USE OF TRITIUM-HELIUM GROUNDWATER AGE IN NITRATE FATE AND TRANSPORT STUDIES

Widespread contamination of groundwater by nitrate has occurred over the past several decades, and now affects the drinking water supply of many areas where land use has changed from agricultural to urban. In studies of nitrate sources and transport, the time marker provided by tritium-helium groundwater age serves to delineate the history of nitrate inputs, to give information on how long it may take before remediation effects are observed in groundwater, and to distinguish pre-anthropogenic from anthropogenic nitrate concentrations.

We use groundwater age in concert with parameters that are useful for determining the source and fate of nitrate in several California basins that are heavily exploited for both irrigation and drinking water supply. Shallow groundwater with high nitrate concentrations and tritium-helium ages of <10 years overlies low nitrate water in several of the study areas. Dilution of nitrate by older groundwater is distinguished from saturated zone denitrification using dissolved excess nitrogen. The significant correction for excess air in the excess nitrogen result is made using the noble gas results. In basins where denitrification is not active, tritium-helium ages allow prediction of when regulatory limits may be exceeded in deep drinking water wells in the future. In areas of significant artificial recharge, identified by very young tritium-helium ages, low-nitrate imported water or denitrified wastewater is replacing high nitrate water from historical agriculture. Nitrate concentrations in tritium-dead groundwater containing low concentrations of radiogenic 4-Helium are used to estimate background conditions, and vary from near zero to about 20 mg/L (as NO3-), depending on local climate and native vegetation.

The Groundwater Ambient Monitoring and Assessment program is sponsored by the California State Water Resources Control Board and carried out in cooperation with the U.S. Geological Survey. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.