10 questions on water quality and agriculture for Prof. Stefan Norra (geoecologist at the University of Potsdam) & Prof. Gunnar Lischeid (environmental scientist at the Leibniz Centre for Agricultural Landscape Research (ZALF) e. V.)
- Which agricultural practices have a particularly strong impact on water quality?
GL: All chemicals that are applied extensively in agriculture represent a potential burden on surface waters and groundwater. These primarily include mineral fertilisers and pesticides, as well as manure from livestock farms. Other chemicals, such as growth regulators (e.g. for shortening cereal stalks) or nitrification inhibitors for stabilising nitrogen fertilisers in the soil, are used in much smaller quantities, but should not be overlooked. Last but not least, the widespread use of antibiotics in animal husbandry and the resulting increase in resistance among a wide variety of bacteria is also a cause for concern.
SN: For a while, compost enriched with paper sludge or sewage sludge was also used to enrich the organic carbon in agricultural soils. However, these sludges can be enriched with so-called perfluorinated and polyfluorinated chemicals (PFAS), which are suspected of being carcinogenic. If these chemicals enter the groundwater, they must be removed at great expense during drinking water production.
In addition, soil compaction caused by heavy agricultural vehicles can affect the subsoil and lead to increased surface water runoff during rainfall, which increases erosion and results in increased particle input into water bodies. This also increases turbidity in water bodies, as well as the concentrations of nutrients and pollutants that may be bound to the particles. In water bodies, nutrients (including phosphate) can promote algal blooms. If cyanobacteria develop in the process, toxic phycocyanins can enter the water.
- How does the use of fertilisers and pesticides affect the quality of groundwater and surface water?
GL: High nitrate levels in groundwater are dangerous to humans. In surface waters, high nutrient inputs lead to eutrophication with corresponding shifts in the species spectrum, as well as temporary oxygen-depleted areas and fish kills in lakes, rivers and areas of the Baltic Sea. Residues of pesticides in surface waters also cause damage to flora and fauna, even if they are not directly lethal, but in combination with other pollutants and stress factors, they weaken the resilience of organisms and ecosystems in the long term.
SN: Under reducing conditions, nitrate inputs into groundwater can lead to the formation of nitrite, which is considered carcinogenic and can cause blue baby syndrome in infants if ingested. Plant protection products protect certain crops by killing other organisms that damage them. Therefore, plant protection products in water bodies also pose a threat to the communities living there. In general, the discharge of fertilisers and pesticides interferes with the biochemical processes in water bodies, resulting in changes to delicate balances and damage to existing ecosystems.
In addition to fertilisers and pesticides, it is also important to mention the medicines and hormones used in animal husbandry, which enter the environment via manure and sewage and can cause damage to the reproduction and development of wild animals.
- In which regions is there particularly high pollution of water bodies from agricultural inputs, and why is this the case?
GL: On the one hand, high levels of pollution are a consequence of high substance inputs. Across Germany, areas with high nitrate concentrations in groundwater correlate with regions where intensive livestock farming is widespread, as there is not enough land available to spread the large quantities of manure produced. On the other hand, geological and meteorological conditions also play an important role. High precipitation levels result in a significant dilution of nitrate concentrations in seepage water and groundwater, for example, while low precipitation levels result in correspondingly high pollutant concentrations. In naturally oxygen-poor aquifers, nitrate is broken down by microbes
SN: Nitrate inputs are particularly evident in regions with intensive livestock farming and the resulting application of manure to agricultural land. This is the case in many areas of Lower Saxony, for example. PFAS inputs are mainly known from northern and central Baden, as paper sludge was used in composting plants here. However, sewage sludge used for fertilisation in previous years can still cause long-term damage today.
- What significance does water protection currently have in agricultural policy?
GL: With the "Farm-to-Fork" strategy published in 2020, the EU formulated ambitious goals to reduce the amounts of pesticides and nutrients used in agriculture by the year 2030. On the other hand, many critics complain that the opportunity to expand environmental requirements was not taken during the almost simultaneous revision of the framework conditions for agricultural subsidies.
SN: Certainly, those responsible for agricultural policy are aware of the importance of water protection, and no farmer deliberately pollutes water bodies. However, the economic and legal framework conditions do not necessarily support a sustainable use of fertilizers and pesticides.
- Which measures – whether technical or nature-based – are considered particularly effective for protecting water quality in agricultural settings?
GL: Riparian buffer strips are very effective in reducing the input of substances that would otherwise be carried from the soil surface directly into water bodies. Inputs from subsurface drainage pipes can be reduced by ensuring that the water does not flow directly into a ditch or stream, but first into retention basins, where a significant portion of nitrogen or phosphorus discharges can be held back. However, neither of these measures has any effect on inputs into groundwater. These can ultimately only be reduced if the amounts applied are significantly decreased.
