Research On Purification Mechanism Of Loessial Soil To Cr And As And Safety Buried Depth Of Protecting Ground Water From Pollution

Groundwater is important water resources in loess land. With increasing of human being's activities on farming and watering, it has great theoretical and practical significance on how to utilizing purification function of loessial soil to protect groundwater from polluting, which is the basis of building item of this paper. On the basis of these, the author studies the following problems and draws corresponding conclusions:(1) By demonstrating physichemical and structural character of loessial soil, the results are obtained that it is having great meaning to utilize widely distributive and profound loessial soil resources to purify sewage for protecting groundwater supplying security.(2) The contaminative elements are analyzed that is penetrated in soil by human being's activities on farming and watering, and Cr and As are decided as the primary researching objects which have great toxicity, quantities and easy migration. There is representative meaning to researching the action of Cr and As in loessial soil.(3) By experiments the paper reveals purification mechanism and ability and its influencing factors of soil to Cr and As. The results indicate:1) The main action to purifying Cr(III) in the loessial soil is that Cr(III) changes into Cr(OH)s. 2) The main absorption to Cr(VI) and As(V) is special absorption. The absorption form of Cr(VI) is HCrO4-. 3) Calculated by Langmuir equation the average absorption quantity of loessial soil layer to Cr(VI) is 162ug/g and to As(V) is 552ug/g. 4) Cr(VI) in loess purified is synthetical effects of adsorption and reduction. The purification mechanism to As( V) is not only absorption , and also includes other physical and chemical actions. 5) The main factors influencing loessial soil purification to Cr(VI) and As(V) are pH, organic matter, calcium, iron, aluminous compounds and contents of mucilaginous grain etc.(4) By the infiltration test of soil column, the author concludes that Cr and As has the following migration patterns by dynamic researches.1) The absorption and migration of Cr(VI) and As(V) in loessial soil relate to content of mucilaginous grain. The order of migration velocity is Cr(VI) > As(V). The purification ability of loessial soil to As( V) is strongger than Cr(VI). 2) The migration form of Cr(VI) in loessial soil is CrO42-, and that of As(V) is HAsO42-. 3) With a certain deepness when layer of soil reaches saturated absorption, the limit of saturated absorption will move to deeper place, with which Cr(VI) and As(V) will move to deeper place too, if infiltration quantity of sewage is continually increasing. 4) When concentration of Cr and As is less than 10.0mg/Lin sewage infiltration fluid, the purification depths of Cr(VI) is 5m, and that of As(V) is 1.5m in loessial soil.(5) The paper discusses interaction of Cr(VI) and As(V) with nutritional element: Ca, Mg, P and finds that Ca and Mg have decrease absorption of loessial soil to Cr(VI) and have increase to As(V).Between Cr and As have possibly promotion to absorption amounts in loessial soil.(6) Utilizing infiltration experiment of soil column, the results show that environmental capacity of Cr and As in loessial soil is 95.9kg/ha and 12.6kg/ha.(7) Exampled by Jiaokou irrigation district in Shaanxi, the author forecasts the influence of irrigation to Cr and As of groundwater by investigating and monitoring irrigation water resource, groundwater and soil. The paper concludes that the safety buried depths of preventing groundwater from being polluted by Cr and As in irrigation district are not less than 5.0m and 1.5m on condition that concentrations of Cr and As in irrigation water are less than 10.0mg/L.