**3. Processes behind the pollution—a word on leaching**

 When speaking of groundwater pollution, leaching is generally the cause of it. Molecules that do not have the positive charge and therefore do not bind well to soil particles get washed through the soil profile as water passes through. Nitrate and many other substances have this exact property which makes them very easy to leach into groundwater. We will only talk about nitrate in the following lines, as it represents the process very well, and is common across the globe, unlike specific pesticides.

Nitrogen as an element is an essential component of plants, so in order for them to grow, we need to supply it to them. For centuries, this was done only with manures, but after the green revolution in the last century, a new source of nitrogen was introduced in form of mineral fertilizers. These allowed for a great increase in agricultural yields (up to 40%), but all the nitrogen from fertilizing did not make it into crops. Because of aforementioned leaching, much of it was lost into ground (or surface) water in the form of nitrate (NO3 <sup>−</sup>). Fertilizers with other forms of nitrogen, like ammonia (NH4 + ) or nitrite (NO2 <sup>−</sup>), are not much "safer" in this regard, as nitrogen forms are prone to quickly transform in nitrification and denitrification reactions. There was a debate in recent years about nitrification inhibitors, compounds used for decreasing of nitrate leaching by inhibiting conversion of ammonium into nitrate. Ammonium, having a positive charge, bounds to soil particles and is not as prone to leaching. But studies have shown that decreasing of nitrification results in ammonia volatilization increase, which, environmentally, is not any better [2, 3].

 Some areas, however, have less trouble with leaching than others. As mentioned, leaching is strongly dependent on soil type, climate, and agricultural practice. Here are some examples: sandy soils that are not as good in water retention are notorious for leaching problems, while clayey soils are better in this regard; water and nitrate are held in root zone for longer, so plants can use more of them. Climate wise, the more problematic are areas with stormy weather and lots of rain during the spring to early summer period, when most of the fertilizer is applied; and thirdly, agrotechnical

**Figure 3.**  *A view of agricultural land on Krško polje plain (photo: Curk, 2018).* 

 practices, which are the only piece of the puzzle we can actually control, also influence leaching significantly. Fertilizing only once per year is way worse than in several small rations, where plants get as much as they need when they need it. Plowing fields in autumn is worse than in spring because winter rains do not leech the nutrients from decaying organic matter we buried in the soil. It is good to use cover crops when soil would otherwise be left bare and use crops with deeper root systems that catch nitrate in deeper layers as well [4–6]. Having all the best factors come together in one area is not realistic though. It is always necessary to compromise—if we have good soil and climate conditions, we could be a bit more "sloppy" with our agrotechnics and not get alarming monitoring results, but if soil and climate are not the most suitable, even the best agrotechnics might result in noticeable leaching. What is important in such cases is dialog between farming and water protection sectors to solve the dilemma of enough food versus clean environment. Most of the nitrate and pesticide problems could probably be alleviated by banning agriculture from vulnerable areas, like sandy shallow-soiled plains above drinking water reservoirs, and environmental side often tries to suggest this option. But in reality, such areas are many times the only areas suitable for intensive agriculture. This is why we adopted threshold values, which are a consensus between health and other benefits. European Union Water Framework Directive therefore recommends a nitrate threshold of 50 mg/L for water used for human consumption, because this concentration is low enough to ensure safe consumption, but high enough that it still allows for use of agricultural land (**Figure 3**).
