The Soil Geochemical And Ecological Response Research Of Chengdu Plain

Chengdu Plain, the core area of Sichuan Basin, is one of areas with dense rivers and thelargest plain in southwestern China, also named Chuanxi Plain or Penxi Plain. It is the mostdeveloped area in economy and society of Sichuan Province. The strata exposing here almostall formed in Quaternary, belonging to piedmont accumulated basin fluvial facies. The materialsources of sedimentary clastic rocks are transported by eight water systems including Pei River,Mianyuan River, Shiting River, Yazi River, Jinma River, West Rive-South River, PujiangRiver and Min River(from Xinjin to Leshan). The differences of exposing rock/stratum units inthe original area decide the chemical components of clastic rocks in the sedimentary area.Therefore, the sedimentary sources of different water system basins have their owncharacteristics. With the deepening of denudation and weathering in the original area, thechemical components of each Quaternary stratigraphic units in the sedimentary area changedgradually from the old to the new. For the stratigraphic units in the same era at the same siteand same valley, the regolith was influenced by geological processes, biological effects andhuman activities, resulting in the differences of clastic materials’ chemical componentsbetween the regolith (the second environment) surface environment and the deep layerenvironment in a certain degree. Based on the basic geological environment conditions above,this thesis made futher study on Quaternary geochemical features of Chengdu Plain and itscauses according to the geochemical component analysis results acquired from field surveyand laboratory testing. With the application of soil geochemical testing results, it madeQuaternary source area division and terrace division, and studied the geochemical response inthe Quaternary agricultural area. Overall, this thesis received main achievements as follows:(1) By collecting and sorting the available geological data of Chengdu Plain, it redefinedthe Quaternary stratum units of Chengdu Plain, and divided into facies belts of differentcauses. (2) Taking typical sections for instance, it made profound discussion on the geochemicalfeatures, geochemical abundance coefficients, marker elements, weathering indexes and theirsignificances of Quaternary soil layer second environment of Chengdu Plain.(3) Taking typical sections as example, it contrasted the geochemical features betweenthe deep environment (the first environment) and the surface environment (the secondenvironment), and discussed the relative abundance, enriching components, losing components,and their causes.(4) Taking typical valley for instance, it contrasted the soil layer’s geochemistry betweenthe high terrace and the low terrace, and made preliminary analysis of correlation ofgeochemical features between original rocks in the weathering area and soil layer in thesedimentary area.(5) Valley as unit, based on general analysis of Chengdu Plain Quaternary soil layers’geochemical features, it analyzed the geochemical correlation between original rocks andQuaternary strata in Longmeng Mountains uplift, taking deep layer environmental backgroundvalue (reference value) and topsoil environmental background value (baseline value) asstandards. The conclusions were as follows: the Quaternary geochemical features were mainlycontrolled by original rocks, while influence of weathering and leaching on soil geochemicalfeatures was mainly correlated with chemical components’ properties and soil layer’sweathering degree.(6) According to the achievements above, it made transverse and vertical Quaternarygeochemical features and anomalies contrast of Chengdu Plain, and found that thegeochemical features’ vertical differences of stratigrafic units in different Quaternary ages ofChengdu Plain were mainly caused by the continuous uplift of Longmen Mountains, theincrease of denudation depth of the original area, and the change of exposing original rocks,while the transverse differences were mainly decided by the difference of water system andsediments’ carrying distance.(7) It studied the soil’s geochemical response to the geology in Chengdu Plain andeco-geochemical response of the agricultural area. The soil texture of Chengdu Plain was fine,and the soil environment quality was relatively high in general. Geochemical componentstesting indicated the main agricultural products had fine overall quality. The soil environmentreached the producing environment quality demand of green or pollution-free agriculturalproducts. But it also found some samples had their testing contents of Cd, Hg, Pb over thepollution-free food limit standard. These samples generally distributed in the polluted soil(Class V and Class Ⅵ) area in soil environment quality comprehensive assessment. In conclusions, this study found out basic Quaternary geochemical features of ChengduPlain, discussed the geochemical relationship between sediments and original rocks in thedenudation area, and explored the geochemical response to geology and geochemical responseof agricultural area, which provided scientific basis for the scientific planning and utilization ofland resources, producing environment protection of characteristic agricultural products,enlarging cultivation development and continuous development plan of economy and society.From this thesis, it also revealed that Quaternary geochemical study of Chengdu Plain shouldnot only focus on three important elements of material source, water system and Quaternarybasement structural shape, but also pay attention to the comprehensive influence of weathering,leaching, elements’ mobility and activities of human and organism. Attributed to absence ofsystemic geochemical component data of original rocks in the original area, some Quaternarygeochemical features of Chengdu Plain are also difficult to explain its cause reasonably, whichneeds further study in the future.