Preliminary Research On Spatial Distribution And Formation Of High As Groundwater In Northern Suburb Groundwater Source Field, Zhengzhou

Northern Suburb Source Field, ZhengZhou is a large concentrated source field bymainly exploiting shallow groundwater, in which arsenic (As), iron and manganesecomposite pollutions in groundwater are the typical water quality problem. This studyhad investigated spatial distribution and mechanism of arsenic mobilization in themicro-confined aquifer by basic theories and methods of application hydrologicalgeology, hydrology geochemical, based on intensive data analysis, field works and labexperiments. This paper was finished by participating in the “typical urban commontechnology research of drinking water safety”. The main achievements having beenreached were shown as follows.(1) The distribution of high As groundwater have been revealed. Distribution ofhigh As groundwater in West Beach and East Beach presented the regional andeast-west banded features, respectively, which of As(Ⅲ) was generally consistent withthe total As. The contents of As in shallow aquifer water (30m to50m) were higherthan those of micro-confined water (250m to350m), and those of deep-intermediateconfined water were not more than the WHO provisional guideline value10μg/L.(2) The key factors influencing enrichment of As in groundwater wereevaluated. According to the cluster analysis and correlation analysis between As andother chemical components, a significant positive correlation was found between Asand Fe, PO43-, DOC, respectively, while a significant negative correlation wasobserved between As and SO42-, DO in understudy water samples, respectively. As(Ⅲ)and Eh showed a negative exponential relationship, with the lower pH in theenvironment, the contents of As(Ⅲ) were Increased in the form of negativeexponential.(3) The sources of As in groundwater were analyzed. The Yellow River waterhad lower levels of As (mean2.75μg/L), which was not the main source of As in thegroundwater. The As combined with pond sediment, surface soil and Yellow River siltwas not easily dissolved out in oxidizing conditions. There were significantdifferences in the contents of As of different strata depths and types of sediment. The contents of As of fine sand in the high arsenic area (10.37mg/kg) were higher thanthose in the low ones (9.53mg/kg), which indicated that the deposits of sand layerwas the direct source of arsenic in the groundwater of the study area.(4) The formation mechanism of high As groundwater was preliminarydiscussed. The external environment of arsenic-rich groundwater was constituted byalternating stagnant hydrological conditions, arid and semi-arid climatic conditions,and the sediments of being rich in high iron/manganese. The internal environment wasmade of reduction environment, iron/manganese oxide reduction, the ions competitionadsorption of phosphate and the mutual transformation of different arsenic forms. Theresults of sediment chemical analysis showed that the content of Fe2O3(12.62%) inthe low-arsenic well B17was much higher than those of the exceeded wells B1(6.38%) and A19(6.17%), which indicated that the strong adsorption of clay inhibitedthe migration of arsenic in the vertical direction of stratum.