Comparative soil CO2 flux measurements and geostatistical estimation methods on Masaya volcano, Nicaragua

We present a comparative study of soil CO2 flux (FCO2) measured by five groups (Groups 1-5) at the IAVCEI-CCVG Eighth Workshop on Volcanic Gases on Masaya volcano, Nicaragua. Groups 1-5 measured FCO2 using the accumulation chamber method at 5-m spacing within a 900 m grid during a morning (AM) period. These measurements were repeated by Groups 1-3 during an afternoon (PM) period. All measured FCO2 ranged from 218 to 14,719 g m d. Arithmetic means and associated CO2 emission rate estimates for the AM data sets varied between groups by ±22%. The variability of the five measurements made at each grid point ranged from ±5 to 167% and increased with the arithmetic mean. Based on a comparison of measurements made by Groups 1-3 during AM and PM times, this variability is likely due in large part to natural temporal variability of gas flow, rather than to measurement error. We compared six geostatistical methods (arithmetic and minimum variance unbiased estimator means of uninterpolated data, and arithmetic means of data interpolated by the multiquadric radial basis function, ordinary kriging, multi-Gaussian kriging, and sequential Gaussian simulation methods) to estimate the mean and associated CO2 emission rate of one data set and to map the spatial FCO2 distribution. While the CO2 emission rates estimated using the different techniques only varied by ±1.1%, the FCO2 maps showed important differences. We suggest that the sequential Gaussian simulation method yields the most realistic representation of the spatial distribution of FCO2 and is most appropriate for volcano monitoring applications.