In matters involving permitting, litigation, or remediation of impacted groundwater, knowing more is way better than knowing less. At Dragun Corporation, we use a multi-technique approach to characterize groundwater and groundwater contamination to develop a more robust and precise site characterization to answer tough questions, withstand rigorous expert review, and/or optimize remediation.
Introducing "Environmental Isotopes"
One technique we have used successfully for decades is "environmental isotope hydrogeology." "Isotopes" are versions of atoms. Think of isotopes as different varieties of apples like Red Delicious or Granny Smith; both are apples but slightly different. The isotopes of an atom have different numbers of neutrons (neutrally charged sub-atomic particles) in their nucleus that cause the isotopes to have different masses. Because they have different masses, the isotopes behave differently in the environment, thus the term "environmental isotopes."
The simplest atom, hydrogen (H), is useful for explaining environmental isotopes and how they can help us. Hydrogen has one proton (positively-charged, sub-atomic particle) in its nucleus. Hydrogen has three isotopes: "protium" (1H) with just one proton in the nucleus, "deuterium" (2H) with one proton and one neutron in the nucleus, and "tritium" (3H) with one proton and two neutrons in the nucleus. The superscript left of the "H" refers to the mass of the atom (both protons and neutrons have one mass unit). While 1H is by far the most common of the three isotopes of hydrogen, all three isotopes occur in natural water molecules.
Solving Groundwater Problems with Hydrogen Isotopes
The nuclear differences in hydrogen's three isotopes can help us solve groundwater problems that cannot be readily solved using conventional groundwater investigation methods. Since hydrogen isotopes occur naturally in precipitation that becomes groundwater, they can be used as a very large-scale, natural tracer test.
For example, we can use hydrogen isotopes to determine whether a deep groundwater supply will be contaminated in the future by leachate from a landfill or from a release of solvents. Another example: farmers have been using nitrate fertilizers more judiciously than in the past, but nitrate concentrations in groundwater and streams remain high. We can use hydrogen isotopes to determine why.
How Old is the Groundwater?
Hydrogen isotopes can tell us if groundwater recharged thousands of years ago, before1950, or more recently. If the groundwater is thousands of years old, it is protected and unlikely to be contaminated by a surface release. If the groundwater recharged before 1950 rather than more recently, its nitrate concentration is a result of activities from long ago, not from recent farming.
Although there are many environmental isotopes that can be used to solve groundwater contamination problems, Dragun typically uses two main groups: (1) isotopes that are in the water molecule and (2) isotopes that are in the contaminants, like nitrate and trichloroethene (TCE). In this article, we will focus on isotopes in the water molecule. In later articles, we will discuss isotopes in nitrate, chlorinated solvents, and fuels.
How Does this Work?
Water molecules consist of two hydrogen atoms and one oxygen atom (H2O), where "O" stands for oxygen. As noted previously, hydrogen has three isotopes. Similarly, oxygen has three isotopes: oxygen-16 (16O), oxygen-17 (17O), and oxygen-18 (18O), with 16O being by far the most common. In hydrogeology, we typically use the ratio of 2H/1H concentrations, the ratio of 18O/16O concentrations, and/or the concentration of 3H to solve problems.
The 2H/1H and 18O/16O ratios in precipitation are temperature dependent because the heavier isotopes preferentially stay in the liquid water while the lighter isotopes preferentially move to the vapor phase. This separation between the heavy and light isotopes increases as temperature decreases. As a result, there can be storm-to-storm, seasonal, and long-term (thousands of years) changes in these ratios in precipitation. The groundwater isotopic signature develops from a mixture of precipitation events over time.
For example, (1) the 2H/1H and 18O/16O signatures of deep groundwater below Detroit/Windsor indicate recharge from precipitation during a very cold climate that occurred thousands of years ago during glacial times, but (2) the shallow groundwater below Detroit/Windsor indicates recharge in a climate like today. We can use the isotopic signatures to infer the hydraulic connection between deep and shallow groundwater on a large scale: near Detroit/Windsor, the deep groundwater is isolated.
What Nuclear Bomb Testing from 60 Years Ago Can Tell Us?
Tritium use is different; there are two factors to consider. First, although tritium is naturally occurring, atmospheric testing of nuclear bombs during the 1950s and 1960s caused the tritium concentration of precipitation (and thus groundwater) to increase at least 100 times. When the atmospheric test ban treaty was signed in 1963, the tritium concentrations in precipitation began to decline. Second, tritium is a radiogenic isotope; its nucleus spontaneously decays with a half-life of about 12.4 years (this means that if you had a pound of tritium in 2006, you would have about a half pound of tritium now). Combining these two factors, one can estimate when groundwater recharged.
For example, if there is no tritium in the groundwater, the groundwater was recharged before about 1950. We can use tritium data to answer questions like, "Can leachate from a landfill started in 1965 be in this groundwater?" This is regardless of the hydrogeologic nuances such as fracturing and sand/silt/clay content.
Solving a Nitrate Conundrum at a Dairy Farm Using Water Isotopes
We recently worked at a dairy site where permitting was required. The dairy must demonstrate that the groundwater nitrate concentration along the downgradient property boundary does not exceed the upgradient (background) groundwater nitrate concentration. We knew that, regionally, the groundwater had considerable legacy nitrates from chemical fertilizer applications over decades. However, the nitrate concentration in groundwater at the designated "upgradient" monitoring well was about 10 times lower than the regional groundwater. Simply accepting the groundwater nitrate concentration in the "upgradient" monitoring well as the background value could seriously affect the dairy during monitoring associated with the permit.
We used a variety of tools to demonstrate that the "upgradient" monitoring well did not accurately represent upgradient background groundwater nitrate concentrations. We used the 2H/1H and 18O/16O ratios in the groundwater as one way to prove our point.
The data we collected from monitoring wells around the dairy indicated the 2H/1H and 18O/16O values for the local groundwater were consistent with those in regional groundwater. However, during the spring, a massive storm event caused flooding in the area around the "upgradient" monitoring well and presented us with an opportunity. Knowing that groundwater is a mixture of recharge from precipitation over seasons (and temperature conditions), there was a good likelihood the individual storm event was isotopically distinct (2H/1H and 18O/16O) from the groundwater (see Sklash et al., 19861 for example).
During the storm event, we sampled surface water from the flooded area and groundwater from the "upgradient" monitoring well and tested these water samples for their 2H/1H and 18O/16O ratios. These data convincingly demonstrated that the ponded surface water from the storm (which proved to be isotopically very distinct from the local groundwater) had infiltrated to the water table and isotopically diluted the groundwater in the "upgradient" monitoring well. The infiltrating surface water not only diluted the isotopic signature of the groundwater, it diluted the nitrate concentration. We corroborated the isotopic findings with some high-resolution groundwater level monitoring that confirmed episodic mounding of the water table at the "upgradient" monitoring well. Ultimately, a much higher background nitrate value, more representative of the true upgradient groundwater, was permitted.
How Can Dragun Help?
We have combined our knowledge of isotopic tracers with traditional and high-resolution site characterization approaches to help our municipal, commercial, industrial, and agricultural clients solve environmental problems. Our future articles will demonstrate how we use these techniques for a variety of problems. For more information, please contact Dr. Michael Sklash.
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1Sklash MG, Stewart MK, and Pearce, AJ. 2006. Storm Runoff Generation in Humid Headwater Catchments: 2. A Case Study of Hillslope and Low-Order Stream Response. Water Resources Research, 22 (8), 1273-1282.
