Use of Tritium in Assessing Aquifer Vulnerability

This fact sheet provides: background information on tritium; information on the applicability of tritium analysis in assessing aquifer vulnerability; guidance on the interpretation of tritium results from public water supply wells; and a list of laboratories that provide analysis for tritium. Tritium concentrations in groundwater provide a useful method for determining the degree of confinement of an aquifer and are identified by the U.S. EPA and Illinois EPA as an acceptable means of assessing the vulnerability of groundwater systems. In summary, tritium levels at or below one tritium unit (TU) are obtained from water supply wells which are "not-vulnerable" to contamination.

Tritium is a naturally occurring radioactive isotope of hydrogen which decays as a beta emitter. It is produced in small quantities in the upper atmosphere where it is incorporated into water molecules and, therefore, present in rainwater and surface recharge to aquifer systems. With a half-life of 12.3 years, tritium can be used to trace and date groundwater, calculate rates of water circulation in the hydrologic cycle, and assess how long a specific groundwater source has been stored out of contact with tritium laden recharge.

In comparison to many other atmospheric radioactive isotopes, tritium is extremely rare and not affected by chemical processes. The naturally occurring tritium level in rainwater (prebomb) is estimated at 5 to 10 tritium units (TU). However, the amount of tritium in the atmosphere was greatly increased as a result of nuclear weapons testing causing recharge waters to be "tagged" with excess tritium beginning in about 1954. Nuclear weapons testing resulted in atmospheric tritium levels in excess of 1000 TU. Modern day values have declined to levels between 50 and 100 TU with the decline attributed to the elimination of atmospheric nuclear weapons testing and radioactive decay.

Relationship to Groundwater System Evaluation

Tritium analysis may be used to estimate the time since recharge to the groundwater system occurred and the susceptibility of the groundwater system to contamination. Groundwater systems with recharge occurring prior to the 1950s will have a tritium level decreased by radioactive decay to levels at or below one TU. These groundwater systems are considered "not vulnerable." Conversely, groundwater systems which have been recharged after the early 1950s will contain tritium levels at, or significantly above, the natural "prebomb" background concentrations and are considered "vulnerable."

Guidelines for Wellhead Protection

Although tritium levels may indicate an aquifer is "not vulnerable," it is important to realize that a potential for contamination may still exist. Water supplies with poorly constructed wells, degraded casings or potential routes which act as conduits for vertical recharge to an otherwise low-vulnerability aquifer may not represent aquifer conditions in general. Aquifers determined "not vulnerable" using enriched tritium analysis should be studied further to make sure that the natural protection has not been breached by potential routes, improperly abandoned wells or poor well construction. Community water supplies that have conducted tritium analysis which indicate low-vulnerability and have followed-up on the issues described above, may be eligible for the maximum reduction in monitoring under the guidelines established for the Illinois Phase II, IIb and V Safe Drinking Water Act monitoring waiver program.

Tritium Samples and Analysis

Sampling for tritium is not difficult since few, if any, chemical processes will alter its concentration. However, since tritium is naturally occurring, it must be assumed that excessive exposure of the sample to the atmosphere might alter the tritium concentration.

One other factor must be considered when sampling for tritium. For the acquisition of meaningful data, the required sensitivity in the analysis is very low (<one TU) and requires enriched tritium analysis. While many labs are equipped to measure tritium levels, few are equipped to conduct the necessary sample enrichment and obtain a one TU detection level. The enriched tritium analysis cost ranges from $150 - $280 and takes about four weeks, so expect a minimum six week turnaround time between sampling and obtaining results. The following laboratories are known to provide enriched tritium analysis at the required level of detection.

If you have any comments or questions regarding tritium and its use in assessing the vulnerability of groundwater systems, please contact Bill Buscher at the Illinois EPA at (217) 785-4787.