Investigation of Tritium in Groundwater Around a Tritium Light Manufacturing Facility

Abstract

Routine environmental monitoring around the SRB Technologies property in Pembroke, Ontario, in 2004-05, found tritium in groundwater at concentrations above the federal/provincial drinking water limit of 7,000 Bq/L. An investigation was ordered to determine the source of these levels, and suggest strategies for source control. Point source leaks were suspected.

The investigation was undertaken by EcoMetrix from December 2005 to October 2007. The scope included gathering data on tritium levels in air, soil water, groundwater and nearby surface water, determining local groundwater uses and associated risks, identifying the tritium source as a basis for source control, predicting future transport of tritium in groundwater and estimating timeframes for improvement of groundwater quality.

Tritium in air was monitored using passive samplers, and modelled from annual emissions using the IMPACT code (CSA N288.1 model). Soil water from boreholes was analyzed for tritium, and concentrations were predicted from modelled air concentrations. Groundwater from 37 monitoring wells was analyzed for tritium, and concentrations were predicted from modelled historical air and soil water concentrations, allowing for decay during vertical transport to the well screen.

It was found that one-third of monitoring wells, mostly within the property boundary, had tritium in groundwater in excess of 7,000 Bq/L. The maximum tritium in groundwater was 50,000 Bq/L. Two water supply wells were hydraulically down-gradient of the stacks. One well at an office building (1,500 Bq/L) was used for drinking, and the other (5,000 Bq/L) was used for truck washing. The nearest residential supply well (450 m north-west) had tritium at 1,400 Bq/L, equivalent to approximately 2% of the public dose limit.

Tritium emissions to air peaked in the year 2000 (1.7E6 GBq/a) (well within licence limits) and have been declining since that time. Tritium in shallow soil water in 2007 was predictable from modelled tritium in air based on 2006 emissions, suggesting a time lag of about one year between air and shallow soil moisture. Tritium in groundwater was consistent with modelled concentrations based on previous years' emissions to air, with a depth-dependent time lag.

Since the tritium in groundwater issue seemed to be related to licensed emissions to air, the control strategy focused on reducing emissions to air. A new emission limit of 67,200 GBq/a was established, as a level that would produce soil water concentrations well below the drinking water limit at the facility boundary. Some deeper groundwater concentrations on site, produced by historical emissions, will continue to exceed the limit for decades. Tritium concentrations in groundwater are expected to be well below the drinking water limit prior to discharge at the nearest surface water body.