Study On Geochemical Environmental Effects Of Metallic Sulfide Deposit And Its Mining In Dabaoshan, Northern Guangdong (China)

The acid mine drainage(AMD), solid wastes and heavy metals produced incourse of mining activities are included among major environmental hazards. TheDabaoshan Mine is a Fe-Cu polymetallic sulfide deposits famous for its giant scale. Itis located in warm and humid area of South China. It is specifically typical inenvironmental effects of sulfide mine and mining activities. It is responsible for thepresent situation of the Shangbacun village which is situated in the lower reaches ofthe mine and famous for its nickname"cancer village"and"poverty village".Symptom of"Itai-Itai disease"also appears in the village.The present study carries systemic field investigations and sampling around themining area and lower reaches of the mine and the revolved Hengshishui river, andanalyzes the chemical contents of mine tailings, the waste water of mining and milling,water system, river sediments, soil and edible leaf vegetables. The species of heavymetals and its mechanisms of releasing, transportation and transform are alsodiscussed,with assessment of the degree of pollution and potential risk in differentenvironmental media, as well as the geochemical environmental effects of miningactivities. A ecological and geochemical transportation model of heavy metals isproposed at the end.It is concluded that:(1) The tailings are the main source of the AMD forming and heavy metalreleasing. They include a great number of sulfide minerals, and are rich in heavymetals. The concentration of heavy metals Pb, Zn, Cd, Cu, Ni, Cr, Mn is up to 637.40,5330.37, 17.76, 421.12, 61.15, 67.54, 1580.24 mg/kg respectively. The Fe-Mn oxides and the residuals are main species, and the other species (exchangeable, carbonates,organics) is respectively 40%, 20%, 40 % and 32% of the total.(2) The water system is characterized by low pH value and high heavy metalsand SO42- concentrations, and negative correlation between the heavy metals and theSO42- concentrations. The heavy metals and SO42- gradually decrease in concentrationwhile the pH value increases from the upper to the lower reaches, which indicates thedirect relationship of pollutions and mining activities. The tailing minerals are crushedand releasing high concentrations heavy metals in the experiment of weathering andeluviating. The high heavy metals concentrations and releasing speed in early timewas decreasing along with increasing experiment times, it is negative correlationbetween the increasing pH value and decreasing concentrations of heavy metals ineluviating solution.(3)The concentrations of Pb,Zn,Cd,Cu in river water is decreasing graduallyfrom upper to lower reaches, but they all exceed the irrigation water quality standard21, 27, 62, 18 times in Tielong reaches, and that is 10, 17, 33, 13 times in Caoduikengreaches, and 7, 11, 16, 4 times in Hengshishui reaches and 7.0, 5.5, 16.2, 4.0 times inShangbacunVillage.(4) The results of water assessment show that the single pollution indexes arevery high, and that of Cd is highest, the compositive pollution index (PI) is heavypollution in Tielong reaches. The pollution of Caoduikeng reaches and Hengshishuireaches are similar to the Tielong reaches, but degree of pollution is lower, the PIpresent middle-light pollution. The single pollution indexes and compositive pollutionindex (PI) inTaipinghe, Lengshuijing and Qiuwuba are all non-pollution.(5) The part of heavy metals of water precipitated and accumulated in sediments.the geoaccumulation index present different level pollution of Pb,Zn,Cd,Cu, and theupper reaches is higher than the lower reaches. The pollution level of Cd isstrong-very strong and that of Pb and Zn is strong and middle-strong respectively. Thesimilar pollution level is in irrigation trench in Shangbacun village.(6) The soils were polluted heavily in agricultural ecosystem. The concentrationof heavy metals exceeded the national soil secondary standard, but the variety inprofile was changed by new water source for irrigation in recent years. Thegeoaccumulation index present the pollution level of Cd is up to strong-very strong,that of Pb and Zn is strong and middle-strong. (7) In whole drainage area, single pollution indexes of topsoil are highabnormally, and that of Cd is highest, the compositive pollution index (PI) presentsmiddle pollution, and the Nemerow Index (PN) is heavy pollution. The singlepollution indexes of Cd decreased and that of Pb,Zn,Cu,Ni,Cr increased, the PIandPN present middle and heavy level pollution respectively when the assessment is baseof the heavy metals concentrations of Chengongwan. The characteristics of pollutionare similar to the water and sediments of river which indicated the direct relationshipof pollutions and mine elements obviously. But the pollution of Cd is relation withhigh background value especially.