Sampling Instead of Source Blending: A Potential Inexpensive Path to Gross Alpha MCL Compliance Jeff Stovall, Ph.D., P.E., Carollo Engineers and Geri Wellborn, Norman Utilities Authority 2017 Southwest AWWA Conference Oklahoma City, October 15 th 17 th
Norman Background Information Wells 1, 2, 3A, and 52 are offline because of gross alpha exceedances. MCL 2
CarolloTemplateWaterWave.pptx 3
MCL for Gross Alpha Activity The maximum contaminant level for gross alpha particle activity (including radium-226 but excluding radon and uranium) is 15 pci/l. 40 CFR 141.66(c) 4
MCL for Gross Alpha Activity The maximum contaminant level for gross alpha particle activity (including radium-226 but excluding radon and uranium) is 15 pci/l. 40 CFR 141.66(c) 5
MCL for Gross Alpha Activity The maximum contaminant level for gross alpha particle activity (including radium-226 but excluding radon and uranium) is 15 pci/l. 40 CFR 141.66(c) Uranium is excluded by subtracting the observed uranium activity. Either directly measured as activity [pci/l], or Converted from mass concentration [ug/l]. Separate MCL for uranium 30 ug/l (40 CFR 141.66(e)) Measured in mass, so utilities most utilities analyze uranium for mass only (and do not measure activity) 6
One Uranium Analysis is as Good as Another Parts is Parts 7
Uranium Analysis Mass? Activity EPA allows (but does not mandate) use of a conversion factor for uranium activity. Utilities do not have to separately analyze for uranium activity. If measured, uranium activities can be directly subtracted. 8
Uranium Isotopes 4 142 9
Secular Equilibrium Activity of Daughter = Activity of Parent Radioactive decay chain Equilibrium reached after ~6 to 7 half-lives Specific Isotope Half-life (years) Activity (pci/ug) U-234 244,500 6,252 U-235 7.04 10 8 2.163 U-238 4.468 10 9 0.336 10
Uranium Conversion Factor If uranium (U) is determined by mass, a 0.67 pci/μg of uranium conversion factor must be used. This conversion factor is based on the 1:1 activity ratio of U 234 and U 238 that is characteristic of naturally occurring uranium. 40 CFR 141.25, footnote 12 Activity = Mass 0.67 11
Uranium Conversion Factor Assumes secular equilibrium of U-234 and U-238 isotopes. U-234 activity can exceed U-238 activity in natural groundwater systems. A slight increase in the proportion of U-234 in groundwater can greatly affect U activity. 12
Uranium Conversion Factor C f = 0.336 1 + A U234 A U238 At secular equilibrium, the activity ratio is 1.0, and the resulting conversion factor is 0.67. If U-234 activity is higher, Ratio is greater than 1.0. Resulting conversion factor will be larger. 13
Example data: Well #5 Sample collected on November 10, 2016 Parameter Result Reporting Limit U-234 (pci/l) 7.18 ± 0.608 0.127 U-235 (pci/l) 0.165 ± 0.100 0.089 U-238 (pci/l) 2.94 ± 0.394 0.074 Total U (ug/l) 9.11 1 GAA (pci/l) 12.4 ± 2.42 1.47 Combined U activity = 10.29 ± 1.10 pci/l. C f = 0.336 1 + 7.18 2.94 = 1.16 pci/ug Default EPA conversion factor (0.67) therefore under-estimates the uranium activity and over-estimates the adjusted GAA in this instance. 14
Summary of Additional Data: Adjusted Gross Alpha Wells 1, 2, and 52 sampled in January and April 2017. Well First Quarter Second Quarter Activity (pci/l) Error (pci/l) Activity (pci/l) Error (pci/l) Well 1 10.5 ±3.7 * 2.8 ±3.0 * Well 2 2.9 ±2.2 * 10.7 ±2.9 * Well 52 3.1 ±3.0 * 2.4 ±3.2 * * Uncertainties add in quadrature (square root of the sum of squares). Samples collected in January and April 2017. 15
Negative Adjusted Gross Alpha Activity Radionuclide results include reported activity and counting error. Reflect randomness of radionuclide decay. Statistical expression of analytical variability. Sample counts are compared to background counts. Naturally occurring radionuclides Detected cosmic radiation. Samples not different from background may be negative when background radioactivity is subtracted. 16
Negative Adjusted Gross Alpha Activity Negative GAA values can occur when most of the gross alpha activity of a sample is associated with uranium. Random errors in the measurement of all four values cause the combined uranium activity to exceed the measured gross alpha activity. 17
Well Reactivation Plan 1. Reactivate wells 1, 2, and 52* Data for first 2 quarters of 2017 show no gross alpha exceedances using direct measurement of activity. Institute regular collection of quarterly monitoring data using isotopic uranium analysis for evaluating gross alpha compliance. 2. Continue quarterly sampling for four consecutive quarters using isotopic uranium analysis. 3. Implement monitoring frequency in accordance with the Radionuclide Rule (3/6/9 year sample interval). *Pending receipt of results from Oklahoma-accredited laboratory. 18
Questions? 19