Plume-Scale Testing of a Simplified Method for Detecting Tritium Contamination in Plants & Soil

Transcription

1 Plume-Scale Testing of a Simplified Method for Detecting Tritium Contamination in Plants & Soil B.J. Andraski 1, K.J. Halford 1, & R.L. Michel 2 U.S. Geological Survey 1 Carson City, Nevada 2 Menlo Park, California

2 OUTLINE Introduction Method development Plume-scale testing Application to long-term performance monitoring

3 INTRODUCTION Subsurface monitoring... soil samples, boreholes, wells Detailed information to assess release & transport Costs can limit number of sampling locations Equipment, installation, maintenance Novel approaches needed for long-term monitoring Cost effective Capable of early warning Accurate & robust enough to assess performance

4 PHASE I METHOD DEVELOPMENT (Andraski et al., 2003, Journal of Environmental Quality) Objectives Develop simplified method for determining tritium contamination in plants Determine if plant data could be used as an indicator of soil contamination

5 Amargosa Desert Research Site Near Beatty, NV Waste facility Low-level radioactive, Chemical, 1970-present Precipitation ~100 mm/yr Creosote bush (Larrea tridentata) Alluvial/fluvial sediments Depth-to-water ~110 m

6 Soil-Water Vapor Sampling

7 Plant-Water Sampling

8 Solar Distillation of Plant Foliage

9 Preparation of Solar Distillate for Tritium Analysis by Liquid-Scintillation Counting (a) Filtration & (b) graphite based solid-phase extraction (SPE) No preparation Filter g SPE Filter + 2-g SPE

10 Plant Tritium as an Indicator of Soil Contamination (e.g., plant water & soil-water vapor from 0.5-m m depth, root zone) 10,000 Soil-water-vapor tritium (Bq L -1 ) 1, r = 0.98 p> r < n = 10 1:1 line ,000 10,000 Plant-water tritium (Bq L -1 )

11 PHASE II PLUME-SCALE TESTING (Work in progress) Objectives Test simplified contamination-detection method for collection & analysis of plume-scale data Gain insight into tritium migration pathways

12 Plant Sample Locations & Delineation of Tritium

13 Geostatistical Analysis of Plant Tritium Data (log (a) Semivariograms spatial correlation & uncertainty (log 10 ) Semivariogram [log10 (plant-water tritium, Bq L -1 )] Sill = 1.36 Range = 380 m Nugget < Separation distance (m)

14 (b) Maps of Estimated Plant Concentrations & Errors Kriging 4,069,200 a Concentrations 4,069,000 Northing (m) 4,068,800 Tritium (log 10 Bq L -1 ) Northing (m) 4,068,600 4,068,400 4,069,200 4,069,000 4,068,800 Kriging Errors b ,068,600 < 0.2 4,068, , , , , , ,000 Easting (m)

15 Field Measurements to Develop Predictive Relations Between Plant- & Subsurface-Tritium Contamination Soil-gas tube Depth (m) Loamy sand Gravelly sand Gravelly sandy loam

16 Prediction of Soil Tritium from Plant Tritium Log10 (soil-water-vapor tritium, Bq L -1 ) Log10 (soil-water-vapor tritium, Bq L -1 ) Root-zone vapor, 0.5 m y = x r 2 = 0.96 P>F < SEE = 0.24 n = Sub-root Log 10 zone (plant vapor, water, 1.5 Bq m L -1 ) y = x r 2 = 0.90 P>F < SEE = 0.35 n = Log 10 (plant-water tritium, Bq L -1 )

17 Maps of Regressed Soil-Water Vapor Tritium 4,069,200 a Root zone, 0.5 m 4,069,000 Northing (m) Northing (m) 4,068,800 4,068,600 4,068,400 4,069,200 4,069,000 4,068,800 4,068,600 b Sub-root, 1.5 m Tritium (log10 Bq L-1) ,068, , , , , , ,000 Easting (m)

18 Tritium-Transect Transect Measurements Preferential lateral movement; upward release to atmosphere Tritium (Bq L -1 ) 10,000 1, Sub-root zone, 1.5 m Root-zone vapor, 0.5 m Plant water Air ( ND, not determined) ND ,000 Distance from w aste facility (m)

19 APPLICATION TO LONG-TERM PERFORMANCE MONITORING (LTPM) The pros based on Amargosa Desert results Simple, cost effective Plant sample collection & preparation requires 1/5 the time of soil-gas samples Identify & map tritium plumes Root zone & sub-root zone Identify presence of preferential pathway Provide data for development of transport models

20 APPLICATION TO LTPM continued Sources of uncertainty Extrapolation from plant- to subsurface-contamination Empirical relations Solid-phase extraction of plant water Organics & colored substances are targeted but the exact mechanisms that interfere with liquidscintillation counting are not completely understood Solar distillation of plant water Isotopic fractionation can slightly reduce measured tritium

21 APPLICATION TO LTPM continued Transferability to other plants & environments Site-specific testing Solid-phase extraction column for plant of interest Accuracy of relations between plant & subsurface tritium Root zone & sub-root zone(?) Depending on objectives & results of testing General indicator or quantitative predictor of subsurface tritium contamination

22 Amargosa Desert Research Site USGS Toxic Substances Hydrology Program USGS Toxic Substances Hydrology Program