Research:

Hanford Site Uranium Contamination

The Department of Energy’s Hanford Site is situated along one of the longest remaining unrestricted stretches of the Columbia River, which has the highest discharge volume west of the continental divide. Decades of nuclear related activities at the Hanford Site have left significant contamination (e.g., nitrate, U, tritium, Cr6+, 99Tc) in the vadose zone and groundwater within the site. Some of this contamination has reached the Columbia River, and there is potential for further contaminant migrating to the river.

Understanding the fate and transport of contaminants has been complicated by the presence of multiple potential sources within relatively small areas. Uranium from nuclear industrial activities covers a wide range of 235U/238U and 236U/238U due to variable combinations of isotopic enrichment and use in nuclear reactors. Natural background uranium in groundwater and vadose zone porewater has essentially constant 235U/238U, virtually zero 236U/238U, but variable 234U/238U due to alpha recoil effects.

The contrasts in isotopic composition between natural and processed uranium, as well as the wide compositional range of processed uranium, provide the means to trace contaminant uranium in the environment and delineate the sources and history of contamination.

Sources of the uranium contaminant

By using high-precision measurements of the isotopic composition of U in environmental samples (e.g. groundwater, sediments, porewater), we can trace the source and history of contamination. For example by comparing U contamination in the vadose zone to a groundwater contamination U plume, we were able to identify the source of contamination (see Christensen et al. 2004; link below). In addition to providing crucial information to guide remediation efforts and decisions, this research also gives insights into the rate, manner and mechanism of U migration within the vadose zone and in aquifers.

Columbia River and its uranium content

A portion of the contamination in the Hanford Site now has reached the Columbia River. We have undertaken a detailed study of the sources and fluxes of this U contaminant. Water samples are collected along depth profiles across the river (upstream and downstream of potential contaminant sources), as well as from along river shores, shore seeps and adjacent groundwater. We use U and Sr isotopic and concentration measurements of the river samples to evaluate sources of U (natural background, Hanford-related, agricultural runoff) and apportion their relative contributions to the river’s U budget. The data also provide constraints on the flux of contaminant U from the Hanford Site to the river. The downstream sample traverse gives us detailed snapshots of the contamination plume within the river resulting from Hanford contamination.

By combining data over time, with data for Hanford groundwater we are producing a dynamic view of the exchange of water/contaminants amongst the river, groundwater and the vadose zone, in response to changes in river stage and precipitation. The U isotopic data also provides a signature for tracing the fate of U downstream as chemical exchange occurs with the sediment load. With the sensitivity of our technique, we can trace the fate of Hanford contaminant U as far as has been sampled -- 350 km downstream of the Hanford Site.

Publication

Christensen, JN, Dresel, PE, Conrad, ME, Maher, K, and DePaolo, DJ (2004) Identifying the Sources of Subsurface Contamination at the Hanford Site in Washington using High-Precision Uranium Isotopic Measurements. Environmental Science & Technology, v.38(12), pp. 3330-3337.