Study On Selenium Transport In Unsaturated Zone Under The Influence Of Rice Irrigation In The Middle-lower Naoli River Plain

Selenium?Se?is an essential trace element to human body.The selenium content and its distribution in unsaturated zone directly affects absorption of crops for selenium,and then affects selenium intake of human body through food chain.Sanjiang Plain is the important commodity grain production area in China.A wide range of selenium-rich soil found in Fujin-Baoqing area of Sanjiang Plain provides an opportunity for the local development of high-quality and high-efficiency agriculture and the improvement of production structure.In recent years,the planting area of rice massively has increased in Sanjiang Plain.The long-term groundwater irrigation has changed the unsaturated zone environment,which also affects the selenium content and its distribution in unsaturated zone.Therefore,whether long-term leaching action for the unsaturated zone will reduce selenium content of the unsaturated zone surface and increase selenium content of deep unsaturated zone and groundwater or not,which is worthy of attention under the influence of rice irrigation.This paper took the typical rice planting area-Daxing farm in the Middle-Lower Naoli River Plain of Sanjiang Plain as study area.The selenium in soil,water and crops was the main research object.Combined with field investigation and sample collection,the hydrogeological conditions,different unsaturated zone structure and distribution characteristics of selenium content in the unsaturated zone in the study area were found out.The transport rule and leaching effect of selenium in the unsaturated zone under the influence of rice irrigation were explored by indoor static experiments and dynamic experiment.And with the long-term leaching action for selenium in the unsaturated zone,the change of selenium content and its influence to the groundwater quality were also analyzed.The purpose of this study was to help soil and groundwater quality evaluation of the study area and provide a scientific basis for the local agricultural production and the rational development and utilization of land resources.The main research results of this paper was as follows:1.With the long-term leaching action of irrigation water for selenium in the vertical unsaturated zone profile of paddy field,the peak of selenium content was not obvious,and the leaching effect was stronger.And the main forms of selenium were strong organic-bound state,humic acid binding state and residue state,which were seldom transported.And the sum of the three forms of selenium was more than 80%of the total selenium content.2.Rice crops could absorb selenium from the unsaturated zone and have the ability to accumulate selenium.The selenium loss caused by absorption of rice crops on selenium in the unsaturated zone was estimated.The results showed that the selenium content in unsaturated zone was reduced about 96.34 g/km².At the same time,the effective selenium content and the selenium content in rice crop organs had the good positive correlation.Increasing effective selenium content in root soil or promoting other selenium forms to transform into the effective selenium will promote the transportation of selenium from the unsaturated zone to the crops.3.The adsorption/desorption experimental results of the unsaturated zone medium on selenium showed that the adsorption ability of loam medium on Se???and Se???was greater than silty sand and fine sand,and Se???was more likely to be adsorbed.The desorption rate of Se???was less than the desorption rate of Se???and the mobility of Se???were weaker.The higher the contents of the physical and chemical properties such as Fe₂O₃,Al₂O₃,CEC,organic matter and so on were,the greater the adsorption effect of medium on selenium was.Moreover,when the pH value of the medium was alkaline,the adsorption on selenium was weaker.4.The indoor soil column experiment results showed that when there was no iron manganese content in initial leaching liquid,the leaching amount of selenium was relatively higher,and the transport ability of selenium was stronger.When the initial leaching liquid was changed to alkaline?pH=8?from acidic?pH=6?,and the selenium content in leachate showed the increasing tendency and the accumulation leaching amount of selenium and leaching rate also increased gradually,which indicated that the mobility of selenium was stronger under the alkaline condition,and the selenium was easily leached from the unsaturated zone.After leaching,the total selenium content in the unsaturated zone was reduced.The strong organic-bound state and residue state were the main selenium forms,while the proportion of water soluble state and ion exchangeable state selenium content in total selenium content decreased.5.Synthesizing the analysis result of field sampling investigation and indoor experiments,the effective selenium in the unsaturated zone was easily absorbed and enriched by the roots of rice crop.In the process of long-term rice irrigation,the selenium in vertical unsaturated profile was leached and the total selenium content decreased.At the same time,the selenium content into groundwater by long-term leaching action of irrigation water is much less than?-?type quality standard of groundwater(?0.01 mg·L^-1)?GB/T 14848-2017?and wolud not affect the groundwater quality.