Mercury Distribution In Vegetation In Water-level-fluctuating Zone Of Three Gorges Reservoir Area And Its Impact On The Behavior Of Mercury In Soil And Water

Three Gorges Reservoir(TGR) is the most distinguished extra-large reservoir, which has very large flooding areas(area of 632km2) called water-level fluctuation zones resulted in long-term wet-dry alternation after completion of impoundment. Water-level fluctuation zones is in dry period from April to September every year showing an special local characteristics that plentiful vegetations(e.g. herbaceous plants) grow in these large areas due to sufficient rainfall and appropriate temperature. On the one hand, submerged soil and vegetation in the process of decomposition of active or passive to release the total mercury and methylmercury into the reservoir ecosystem; on the other hand, in the bottom of the reservoir formation of anaerobic environment, provides a reaction sites of mercury methylation. Organic matter of soil and vegetation decomposition releases a large amount of nutrients, and provides a sufficient food source for the microorganisms, which makes the sulfate reducing bacteria and other bacteria multiply, and promote the transformation of inorganic mercury to methylmercury. Three Gorges Reservoir area, there is a long dry and dry wet alternate state, and the physical and chemical properties of the soil are changed.The vegetation growth and the regeneration process of the vegetation in the riparian zone, and the organic matter in the soil every year, will be added. Plants also have a strong mercury enrichment ability, the plant from the atmosphere, soil absorption of mercury. Three Gorges reservoir is a typical "Mercury sensitive ecosystem", and the effect of plant on the migration and transformation of mercury in the Three Gorges reservoir is not negligible.The mercury(Hg) distribution and storage in plants at Water-Level-Fluctuating Zone(WLFZ) in the Three Gorges Reservoir were investigated by analyzing the total mercury(THg) and methylmercury(Me Hg) levels in different parts of plants collected from three typical sites including Shibaozhai, Zhenxi and Hanfeng lake in WLFZ. The results indicated that THg and Me Hg concentrations in plants ranged from 1.62±0.57 to 49.42±3.93 ?g·kg-1 and from 15.27±7.09 to 1974.67±946.10 ng·kg-1, respectively. In addition, THg levels in different plant parts followed the trend: root> leaf > stem, and similar trend for Me Hg was observed with the highest level in root. An obvious spatial distribution was also found with the THg and Me Hg levels in plants in Hanfeng higher than that in same plants in other two sampling sites(Shibaozhai and Zhenxi), as well as a difference of THg and Me Hg storage in plants appeared in various attitudes. The corresponding THg and Me Hg storages were 145.3, 166.4, 124.3 and 88.2 mg·hm-1,and 1.9, 2.7, 3.6 and 3.2mg·hm-1 in 145-150, 150-160, 160-170 and 170-175 m attitudes. The accumulation ability of dominant plants in WLFZ for THg(bioaccumulation factor, BAF<1) was weaker than that for Me Hg(BAF>1).The soil samples were collected monthly in fall dry period and sediments and overlying water in Storage Period for measuring Different Forms of Mercury( including THg, Me Hg and Bioavailable mercury in soil and Reactive Hg(RHg), THg, Me Hg, Dissolved mercury(DHg) and Dissolved mercury(DMe Hg) in water)in two types of bare land and grass land Fluctuating Zone of Reservoir River and its tributaries from May,2014 to April,2015.Resulst showed that the mercury concentration varies in the different soil types at the same altitude and the different altitude at the same soil type. In fall dry period of resevior river, The thg of 0-5 layer soil in bare land and grassland were(75.90±4.06)?g·kg-1 and(86.97±1.73)?g·kg-1,respectively and in storage period,(86.00±4.00) ?g·kg-1 and(98.42±2.62)?g·kg-1, respectively. The soil thg decreased with the increasing of altitude and presented the trend of migration from topsoil to subsoil(below), especially in bare land, which followed the same rule both in River and tributaries. Both bare land and grass land reached the high peak of Me Hg in 0-5cm soil. In bare land, Me Hg of topsoil were the lowest at altitude of 150~155m(SB) with(0.37±0.02) ng·kg-1, and in grassland the highest at altitude of 160~165m(SB) with(0.67±0.03) ng·kg-1. Afer water storage, Me Hg increased in each soil layer, however, the upper layer were higher than lower layer in Me Hg concentration. Biovailable Hg of soil account for a lower propotion of THg(only 1.48%~4.66%) in river and tributaries, with bare land lower than grassland and water storage period lower than fall dry period in bioavailable Hg.THg of river and tributaries were 14.91±1.06 ng·L-1 and 