2:21 PM - 2:39 PM
[BPT01-04] The spatial distribution and the origin of heavy metals in bottom sediments from Lake Shinji and Nakaumi
Keywords:Lake Shinji and Nakaumi water system, surface sediment, heavy metals, spatial distribution, anthropogenic pollution, accumulation process
The elemental composition of the surface sediment of the Hii River system estuary (Lake Shinji / Nakaumi), which straddles Shimane and Tottori prefectures, was clarified. Based on the results, mainly targeting the heavy metal elements, the spatial distribution and factors were investigated.
Using an Ekman-Birge grab, the surface sediment samples were collected from the bottom of Lake Shinji and Nakaumi and the related water systems during August and September 2016. The surface layers of the samples were separated by 2.3 cm square cube for the following analysis. Afterwards, the homogenized samples were lyophilized in order to be acid-decomposed with nitric acid and hydrochloric acid. Ultimately, the element concentration was measured by an Agilent 7700x ICP mass spectrometer.
Most of the surveyed elements show that it tends to be distributed at relatively high concentrations near the center of Lake Shinji and Nakaumi. On the contrary, low concentrations appear near the Ohashi River and its estuary and the Sakai Channel. This result has a good inverse correlation with the sand content of the sediment and is consistent with the research result by Matsumoto (2009). This tendency is thought to be due to the fact that debris brought from the catchment areas was separated according to particle size and specific gravity by the deposition process, and then particles containing each element accumulated near the center of the lake.
The spatial distribution of heavy metals in the surface sediments of the Lake Nakaumi obtained in this research is compared to the vertical change of the core sediments excavated in the center of the Lake Nakaumi (Aoki, 2019) and the spatial distribution in previous studies (Nakao et al., 1971; Kondo et al., 1986, etc. ). Most of the heavy metal elements reflect the load of former mining activities, which were brought through the Ohashi and Iu rivers. Compared to the past results, the concentrations of heavy metals have been declining in recent decades, but still show a relatively high level in the western part of the Nakaumi. On the other hand, the sample of Yonago Bay in the eastern part of Nakaumi retains a relatively high concentration, among which the concentration of Mo is particularly remarkable. It is considered that Mo remains in the sediment as sulfide deposition. Possible causes include the effects of organic matter due to production activities and waste water from daily life around Yonago Bay, as well as the reducing environment caused by the obstruction of the bay and the depressions after dredging. Moreover, Mn shows a relatively high concentration from the central part of Lake Nakaumi to the area around Daikonshima and Ejima, and a relatively low concentration in Yonago Bay. It is considered that the reason why Mn is concentrated in the center of the lake rather than fixed in the sediment is probably that Mn does not form sparingly soluble sulfide.
Using an Ekman-Birge grab, the surface sediment samples were collected from the bottom of Lake Shinji and Nakaumi and the related water systems during August and September 2016. The surface layers of the samples were separated by 2.3 cm square cube for the following analysis. Afterwards, the homogenized samples were lyophilized in order to be acid-decomposed with nitric acid and hydrochloric acid. Ultimately, the element concentration was measured by an Agilent 7700x ICP mass spectrometer.
Most of the surveyed elements show that it tends to be distributed at relatively high concentrations near the center of Lake Shinji and Nakaumi. On the contrary, low concentrations appear near the Ohashi River and its estuary and the Sakai Channel. This result has a good inverse correlation with the sand content of the sediment and is consistent with the research result by Matsumoto (2009). This tendency is thought to be due to the fact that debris brought from the catchment areas was separated according to particle size and specific gravity by the deposition process, and then particles containing each element accumulated near the center of the lake.
The spatial distribution of heavy metals in the surface sediments of the Lake Nakaumi obtained in this research is compared to the vertical change of the core sediments excavated in the center of the Lake Nakaumi (Aoki, 2019) and the spatial distribution in previous studies (Nakao et al., 1971; Kondo et al., 1986, etc. ). Most of the heavy metal elements reflect the load of former mining activities, which were brought through the Ohashi and Iu rivers. Compared to the past results, the concentrations of heavy metals have been declining in recent decades, but still show a relatively high level in the western part of the Nakaumi. On the other hand, the sample of Yonago Bay in the eastern part of Nakaumi retains a relatively high concentration, among which the concentration of Mo is particularly remarkable. It is considered that Mo remains in the sediment as sulfide deposition. Possible causes include the effects of organic matter due to production activities and waste water from daily life around Yonago Bay, as well as the reducing environment caused by the obstruction of the bay and the depressions after dredging. Moreover, Mn shows a relatively high concentration from the central part of Lake Nakaumi to the area around Daikonshima and Ejima, and a relatively low concentration in Yonago Bay. It is considered that the reason why Mn is concentrated in the center of the lake rather than fixed in the sediment is probably that Mn does not form sparingly soluble sulfide.