Quantifying contaminant transport through groundwater systems is challenging, but using environmental tracers can reduce uncertainties. Environmental tracers are naturally occurring constituents in groundwater that can be analyzed to help reveal flow and transport behavior. This Delve explains what environmental tracers are and how they are scientifically applied.  

Why is it Difficult to Quantify Groundwater Flow and Transport?  

There are many factors that make quantifying groundwater flow and contaminant transport challenging, including the following:  

• Subsurface heterogeneity: Aquifers contain layers of sand, gravel, clay, fractures, and pore distributions that cause groundwater and contaminants to move at different speeds and directions. 

• Variable flow paths: Groundwater follows paths influenced by complex geology, topography, and hydraulic gradients.  

• Chemical and biological reactions: Contaminants may sorb (stick) to soil particles, degrade biologically, or react with minerals. These processes alter concentrations and mobility in ways that are hard to model precisely. 

• Temporal variability: Recharge events (following rainfall or snowmelt), pumping, or seasonal energy balance changes (like evapotranspiration) can alter groundwater flow patterns.  

• Limited observability: Because groundwater is hidden beneath the surface, data collection often relies on monitoring wells, which provide only point measurements rather than a full picture. 

What are Environmental Tracers?  

A tracer is any constituent in water that can be measured and used to interpret environmental processes. Tracers are chemical molecules, elements, or isotopes that have well-known input concentrations and are either conservative (i.e. nonreactive), or have well-known reaction rates (e.g., radioactive decay rates) that are often conservative on timescales of interest. Tracers can be measured and used to determine sources of groundwater recharge and discharge to rivers, as well as the timescales of groundwater movement and mixing. 

There are artificial and environmental tracers. Artificial tracers are purposely released or injected into water to gather groundwater flow and transport information. These techniques are limited to small spatial and temporal scales due to the slow movement of groundwater (often millimeters to meters per year).  

Environmental tracers are naturally or anthropogenically applied to the hydrologic cycle over broad spatial scales and time periods. Environmental tracers are further divided into geogenic (natural to the Earth) and anthropogenic (human created) isotopes or compounds.  

Environmental tracers are usually more effective than artificial tracers for understanding large and complex groundwater systems because they can provide information on processes occurring over large areas and timescales.   

What can Environmental Tracers Measure?  

Environmental tracers can reveal and measure many groundwater characteristics:  

• Age: Tracers like tritium and carbon-14 can help determine how long water has been underground, or the time since recharge occurred. Groundwater age provides a direct measure of groundwater transport velocities. 

• Mixing: Tracers can distinguish between different water sources and show how they interact. 

• Recharge rates, sources, and processes: Stable isotopes of oxygen and hydrogen and dissolved noble gases can show whether groundwater originates from rainfall, snowmelt, or river infiltration, in addition to the locations of recharge (e.g., watershed elevation). 

• Discharge: Stable isotopes of water, dissolved helium isotopes, chlorofluorocarbons, and radon can be used to detect the location and source of groundwater that discharges into rivers and lakes.  

• Flow paths and velocities: Chemical tracers, like chloride and bromide, and isotopes can track how water moves through aquifers. 

• Geochemical processes: Isotopes and dissolved gases reveal interactions with rocks, mineral dissolution, or microbial activity that alter water chemistry.