[HTT23-P09] Multi isotopic analysis of rainwater and inland water in acid sensitive areas
Keywords:strontium isotopic ratio, lead isotopic ratio, water isotopic ratio
Method: Study was conducted in 2 catchments and 6 lakes shown in the figure. We collected rainwater, soil solution and streamwater in catchments and surface water in lakes. We determined Sr isotopic ratio by thermal ionization mass spectrometer (TRITON, Thermo Scientific), Pb isotopic ratio by double focusing type multi-collector ICP-MS (NEPTUNE, Thermo Scientific), oxygen and hydrogen of water isotopic ratios by water isotopic analyzer (L2120-i, Picarro).
Results and discussion: Sr isotopic ratio was determined from Dec. 2013 to Sep. 2015. In both catchments, 87Sr/86Sr of rainwater showed seasonal variation in range of 0.709±0.001 by contribution of soluble minerals (0.711±0.001) contained in continental dust in spring and sea salt (about 0.709) in winter. On the other hand, streamwater ratios were stable around 0.707 in Kajikawa and around 0.715 in Ijira, and these were widely different from rainwater. Soil solution indicated middle value of rainwater and streamwater. Sr concentration of rainwater were several μg/L even though in high concentration period, but streamwater were stable around 20 μg/L even though in lower concentration catchment Ijira. These results suggest that Sr leaching into streamwater is strongly contributed by geological Sr. In overviewing lakes data based on these results, Sr concentration of Ohataike, Oike and Banryu were around 15 μg/L, so these suggests that geological contribution is strong in these lakes. On the other hand, Sr concentration were around 3 μg/L in Yashagaike, Meike and Sawanoike which seems to be acid sensitivity is particularly high. In the case of these lakes, effect of bedrock weathering may be small and contribution of atmospheric deposition may be relatively large.
Pb isotopic ratio was determined from Apr. 2014 to Aug. 2015. In the case of 206Pb/207Pb and 208Pb/207Pb, contribution of rainwater clearly appeared in streamwater ratios and response speed was also fast. Particularly in Kajikawa which catchment area is small, response time rag was shorter than one month. In plotting with two components of 206Pb/207Pb and 208Pb/207Pb, all values of our study sites fit in the range of previous reported values of aerosols in East Asia (Nakano et al. 2006),especially plotted near range of Russia and Mongolia values.
Oxygen and hydrogen isotopic ratios of water were determined from Jun. 2014 to Jul. 2015. In both catchment areas, rainwater δ18O and δD showed seasonal variations, but streamwater ratios were stable around yearly average of rainwater, so it suggests that streamwater may leach from rainwater after recharged and averaged in catchments in calm water situation. In both catchments, d-excess values of rainwater showed similar trend with variation range from 5 to 35, and effect of dry continental air masses appeared clearly in winter.
In poster presentation, we also discuss about mutual relationship between these isotopic ratios.
Acknowledgment: Samples used in this study were collected as a part of long-term monitoring of transboundary air pollution and acid deposition by MOEJ. Isotopic analysis was conducted as Environmental Isotope Study in Research Institute of Humanity and Nature. The authors thank relevant persons and organizations.
Reference: Nakano, T. et al. 2006. Determination of seasonal and regional variation in the provenance of dissolved cations in rain in Japan based on Sr and Pb isotopes. Atmospheric Environment 40, 7409–7420.