Research On Pollution Risk Of Heavy Metals In Soil And Groundwater Under Reclaimed Water Irrigation

The objective of ths study is to ascertain pollution risk of heavy metals in soil andgroundwater of reclaimed water irrigated district in the surth-easten area of Beijing, China,based on district investigation, field irrigation experiment and in situ infiltration experiment.The spatical distribution and origin of heavy metals in soil and groundwater of the long-termreclaimed water (or wastewater) irrigation were determined. The characteristic of migrationand influencing factors of heavy metal in soil-groundwater system at the process of watertransportion and irrigation were found in this study. The groundwater risk zoning strategy wasproposed.The heavy metal monitor system of long-term reclaimed water irrigated district wasestablished. The spatical distribution, origin and affect factor of heavy metals in soil andgroundwater of such area were determined by the investigation. Compared with other sewageirrigation areas, both domestic and international, the mean contents of As, Cd, Cr, Cu, Hg, Pb,Zn, Se, and Mn in soils were still at a relatively low level in the study area, and the contents ofall of the examined heavy metals were below national standard limits (GB15618-1995). Thelonger time of wastewater irrigation led to the accelerated enrichments of Hg, Cu and Pb.Significant heavy metal enrichments occurred in the areas around reclaimed water outfalls inthe irrigation area and canals, in which the wasterwater could be more easily lifted than theinterior areas., The TFe2O3content, OM, CEC and particle components of the soils werefound to be the major influencing factors affecting the accumulation and absorption of heavymetals in the soils. Our comprehensive analysis, combining the assessment of various sourcesof heavy metals, enrichment levels and the toxicity levels of heavy metals, suggested that Hg,Cu and Pb are the pollutants that should be given priority in control measures implemented inthe sewage irrigation district. Spatial distribution characteristics and sources of heavy metalsin the shallow groundwater in the district were detected. The longer time of wastewaterirrigation led to the accelerated enrichments of Hg and As. Fe and Mn in soil were relativelyhigher correlated with that in the shallow groundwater (RFe=0.497、RMn=0.375). As and Cd inthe shallow groundwater relatively were higher correlated with that in the surface water(RAs=0.554、RCd=0.485).The detect method of heavy metal pollution was established, which contained thelong-term reclaimed water irrigation experiment and in situ mornitoring infiltrationexperiment. Compared with the the groundwater treatment, the significantly higher levels ofTotal metal (Cd, Pb, and Hg) were found at50~70cm,30~40cm, respectively (p<0.05), and the significantly higher concentrations of DTPA-extractable metals (Cd, Cu, Fe) at deeperdepths of50~90cm (p<0.05). Hg, Cd, Cu, and Pb are the pollutants that should be givenpriority in control measures implemented which from high risk to low with the long-termreclaimed water irrigation. The contents of pH, OM, clay, sand, and SAR were the mainlyfactors which affect the accumulation and transfer of heavy metals in soil profiles.The inputof extra metals and nutrients by the process of reclaimed water irrigation has led to thesignificantly difference of the correlation between metals and other elements the soilsirrigated with reclaimed water and groundwater. Except for Fe and Mn, the other heavymetals were at a low level in the deffrent shadow groundwater depths which were at distanceof5m,15m,35m longitudinal to the reclaim water river. The vadose zone and the shallowaquifer in this study area contain a large amount of clay and sand-clay interlayers, which hascertain anti-pollution properties.Four metals parameters were selected as representative contaminants for groundwaterpollution risk assessment under reclaimed water irrigation. Groundwater heavy metalpollution risk zoning for the groundwater of study area was proposed and the district wasdivided into relatively low-, intermediate-and high-risk areas. with the area of775km2(40%of total area),1,040km2(53.7%) and122km2(6.3%) respectively. The strategies andmeasures are proposed in order to prohibit groundwater pollution in developing reclaimedwater irrigation.