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Using radium isotopes to characterize water ages and coastal mixing rates: A sensitivity analysis
Author(s) -
Knee Karen L.,
GarciaSolsona Ester,
GarciaOrellana Jordi,
Boehm Alexandria B.,
Paytan Adina
Publication year - 2011
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.4319/lom.2011.9.380
Subject(s) - environmental science , isotope , submarine groundwater discharge , atmospheric sciences , groundwater , geology , aquifer , physics , geotechnical engineering , quantum mechanics
Numerous studies have used naturally occurring Ra isotopes ( 223 Ra, 224 Ra, 226 Ra, and 228 Ra, with half‐lives of 11.4 d, 3.7 d, 1600 y, and 5.8 y, respectively) to quantify water mass ages, coastal ocean mixing rates, and submarine groundwater discharge (SGD). Using Monte Carlo models, this study investigated how uncertainties in Ra isotope activities and the derived activity ratios (AR) arising from analytical uncertainty and natural variability affect the uncertainty associated with Ra‐derived water ages and eddy diffusion coefficients, both of which can be used to calculate SGD. Analytical uncertainties associated with 224 Ra, 226 Ra, and 228 Ra activities were reported in most published studies to be less than 10% of sample activity; those reported for 223 Ra ranged from 7% to 40%. Relative uncertainty related to natural variability — estimated from the variability in 223 Ra and 224 Ra activities of replicate field samples — ranged from 15% to 50% and was similar for 223 Ra activity, 224 Ra activity, and the 224 Ra/ 223 Ra AR. Our analysis revealed that AR‐based water ages shorter than 3–5 d often have relative uncertainties greater than 100%, potentially limiting their utility. Uncertainties in eddy diffusion coefficients estimated based on cross‐shore gradients in short‐lived Ra isotope activity were greater when fewer points were used to determine the linear trend, when the coefficient of determination ( R 2 ) was low, and when 224 Ra, rather than 223 Ra, was used. By exploring the uncertainties associated with Ra‐derived water ages and eddy diffusion coefficients, this study will enable researchers to apply these methods more effectively and to reduce uncertainty.