Anthropogenic nitrate traced through Chinese groundwater
Nitrogen and oxygen isotopes were used to identify nitrate sources in groundwater in Guiyang, Southwest China.
Tracing nitrate contamination to its source often proves difficult and requires sophisticated methods. New research published today on ES&T’s Research ASAP website (DOI: 10.1021/es0610129) combines multiple isotope ratios to identify local sources of nitrate in an aquifer in southwest China. The method allowed scientists to tease apart two very different sources of nitrate in groundwater by considering which one dominates in winter and summer. They could also conclude that karst aquifers cannot recover quickly from human sources of nitrogen.
Nitrate is highly water-soluble and can easily enter groundwater, which is a major source for drinking water. Elevated concentrations of nitrate in drinking water have been of concern because of its human health effects. “Nitrate content of only 5 times above that of natural background [levels], [at] about 2 milligrams per liter [mg/L], can be toxic to infants and create long-term health problems in adults,” says Samuel Panno, a senior geochemist at the Illinois State Geological Survey. Long-term exposure through drinking water can lead to human health impacts that include diuresis and hemorrhaging of the spleen, for example. Therefore, Panno says, “understanding the nitrate cycle is important for understanding where nitrate is coming from, how it is being transported, and how it evolves and reacts in the environment.”
The U.S. EPA has set the maximum contaminant level for nitrate in drinking water at 10 mg/L. The limit is 20 mg/L in China, where on average about 20% of drinking water comes from groundwater. In northern parts of the country and in urban areas, the percentage is much higher, says lead author Congqiang Liu of the State Key Laboratory of Environmental Geochemistry in the Chinese Academy of Sciences.
Liu and colleagues focused on identifying the local sources of nitrate within and around the rapidly developing city of Guiyang, in southwest China. Karstic landforms such as carbonate limestone surround Guiyang, and these rocks are well known to be sensitive to pollution. The limestone is dissolved by rain and groundwater, so that sinkholes and underground conduits are created through which surface water can flow relatively unhindered.
To fingerprint the nitrate in the karst groundwater, Liu’s team analyzed nitrogen and oxygen isotopes, which differ according to their source, whether from biological activity, atmospheric deposition, or waste. From the isotope ratios, they conclude that the main nitrate sources during summer are agricultural activities, whereas during winter, the main input comes from untreated municipal sewage.
The researchers also measured concentrations of chloride and other elements, such as magnesium and calcium, comparing these levels in surface water and groundwater to those in an untreated wastewater sample. The chloride concentrations, in particular, were similar in all three source types. The nitrate/chloride ratio in groundwater mirrored surface-water ratios in the summer; the similarities indicate a possible direct and immediate connection between water above and below ground, or later mixing of fertilizer-bearing rain with groundwater, the authors say.
The group has conducted research on nitrate cycling in Guiyang for about 4 years and has found no significant variation during this period. “The big change in nitrate content in the groundwater is just controlled by the local environment of the sampling sites,” Liu says.
Biological denitrification—in which microbes break down nitrate—can occur, to some extent, in aquifers. However, the new research “shows convincingly that the reduction of nitrate loads by denitrification is quite insignificant in the karstic terrains,” remarks Bernhard Wehrli, a professor in aquatic chemistry at the Swiss Federal Institute of Aquatic Science and Technology (Eawag).
Treating groundwater that is very contaminated with nitrate can be quite expensive, says Wehrli. “It requires the application of ion-exchange technologies. Another common approach is to dilute nitrate-rich water with clean water from other sources in order to reach the water-quality goals,” he explains. And human activities, Wehrli notes, such as “the production of nitrogen-[based] fertilizers and the oxidative nitrogen fixation by burning fossil fuels, have doubled the production of bioavailable nitrogen species on a global scale.”
