Study On Contaminants Transport Along With Runoff From Sloping Plots Amended With Sewage Sludge On A Lateritic Red Soil

The loss of some contaminants in a sewage sludge,nitrogen,phosphorous,copper, nickel,chromium,cadmium,lead,mercury and arsenic,along with slope runoff under different slopes,methods and rates of sewage application was studied by anthropogenic rainfall on a hill of lateritic red soil located in the southern tropical region.The main results were as follows:1.The accumulative runoff amounts,the accumulative loss amounts of particulates and the total suspended solids varied greatly when the sewage sludge was applied by different methods.The accumulative runoff amounts increased in the order of hole-application(HA)＞control(CK)＞broadcast and mixing with surface soils(BM)＞broadcast with hay cover(BH).The accumulative loss amounts of particulates increased in the order of CK＞HA＞BM＞BH while those of the total suspended solid(TSS)increased in the order of BM＞HA＞CK＞BH.Both the accumulative runoff amounts and the accumulative loss amounts of particulates and total suspended solids decreased with the increasing rate of sewage sludge for BM treatments.The rate of sewage sludge did not affect apparently the accumulative runoff amounts,the accumulative loss amounts of particulates and the total suspended solids for the HA treatments.When the sewage sludge was broadcasted on the plots with different slopes,the accumulative runoff amounts increased in the order of 20°＞15°＞10°＞25°while both the accumulative runoff amounts and the accumulative loss amounts of particulates and the total suspended solids increased in the order of 25°＞20°＞15°＞10°.2.The concentrations and the loss amounts of MTC(total contaminants of mixing samples)and STC(total contaminants of settled sample)(C = contaminants,including N,P, Cu,Ni,Cr,Cd,Pb,Hg,As)in the slope runoffs reached the peak values 1 day or 18 days after the application for the BM treatment,which were 1.2～15.2 and 1.1～37.2 times of the corresponding peak values of the same contaminants for the HA and BH treatments, respectively.Both the concentrations and the loss amounts of the above contaminants resulted from the BM treatment diminished gradually with time.During the whole experimental period,the mean concentrations and the mean loss amounts of the contaminants in the slope runoff for BH were 6.7%～95.5%and 2.1%～36.0%of those for the BM treatment,respectively.The mean concentrations of the contaminants in the slope runoff for HA were 2.5%～77.8%of those for the BM treatment.The mean loss amounts of N,P,Cu,Ni and Cd by the slope runoff for HA treatment were 2.5%～89.8%of those for BM treatment whereas the mean loss amounts of Pb,Hg,As by the slope runoff for HA treatment were nearly identical or higher than those for the BM treatment.3.The mean concentrations of most of the contaminants in the slope runoff for the BM treatment increased with the increase of sludge application rate to different degree whereas it was not certainly the case for the mean loss amounts of the contaminants.The mean loss amounts of TPN(total particulate nitrogen),NH₄⁺-N,Cu,Ni for the BM with the rate of 60 t/hm² were 52.4%～89.6%and those of N(MTN,STN,TSN,TDN and NO₃^--N),P(MTP,STP,TPP and TSP),Cr,Cd,Pb,Hg and As for the BM with the rate of 60 t/hm² were 1.0～2.4 times of those for the BM with the rate of 120 t/hm².Sludge application rate had no significant effects on the mean concentrations and the loss amounts of the contaminants in the runoff for the HA treatment.4.The concentrations and the loss amounts of the contaminants in the slope runoff for the BM treatment increased with the slope degree during the early period of the experiment (1 day or 18 days after the sludge application)as a whole.As the diminution of the concentrations and the loss amounts with time,the differences between the different slopes were narrowed.The concentrations and the loss amounts of MTN,STN,MTP,STP,MTCu, STCu,MTNi,STNi,MTCr,MTCd,MTPb,MTHg,MTAs in the slope runoff for the BM treatment both reached their peak values during the early experiment period,showing the greatest risks to the surface waters.The concentrations and the loss amounts of most of the contaminants increased with the slope degrees with those of the MTC(C = N,P,Cu,Ni, Cr,Cd,Hg,As)for 25°treatment being 2.1～5.7 times of those for 10°treatment.The mean concentrations and the loss amounts of the slope runoff of MTN,STN,MTP,STP,MTCu, STCu,MTNi,STNi,MTCr,MTCd,MTHgå’ŒMTAs generally increased with the slope degree while those of MTPb,STCr,STCd,STPb,STHg,STAs did not show significant relationship with the slope.5.The loss of the contaminants along with the slope runoff on the lateritic red soil depended greatly on the ratio of sludge particulates to soil particulates,and the loss of the particulates.P,Cu,Ni,Cr,Cd,Pb,Hg and As were mainly transported with the particulates.Nitrogen was largely lost in dissolved species while large portion of NH₄⁺-N was lost with particulates.6.Nitrogen,phosphorous and nickel showed greater pollution risks to surface waters than the other contaminants.The pollution risks to the surface waters of all the treatments decreased in the order of BM(25°,120 t/hm²)＞BM(20°,120 t/hm²)>BM(15°,120 t/hm²)＞BM(10°,120 t/hm²)＞BM(15°,60 t/hm²)＞HA(15°,60 t/hm²)＞BH(15°)≈HA(15°,120 t/hm²)＞control(15°).To control slope degree of sludge application area,using BH or HA trament can reduce the risk of pollution on surface waters from sloping plots amended with sewage sludge to different extent.