FT-ICR MS reveals that rainstorm runoff significantly increases CDOM and DOC input from lakes and reservoirs

- Dec 04, 2020-

Reservoir is an important source of drinking water, and its water quality and dissolved organic matter concentration and composition are of great significance to the safety of drinking water supply to the reservoir.

The rainstorm runoff process directly causes a large amount of suspended solids and colored soluble organic matter (CDOM) to be imported into the reservoir. CDOM is the part of dissolved organic matter (DOM) that can strongly absorb ultraviolet radiation. The presence of high concentration of CDOM makes the water sour and pungent, and in the process of water treatment and distribution, the rust filtration system releases carcinogenic companion organisms and increases the amount of disinfectant in the drinking water pipe network. Qiandao Lake is an important source of drinking water in the eastern part of my country. It is located in the Xin'an River Basin in the upper reaches of the Qiantang River. It is located in the subtropical monsoon climate zone and the precipitation is unevenly distributed throughout the year. During the storm runoff process, a large amount of suspended solids, nutrients and CDOM entered the Qiandao Lake, threatening the safety of the lake's water supply. The impact of storm runoff process on the source composition and bioavailability of CDOM in downstream lakes remains to be studied. Traditional half-month, month-by-month and quarterly observations are not easy to effectively monitor the impact of storm runoff on the source composition of CDOM. At present, there are few reports on the impact of high-frequency monitoring of storm runoff process on the CDOM composition of drinking water supply lakes and reservoirs. In view of this, with the support of the Chinese Academy of Sciences’ strategic leading science and technology project and the National Natural Science Foundation of China’s key projects, the research team of Zhang Yunlin, a researcher at the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences Jinshan deployed buoys to conduct daily observations combined with in-situ high-frequency sampling observations to discuss the variation characteristics of CDOM source composition and bioavailability with the process of storm runoff. Related research results were published on Water Research (Zhou et al. 2020).

The monitoring results of the buoy high-frequency fluorescent probe show that the rainstorm runoff process increases the fluorescence signal intensity of terrestrial humic acid at the upper reaches of Qiandao Lake and Xiaojin Mountain, which is more significant at the estuary where the Xin'an River enters the lake. (figure 1). The researchers found through point plotting that the mean value of the fluorescence signal of terrestrial humic acid input from the water column profile to characterize soil leaching and the upstream water inflow showed a significant positive correlation at Jiekou and Xiaojinshan stations (Figure 1).

   Fourier Transform Ultra High Resolution Mass Spectrometry (FT-ICR MS) technology can effectively reveal the source composition of DOM in different water ecosystems from the molecular level. Using this technology, researchers found that the process of heavy rain caused the upstream influent DOM molecular mass-nucleus ratio m/z to decrease, and the relative abundance of condensed aromatic hydrocarbon components increased accordingly. The biological culture experiment found that after 28 days of biological culture, the aliphatic components in the DOM at Jiekou Station were rapidly degraded. Through the Spearman correlation coefficient, the researchers found that the relative abundance of aliphatic components is positively correlated with the CDOM spectral slope S275-295 and negatively correlated with the humification index HIX, while the condensed aromatic hydrocarbon components are negatively correlated with S275-295 and have a negative correlation with S275-295. HIX is positively correlated. Since S275-295 decreased with the increase of CDOM humification, HIX was positively correlated with humification. This means that the CDOM spectrum and the CDOM composition related information reflected by the mass spectrum composition are highly consistent. The correlation analysis between the relative abundance of mass spectrometry molecules and the upstream water inflow shows that the process of storm runoff increases the relative abundance of colored, hydrophobic and aromatic hydrocarbon-rich organic matter. The UV absorption of SUVA254 can effectively reflect the aromaticity level of CDOM, and the two are positively correlated; field observations in three different high and dry scenarios show that a large number of CDOMs with higher aromatic levels in the high water scenario flood into Qiandao Lake with the upstream water (Figure 3) This is highly consistent with the results of ultra-high resolution mass spectrometry. These results indicate that the process of storm runoff enhanced the leaching of organic matter in the upstream soil and increased the input of CDOM with a higher degree of humification in the upstream Xin'an River.

   The organic carbon balance estimation results show that Qiandao Lake itself is a dissolved organic carbon (DOC) carbon sink with a sink flux of 0.82×104 t C yr1, of which 0.30×104 t C yr1 is a component with strong biological activity. According to statistics, in the past 60 years, the frequency of heavy rain (≥25 mm d -1) and heavy rain (≥50 mm d -1) in the Qiandao Lake Basin has gradually increased. The future heavy rain scenario will cause a large amount of organic matter to flood into Qiandao Lake. Department's attention.