Diffuse Pollution Pathways are not the same for N and P Compounds

Macronutrient fluxes from diffuse sources can affect aquatic ecosystems' health and drinking water quality. These are critical issues covered in the sixth gal of the 2030 Agenda for Sustainable Development to ensure the availability and sustainable management of water and sanitation for all [1]. To promote water quality protection, we need a better understanding of the hydrogeochemical, ecological and anthropogenic factors affecting diffuse pollution by Nitrogen (N) and Phosphorus (P) compounds generated by human activities.

N and P are macronutrients that play essential roles for all living organisms. They are the main growth-limiting nutrients in aquatic ecosystems, but their concentrations in the environment can be greatly modified by human action. Excess N and P inputs lead to the eutrophication of water bodies, while high levels of nitrate in drinking water are considered dangerous to human health [2].

Macronutrient conditions in European surface waters have improved in recent decades, but there has been no overall decrease in nitrate concentration in groundwater [3]. Agricultural diffuse loss is currently the dominant source of pollution with N and P in most of the European Union´s (EU) water resources, partly as a result of the efforts made to reduce point source pollution during the past few decades (by the improvements in wastewater treatment and the reduction of P in detergents; [3,4]). More specifically, groundwater pollution by N and P mostly comes from diffuse sources, while surface water pollution is still attributable to both point and diffuse sources [4]. In the EU, the estimates of agricultural diffuse losses of N and P range from about 0-30 kg N ha-1 and 0-1 kg P ha-1, while the background losses are around 1-2 kg N ha-1 and 0.1 kg Environment P ha-1 [5]. The European Environment Agency [2] has warned that EU countries still maintain a surplus of N unacceptable in agricultural land, reflected in increasing N-leaching affecting nitrate levels in many groundwaters [3]. Although less studied than N, it has been documented that the high amounts of P applied for many years in EU countries have increased P-leaching in sandy soils [6,7].

The limit values of N and P established to reduce and prevent eutrophication vary considerably across the EU countries, suggesting that setting consensus criteria is not an easy task. A thorough review of the nutrient criteria for surface waters under the European Water Framework Directive by Poikane S, et al. [8] reported values for good/moderate threshold concentrations of total-N varying from 0.25 to 35 mg L−1 and of total-P from 0.008 to 0.660 mg L−1 for EU rivers. These authors observed the relatively widespread use of the concentration of 11.3 mg L−1 as a limit value for total-N in most EU countries, probably attributable to the nitrate guideline value fixed by the Nitrates Directive (91/676/EEC; [9]). The Nitrates Directive establishes that both surface freshwater and groundwater are affected by nitrate pollution when their concentration exceeds 50 mg L−1 (=11.3 mg N L-1). Unlike N, a guideline limit value for P in groundwater remains to be established, perhaps because the transport of P is not as well understood as that of N [10]. Until now, the threshold value of 0.035 mg L-1 of total-P proposed by OECD [11] has been widely used in the scientific literature to delimit the transition from mesotrophic to eutrophic state in temperate zones.

N and P in water are generally present in soluble inorganic forms and soluble and particulate organic forms. Nitrate is usually the dominant N form in groundwater and the soil solution [7], being very soluble and highly mobile. Nitrate leaching depends on natural factors such as soil and vadose zone characteristics and climate, as well as human factors such as crop type, irrigation method and doses and the utilisation regime of fertilisers [12]. Orthophosphate and organic-P are the most common forms of P found in groundwater, while orthophosphate is generally the main form in the soil solution [7]. Since nitrate is highly mobile, it tends to accumulate in zones with the lowest phreatic level elevation of the aquifers, generally flat areas where groundwater flow is very slow. P, however, does not move quickly through the soil due to its low solubility, so it does not tend to accumulate in the same areas as nitrate [7]. The application of P to soil (as fertiliser, manure or in sewage effluent) results in an immediate rise in the level of water-soluble P, which declines rapidly with time due to adsorption and precipitation reactions taking place in the soil [13]. These processes mainly depend on the soil texture and soil hydraulic properties. Therefore, the actual level of water-soluble P in soil is usually low, and the movement of P through the soil is very restricted, with runoff being considered the main route for phosphate transport to surface waters [13]. Even so, it has been documented that the application of high amounts of P can result in soils saturated with P and, hence, significantly increased P-leaching through preferential flow in coarse/sandy soils [6,13,14] and cracked heavy-clay soils [15].

In short, it seems clear that the impact of nitrate pollution from diffuse sources on groundwater is much higher than that of phosphorus. The low mobility of phosphorous can be considered the main factor explaining its lower impact on groundwater quality. However, although runoff is the main pathway for P transport, special attention to the risk of P-leaching must be paid when factors such as (1) coarse/sandy or cracked heavy-clay soils, (2) high P surpluses from agriculture and (3) high levels of soil moisture converge [7].

In the EU, the Nitrates Directive requires the Member States to designate Nitrate Vulnerable Zones (NVZs; areas that drain into waters polluted or at risk of pollution by nitrate), in which action programs must be implemented to minimize nitrate leaching and runoff (by limiting N-fertilization and animal manure application and optimizing irrigation strategies and tillage work). However, several studies have found a link between poor NVZs designations and the persistence of nitrate pollution in groundwater bodies [16,17]. In this regard, Arauzo [18] suggests fine-tuning and unifying the criteria for the NVZs designation, using a scientifically robust and versatile methodology for the evaluation of groundwater vulnerability under varying hydrogeologic and hydro-climatic conditions based on a source-pathway-receptor approach. In addition, Cameira MR, et al. [19] highlight the need to apply efficient methods to determine the effectiveness of N mitigation measures in reducing groundwater nitrate pollution and, where necessary, fine-tune the action programs within the NVZs.

The authors thank the Journal of Biomedical Research & Environmental Sciences for the invitation to publish this mini-review.

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