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Title: Nitrate vulnerability projections from Bayesian inference of multiple groundwater age tracers

Nitrate is a major source of contamination of groundwater in the United States and around the world. We tested the applicability of multiple groundwater age tracers ( 3H, 3He, 4He, 14C, 13C, and 85Kr) in projecting future trends of nitrate concentration in 9 long-screened, public drinking water wells in Turlock, California, where nitrate concentrations are increasing toward the regulatory limit. Very low 85Kr concentrations and apparent 3H/ 3He ages point to a relatively old modern fraction (40–50 years), diluted with pre-modern groundwater, corroborated by the onset and slope of increasing nitrate concentrations. An inverse Gaussian–Dirac model was chosen to represent the age distribution of the sampled groundwater at each well. Model parameters were estimated using a Bayesian inference, resulting in the posterior probability distribution – including the associated uncertainty – of the parameters and projected nitrate concentrations. Three scenarios were considered, including combined historic nitrate and age tracer data, the sole use of nitrate and the sole use of age tracer data. Each scenario was evaluated based on the ability of the model to reproduce the data and the level of reliability of the nitrate projections. The tracer-only scenario closely reproduced tracer concentrations, but not observed trends in the nitratemore » concentration. Both cases that included nitrate data resulted in good agreement with historical nitrate trends. Use of combined tracers and nitrate data resulted in a narrower range of projections of future nitrate levels. However, use of combined tracer and nitrate resulted in a larger discrepancy between modeled and measured tracers for some of the tracers. In conclusion, despite nitrate trend slopes between 0.56 and 1.73 mg/L/year in 7 of the 9 wells, the probability that concentrations will increase to levels above the MCL by 2040 are over 95% for only two of the wells, and below 15% in the other wells, due to a leveling off of reconstructed historical nitrate loadings to groundwater since about 1990.« less
 [1] ;  [2] ;  [3] ;  [3] ;  [4] ;  [5] ; ORCiD logo [1] ;  [3]
  1. The Catholic Univ. of America, Washington, D.C. (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); California State Univ., Hayward, CA (United States)
  3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  4. Univ. of Bern, Bern (Switzerland)
  5. California State Univ., Hayward, CA (United States)
Publication Date:
Report Number(s):
Journal ID: ISSN 0022-1694
Grant/Contract Number:
AC52-07NA27344; LLNL-JRNL-677921
Accepted Manuscript
Journal Name:
Journal of Hydrology
Additional Journal Information:
Journal Volume: 543; Journal Issue: PA; Journal ID: ISSN 0022-1694
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
Country of Publication:
United States
54 ENVIRONMENTAL SCIENCES; 58 GEOSCIENCES; Nitrate vulnerability; Age tracers; Bayesian inference; Uncertainty analysis; Lumped parameter models; Residence time distribution
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1396552