Characteristic And Ecological Environment Effects Of Heavy Metals Pollution In Typical Wetland Ecosystem Of Sanjiang Plain,Northeast China

Sanjiang Plain is the largest region of swamps in China,and the wetlands in the Sanjiang Plain have high ecological significance regionally,nationally,and globally.Heavy metals reach natural water via a range of environmental process including hydrological processes,atmosphere deposition,physical erosion,and agricultural runoff,related to the excessive use of fertilizer and pesticides,coal burning,and mineral extraction.Degradation of wetland habitat due to heavy metal pollution is an important international issue.Thus,the pollution levels and spatial distributions of heavy metals in the aquatic environment of a typical wetland in Sanjiang Plain were investigated and evaluated,by establishing the multi-medium monitoring network.Bioaccumulation of heavy metals in the wild fish was investigated,and the human health risk related to wild fish consumption was evaluated.The inventory of heavy metals in snow cover was estimated,under the certain snow density and thickness,and the atmospheric deposition flux of heavy metals during the sampling period was revealed.Though establishing the exposure risk assessment model,the heavy metal exposure risk to migratory birds at different developmental stages was evaluated,via water,sediment,and wild fish consumption,and the protection scheme for migratory birds was given.The detailed results as followed:（1）The decreasing trends of average concentration of heavy metals were Zn（275.77±69.24）>Cr（80.26±11.40）>Ni（38.31±4.14）>Cu（23.4±5.73）>Pb（20.35±9.18）>Cd（0.16±0.05）in sediments（mg/kg）and Zn（117.44±51.98）>Cr（6.96±4.98）>Pb（3.83±1.63）>Ni（3.41±2.13）>Cu（2.76±2.72）>Cd（0.17±0.09）in water（μg/L）.A large coefficient of variation occurred across the water samples compared to those measured in sediment samples,due to dilution of water,inputs from the upstream or atmospheric deposition.Meanwhile,during the dry season,stable hydraulic conditions and relatively higher p H were beneficial to heavy metal deposition.Thus,there exhibit an opposite spatial distribution of heavy metals in the water body and surface sediments.（2）Concentrations of Cr were significantly elevated compared to the threshold of Aquatic Life Water Permissible Limits（AWPL）and Sediment Quality Guidelines（SGQ）proposed by the Canadian Council of Ministers of the Environment（p<0.01）,thus may pose negative impacts to the aquatic ecological system.Values of comprehensive water pollution index（WPI）indicated‘Clean’status for more than 85%of sampling sites with Zn accounting for 58%of the value of the WPI.Most of the sediment sampling locations were deemed to be‘Clean’or‘Low’status as defined by geo-accumulation index(I_geo)and improved Nemerow pollution index（P_N）with Zn and Cd（mainly derived from agricultural sources）identified as the dominant indicators of pollution.Applying the potential ecological risk index（RI）indicated a‘moderate ecological risk’level in sediment with an average value of RI was 80.6,which was significantly higher than that of water.（3）The decreasing trend of the average concentration of heavy metals in snow cover was Zn（103.46±39.16）>Pb（13.08±4.99）>Cr（11.97±2.82）>Ni（9.55±4.96）>Cu（6.19±1.79）>Cd（0.55±0.25）,μg/L,with the concentrations of Cu,Ni,and Cr were lower than the standard of Chinese surface water quality I,and the Cr and Zn content were relatively higher but lower than the standard of Chinese surface water quality II,while the content of Pb was much elevated than the threshold of quality II.Except for Zn,all the heavy metal contents in snow cover from the Qixing River wetland were much higher than those of the Tibet Plateau area but lower than those of northeast China.Values of WQI indicated‘clean’status for more than 61%of sampling sites.The inventory of heavy metals in snow cover following the decreasing trend of Zn（2313.57±1194.67）>Pb（275.35±111.91）>Cr（266.56±109.02）>Ni（216.69±139.24）>Cu（134.41±52.68）>Cd（13.91±10.45）,μg/m².The heavy metal inventory was not only effect by pollution emission,but also the snow thickness and density.The higher metal inventory areas were occurred in the buffer zone,for the higher vegetation coverage leads to a deeper snow thickness.（4）Except for the concentrations of Ni in Perccottus glenii samples were below the detection limit,all the heavy metals can be detected in the wild fish species.And‘moderate accumulation’of Zn was found in Cobitis granoe,while the four other heavy metals in wild fish species were in‘low accumulation’status.The single risk factor（CF）which was taking the Aquatic Life Water Permissible Limits（AWPL）and Sediments Quality Guideline（SQG）as the reference value indicated a‘high risk’threat to the aquatic organisms as affected by Cr alone,and Cr contributed most to the comprehensive risk index（PLI）.The target hazard quotients（THQ）and hazard indexes（HI）of these five heavy metals were lower than 1,indicated that there was no non-carcinogenic risk via consumption of wild fish;while the carcinogenic risk metrics（CR）of Cd exceeded the cancer risk threshold(10^-5)indicative of elevated cancer incidence in local consumers.（5）Risk of the five heavy metals decreased in the following order:Cr>Zn>Cu>Ni>Cd,which was different from the results of the pollution assessment for sediment.Despite the majority of heavy metals（except for Zn）being assigned‘Clean’or‘Low’pollution status,the elevated hazard quotient（HQ>3）associated with Cr enhanced overall values of integrated hazard index（HI）,especially for nestlings,which were in‘high risk’status（HI>3.13）,indicating that current pollution control measures are insufficient for the protection of migratory birds.Among the heavy metal exposure pathway,the drinking water exposure pathway can be ignored,while the sediment and wild fish consumption contributed 21.44%-93.2%and 6.77%-78.53%to the exposure risk.The protection scheme for migratory birds can be further improved by determining the threshold of safe feeding water depth and guaranteeing sufficient food supply in the water and maintaining or restoring the biodiversity.The exposure doses of heavy metals via snow ingestion were 3–4 orders of magnitude lower than the tolerable daily intake doses,indicating that the exposure risk to overwintering birds can be ignored.