Isotope Hydrology

New Approaches to Sustainable Water Management

Across California—from farms, to communities, to industry—water plays a vital role.

Yet managing this finite resource is a complex process that involves understanding where water comes from, how it moves through the environment, and how this precious resource is replenished. Information about this dynamic, complex system enables policymakers, water agencies, and other stakeholders to implement solutions aimed at meeting California’s need for clean, fresh water.

LLNL combines cutting-edge analytical techniques and expertise to deliver timely information regarding water management options to government entities, resource managers, industry partners, and other stakeholders.

LLNL scientists use isotope hydrology to trace the movement of water, which enables them to identify the sources of local fresh water, how quickly it is replenished, and whether it is at risk of contamination.

About Isotope Hydrology

How it works

Isotope hydrology involves the use of environmental isotopes to trace the movement of water throughout the hydrological cycle, from precipitation, through surface runoff and evaporation, to the flow of surface water and groundwater.

As water molecules move through this cycle, they are naturally tagged with isotopic “fingerprints,” offering clues about the water’s origin and its history of movement.

Hydrologists can use these naturally occurring isotopic tracers to map sources of groundwater, identify how quickly water resources are recharged (replenished), and determine the risk of saltwater intrusion or other contamination. In addition, noble gas tracers can be introduced into managed aquifer recharge facilities or aquifer storage and recovery wells to provide detailed information regarding the flow rates in the aquifer and the recovery of recharged water.

What we can learn

Isotopic analysis of surface water and groundwater can generate a broad range of data to facilitate informed decision making regarding how to address water management challenges, such as:

  • How fast water travels through an ecosystem, which can help identify where the geology supports fast flow paths that are beneficial for intentional, managed recharge of aquifers.
  • The age of existing groundwater, which helps explain trends in groundwater flow and geochemistry, and provides insight regarding future trends.
  • How groundwater interacts with surface water, including where and when those interactions occur, providing insight regarding the vulnerability of ecosystems that are dependent on groundwater.
  • How potential changes in groundwater management, such as altering flow paths, might impact the availability of future water resources.
  • The cause of any contamination or pollution detected in water sources.

Answering key questions

Stakeholders need actionable information that can help them answer questions like those provided below—and make informed decisions regarding managing future water needs.


  • What data do I need when making decisions regarding water management policies and legislation?
  • How will water management decisions impact groundwater reserves, wells, reservoirs, and other resources?

Water Resource Managers

  • Where does local groundwater come from, and how quickly is it replenished?
  • What risks threaten the local water supply, and how can I mitigate those risks?
  • Can I use managed aquifer recharge techniques to augment the local water supply?

Agricultural Stakeholders

  • How sustainable are water sources used for irrigation in a changing climate?
  • Over what time scale can we expect water management practices to improve water quality?

LLNL’s Unique Capabilities

LLNL’s comprehensive suite of analytical techniques enables our scientists to study a broad range of water management challenges across the hydrological cycle. Our experts collect water samples and use one-of-a-kind, high-precision mass spectrometry and nuclear counting facilities at LLNL to conduct isotopic analyses of the samples. We also develop new isotope tracer techniques to understand evolving water management challenges.

LLNL Analysis of San Joaquin Valley Groundwater Recharge

With funding from the California State Water Resources Control Board, LLNL scientists used isotope hydrology to study a groundwater basin in the south-eastern portion of the San Joaquin Valley and understand how this important water resource is replenished.

Investigators learned that river water provides nearly 50% of the groundwater recharge in this predominantly agricultural area. However, this recharge mechanism fluctuates significantly throughout the year, as the major source of river water is snowmelt from the Sierra Nevada mountains. In addition, all of the region’s major rivers are dammed, and water is diverted for agricultural irrigation and drinking water in other parts of California.

Based on their findings, LLNL scientists recommended groundwater banking of seasonal surface water, along with expansion of managed aquifer practices, to increase the resilience of San Joaquin Valley’s water system.

Isotope study results


GAMA Special Studies Project Reports

LLNL was the project technical lead of the Special Studies Project, as part of the California Waterboards Groundwater Ambient Monitoring and Assessment Program. The Special Studies Project focused on specific groundwater quality studies, using state-of-the-art scientific techniques and methods that help researchers and public policy planners to better understand how groundwater contamination occurs and behaves. Since 2004, studies have focused on sources of nitrate, wastewater indicators, groundwater recharge, detection of pharmaceutical compounds and personal care products using low-level anthropogenic compounds as tracers, and isotopic composition as a contamination source tool. Expand the section below to view LLNL Special Studies reports to the Waterboards.


LLNL’s Isotope Hydrology Group includes experts in hydrology, reactive transport, radiochemistry, nuclear counting, and mass spectrometry. Rooted in more than five decades of national leadership in environmental radiochemistry, our multidisciplinary team studies water management scenarios and delivers timely answers to sponsors, academic and industry partners, and other stakeholders.