SN: The most effective protection of water bodies is the renunciation of fertilizers and pesticides. However, this requires the use of biological plant protection agents, which cannot always guarantee the effectiveness of chemical pesticides. Since nutrients are removed from the soil through harvest, fertilization cannot really be dispensed with. The challenge here is to return only such high amounts of fertilizer to the soil that ensure the growth of crops in the following year, without causing discharges into the groundwater. Guaranteeing this without economic losses is the real challenge for agriculture.
- How widespread is awareness of the impact of agriculture on water quality within agricultural practice?
GL: If only because of the extensive regulations and required certifications, farmers cannot avoid taking the environmental problems of agriculture seriously. In addition, many have their own interest in minimizing harmful impacts. However, increasingly bureaucratic regulations, which are not always seen as effective, are met with growing resentment.
SN: Farmers depend on functioning ecosystems in the exercise of their profession — this is clear to everyone. Therefore, it can be assumed that farmers have no interest in endangering their environment and, with it, water resources. However, political and economic goals stand in opposition, as they demand high agricultural yields and thus encourage the unsustainable use of fertilizers and pesticides in agriculture.
- What challenges exist in measuring and assessing agricultural impacts on water quality?
GL: After decades of high inputs from agriculture, corresponding pollution can be found year-round and across the board in groundwater and surface waters throughout Central Europe, often without any longer being directly attributable to a specific application. Nutrients as well as active substances and their metabolites can still be mobilized in the soil years after application under suitable weather conditions, and can then repeatedly be detected in groundwater or surface waters. In addition, there are burdens from outside agriculture: the eutrophication of soils and waters is also exacerbated by high nitrogen inputs from combustion engines and industrial plants. Trifluoroacetate (TFA) is released not only through the breakdown of certain pesticides but also from industrial facilities. Glyphosate, an active ingredient of a widely used pesticide, is also formed through the transformation of components of detergents in the environment and thus enters surface waters.
SN: Laboratory analyses, as well as in-situ monitoring with corresponding sensors in the waters, are always associated with costs that someone must bear. Even though analytical methods are highly developed, for fiscal reasons the optimum level of environmental monitoring is not always implemented. Another challenge lies in the enforcement of the consequences that should follow from the measurements. For example, nitrate pollution is well known — but it continues to persist.
- To what extent does climate change alter the relationship between agricultural use and water quality?
GL: Longer dry periods reduce not only plant growth but also the uptake of nutrients, which are then later leached unused from the soil. On the other hand, the risk of leaching nutrients, pesticide residues, and other substances from the topsoil is increased by increasingly frequent and intense heavy rainfall events. Lowering groundwater levels, by contrast, has an apparently positive effect, since the residence time of percolating water and the dissolved pollutants it carries in the soil increases, allowing them to be broken down more effectively by microorganisms or adsorbed onto soil particles.
SN: The amount of water must be considered first. Periods of drought create a greater need for artificial irrigation, which can lead to the overuse of groundwater resources. Soil compaction caused by agricultural machinery can in turn result in increased surface runoff, which leads to erosion and the entry of applied fertilizers and pesticides into water bodies. Climate change–induced longer dry periods also reduce the soil’s capacity to absorb rainfall after drought, thereby further increasing surface runoff. Higher temperatures also lead to greater evaporation, which must, for example, be compensated for through irrigation. Additionally, climate change with its rising temperatures promotes the spread of pests from warmer regions, which are then again controlled with pesticides — some of which ultimately end up in water bodies.
- What role does food retail or consumer behavior play in promoting more environmentally friendly agriculture and better water protection?
GL: Consumer behavior cannot compensate for misguided incentives of the market economy. However, by consciously favoring products from farms that operate in a more environmentally friendly way, consumers have made it possible that today already 11% of agricultural land is managed organically — that is, without the use of mineral fertilizers and pesticides.
SN: A more environmentally friendly agriculture does not come free of charge; it is linked either to higher costs or to changes in individual consumption behavior, which are ultimately passed on to consumers. In our societal system, food retail follows market economy principles. Consumer behavior, on the other hand, can be shaped by social role models, which would need to be supported accordingly. Even a partial shift away from meat consumption toward a vegetarian diet could help advance more environmentally friendly agriculture.
- Which measures are considered particularly promising for protecting water quality in the long term from agricultural pollution?
GL: In Central Europe, intensive agriculture with massive use of fertilizers and pesticides has been practiced for about 70 years. This has now led to widespread and still increasing pollution of groundwater and surface waters, even outside arable farming areas. In the long term, there is no way around significantly reducing the use of agrochemicals.
SN: Ultimately, political willingness is a prerequisite for implementing measures in the affected areas with all their consequences. Where water quality is impacted by agriculture, the use of the respective chemicals would need to be reduced or prohibited accordingly. The resulting economic consequences for farms — or the higher prices for agricultural products — must then either be accepted by society or absorbed by the state, which gives political decision-makers a decisive role.