Background
With the change of the national energy development layout, Qingyang has seen a situation where oil exploitation and agriculture go hand in hand, which may lead to local soil pollution if not taken seriously.
Objective
To evaluate the distribution characteristics, possible sources, and ecological risks of heavy metals in farmland soils around the main production areas of Changqing Oilfield.
Methods
A total of 60 farmland soil samples were collected from Zhengning County, Zhenyuan County, and Qingcheng County of Qingyang City, and the contents of heavy metals such lead (Pb), mercury (Hg), chromium (Cr), cadmium (Cd), and arsenic (As) in farmland soil were detected according to GB 15618-2018 Soil environmental quality—Risk control standard for soil contamination of agricultural land(on trial). The soil background value of Gansu Province was used as the denominator in the calculation of pollution index, and the pollution characteristics and ecological characteristics of selected five heavy metals in farmland soil were evaluated by single-factor pollution index (Pi), Nemerow comprehensive pollution index (PN), and potential ecological risk index.
Results
The levels of Pb, As, and Hg in farmland soils around Changqing Oilfield, the levels of Cr and Cd in Qingcheng County, and the level of Cd in Zhengning County were higher than the corresponding soil background values of Gansu Province, but lower than the national soil environmental quality standard. The single-factor pollution indexes (Pi) were: Hg, 2.14; Pb, 1.24; As, 1.13; Cr, 0.78; Cd, 0.67, which indicated that Hg were graded as moderate pollution, Pb and As were slight pollution, and Cd and Cr were not polluted. The Nemerow comprehensive pollution indexes (PN) were: Cr, 0.92; Cd, 1.08; As, 1.20; Pb, 1.68; Hg, 3.85, which indicated that Cr was graded as no pollution, Cd, Pb and As were mild pollution, and Hg was severe pollution. The variation coefficients of Hg and Cd in Zhenyuan County and that of Hg in Qingcheng County were 60.00%, 50.00%, and 50.00%, respectively, which were all greater than 50%, indicating that the pollution of above heavy metals in the location was subject to human activities. The potential ecological risk indexes (Er) of Pb, Cr, Cd, As, and Hg were 6.20, 1.55, 20.05, 11.28, and 81.64, respectively, indicating that Hg was graded as strong ecological risk, and the other four heavy metals were mild ecological risk. The comprehensive potential ecological risk index (RRI) was 124.48. Combined with the potential ecological risk index of Hg, the comprehensive potential ecological risk of the five heavy metals in local farmland soils was considered to be at a strong ecological risk level.
Conclusion
Although the average values of selected five heavy metals in farmland soils surrounding the main production areas of Changqing Oilfield are qualified with the national soil environmental quality standards, they exceed corresponding soil background values of Gansu Province, and there are signs of human influence and potential ecological risks of different degrees.
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