Isotope Hydrology in Groundwater Investigations

Figure 1: Preferential evaporation of H2O with 16-O over H2O with an 18-O atom (uoregon.edu)

Isotope techniques provide a powerful tool to compliment traditional techniques in groundwater investigations. They are also often cheap and easy to implement in a qualitative way to give information about the source and processes affecting water. Spatial and temporal variations in isotopic signature of an aquifer can indicate how homogenous the aquifer is, how stratified the flow is, and whether mixing occurs with other aquifers or with local surface recharge. They can also indicate potential processes that have acted upon the water, such as evaporation and high temperature exchange with bedrock producing heavier isotopic signatures. Combining the two, it is possible to trace long flow paths and discern groundwater age – a.k.a. the where and when of aquifer recharge.

Almost all elements have multiple isotopes, i.e. atoms with the same number of protons but a different number of neutrons – resulting in atoms of the same element but different atomic masses. This small difference in mass leads to a small, but significant difference in behavior. Many common elements have multiple stable isotopes, the ratios of heavy to light (e.g., 18O vs 16O) of which can be measured in liquids and gasses. The preferential evaporation of “light” H2O molecules is a good example. It is more energetically favorable for an H2O molecule with 16O to change states from a liquid to a gas than for an H2O molecule with an 18O atom (Figure 1).

The same is true for 1H vs 2H. Over time, this process leaves more “heavy” molecules in an evaporatively-enriched pond than you would find in typical rainwater. The source of water and its dissolved constituents, together with processes that act on them, leave a unique isotopic signature that can be determined by taking a sample of water.

Some of the isotopes of common elements such as hydrogen and carbon are not stable (e.g., 3H, 14C, etc.) and decay over time. This means that they become a different element by losing protons via alpha decay or converting protons via beta decay. The most famous example is the decay of carbon-14 to nitrogen-14, used in carbon dating. Two decay chains used for groundwater age dating are the decay of carbon-14 (14C) to nitrogen-14 (14N) and hydrogen-3 (3H or tritium) to helium-3 (3He). Carbon dating can be a quantitative method if you assume that the groundwater system is closed and only advection occurs rather than mixing and advection. Tritium dating is more of a qualitative method, indicating some portion of post-1945 recharge if it is detected in groundwater. Naturally-occurring tritium is exceedingly rare, and only entered our atmosphere in great volumes as a result of open-air nuclear testing beginning in 1945.

Measurements of stable isotope ratios 2H/1H and 18O/16O, and concentrations of 3H and 14C in groundwater can be cheap and valuable tools in aquifer studies. Oxygen and hydrogen stable isotope analysis of water typically has a one-month turnaround. Radioactive isotope measurements are considerably more expensive with longer turnaround times.

A major advantage of stable isotope and groundwater dating techniques is that these isotopes are naturally occurring. Tracer studies using dyes or labeled solutions are limited to tracing flow paths with a shorter travel time than the study, typically less than 5 years. Tritium can fingerprint waters younger than 1945 and 14C can date water with an average age from 10 up to 40,000 years in some cases (Beta Analytic). Stable isotopes of water can provide information about the source(s) of the water with higher elevation and/or winter precipitation producing lighter isotopic signatures.

Figure 2: SINTACS vulnerability map showing increased vulnerability over time with the urbanization of Porto City, Portugal (Afonso et al., 2016).

One of the most powerful applications of stable and unstable isotope techniques lies in predicting aquifer vulnerability preemptively and in groundwater contamination. Determining the vulnerability of groundwater resources to human activity is a key component of effective management (Wachniew, 2015). Vulnerability assessments are particularly useful in urban and rapidly-developing areas (Figure 2) (Afonso et al., 2016).

Classic hydrogeologic techniques are aided by “fingerprinting” the source and age of water in the aquifer(s) of concern. Groundwater flow models can predict groundwater movement, but errors stemming from incorrect assumptions propagate as you get farther from the time of origin. If long-term groundwater flow paths are known, steps can be made to mitigate the effects of potential or existing groundwater contamination. Despite the advantages of isotopic methods in groundwater investigations, they are not commonly used to assess groundwater vulnerability (Wachniew, 2015).

HOW CAN LWS HELP?

Lytle Water Solutions can apply conventional or novel techniques to groundwater hydrology issues, including isotope hydrology, groundwater modeling, groundwater contamination related to fate and transport, and water rights. If you have concerns about the sustainability or vulnerability of your groundwater resources, just give our team at LWS a call or send us an email.

Phone: 303-350-4090

Bruce Lytle, bruce@lytlewater.com

Ben Bader, ben@lytlewater.com

Anna Elgqvist, anna@lytlewater.com