Mercury Dynamic And Release Characteristics Of Several Plants Collected From The Water-Level Fluctuation Zone Of The Three Gorges Reservoir Area During Flooding

Characterized by activation effects of mercury, reservoirs are typical mercury sensitive ecological zones. Soil and vegetation inundated with water in reservoirs essentially account for the increasing of methyl mercury (MeHg) in water and fishes bodies. As a remarkable huge reservoir, Three Gorges Reservoir has large submerged land area, forming a large area of water-level-fluctuating zone with long-term alternating wet and dry cycles. As a collection area of the runoff in the reservoir district, it naturally becomes a collection area of mercury and other pollutants. On the other hand, the vegetation and soils in the water-level-fluctuating zone, as the last buffer of the reservoir, would ultimately affect the water quality caused by its collection of mercury and other pollutants. Therefore, for reservoir the water-level-fluctuating zone is a mercury-collecting area but also a source of mercury. Each year from April to September, the water-level-fluctuating zone is exposed out of water. Due to abundant rain and heat, a large area will grow lush herbaceous vegetation, and part of the water-level-fluctuating zone will be used by local farmers to grow crops. Presumably, plant growth in the water-level-fluctuating zone is bound to affect chemical and environmental behaviors of mercury in the water-level-fluctuating zone. Plants generally have a strong mercury enrichment capability and will be decomposed after being submerged under the high water level. Therefore, the mercury absorbed by the plants will be released into the water and is more easily transformed into methylmercury under an anaerobic environment with abundant organic matters, which can bring higher environmental risks. However, this presumption has not verified by any relevant researches or reports and the increased risks of mercury pollution caused by vegetation regenerated in the water-level-fluctuating zone under the low water level are still not clear. Therefore, several typical vegetation were collected from the Chongqing section of the Three Gorges Area to investigate the mercury dynamic changes and release from the vegetation under the low water level in the water-level-fluctuating zone through on-site survey sampling and analysis, and indoor-outdoor simulation test. Results of the study indicated:(1) The average total mercury concentration of several typical vegetation in the water-level-fluctuating zone was 16.64±2.84 ng·g-1 with a range of 9.54±.74 ng·g-1; the mean methylmercury (MeHg) concentration was 0.27±0.09 ng·g-1 with a range of 0.18-0.37 ng·g-1; methylmercury (MeHg) took a range of 1% to 3% of total mercury.(2) Results of laboratory simulation of flooding experiment showed that with the increase in submersion time the plants continued decomposing with a decomposition rate changing from fastest to slowest. To the end of the experiment, there were significant differences among quality loss of [Echinochloa crusgalli (L.) Beauv.],[Cynodon dactylon (L.) Pers.] and (Culmus Zeae mays L.), and the mass loss rate were 53.35%,37.41% and 32.33% respectively.(3) In the laboratory simulation of flooding process, several plants of total mercury concentration showed an overall downward trend, indicating a net total plant mercury release. By the end of the test, the total mercury concentration declined by 55.96%[Echinochloa crusgalli (L.) Beauv],41.57% [Cynodon dactylon (L.) Pers.]and 35.81%(Culmus Zeae mays L.). The MeHg concentration in plants showed an overall upward trend, indicating an occurrence of in situ methylation. The MeHg concentration of Barnyard grass, Bermuda grass and corn stover were 3.07,3.04, and 6.63 times the initial concentration. On the other hand, the concentration of dissolved mercury (DHg) in overlying water showed a sustainable increasing trend to the end of the test. The dissolved mercury (DHg) concentration increased by 232.15% [Echinochloa crusgalli (L.) Beauv.],142.41%[Cynodon dactylon (L.) Pers.], and 103.23%(Culmus Zeae mays L.). The increase of concentration of the dissolved mercury (DHg) in overlying water was almost entirely caused by the decomposition of plant and mercury release. Meanwhile, the concentration of the dissolved methylmercury (DMeHg) in the overlying water also showed rapid increasing trend. The concentration values were 16.05,14.84, and 15.47 times the initial concentration for [Echinochloa crusgalli (L.) Beauv.], [Cynodon dactylon (L.) Pers.] and (Culmus Zeae mays L.) respectively. As calculated, the concentration increase of the dissolved methylmercury (DMeHg) in the overlying water excessed the complete release of methylmercury from plants decomposition, indicating that methylation of biotic and abiotic systems occurred.(4) The laboratory simulation experiments showed the concentration of methylmercury (MeHg) in the overlying water and plant residues significantly correlated to DOC, DO, and pH, suggesting that these factors played an important role on the change of the concentration of methylmercury (MeHg) in the overlying water and plant residues.(5) By the end of the field simulation experiment, the mass loss rates of [Cynodon dactylon (L.) Pers.], [Alternantheraphiloxeroides (Mart.) Griseb.] and(Polygonum lapathifolium L.)were 61.46%,69.56%, and 85.03% respectively; the total mercury concentration of plant residues declined by 63.68%,76.92%, and 80.20% respectively; the methylmercury levels were 2.21,3.03, and 2.22 times the initial concentration, respectively. The ratios of methylmercury to total mercury were 9.51,9.64,20.23 times that before flooding, which showed the submersion condition was conducive to plant mercury methylation.(6) In process of field simulation flooding, the unit mass of total mercury emissions of the several plants gradually increased. Until the end of the experiment, the total mercury emissions of [Alternantheraphiloxeroides (Mart.) Griseb.], (Polygonum lapathifolium L.) and [Cynodon dactylon (L.) Pers.] were 19.79,18.28, and 11.79 ng·g-1, accounting for 76.89% 88.92%,68.10% of the initial total mercury concentration, respectively; the release rate of total mercury of the several plants increased rapidly in the first 5 days, and then gradually declined; to the end of the experiment, the total mercury release rates were 0.26,0.24,0.16 ng·(g·d)-1, respectively. Combined with aboveground biomass data of vegetation in the water-level-fluctuating zone, the total mercury average release load per unit area of several plants was estimated to be 31.84 mg·hm-2. In addition, the average release load per unit area of the total mercury after complete decomposition of vegetation in the water-level-fluctuating zone was estimated to be 42.21 mg·hm-2.