Risk assessment of soil heavy metals associated with land use variations in the riparian zones of a typical urban river gradient

Urbanization induced land use changes in riparian area alter soil and water regimes in complex ways, which may also affect the migration and transformation of soil heavy metals and increase the risk of release. In this study, soil samples from the riparian zone of Beiyun River, which located in the rapidly urbanized Beijing metropolis, were collected and analyzed for heavy metals (As, Cd, Cr, Cu, Mn, Ni, Pb, and Zn). Then their zoning distribution pattern along river (section 1 to section 4 from upper to low reaches) and the correlation of heavy metals between riparian soils and riverine sediments were investigated. Results showed that the average soil heavy metal concentrations of Cd, Cr, Cu and Zn in riparian zone were approximately 2.2, 1.7, 1.9 and 2.0 times higher than the background values. Sectionally, the concentrations of Cd, Ni, Pb and Zn displayed a decreasing order with section 2 > section 3 > section 4 > section 1, while the highest values of Cr and Cu were found in section 3. The concentrations of all heavy metals except Cr in artificial garden land were higher than those in other land use types, and the concentrations of Cr among five land use types were in the order of grass land > farmland > artificial garden land > forest land > forest-grass land. Generally, most of the heavy metals in the riverine sediments had higher contents than those in riparian zones, especially Cu and Zn. There was a decreasing order for the average geo-accumulation index (I-geo) of measured heavy metals in the soils of riparian zone: Zn (0.15) > Cr (0.08) > Cu (0.07) > Cd (-0.08) > As (-0.57) > Pb (-0.67) > Mn (-0.75) > Ni (-0.86), whereas they had different high-low orders in different land use types. The I-geo index indicated most regions of riparian zone were moderately polluted with Cd, Cr, Cu and Zn, especially in grass land and forest land. Also, Cd, Cr and Zn in riparian zone have positive relationships with the concentrations in riverine sediments. Health risk assessment showed that the contribution of ingestion HQ to HI was the highest among the three exposure pathways (ingestion, inhalation and dermal contact), and children had higher non-carcinogenic risk and carcinogenic risk index than adult. Our findings suggest that land use and soil in riparian zone should be protected and managed scientifically to control the riverine pollution and ensure human health.