8.  Analyte Selection for the Baseline Study

There were a number of factors that influenced HþME’s analyte selection.  We had to balance the costs against the need to address potential source term radionuclides and non-radioactive contaminants from the Nevada Test Site, the U.S. Ecology site, and potential Yucca Mountain Repository contaminants.  Amongst the potential radionuclides and non-radioactive elements, we gauged their potential hazard, based largely upon:

·         the US EPA Maximum Contaminant Levels (MCLs)[1],

·         the mass and/or curies of each potential contaminant, and

·         the expected mobility within the regional aquifer.  

Mobility studies by the Department of Energy[2], while useful, are still in question,[3] and may change upon future well water analysis.  Therefore, HþME preferred to exercise the “precautionary principle” in light of these uncertainties, which is particularly germane when considering the toxic lifetimes of many of the radionuclides and non-radioactive contaminants.

We adopted the approach of analyzing for many elements that are complimentary to existing groundwater studies, mainly those conducted by Nye County as part of its early warning system.  In this way, HþME has to some extent augmented Nye County’s groundwater data.  We did perform some of the same tests as Nye County, to provide some overlap for comparison, such as gross alpha and beta, which we expected to reveal an effective background radiation comparison.  However, the cost of performing more of the same standard water analysis that Nye County has done would not have allowed us to examine other potential longer term hazards not specifically addressed by Nye County or the DOE NTS studies. 

In order to facilitate analyte selection, HþME constructed an analyte assessment table (see Appendix 12.1).  The table lists principle source term species from the NTS and proposed Yucca Mountain Repository.  Each specie received a check if it was a priority for the NTS study[4], the DOE Yucca Mountain analysis[5], required for the EPA Safe Drinking Standard, and of interest to HþME, which are the columns in the table labeled as YMP, NTS2, 40CFR141, and Home anal. respectively.  Also listed are available MCLs for each specie, and a rough measure of solubility[6].  However, the more specific solubility and radionuclide transport analysis in references 128 and 129 were prominent in our evaluation.  

We then created a ranking of 1, 2, or 3 (1 being the highest priority) for each analyte.  Those that received the most check marks (Y) in the four categories discussed above were given the highest priority.  Thus, those analytes that had four check marks were initially given a rating of 1 and were our highest priority for the first cut, and those that had three were initially given a rating of 2 and so on.  We then scrutinized the ranking, making adjustments based on HþME’s project goals.  Specifically:

• Some of the Yucca Mountain Project selections were included, such as Thorium-229, since it is a significant intermediate daughter product of Plutonium-240, 239, & 238.  Uranium-232 & 233, Americium-241, and Neptumium-237 decay with fairly large half lives (on the order of thousands of years).  They are expected to contribute to long term doses, which necessitated including them in the first rank list.

• Lead-210 is also a daughter intermediate product from the Uranium-238 decay chain; however relatively short lived (21 year half life).  Other lead isotopes are probably better to focus on like Lead-206 and 208, which are the end products of uranium and plutonium decay.

• Actinium-227 was on the DOE list, most likely included because of the potential disruptive and human intrusion scenarios.[7]  This isotope is very toxic, so it could present a substantial hazard if either of these scenarios were to occur.   Otherwise, it is not likely to be a hazard, since the inventory of Actinium-227 is not large, has a short half life (2.2 years), and its mobility in groundwater could also be rather small.   For this reason, we decided not to grant it a rating of one.

• Technecium-99, Neptunium-237, and Americium-241 & 243 are all good indicators of repository and NTS source contamination.  Nevada Test Site underground testing resulted in very small amounts of Americium-243, so this isotope could differentiate the two sources.  Calcium-41 could be used as a NTS tracer, as it has a relatively long half life; although it is likely not to be very mobile in the water system, and other isotopes were considered more important to the baseline goal., so it was not included.

• Europium-154 contributes much of the short term radioactivity.  It may not reach water supplies or will have decayed between exposure and illness realization, so Gadolinium-154, the stable daughter product, is worth analysis.  Gadolinium-154 could also serve as a YMP/NTS tracer.  The cost of analysis of this stable isotope required that we drop it from the analysis list, as there are other isotopes that can serve as tracers, and could also contribute to the long term dose.  The same could apply to Strontium-90 and Cesium-137, which have comparable lifetimes, but exist in very large mass amounts as well as curies.  Thus, it is prudent to analyze for Barium-137 and Zirconium-90, the stable daughter products of Cesium-137 and Strontium-90.  It should be noted that not many analytical laboratories can analyze for these stable isotopes, and the precise isotopic analysis that is necessary to discern these stable isotopes is very costly, so we could not analyze for these.

• The communication from Paz cites many references within that reveal a need to assess concentrations of a number of heavy metal elements that are or suspected to have intrinsic toxicity, and which could exacerbate radiotoxic effects.[8]  Chromium, molybdenum, titanium, nickel, and zirconium will exist in very large quantities as part of the Yucca Mountain Repository as canister corrosion occurs[9].  They will most likely appear in the water system at potentially high enough levels to cause health impacts, and are therefore included in the analyte list.  Chromium toxicity is well known, as specifically considered in the EPA Safe Drinking Water Standard.  

• Tritium (H-3), potassium-40, and carbon-14 are all quite mobile, especially tritium, which has also been produced to very large quantities.  All are easily ingested through water and air.  All already exist in the background at some level, and so are important to the baseline goal.    Potassium-40 could also be a marker for the repository, and not the NTS, so elevated levels of this isotope could serve to distinguish between contamination coming from the repository versus NTS.

• Uranium and to a lesser extent thorium are naturally occurring to various extents in the region, as well as many other parts of the Great Basin, and should be part of the baseline.  An isotopic breakdown is preferred for these isotopes, which we attempted to do, cost permitting.

• Iodine-129 would be present in the repository in large amounts and is also expected to be quite mobile.  It is expected to contribute to the long term dose from the repository, but it is also a potential contaminant from the NTS.

• A gamma spectrum analysis was fairly inexpensive, and many isotopes can be analyzed simultaneously without much added cost.  For example, cobalt-60 could be analyzed with the cesium-137, so we included it.

Considering all of the above along with cost constraints, we narrowed the list down further.  In making our final choices, we saw it as important to have species on our analyte list which were distinct markers for the Yucca Mountain Repository and the NTS.  In other words, while potential contaminants from each location have some analytes in common, we wanted to identify and establish baseline data for analytes that could only have come from one source or another.  HþME’s final choices are seen in Tables 10.2 and 10.3, showing analysis results.