Investigation of Strontium-90 in Groundwater Discharging to the Ottawa River

Abstract

In 2007, an area of Ottawa Riverbed (about 250 m² in extent) was found to be a groundwater discharge zone with above-background levels of tritium (as tritiated water - HTO). The area was adjacent to the shoreline along the property boundary of Atomic Energy of Canada Limited’s Chalk River Laboratories. The probable source of the HTO has been identified as a groundwater plume originating from a facility undergoing decommissioning. A discharge of strontium-90 (⁹⁰Sr) that occurs within a small part of the groundwater discharge zone in the study area has also been identified and is likely coming from the same groundwater plume as the HTO. Based on information from years of ongoing site and plume monitoring, a more detailed investigation was conducted to provide specific information on where the ⁹⁰Sr plume was emerging to the surface and to estimate contaminant flux to the river.

Work in 2009/10 revealed discharge of ⁹⁰Sr within a small portion of the groundwater discharge area. A land-based survey using a portable gamma spectrometer showed up to 550 total gamma counts per second above the plume’s path. Based on a ¹³⁷Cs survey, these gamma counts were not attributable to ¹³⁷Cs and are likely due to the presence of ⁹⁰Sr and its decay product, yttrium-90 (⁹⁰Y). A vegetation survey of alders along the shoreline revealed gross beta levels of 1 – 70 Bq/g. Alders are known to root in wet ground and are likely pumping ⁹⁰Sr from the contaminated groundwater to the surface. Groundwater samples obtained from the river using mini-piezometers had gross beta levels of 3.3 – 590 Bq/L, with the highest values obtained near the area of the most contaminated vegetation. These same groundwater samples also contained elevated tritium. Seepage meters were used to calculate discharge rates to the river.

Although actions were taken to cut the source of ⁹⁰Sr to the aquifer, the residual ⁹⁰Sr plume continues to advance toward the Ottawa River. The primary goal of the current project is to quantify the discharge of ⁹⁰Sr that moves to the river annually at this location. The groundwater plume had been defined previously using widely spaced onshore monitoring wells 20 – 40 m from the shoreline. However, in this study, closely spaced mini-piezometers showed actual locations of ⁹⁰Sr discharge to the river and seepage meters provided direct measurements of discharge rates. As the project nears completion, the resulting data will be useful if remedial action is required.