Study On Spatio-temporal Variability Characteristics And Quality Safety Assessment Of Heavy Metals In Soil And Rice

In order to clarify the characteristics of spatio-temporal variations of heavy metals in soil and rice under farmland scale and to deepen the research on soil improvement and early warning of rice pollution,the paper takes 80 scaled-cultivation districts which located in the central section area of Jiangsu Province as the research objects.With the classical statistics,geostatistics and GIS methods,the sampling plots of the soils cultivation layers(0-20 cm)and the corresponding rice plants in the study area were collected in two consecutive years from 2015 to 2016.We determined the total arsenic,total cadmium,total chromium,total copper,total lead,total zinc and other heavy metals in soils and rice organs(stems,leaves,ears of rice,glume shells and rice),took the same expriment in 2014 as a reference,and applied the single factor pollution index method and the Nemero comprehensive pollution index method to study the spatio-temporal variation characteristics and quality safety evaluation of multiple heavy metals in soils and rice at the farmland scale.The main results are as follows:(1)In 2015,the average contents of total arsenic,total cadmium,total chromium,total copper,total lead and total zinc in soil were 18.7,1.07,37.8,4.76,11.4 and 44.6 mg/kg.Compared with 2015,total arsenic decreased by 39.6%,total cadmium,total chromium,total copper,total lead and total zinc were increased by 150.1%,5.50%,9.20%,17.5%and 4.70%respectively..According to the secondary standard(pH>7.5)of?Soil Environmental Quality Standard?(GB 15618-1995),the single factor pollution index of six heavy metals in two years are:total cadmium>total arsenic>total zinc>total chromium>total copper>total lead.Total arsenic in 2015 and 2016 was 31.3%and 2.50%exceed the standard.Total cadmium was 100%.The other four heavy metal elements were all up to the standard.Nemero's comprehensive pollution index for the two years in the study area was 1.12 and 1.30,respectively.In 2015,all of the 80 samples were lightly polluted.In 2016,5.00%were lightly polluted,18.75%were moderately polluted and 76.25%were heavily polluted.(2)According to the semi-variance model of soil heavy metal,the C0/(C0+C)of heavy metals of soils in 2015 and 2016 were total chromium>total zinc>total cadmium>total lead>total copper>total arsenic and total arsenic>total copper>total chromium>total lead>total zinc>total cadmium respectively.Total arsenic in 2015 and total cadmium in 2016 were strongly spatially correlated,while other elements were moderately spatially dependent.The minimum range is 177.9m,indicating that the measured data of each soil sampling point in this study is valid at least within a radius of 177.9m(9.9hm2).Based on the spatial distribution of heavy metal maps by Ordinary Kriging,the total arsenic concentration tends to concentrate in the northwest from the southwest corner and the northeast corner.The content of total cadmium in the study area gradually increases.The total chromium has a shift from southeast to northwest The total copper has a high value point of the original proliferation of a central and northern part of the region;the total lead is more stable,the high value region is always in the southwest corner;total zinc from the original number of potential fields,concentrated in the northwestern region,the content of total zinc was less and less in south central.(3)Bioconcentration factor indicates the absorption and accumulation characteristics of different heavy metal elements in different organs of rice.In all organs of rice,copper is the most abundant and cadmium is the least.For the accumulation of the same heavy metal in different organs,it was positively correlated with the metabolism of various organs of rice,the most in stems,the least in grain.the migration coefficient of rice significantly decreased compared with bioconcentration factor,The migration coefficients of descending order are total copper>total zinc>total arsenic>total chromium>total lead>total cadmium in 2015,which is as follows:total arsenic 7.07%,total cadmium 2.56%,total chromium 4.77%,total copper 27.6%4.50%,and 13.8%of total zinc,but different in 2016:total copper>total zinc>total arsenic>total chromium>total cadmium>total lead.This is related to the underground part of the rice,that the root system has strong absorption and fixation on the heavy metals in soil.(4)The contents of heavy metals in rice were the lowest and the highest in stems.In 2016,total cadmium in the ear of rice and grain doubled comparing with that in 2015,while the contents of heavy metals in other organs decreased.In two years,six kinds of heavy metal elements in different organs of rice were moderate variability,and were greater than soil heavy metals.According to the spatial distribution maps of heavy metals in various organs,there are more heavy metals in the northwest corner and less in the northwest.According to the?Principles and Methods of Chemical Risk Assessment in Food?writed the Food and Agriculture Organization of the United Nations,rice has different levels of pollution in 2015,light,moderate and severe pollution accounting for 8.75%,30.0%and 61.3%respectively;In 2016,the improvement of grain,light,moderate and severe pollution accounted for 17.5%,23.8%and 57.5% respectively.