ABSTRACT

The concentration and carbon isotopic ratio (δ¹³C) of dissolved inorganic carbon (DIC) in natural waters is often altered by microbial activity, even over short durations, which can affect sample preservation. Hence, the influence of these biogenic processes should be limited to properly investigate DIC in natural waters. This study investigates the effectiveness of filtration and exposure to benzalkonium chloride (BAC) and sodium chloride (NaCl) for preserving natural water samples (seawater, groundwater, river, pond, hot spring, and brackish waters) in order to analyze the concentration and δ¹³C of DIC. We assessed six filter materials with pore sizes of 0.2–5.0 μm. In the waters filtered by polyether sulfone, polyvinylidene fluoride, or cellulose acetate filters, the number of microorganisms still increased during preservation. However, the 0.45-μm polytetrafluoroethylene filter provided effective microbe removal, yielding constant DIC concentration and δ¹³C even after three weeks of preservation. Although none of the filters studied could offer complete microorganism removal and constant δ¹³C, biogenic DIC changes in the water were successfully reduced by the use of filters with pore sizes of 0.45 μm or below. Exposure to BAC or NaCl promoted microbial exclusion in the freshwater samples, with the DIC concentration and δ¹³C maintained during preservation over the one month. These effects were degraded in the seawater and brackish water samples, with the DIC concentrations and δ¹³C only constant for 10–15 days when exposed to BAC, and 1–3 days when exposed to NaCl. Although NaCl diminished biogenic activity in the freshwater samples, its influence on DIC concentration must be corrected for, which requires accurate knowledge of the NaCl concentration. We also applied the BAC solution in a field campaign in eastern Thailand and compared the DIC concentrations and δ¹³C values with those of samples treated with HgCl₂. The results of the field trial showed that, except for brackish water from a rivulet in a mangrove forest and seawater samples, BAC-treated samples were not significantly different from HgCl₂-treated samples. BAC can be considered an effective disinfectant for natural water samples, with less efficacy in seawater and brackish water.