Occurrence and geochemistry of Polonium-210 in Groundwater

USGS discovered that gross alpha radioactivity in numerous wells was substan­tially higher than could be explained by the presence of uranium during a groundwater investigation conducted in 2001 near Fallon, Churchill County, Nevada.  A subsequent investi­gation in 2007-2008 showed that the excess radioactivity in drinking-water wells was due to polonium-210 (²¹⁰Po), and that ²¹⁰Po levels in the area were among the highest known in the United States.  Polonium-210 is rarely found in groundwater because it strongly binds to sediments, and its occurrence at high levels is a cause for concern because it is a known human carcino­gen.
Data from approximately 30 wells sampled in 2007-2008 indicate the highest ²¹⁰Po levels are in agricultural areas south-southeast of Fallon.  Nonetheless, many wells with ²¹⁰Po levels exceeding drinking-water standards established by Canada (5.4 pCi/L)  and the World Health Organization (2.7 pCi/L) are located in densely populated suburbs north of Fallon.

Objectives

The objectives of the investigation are (1) to describe the distribution of elevated ²¹⁰Po in the valley so State and County officials can take appropriate measures to protect public health, and (2) to identify processes that lead to elevated ²¹⁰Po levels in the groundwater. 

Strategy and Approach

The project will be done over a three year period between October 2008 and September 2011.  In FY 2009, starting in Fall 2008, about 25-30 wells will be sampled to better define the geographical distribution of groundwater that exceeds the Canadian standard.  In FY 2010, 25-30 new wells will be sampled in areas where additional data is needed. 
Most wells will only be analyzed for ²¹⁰Po, sulfate, and uranium and will be used for mapping ²¹⁰Po distribution.  A few wells will be sampled for major ions, trace elements, several radionuclides and stable isotopes.  These data will be used to identify the processes responsible for mobilizing ²¹⁰Po in Churchill County groundwater.  Experi­ments will be done to determine the species of ²¹⁰Po present in the water and aquifer sediments will be analyzed for radionuclides.  Samples of raw and treated water from supply wells will be analyzed to evaluate whether public water supplies meet drinking-water standards.  Samples will be collected from Carson River water to evaluate potential exposure of crops to ²¹⁰Po.  A few samples will be collected from household reverse-osmosis systems to determine whether they efficiently remove ²¹⁰Po from well water.

Preliminary results

Fifty-eight wells in the study area have been sampled for ²¹⁰Po since 2007, 28 since October 2008.  ²¹⁰Po concentrations range between <0.1 and 76 pCi/L (average 14.5 pCi/L).  ²¹⁰Po levels were <0.25 pCi/L in samples from two municipal wells in the basalt aquifer and one in the intermediate aquifer: only domestic wells in the intermediate aquifer had ²¹⁰Po concentrations >1 pCi/L.  Carson River water had very low ²¹⁰Po concentrations, <0.2 pCi/L.
The stability of  ²¹⁰Po concentrations in groundwater over periods of time much longer than its half-life indicates it has a natural source and is not the result of an underground bomb test near Fallon in the 1960s.  Concentrations of ²¹⁰Po in aquifer sediments do not vary significantly across the valley or with depth, indicating the principal control on how much ²¹⁰Po is present in the water depends on the chemistry of the water and not on the amount of ²¹⁰Po present in the sediments.  The ²¹⁰Po is principally dissolved in ground­water and passes through 0.45 micron filters, but is efficiently removed from ground­water by household reverse-osmosis systems.

Relevance and Benefits

This study supports the USGS science strategy in providing scientific knowledge and information to improve our understanding of environmental contributions to disease and human health.  Polonium-210 is a known carcinogen and consumption of it in drinking water may have adverse health effects on exposed populations.  The project will also provide data to the U.S. Environmental Protection Agency, which has stated further information about the occurrence of ²¹⁰Po is needed.  The data gained from this project will provide greater understanding of naturally occurring radionuclides in ground water and study results also may have transfer value to areas in the U.S.

Additional Project Information

Reports and Journal Articles

USGS Open-File Report 2007-1231 — Methods and data used to investigate Polonium-210 as a source of excess gross-alpha radioactivity in ground water, Churchill County, Nevada. Updated April 2008: Version History

Seiler, R.L., 2004. Temporal changes in-water quality at a childhood-leukemia cluster, Groundwater, 42, 446-455.

Frequently Asked Questions and Fact Sheets

Centers for Disease Control and Prevention (CDC) Polonium-210 FAQ: http://www.cdc.gov/nceh/radiation/fallon/Polonium_faqs.pdf

U.S. Food and Drug Administration (FDA) Questions and Answers: Polonium http://www.cfsan.fda.gov/~dms/po-qa.html

U.S. Geological Survey (USGS) FAQ about USGS data on polonium-210 in wells in Lahontan Valley  http://nevada.usgs.gov/water/polonium/FAQ.pdf

Frequently Asked Questions about USGS data on Polonium-210 in Wells in Lahontan Valley (PDF)

Information from the Centers for Disease Control (CDC):

Frequently Asked Questions

Fact Sheet — Facts about exposure to Polonium-210 from Naturally Occurring Sources.