(8) The heavy metals exchangeables are the main species absorbed by plants.The exchangeables concentrations of all topsoils are high and increasing with the totalconcentrations. The exchangeables decrease downwards profiles and present negativecorrelation with the change of pH value in profiles. The residuals and Fe-Mn oxidesspecies are large amount of totals except Pb in Tielong dam and Shuilouxiacun andShangbacun village, but the biovalidity of exchangeables and carbonates and organicsare high.The mobilityof Pb and Cd are strong.(9) Obvious correlation of concentration did not present between rhizospheresoils and vegetables. Pb concentration of rhizosphere soils did not exceed the nationalsoil secondary standard but the Pb of vegetables exceeded the food hygiene limitstandard. Cd of rhizosphere soils all exceeded the national soil secondary standard;Cd of vegetables exceeded the food hygiene limit standard. Only a few of Zn, Cu, Niof rhizosphere soils and vegetables exceeded standard. Pb mobility of soils is strongand absorbed byplant easilyin local.(10) Potential risk index (RI) assessment shows that potential ecological risk isstrong in whole drainage area. The average RI of three Shangbacun topsoil approachto 600 which present strong risk, the others are strong risk which RI exceeded 600.The RI is middle risk in Chengongwan because high Cd pollution. The RI ofrhizosphere soils are strong risk in Huawucun, Liangqiaocun and Shuilouxiacunvillage, and that of Shangbacun village is middle risk, the risk level decreasesgraduallyfrom upper to lower reaches.(11) The four reaction interfaces of releasing and transportation from tailings toecological system control ecological environmental geochemical transportation ofheavy metals. The interface of tailings/water decide the releasing of heavy metals from main source of tailing; the interface of water/sediments decide the equilibrium ofprecipitation and releasing between water and sediments; the interface of soils/porewater decide the biovalidity of the soils; and the interface of soils/plant decide theabsorption and utilization of livings. The interface reaction can be prevented throughchanging the environmental conditions of interfaces, which help to afford referenceframe for environmental restore.(12) The interaction of different mediums pictured the transportation model ofexposition from deposits and releasing, transportation, transform and end-result insurface environment. The transportation of heavy metals from mining area formedheavy metals accumulation around mining area and lower reaches, produced moreecological press. The heavy metals transport to plants and animals through food chainand to human at last, harm life health and ecological safe.Inclued among the main initial points of the studyare that:(1) Select the Dabaoshan Mine which has caused severe hazard, and isspecifically typical as studied subject, to reveal the geochemical environmental effectsof mining activities, and thus provide a model to study metal sulfide mine andenvironmental restore in warm and humid areas.(2) Analyze the ecosystem of Dabaoshan Mine, revealing the direct relationshipof pollutions and mine elements, the pollution of Cd is relation with high backgroundvalue especially.(3) Reveal the interaction of different mediums, and pictured that the fourreaction interfaces control ecological environmental geochemical transportation ofheavy metals releasing and transportation from tailings to ecological system. Theinterface of tailings/water decide the releasing of heavy metals from main source oftailing; the interface of water/sediments decide the equilibrium of precipitation andreleasing between water and sediments; the interface of soils/pore water decide thebiovalidity of the soils; and the interface of soils/plant decide the absorption andutilization of livings. The interface reaction can be prevented through changing theenvironmental conditions of interfaces, which help to afford reference frame forenvironmental restore.