Popular Science: Intensive agriculture keeps contaminating groundwater by nitrates
Prague’s drinking water comes from two sources: the Želivka water reservoir to the south-east and from the Central Bohemian Water Supply System to the north. The Káraný waterworks, which manages this system, supplies about one-third of all the drinking water to Prague’s inhabitants. The waterworks use a system of bank and artificial infiltration: the water is naturally cleaned in sandy sediments, where it is infiltrated either naturally from the Jizera river basin or artificially through infiltration ponds with sandy bottom. In both cases, the water is then pumped out and further treated by the waterworks.
With nearly 500 wells located at regular intervals of 22 km by the banks of the Jizera river and nearly 80 years of continuous monitoring of nitrates, this bank and artificial infiltration system in Káraný is a unique field laboratory where the adverse effects of intensive agriculture on groundwater can be studied.
Before the use of chemical fertilizers (1938–1940), the concentration of nitrates in gravel-sand sediments was 15–20 mg/l. Recently, intensive agriculture increased the average concentration in the same wells up to 104 mg/l. However, this concentration changes over time, as it is a mixture of the infiltrated water from the Jizera river with low nitrate concentrations and the heavily polluted water coming from the wider surroundings of the river basin with nitrate concentrations up to 250 mg/l. In dry seasons, the river water predominates, and nitrate concentrations in the boreholes are declining, whereas in precipitation-rich years, nitrate concentrations are rising due to water infiltration from local fields. To use the collected water for drinking purposes, it is necessary to dilute it with water with a low content of nitrates from other sources, for example from the mentioned bank or artificial infiltration.
The lowest content of nitrates is in those water collection wells where the proportion of water from the river is high, such as near the infiltration sites and above the weirs on the river, or in areas where the sediments are partially fed by groundwater from deeper Cretaceous sandstone collectors, where the presence of pyrite mineral and organic matter leads to the alteration of nitrate into harmless nitrogen.
In a part of the studied area (in the vicinity of the villages of Sojovice and Skorkov), an increase in nitrate concentration in the groundwater can be observed, reflecting two periods of increased use of fertilizers: in the 60s and 90s. In the 1990s, the use of chemical fertilizers increased by about twice as much as in the period of 1970 to 1990 and the concentration in several wells continues to rise.
The occurrence of nitrates in groundwater is demonstrably the result of very intensive agriculture, and at least in some places it is evidently rising. In order to reduce this extremely high nitrate abundance in groundwater resources, it is urgently needed to reduce the amount of nitrogen fertilizers use.
Bruthans, J., Kůrková, I., & Kadlecová, R. (2019). Factors controlling nitrate concentration in space and time in wells distributed along an aquifer/river interface (Káraný, Czechia). Hydrogeology Journal, 27(1), 195-210.