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As a key geochemical factor in earth system, dissolved organic matter (DOM) plays an important role in controlling environmental quality of watersheds. In this study, a typical agricultural watershed of Three Gorges reservoir areas, Wangjiagou watershed in Fuling district of Chongqing, was selected to characterize DOM in waters through fluorescence and UV-Vis spectroscopy, while the effect of land-use types in this watershed was discussed. The results showed large spatial variances of aquatic DOM in this watershed, with significant differences in compositions and sources. After calculation of a g * (355) for indicating proportion of chromophoric DOM in bulk DOM, the order of DOM was paddy rice field> ditch> pond> well> outlet point. DOM samples from paddy rice field and ditch showed higher SUVA 254 suggesting higher aromaticity. DOM from this watershed showed 2 typical types (4 peaks A, C, B and T) of fluorescent components including humic-like and protein-like components. Dual contributions from autochthonous (e.g., microbial or alga production) and allochthonous both heavily affected the DOM characteristics. Besides active microbial activities due to high organic and nutrients inputs from agricultures, discharge of sewage and water used in agricultural production could contribute proteins possibly inducing ascending proportion of protein-like component as shown in fluorescence analysis. DOM samples from the same sampling points but in different crop plantation seasons were collected to compare, for understanding the differences between two planting seasons. It clearly suggested protein-like component was the key variable to control the DOM dynamics. After land-use changing from rice/corn into mustard plantation, all of DOC, CDOM and r (T/C) showed significant differences, but no such observations were observed in FI, BIX and r (A/C) .

Phthalic acid esters (PAEs) have received increasing attention in recent years due to their widespread use and hazards to human health and fertility in the environment. In order to understand the migration and release processes of organic pollutants in huge fluctuating zone soil, Dibutyl-phthalate(DBP) was chosen as a typical substance, and its migration and release characteristics in the fluctuating zone of the Three Gorges Reservoir to overlying water and the impacts of DBP concentration in the soil, ionic strength and the concentration of organic mater in overlying water on the process were studied using static flooding method. The results showed that DBP migrated from the soil to the overlying water in the early days after flooding, and the release process of DBP was divided into two phases: one was quick release with a relatively short releasing time and a rapid releasing rate; the other was slow release with a relatively long releasing time and a slow releasing rate. The migration and release processes were well fitted by two-compartment first-order kinetics. After different concentrations DBP were added into soil, the rate of quick release increased with the increasing DBP concentrations in soil while the percent of quick release decreased with the increasing DBP concentrations. The results of rate of slow release and the percent of slow release were on the contrary. The water conditions of overlying water could impact the migration and releasing process of DBP when the soil in fluctuating zone was flooded. The amount of DBP released into the overlying water would increase when the ionic strength in the water increased. At the same time, when the ionic strength increased, in spite of the decreasing quick release rate, the percent of quick release increased. The higher concentration of organic matter in overlying water, the more the amount of DBP released into the overlying water. At the same time, all of the rates of quick release, slow release and the percent of quick release would increase with the increasing concentrations of organic matter, while there was almost no influence on the percent of slow release of DBP.

The temporal and spatial distribution of mercury (Hg) in four different sources of stream (aquaculture wastewater, bare land, forestry land and agro-forestry land) and well water of an agro-forestry and livestock compound watershed in the Three Gorge Reservoir region was studied during the period from March 2013 to March 2014. The total mercury (THg) concentrations ranged 9.95-15.26 ng x L(-1) with an average of (11.95 +/- 1.87) ng x L(-1), and the THg concentration decreased in the order of bare land > aquaculture wastewater> forestry land > agro-forestry land > well water. The total methylmercury (TMeHg) concentrations ranged 0.120-0.441 ng x L(-1) with an average of (0.232 +/- 0.099) ng x L(-1), and the TMeHg concentration decreased in the order of aquaculture wastewater > agro-forestry land > forestry land > bare land > well water. THg and TMeHg in well water were both dominated by the dissolved fraction, whereas for other sources of stream, the particulate phase accounted for the major fraction of THg and TMeHg. The THg concentrations in winter and spring were significant higher than those in summer and fall, however, no obvious seasonal trend of TMeHg distribution was observed. Comprehensive analysis showed human activities and weather conditions such as air temperature and precipitation etc. were the main reasons for the difference of temporal and spatial distribution of THg and TMeHg.

The distribution of mercury (Hg) in four different water bodies (streams, pond, precipitation and wells) of a small typical agricultural watershed in the Three Gorges Reservoir region was studied during the period from November 2011 to September 2012. Total mercury (THg) and methylmercury (TMeHg) concentrations ranged from 1.12 ng x L(-1) to 64.04 ng x L(-1) and from 0 (undetected) to 4.24 ng x L(-1), with an average of (13.54 ± 10.55) ng x L(-1) and (0.22 ± 0.42) ng x L(-1), respectively. Particulate mercury (PHg) accounted for a major fraction of THg in these four water bodies, whereas for TMeHg dissolved fraction dominated in wells and streams, but not in pond and precipitation. An obvious spatial pattern of Hg distribution was observed with THg concentration in the order of precipitation > pond > streams > wells and streams > pond > precipitation > wells for TMeHg. The THg temporal distribution showed that THg concentrations in cold season were higher than that in warm season, but the temporal distribution of TMeHg varied with the water types. Comprehensive data analysis found that wet deposition was a significant source of Hg in this agricultural watershed and the particulate transport was the main way for Hg and MeHg transport in this watershed. In addition, surface runoff was an important process for Hg contribution from watershed to the Three Gorges Reservoir, but played a minor role for TMeHg load to the reservoir.