Characterization of dissolved organic matter in groundwater from the Coastal Dagu River watershed, China using fluorescence excitation-emission matrix spectroscopy.

The fluorescent properties of dissolved organic matter (DOM) from groundwater in the coastal Dagu River watershed, North China were determined using excitation-emission matrix spectroscopy (EEMs) analysis. Surface water DOM samples were also investigated for comparison. Two humic-like components (C1: 250, 355/472 nm; C2: < 240, 325/400 nm) and one protein-like component (C3: <240, 280/340 nm) were identified using parallel factor analysis. Low intensities for all components were observed in groundwater DOM from the upper and lower reaches of the study area. However, higher abundances of these components occurred in the middle reaches, reflecting the combined effect of seepage of surface water with strong anthropogenic pollution and the alteration of groundwater circulation due to cutoff as a result of the construction of a cutoff wall since the late 1990s. The humic-like components were dominant in groundwater DOM, with the average percentage of the protein-like component being only 15%, which was less than half of the corresponding percentage in surface water DOM. The freshness index in groundwater DOM was lower than the surface water samples, while the fluorescence index and humification index were higher than in the latter. These indices demonstrated the much higher degree of humification for groundwater DOM, which may be related to the longer residence time of groundwater and greater contribution of microbial degradation in the aquifer environment. This study demonstrated that EEMs could distinguish between the effects of natural background and human activities on the quantity and characteristics of the groundwater DOM, and thus could be a useful tool for studing the carbon dynamics and the controlling factors in groundwater systems.

[Distribution, Migration, and Transformation Mechanism of Labile Phosphorus in Sediments of Xixi River Estuary, Xiamen].

To explore the effect of manganese, iron, and sulfur geochemistry on the distribution of labile phosphorus in different estuarine areas, the diffusion gradient in thin-film (DGT) sampling technique was used for in-situ high-resolution monitoring of available phosphorus (DGT-P), manganese, iron, and sulfur in sediments from Xixi River estuary in Xiamen. The results showed that the distribution of DGT-P in the vertical profile was closely related to the redox transformation of iron and sulfur and the background value of active phosphorus in sediments. The passivation/activation of phosphorus was mainly controlled by the oxidative adsorption/reductive dissolution of phosphorus by iron oxides and the activation of phosphorus induced by sulfate reduction and sulfide accumulation. Along the sampling sites, the average concentration of DGT-P varied greatly (0.075-0.80 mg·L-1), which was not related to salinity but closely related to redox conditions, that is, the deeper the oxidation zone, the lower the average concentration of DGT-P. The simulation results showed that the phosphorus resupply capacity from surface sediments to porewater was correlated with DGT-P concentration and redox conditions, that is, the oxidative environment was unconducive to the desorption and resupply of sediment phosphorus, whereas the coupling with iron and sulfur geochemistry in the reducing environment was conducive to the maintenance of high labile phosphorus concentration and the continuous release of phosphorus.