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SUMMARY Induced calcite precipitation is used in geotechnics to modify the mechanical and hydrological properties of the underground. Laboratory experiments have shown that spectral induced polarization (SIP) measurements can detect calcite precipitation. However, the results of previous studies investigating the SIP response of calcite precipitation were not fully consistent. This study aims to investigate how the SIP response of calcite depends on solute concentration to explain the differences in SIP response observed in previous studies. A four-phase experiment with SIP measurements on a column filled with sand was performed. In phase I, calcite precipitation was generated for a period of 12 d by co-injecting Na2CO3 and CaCl2 solutions through two different ports. This resulted in a well-defined calcite precipitation front, which was associated with an increase in the imaginary part of the conductivity ($\sigma ^{\prime\prime}$). In phase II, diluted solutions were injected into the column. This resulted in a clear decrease in $\sigma ^{\prime\prime}$. In phase III, the injection of the two solutions was stopped while calcite precipitation continued and solute concentrations in the mixing zone decreased. Again, this decreased $\sigma ^{\prime\prime}$. Finally, the injection rate of the Na2CO3 solution was reduced relative to that of the CaCl2 solution in phase IV. This resulted in a shift of the mixing zone away from the calcite precipitation front established in phase I and an associated decrease of $\sigma ^{\prime\prime}$. These results imply that the SIP response of calcite is highly sensitive to the solute concentration near the precipitates, which may explain previously reported conflicting results.

Landfill sites are significant sources of methane gas globally. Understanding the temporal variabilities of methane emissions from landfill sites is necessary for estimating such emissions. In this study, an automated monitoring system was used to monitor methane emission flux and concentration on daily and hourly time scales at a landfill site. Measured methane emission fluxes were almost negligible in the studied area. However, methane concentration at landfill surface at nighttime was significantly higher than those in the daytime, which demonstrates the importance of investigating methane emissions at an hourly time scale, including during nighttime. The daily and hourly variations in methane concentration were well correlated with either soil temperature or volumetric water content near the surface. The obtained relations indicate that the automated monitoring system measurements can facilitate a more comprehensive understanding of the methane emission mechanisms at different time scales.