water pollution via atmospheric deposition), and wastewater from rural dwellings not connected to central sewage treatment. Typically, diffuse sources are more variable in space and time than point sources, and quantifying them may be more challenging.

Pathways are the means or routes by which nutrients can migrate or are transported from their various sources to the waterbody. Following release, they may be directly emitted to a waterbody or reach it after being transferred to and stored within environmental media, including soil and impermeable surfaces. Typical pathways of wastewater from industrial or urban sources to a waterbody are sewer systems and wastewater treatment plant effluents or groundwater in unsewered areas. Pathways transporting nutrients from agricultural areas and other surfaces follow the hydrological pathways as surface run-off, interflow (a subsurface run-off component that does not reach the groundwater), tile drainage (artificial pipe installations that drain agricultural areas to avoid soil being too wet) and groundwater. While nitrogen in form of nitrate is very soluble and readily reaches waterbodies via drainage, phosphorus supplied to soil in higher amount than needed by the crop is usually adsorbed to a high extent to soil particles. Erosion transports such particles over the land surface to waterbodies. Aerial emission is an important pathway for nitrogen and can result in subsequent direct deposition on the surface of a waterbody or indirect entry via soil or a sewer system.

The differentiation between sources and pathways is useful because measures to reduce nutrient emissions may either directly address the sources of nutrients (e.g., reduced fertilisation or livestock, improvement of industrial production processes, P-free detergents) or intercept the pathways of nutrients to the waterbody (as, for instance, erosion abatement by riparian buffer strips), and because, as discussed above, some pathways differ for N and P.

Besides external loads, processes within surface waters determine the nutrient concentrations in the water. These processes include a wide range, for example, sorption onto suspended particles, plant uptake, desorption or - for nitrate and ammonium - denitrification. Retention is a broad term used to describe the outcome if loads entering surface water remain there, without, for example, being discharged to coastal waters or - in case of nitrogen be lost to the atmosphere through denitrification (see section 4.3.2), a process relevant particularly in shallow lakes at elevated temperatures. The fractions that are retained by sedimentation in the river, along riverbanks or in sediments of lakes and reservoirs, can potentially be mobilised in future; however, this is not always the case. The extent of their retention depends on the nutrient (N or P) as well as hydromorphological conditions of the waterbody (Behrendt & Opitz, 1999; EC, 2012).

While nitrogen largely reaches waterbodies as dissolved inorganic N, for phosphorus, loads can occur in different binding forms. As discussed in Chapter 4, for limiting cyanobacterial biomass in the waterbody, it is important to assess not only the concentration of soluble reactive phosphorus (SRP) but rather that of total phosphorus (TP). P binding forms are also relevant for assessing P transport: some of the pathways discussed below transport a high share of P as SRP (groundwater, treated wastewater). Via other pathways, P is transported primarily in particulate forms, that is, P adsorbed to soil particles from erosion or P in organic material from raw wastewater. Whether particulate P may become available for the growth of cyanobacteria and algae depends on P forms in the particulate matter and the physiochemical conditions in the respective waterbody, which determine the fate of the respective P forms: for instance, P in apatite (as part of soil material) will rapidly settle to the sediment and not become available even over long periods of time, while P bound in organic matter will become available as organic matter decomposes, and P bound to iron salts may dissolve in anaerobic zones of the sediment (Psenner et al., 1988). On the other hand, if potential binding partners for phosphorus, such as iron- and aluminium oxides and hydroxides as well as certain clay minerals, are available in a waterbody or reach it together with the P load, dissolved phosphorus may adsorb to these binding partners, and if these complexes settle to the sediments, they will contribute to removing phosphorus from the productive water layers. Consequently, either they may be buried under younger sediment layers and thus be permanently removed from the system, or they may be mobilised again later on by desorption, particularly during events of sediment resuspension, increasing the concentration of dissolved P forms in the water system. Therefore, availability of P is not only a question of its emission pathway but also a question of complex biological and chemical processes of the P cycle within the waterbodies.

Similar processes of interaction between nutrients and soil also apply on land. If agricultural soils with increased P concentrations erode, P is transported together with soil particles and eventually emitted to surface waters. Depending on soil properties and soil saturation with P, P might be transported in soluble form and reach surface waters with surface run-off, tile drainages (i.e., drainage from fields and meadows), interflow or ground- water. In most settings, transport with erosion dominates. Losses of P from agricultural soil are impacted by many factors. Fox et al. (2016) give a review of these processes, including a discussion of “legacy P” accumulated in soils on land with literature indicating that this may be released for years or even centuries after it has been deposited.

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