Environmental isotopes can be used to solve difficult groundwater problems. Isotopic methods are a totally different and independent approach from typical groundwater investigations, making them a valuable tool in convincing adjudicators.
An isotope is a variety of an atom in the water molecule, you can all sorts of varieties, and this is due to the makeup of the nucleus of the atom. What I can do with hydrogen and oxygen 18 for example, is I can tell whether water in the ground is recent water or water that has been stored in the ground for years. When you look at water that is in a stream, did that water come from rainfall, snowmelt or was it stored water? When you had rain storm on a farm field and you’ve got nitrate in the stream, where did that nitrate come from? Did it come from the rainfall, running off the ground or did it come from the groundwater? The implication of that is, if you’re an agricultural producer, and the regulators are asking you to do something to decrease the nitrate concentration of runoff, will that make a difference?
Studies in larger watersheds, show that water that’s in the stream could be 20 years old or 30 years old. If I stop applying fertilizer, today, that’s really not going to affect anything in the surface water for years and years and years.
One of the things you can use to differentiate the sources of nitrates, is using nitrogen 15 isotope. So with nitrogen 15, we can differentiate between chemical sources of nitrate and animal sources of nitrate. If you run a dairy, for example, you have to show the nitrate leaving the groundwater in your site is less than nitrate concentrations entering your site, otherwise, the input is from your operation.
Tritium is an isotope of hydrogen that has two neutrons in the nucleus, and it’s radiogenic – it spontaneously breaks down. The input function of tritium is kind of interesting. In the 1950s, we were basically at natural levels of tritium, very low concentrations, due to natural causes. There was a lot of testing of nuclear weapons, in the atmosphere. That increased the production of tritium by many orders of magnitude – thousands of times. That got into the atmosphere and into the precipitation.
Then in 1963, with the atmospheric test ban treaty, the input function of tritium in the atmosphere declined. So we see the 1950s being very low, 1963 being a peak and then a gradual reduction.
The first landfill project I worked on with Jim Dragun, there was a question about how far the contamination from a landfill moved out with the groundwater. One of the things we did was we looked at tritium. And tritium would tell us where the 1965 groundwater was, in 1965 is when the landfill started.
So, wherever we found groundwater older than 1965 that meant that any of the chemicals there could not have come from the landfill. This was a large lawsuit, the landfill owners, which were some municipalities and the other side were some regulators and there was also a wrongful death suit.
We put together what we call a conceptual site model. The state and some private parties were suing our clients, which were a number of municipalities who had operated the landfill. The outcome of this investigation was very favorable for our client.
