Summary and Recommendations
In 1997, while clean-up and closure work continued at the Rocky Flats site, a Corrective Action Decision/Record of Decision (CAD/ROD) was issued for Offsite Areas (DOE, 1997). Based on the investigation results and risk assessment, the CAD/ROD found risks to be lower than minimum thresholds set by EPA and CDPHE and concluded that the appropriate remedial action for Offsite Areas was no action (DOE, 1996 and EPA, 1997). Calculated maximum increased lifetime cancer risks ranged from 3x10-6 (three in one million) to 6x10-8 (six in 100 million). Risks based on recreational exposure were even lower. These findings were supported by an external review of available data conducted by ATSDR (ATSDR, 2005). The investigation evaluated surface soils, subsurface soils, groundwater, air, sediments, and surface water in offsite areas (OU3). The offsite area of focus for the OU3 investigation was approximately 38 square miles of land and surface water east and southeast of the site.
Relevant major studies were reviewed to gain an understanding of current human health risks in OU3 from plutonium-239,240 and americium-241 originating from Rocky Flats. For each medium noted above, findings for plutonium-239,240 and americium-241 are as follows:
Trace contamination by plutonium-239,240 and americium-241 (above background nuclear fallout levels) exists in surface soils east and southeast of the site
This contamination is due primarily to the spread of soil contamination from the 903 Pad on the Rocky Flats site prior to 1970.
The highest offsite surface soil concentrations of plutonium-239,240 and americium-241 are due east of the Rocky Flats site boundary, within a few hundred yards of Indiana St.
General magnitudes and distribution patterns of plutonium-239,240 and americium-241 in surface soils have been confirmed by subsequent sampling by entities unassociated with DOE.
A conservative assessment of risk due to surface soil contamination in these areas showed a maximum increased lifetime risk, for living on a plot with the highest observed soil concentrations, of getting cancer of three in a million.
This scenario assumed future residential use, including children, of the currently uninhabited land with the highest concentrations.
The scenario also assumed inhalation, ingestion, external radiation exposure, consumption of vegetables grown in the soil, and consumption milk and meat raised at the same location.
Subsurface Soils and Groundwater
There is no indication of contamination of subsurface soils or groundwater from plutonium-239,240 and americium-241 from the Rocky Flats site.
This finding is based on onsite and offsite observations of groundwater and subsurface soils.
This finding is in agreement with the scientific understanding of fate and transport mechanisms for plutonium-239,240 and americium-241.
(Plutonium-239,240 and americium-241 form insoluble compounds that are not subject to dissolved transport by water (Clark et al., 2006).)
Air concentration data, including ultra-high-volume air sampler data collected at three locations around Standley Lake, show no concentrations of plutonium-239,240 or americium-241 approaching levels of public health concern.
Per ATSDR (2005), “Ambient-air monitoring data collected since 1989 quite clearly indicate that the air near RFETS [Rocky Flats] has been safe to breathe.”
Surface and subsurface sediment have been sampled several times in Standley Lake and Great Western Reservoir (1970s, 1980’s, and 1990’s) and once in Mower Reservoir (1990’s).
Results show that low levels of plutonium-239,240 and americium-241 contamination are present in buried sediments in all three water bodies.
The buried contamination is likely from windblown contamination from the 903 Pad before 1970 and contaminated sediment transported through Walnut Creek to Great Western Reservoir in the early 1970’s.
Sampling results also show that contamination from buried sediments is not migrating into the surface water or groundwater, matching the scientific understanding of how plutonium-239,240 and americium-241 can move in the environment.
Risk calculations show that even if reservoirs were drained and converted to recreational or residential use, existing sediment contamination levels would pose no public health threat.
Shoreline sediments are at concentrations similar to background concentrations for plutonium-239,240 and americium-241.
Surface water was sampled at 29 locations in Standley Lake, Great Western Reservoir, and Mower Reservoir in 1992 for plutonium-239,240 and americium-241 as part of the OU3 characterization.
Results showed no apparent contamination of surface water in these reservoirs by plutonium-239,240 and americium-241. Sampling results were all at or below background concentrations and well below minimum risk thresholds.
Results showed no migration of contamination from buried sediment to surface water. Sampling included near-bottom sampling during stratified conditions in Standley Lake.
The City of Westminster routinely samples water from Standley Lake for gross alpha contamination (both plutonium-239,240 and americium-241 are primarily gross alpha emitters).
Standley Lake is subject to highly conservative and protective standards for plutonium-239,240, americium-241, and gross alpha contamination.
Runoff from Rocky Flats does not enter Standley Lake. This water has been diverted away from Standley Lake since the 1980’s. Since 1995, the Woman Creek Reservoir has been in place, just upstream of Standley Lake and downstream of Rocky Flats, diverting water from Rocky Flats around Standley Lake, via Walnut Creek.
Data indicate that Standley Lake is a safe source of raw water, in terms of contamination concerns related to Rocky Flats, for treatment and distribution as drinking water.
Several epidemiological studies have been conducted to assess whether there is evidence of increased cancer incidence among communities surrounding Rocky Flats, presumably due to exposures to contaminants from Rocky Flats. An early study (Johnson, 1981) reported a link; however, subsequent studies challenged this result and the methods used. More recent epidemiological studies do not shown a definitive link between cancer incidence and proximity of populations to Rocky Flats. This finding agrees with the RFI/RI estimations of very low risk based on soil, air, and water characterization results.
In summary, extensive sampling results from soils, sediment, water, and air indicate that trace amounts of plutonium-239,240 and americium-241 present in surface soils and buried sediments in OU3 from previous Rocky Flats releases do not pose a public health concern to those living in or recreating in this area. All conservatively-estimated risks are well below EPA significance thresholds, and the offsite areas are suitable for all uses, without restriction.
Based on the findings of this review, no additional sampling is recommended at this time. It may be prudent, largely in the interest of being responsive to public concern (as opposed to in anticipation of actual risk), to consider additional soil sampling prior to any major earthwork or dredging project.