Ecological risk assessment of heavy metals in soil in Silong and Beiwan towns, Baiyin city, Gansu Province

This paper is the result of environmental geological survey engineering.

Objective The mining industry in the Baiyin region of Gansu Province is well−developed, leading to severe heavy metal soil contamination. Grasping the influence of heavy metal contamination of the soil and its potential risks to ecological health is significant in protecting the environment.

Methods Taking the surface soils in the four−dragon town−north bay town farmland area of Baiyin region of Gansu Province as the research object, four methods, namely the single−factor index, Nemerow comprehensive index (P_Z), geochemical accumulation index, and potential ecological risk index (RI), were used to analyze and evaluate the heavy metal (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) contamination in the region.

Results The single−factor index results showed that the soil pollution in the study area was mainly Cd and As, in which the levels of P_Cd were at firstclass cleanliness (89.21%), slight second−class pollution (8.63%), third−class light pollution (0.72%), fourth−class medium pollution (1.44%), and the levels of P_As was at first−class cleanliness (94.24%), slight second−class pollution (5.04%), fourth−class medium pollution (0.72%). P_Z ranged from 0.339 to 2.869, with an average of 0.603, indicating that 85.61% of the soil was clean, 7.91% was slightly polluted, 5.04% was lightly polluted, and 1.44% was moderately polluted. The geochemical accumulation index results showed that the heavy metal contamination of the soil in the study area was generally at the level of 0−2, mainly Cd, Hg, As, and Pb. In the potential ecological risk area, Cd and Hg were the main pollution factors. Cd had the highest potential ecological risk, with 10.07% belonging to medium potential risk, 5.04% to relatively high potential risk, and 1.44% to very high potential risk. The average value of the RI was 211.80, indicating that the soil in the area was generally at a medium danger level. The four evaluation methods were consistent overall, indicating that the study area was mainly contaminated by Cd, As, and Hg and should be strengthened for monitoring.

Conclusion Through the divisional study of the survey area with the Yellow River as the boundary, the results showed that the soil of the north bank of the irrigation and silt area (Area B) was the most polluted, and the pollution level was the highest at the junction of East Dagou River. The heavy metal analysis results of the water and sediment samples also confirmed that the pollution of East Dagou River was more obvious, further confirming that the north of the study area was mainly polluted by the East Dagou River basin of Baiyin mine, while the soil on the south bank of the Yellow River was relatively clean, but was still affected to a certain extent. In recent years, soil pollution has been alleviated to some extent with the measures of Yellow River irrigation and mining environmental governance. However, the heavy metals accumulated in the soil need long−term natural degradation or migration, and monitoring and governance should be strengthened for the high−risk areas.