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EEMs Characteristics of Dissolved Organic Matter and Their Implication in High Arsenic Groundwater of Jianghan Plain
Abstract
Dissolved organic matter (DOM) is one of the key factors controlling the release of arsenic from sediment to groundwater. There are abundant surface water systems with rivers, lakes, streams and ponds in the Jianghan plain, densely covered by irrigation canals and ditches. The interactions between surface water and shallow groundwater may significantly affect the characteristics and strength of DOM in water. In order to elaborate the roles of DOM on arsenic mobilization in groundwater of Jianghan plain, this study analyzes surface water and groundwater samples using excitation-emission matrix spectroscopy (EEMs) and characterizes the dissolved organic matter of its functional groups, molecular, fluorescence using PARAFAC (parallel factor analysis). It is found that dissolved organic matter in Jianghan plain mainly includes three different components, of which C1 and C2 are humic-like substances and C3 is characterized by protein-like substances. The main components of dissolved organic matter in groundwater are C1 and C2, whilst the dissolved organic matter in surface water is mostly C2 and C3. Dissolved organic matter in groundwater dominated by terrestrial materials, promotes the mobilization of arsenic in two ways. (1) The humic substances act as an electron shuttle for microbial communities, promoting the microbially mediated oxidation of organic matter and reduction of Fe oxides with the release of arsenic and the production of large amounts of DOC (dissolved organic carbon)and HCO3⁻; (2) Complexation of As with DOM via the bridging of metal cations (such as Fe) enhances the mobility of As in groundwater. © 2017, Editorial Department of Earth Science. All right reserved.
... Considerable proportion of oxidizable As (avg. 13.4%) was generally found in samples as the proclivity of HAsO 4 2 − and As(OH) 3 compounds for complexation with organic matters at high pH (Saada et al., 2003), and bridging of metal cations which may enhance the mobility of As (Lu et al., 2017;Li and Thornton, 1993). ...
Elevation in toxic trace metal concentration found in coastal sediments in recent years (2013-2016) increased the risk to the aquaculture industry in south Hangzhou bay. This study assessed the main factors controlling the metal distribution and mobility in sediments by sampling from 20 sites along the bank. Spatial distribution and cluster analysis indicated that Cd, As, Hg and Sb attributed to anthropogenic terrestrial sources; while Cr, Co, Cu, Ni, Zn, and Pb, carried by fine-grained sediments and accumulated on tidal flat, were inputted from marine sources. High mobility of Cd was expected because of its considerable proportion (~50%) associated with the acid extractable fraction. Pb, Cu and Co in redox sensitive fraction should also be taken into concern in management of reclaimed area affected by tide. Risk assessments by potential ecological risk index (PERI) emphasised the importance of further monitor and proper treatment of 4 terrestrial generated metals in sediments.
Nitrogen (N) pollution in shallow groundwater within the Jianghan Plain—a depositional basin along the Yangtze River in China—is a serious issue affecting the local water supply. There is little concern regarding temporal variation patterns of groundwater N in regions with intensive groundwater/surface-water interactions thus far. In this study, a 2-year field monitoring program and a set of incubation experiments were jointly applied to characterize the seasonal variation mechanisms of groundwater N. Groundwater monitoring suggested that temporal N variations in monitoring wells of 25 and 10-m depth were correlated with fluctuations in water levels, with lower NH4-N and higher NO3⁻-N concentrations corresponding to groundwater discharge during the dry season, and with higher NH4-N and lower NO3⁻-N concentrations corresponding to groundwater recharge during the wet season. Batch sediment incubation experiments showed that buried organic carbon in sediments from the field site could not promote NH4-N release, but exogenous organic carbon and oxidant input could catalyze organic N mineralization, resulting in NH4-N release. Nevertheless, NO3⁻-N release was limited in all incubations. Thus, the temporal variation patterns could be divided into (1) groundwater N in type-1 wells (25 and 10 m), controlled by the annual reversal of groundwater flow gradient, which determined whether there was input of exogenous organic carbon or oxidants, ultimately affecting organic N mineralization, denitrification and other transformation processes, and (2) groundwater N in type-2 wells (<4 m depth), which showed no significant seasonal variation, implying the process is mainly controlled by surface inputs in the shallower subsurface.
Landfills have been the most common methods of organized waste disposal in China posing an incredible groundwater pollution threat. Dissolved organic matter (DOM) can be used to trace the source, species and migration of contaminants in groundwater, and the investigations of its composition, structure, and distribution play a role in environmental protection. This study investigated the DOM source, composition, and molecular structure in groundwater at landfills for different years of operation, and explored the dynamics of groundwater DOM evolution over time, usingmodern spectroscopy in combination with multivariate statistical analysis. The results showed that DOM in landfill groundwater was initially dominated by outputs from microbial activities, and this was followed by autogenous terrigenous input. In the early stages of landfilling, the DOM of microbial origin was significant; however, towards the late stages of landfilling, the presence of microbial DOM has weakened. The groundwater DOM with short landfill times were mainly composed of newly produced tryptophan and tyrosine, which had low humification, aromaticity, and molecular weight. Microbial activity was strong, and while there were initial, significant differences between sampling points, evidence of its presence could be used for early warning of contamination and monitoring should be conducted for its presence. Microbial activity weakened with longer landfill operation time, landfill waste tended towards stability, and the DOM in groundwater with high humification, aromaticity and molecular weight, was able to reduce the landfill impact on groundwater.
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