The Use of Isotopes and Caffeine

To Identify Sources of Nitrate

By Steven Winkley, NYRWA

Introduction

Levels of nitrate are commonly above 2 mg/l in some areas of New York. In some instances, levels of nitrate are higher and even approach the MCL of 10 mg/l. The question that is always posed is: what is the source of the nitrate? Is it from septic systems or agriculture (commercial fertilizers and/or animal wastes)? Knowing the answer to this question is often critical to reduce nitrate levels and protect drinking water.There is no way of knowing the source of nitrate contamination by simply looking at nitrate concentrations alone. Although tracking nitrate levels over time may yield some patterns, it is impossible to identify the source of nitrate. Now with new analytical tests, there is a way to identify the source of nitrate! These tests involve a combination of nitrogen isotopes, caffeine, and pharmaceutical substances.

Nitrogen Isotopes

Isotopes are atoms of a particular chemical element that have different numbers of neutrons. Nitrogen isotopes are useful because potential sources of nitrogen contamination are evident by their ratio of N¹⁵ to N¹⁴ isotopes (the N¹⁵/ N¹⁴ ratio). Studies have shown that human or animal waste nitrate can be distinguished from commercial fertilizer or naturally occurring nitrate on the basis of nitrogen isotopic ratios (Howarth, 1999). However, nitrogen isotopes cannot distinguish between human and animal sources of nitrate.

Caffeine

Caffeine is found in beverages, food products, and medications used by humans. Caffeine is excreted by humans and has been found in very low concentrations in wastewater discharges. As such, it is an excellent indicator of organic contamination associated with human wastewater. For example, if a groundwater sample is taken in an unsewered area and found to contain caffeine, it indicates that the aquifer is receiving some effluent from septic systems.Whereas the average cup of home-brewed coffee contains about 350 parts per million of caffeine, caffeine concentrations in water in the environment have been found typically in the range of 0.01 to 0.1 µg/L (microgram per liter) (Seiler, 1999). Thus, laboratories must be able to measure very low caffeine concentrations. A recent study in Nevada by the USGS found the highest levels in wells at 0.23 µg/l.

Other Substances

In addition, it should be noted that prescription medicines and other pharmaceuticals have been found in ground water. These substances can also be used as an indicator of sewage contamination. Antibiotics are also heavily fed to livestock and potentially could be used indicators for agricultural contamination. However many of the same chemicals are taken by humans as well.

Closing Thoughts

The use of isotopes, caffeine, and pharmaceuticals to distinguish the source of nitrate in water is only in its initial stages of use. These analytical techniques require sophisticated laboratory procedures and perhaps only a few laboratories currently are able to conduct these types of analyses. The cost of these tests is likely to be high. However, these types of analyses could prove to be invaluable and I hope to see these tests become more widely used. In the future, I will research further the costs of these analyses and where water systems can have their water tested for caffeine and nitrogen isotopes. In the meantime, let me know if you have any questions.