Environmental Geochemistry Of Irrigative Water And Soil Heavy Metals In Key Areas, Tianjin

Farmlands surrounding Tianjin city are chosen as research objectives in this paper. On the basis of field investigation and indoor measurement, environmental geochemistries of irrigated water and soil heavy metals in research areas are studied in detail. Irrigated water is mainly weak salt water from the shallow stratum and a little freshwater from the deep stratum under the Earth, at the same time, mixed with sewerage, belonging to the slight pollution, and the contaminations are fluoride and chloride in the northwestern areas. Shallow groundwater with high mineralization can not be used to irrigate and deep groundwater has been overused in the middle and southeastern areas. Agricultural soil had to be irrigated by using sewerage in this area, thus belonging to be of seriously polluted areas. The contaminations include chloride, salt, sulfide, fluoride and nirtrogen, leading to soil salinization and second pollution. Soil heavy metals in research areas are higher in content than the average of Tianjin City, especially Hg and Cd elements are obviously enriched relative to other elements. Many element contents gradually decrease from in the vegetated soil, to in the irrigated farmland, to in the irrigable land, then to in the dryland, finally to in the garden. Cr, Zn, Cd and Cu are high and similar in content in the irrigated farmland and vegetated soil, indicating the great effect of sewerage irrigation and fertilizer-pesticide on these soils. Hg content is higher in the vegetated soil than in other soils, related to the overuse of fertilizer-pesticide in vegetated soil. Pb is the highest in town soils, associated with releasing of gasoline. As and Ni contents change little in various soils. Heavy metals such as Cd, Hg, Cu, Pb and Zn are rich about 0—40cm deep under the soil surface, but rapidly decrease and finally close to the background values under 40cm deep soils. As, Ni, Cr and Mo are not significantly enriched in the soil surface. Cd content is 1.753mg/kg in the soil of research areas, and mainly occurs in ion-exchange and carbonate phases, thus is very easy to be absorbed by plants and greatly harmful to plants. Cu, Pb, Zn and As are low in the ion-exchange and water-dissolution phases, but their contents in humic acid and Fe-Mn oxides phases reach 50% relative to in other phases, thus are potentaly harmful. Cr content is the highest in the residues, perhaps related to soil nature. Hg is little harmful because it mainly occurs in the residue and organism phases which it is difficult for plants to absorb. Se is closely related to organic matter. Heavy metal pollution of soil is tightly related to heavy metal enrichment of vegetables in the research regions, It is the most obvious for Cd, second for Hg, Cu, Cr, Zn and Se. Pb contents are not correlated between vegetables and associated soils. Among cabbage, celery and naked oats, heavy metals are the best correlated between celery and its soil. Enrichment coefficients of Cd and Se are the highest, those of Zn and Cu are middle, and those of Hg, Cr, Pb and As are the lowest in the vegetables.