Review of measures to decrease nitrate pollution of drinking water sources
|Oene Oenema, Meindert Commelin, Piet Groenendijk, John Williams, Susanne Klages, Isobel Wright, Morten Graversgaard, Irina Calciu, António Ferreira, Tommy Dalgaard, Nicolas Surdyk, Marina Pintar, Christophoros Christophoridis, Peter Schipper, Donnacha Doody
|»Oenema, O. et al. 2018. Review of measures to decrease nitrate pollution of drinking water sources. FAIRWAY Project Deliverable 4.1, 125 pp
Sufficient safe drinking water is vital for human health, public welfare and an important driver of a healthy economy. This drinking water is extracted from groundwater (aquifers) or surface waters, and in many countries purified before consumption. About 2 billion people in the world lack sufficient safe drinking water, mostly in Africa and Asia. In the European Union about 65 million people are exposed to drinking water resources which quality cannot be guaranteed. Further, many drinking water resources run the risks of pollution by nitrates and pesticides, resulting from the intensification of agricultural production. In response, drinking water authorities have taken a range of measures around their drinking water resources to reduce the pressures from pollution, and have invested in various purification steps, or in the closure of wells when contamination was unacceptably high. In addition, various policy measures have been implemented as a blanket in the European Union from the early 1990s onwards to decrease the pollution of drinking water resources with nitrates and pesticides. The current view is that not all measures are equally effective, and that the protection of drinking water resources has to be improved.
In this section of FAIRWAYiS we review and assess measures to decrease nitrate pollution of drinking water resources. The work builds on insights and results gathered in EU-wide and global projects and studies. It provides an overview and assessment of the effectiveness and efficiency of measures aimed at decreasing nitrate pollution of drinking water reservoirs.
»Background and objectives of the review shows that various reviews on measures aimed at decreasing nitrate leaching have been published already, but either these reviews focussed on single measures or were rather qualitative and descriptive in nature. The novel aspect of this review is that the accessible literature has been screened for experimental data related to the effectiveness of most measures to reduce nitrate pollution of groundwater and surface waters, in a coherent and quantitative manner, using statistical analyses.
»Review methodology describes the two surveys that were conducted.
- Firstly, a survey of practical guidelines and measures, also at the case study sites, and earlier inventory reports yield gross lists of some 40 measures. All these measures were uniformly and concisely described and are to be found in Annexes 1 and 2 of »Review of measures to decrease nitrate pollution of drinking water resources.
- Secondly, a survey of published literature was conducted to identify papers that reported experimental results on the effectiveness of measures to decrease nitrate leaching, using the ISI-Web of Science and Google Scholar from 1980 to 2017. The reviews were conducted by different review teams covering different geographical regions using an approved protocol. Results were stored in a database and analysed statistically.
»The nitrogen cycle and nitrogen transformation processes provides background information about the sources of nitrate nitrogen in agriculture and about the processes and factors that contribute to the pollution of groundwater and surface waters with nitrates. The nitrogen cycle has been characterised as leaky and complex. Main sources are animal manures and synthetic fertilizers, but also residues and wastes, and the mineralization of soil organic matter following land use change can be sources regionally. Estimates suggest that some 60% of the amounts of nitrogen entering the aquatic system originates from diffuse agricultural sources in EU-28, which is about 6 Tg (1 Tg is 1 million ton is 1012 g), and equivalent to 60% of the N fertilizer use in EU-28.
»Agriculture in EU-28 and the use of nitrogen presents background information about agricultural systems and land use in EU-28 and about management factors that influence nitrogen use in agriculture. The nitrogen input-output balance is a synthetic manner for summarizing N use at farm level but also at regional and national levels. We also discuss the difficulties of optimizing N fertilization due to site and temporal variations in N demands by growing crops.
»Processes and factors that transfer nitrates to drinking water resources describes the hydrological cycle and pathways of N transfers from land to groundwater and surface waters. The potential risks of runoff and leaching of nitrate and nitrogen to surface waters is determined by a combination of pedo-climatic factors and the amounts of nitrate and nitrogen in the top soil. Important pedo-climatic factors are:
- rainfall amount and distribution, especially heavy rainfall events, and
- water infiltration rate into the soil.
The latter is determined by slope, soil texture, soil structure, soil depth to underlying rock, vegetation cover, snow and frost and freeze-thaw cycles, and the presence of terraces, tree-lines, buffer zones, riparian zones, which all contribute to intercepting overland flows. Soils with a high nitrate leaching vulnerability have a high infiltration rate and a high hydrological conductivity, such as coarse-sandy soils and shallow soils overlying karst formations.
»Overview of measures and practices that decrease nitrate losses describes how the mean cost-effectiveness of most measures aimed at decreasing nitrate losses from agriculture to ground and surface waters roughly ranged from 1 to 5 euro per kg N, but the uncertainty in the cost-effectiveness is large, and some measure had higher costs. At farm level, the cost of the measures ranged from a net gain to a cost of more than a few thousands euro per year.
»Further characterisation of key measures that decrease nitrate losses examines in further detail the rational and effectiveness of 11 key measures and serves as basis for a further quantitative analysis in the next article.
In »Quantitative analysis of measures and practices a total of 84 papers with 228 experimental comparisons are examined and utilized for statistical analyses; these papers report experimental data related to measures aimed at decreasing nitrate leaching losses. Most measures were on effective overall, but some not as effective as others. Effective measures were
- N input control,
- adjustment of crop type and/or crop rotation,
- growth of cover crops,
- minimum tillage and surface mulching, and
- nitrification inhibitors.
Somewhat surprisingly, fertilizer type and time and method of application turned out to be not effective. These initial results need further underpinning. Moreover, the effective measures do show a wide variation; the 95% confidence interval of the mean response ratio was often very large, which is probably related to site-specific variations in socio-economic and environmental conditions.
»Implication of review findings discusses the importance decreasing nitrate pollution and considers the effectiveness, cost-effectiveness and applicability and adoptability of different measures. Our findings largely confirm the observations of earlier reports, but some meta-analysis studies provide additional and different results.
In summary, the variability in the effectiveness of measures to decrease nitrate leaching losses across sites is possibly one of the reasons for the widespread recording of nitrate concentrations exceeding 50 mg/L in groundwater and surface water monitoring stations, despite the implementation of series of measures during the last 2 to 3 decades. It shows a need for farm-specific packages of measures. This review and the subsequent »Report on most promising measures are important scientific building blocks for the further development of innovative measures and governance approaches for a more effective drinking water protection, together with local, regional and national actors.