A study on the water environment of around Mt. Ontake after eruption (140927) (6)

Ⅰ Introduction<br>The steam explosion eruption of Ontake in 2014 released the volcanic ejecta and affected the lake water in the summit area and the surrounding river water used as drinking water. Therefore, we compared water quality of lakes and rivers after eruption with water quality before eruption or eruption in 1979, grasp how the water quality changed due to this eruption, continue to measure the water quality, We attempted to follow the change in water quality accompanying it. In the spring meeting of 2017, we reported the results until February 2017, but this time we will report on the results until February 2018, centering on the data from October 2014 to October 2017.<br><br>II Research method<br>The survey has been conducted 32 times so far, and the field survey items are AT, WT, pH, RpH, EC etc. In addition to investigating rivers and lakes, it also collects rainwater. Samples collected at the site were processed in the laboratory and analyzed for TOC and major dissolved components.<br><br>Ⅲ Results and Discussion<br>1. Until one year after the eruption<br>The Otaki River after the confusion of the Nigorikawa River, which was strongly influenced by the volcanic ejecta, became opaque, the pH was low and the value of the electric conductivity (EC) was high. After that, the pH increased, the EC value decreased, the value stabilized at the end of January 2015. Meanwhile, in Ontake Lake in Otakitagawa River, turbid water spread throughout the lake due to the whole circulation, a phenomenon in which the pH was lowered due to time lag due to water discharge and EC increased was observed. The impact of snowmelt began to appear in February and peaked at the end of April. Contrary to the initial hypothesis, since the value of EC decreased with decreasing pH, the influence of the volcanic ejecta deposited in the mountainous body did not appear in the water quality, and EC was thought to be diluted by low pH snow melting water. Since June, after the rainfall caused by the rainy season or typhoon, the decrease in pH and the rise in EC value were measured, so it is considered that the volcanic ejecta that had accumulated flowed and the water quality changed again.<br>2. Until two years after the eruption<br>The typhoon period passed and the water quality stabilized at the end of October. After November, the influence of the property of groundwater on the water quality of river water was observed. In February 2016, the EC value declined with pH and it was shown that the dilution effect prominently at the snow melting season, but the EC value did not decrease as much as the first year. Compared to the same period one year ago, the amount of snow was overwhelmingly small, and the difference in snowfall amount appeared also in the magnitude of the dilution effect. After June the influence of the volcanic ejecta appeared in the water quality due to rainy season and typhoon, especially pH and EC value fluctuated mainly in the turbid river.<br>3. Until three years after the eruption<br>As in 2015, water quality became stable at the end of October, and after November the properties of groundwater appeared in river water quality. Even in 2017, after the rain, pH decrease and EC rise are measured mainly in the turbid river, and the influence of the volcanic ejecta is presumed to continue for a while.<br>Comparison with 1979 eruption<br>The water quality composition about one month after the eruption was very similar between 1979 and this time, and the distribution was consistent. However, when comparing the water quality composition of the River River immediately after the eruption, it was a calcium sulfate type in 1979, whereas in this case there was a difference in the sodium chloride type, and the concentration of each component was also different.<br><br>View PDF for the rest of the abstract.