14.89±0.32 ng·L-1, respectively, which were far higher than that of other nature water in the world. Compared to other altitude, THg of underlying water in bare land area was lower at the altitude of 150-155m(SB) with concentration of 17.96±0.84 ng·L-1 and in grassland area was higher with concentration of 22.11±1.79 ng·L-1.Me Hg of river and tributaries were 14.91±1.06 ng·L-1 and 14.89±0.32 ng·L-1, respectively. Me Hg mean concentration of bare land and grassland type in river fluctuating reservoir area were(0.25 ± 0.03) ng ? L-1 and(0.33 ± 0.02) ng ? L-1; tributaries were(0.46 ± 0.02) ng ? L-1 and(0.49 ± 0.03) ng ? L-1, water Me Hg concentration gradually increased from the surface to the bottom, and bottom water Me Hg were significantly lower than the grass in bare land. Mean dissolved mercury concentration of river water body were(6.88 ± 0.76) ng ? L-1, thre ratio of dissolved mercury and total mercury(DHg/THg) in river and tributary waters were 40% ± 13% and 36% ± 11%, It showed that THg in water body were mainly existing in the form of particulate mercury. DMe Hg mean concentration of bare land and grassland type in river fluctuating reservoir area were(0.13 ± 0.01) ng ? L-1 and(0.14 ± 0.02) ng ? L-1, ratio of total dissolved Me Hg and methylmercury(DMe Hg/TMe Hg) were 48% ± 11%, which was consistent with the overall situation of mercury, that particulate Me Hg dominated the distribution ratio of methylmercury. Mercury activity in water concentrations of mainstream and tributaries in storage time were 2.04 ± 0.28 ng ? L-1 and 2.13 ± 0.23 ng ? L-1, a high mercury content of the mercury activity existed in surface and bottom, while low in the middle layer. There were no different significant difference of mercury activity content in various elevation of covered fluctuating bare and grass lands(P> 0.05), a higher concentration of active mercury existed in surface and bottom layers.Plentiful plants in the water-level-fluctuation-zone(WLFZ) of Three Gorges Reservoir(TGR) grow vigorously during the non-flooded period, especially the herbaceous ones. Then, the WLFZ is submerged gradually from the end of September. Soil-plant system that under a long time flooded condition may change the form of mercury, thus resulting in a secondary pollution of the water environment in TGR. To understand the characteristics of mercury species in soils and water after submerged, four kinds of typical plants from TGR were conducted in the lab under submerged condition. Results indicated that the plants can promote the formation of soil methylmercury(Me Hg), and have a significant effect on the different forms of mercury concentrations of the overlying water during inundation. Cynodon dactylon as the dominant species in WLFZ,because of its higher content of total mercury(THg) and methylmercury, the effect on Me Hg and the other forms of mercury in the soil and the overlying water system was obvious.After 90 days, the soil Me Hg level was highest in Cynodon dactylon&soil&water treatment(B1)(1135.86±113.84 ng·kg-1).It was approximately 2 times less than that of the soil Me Hg in soil&water treatment(CK2).The variation character of total mercury(THg), reactive mercury(RHg), dissolved mercury(DHg), total methylmercury(TMe Hg) and dissolved methylmercury(DMe Hg) of overlying water were all showed a parabolic shap with a peak skewed to the left, which reached the peak at the 30 th day. Meanwhile, TMe Hg, THg and DHg in B1 treatment were the highest, which were 3.77±2.88 ng·L-1,0.29±2.42 ng·L-1 and 35.51±0.06ng·L-1 respectively, and TMe Hg and THg in the overlying water are mainly in the form of dissolved state. Therefore, it can be inferred that the water consumption of the Three Gorges reservoir will increase the mercury pollution load of the reservoir.We can estimating that mercury reserves of plant in WLFZ of Three Gorges Reservoir area was183 kg, The bare and grass land riparian soil approximately were 3310 kg and 3799 kg. After impounding the natural growth of vegetation fluctuating with water to release the amount of mercury is about 287 kg, while weeding by man with the release of mercury to the water about 184 kg, decrease of about 36%.Geoaccumylation index and the potential ecological risk factor for the reservoir area of bare land and grassland Fluctuating two types of soil mercury assessment, evaluation results show that the former River area Fluctuating Zone reach light pollution, The latter is displayed at the reservoir area of bare land and grassland River and its tributaries Fluctuating Zone there are moderate risk of contamination. Thus, vegetation Fluctuating Zone of the reservoir not only increased the pollution load of mercury, also increases the risk of contamination with mercury Fluctuating.