Abstract
Total organic carbon (TOC), black carbon (BC), total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) were determined in 73 surface (0-2 cm) and subsurface (5-20 cm) soil samples taken from a 142 km2 area in Central London, UK. Soils were assessed to provide a baseline chemistry for site owners, developers, occupiers and regulators involved in understanding the potential risk to human health and the environment. TOC range was 1.75-11.85 % (mean 5.82 %), BC 3.72-32.71 mg.g-1 (mean 13.8 mg.g-1), TPH 72-4673 mg.g-1 (mean 443 mg.g-1), Σ16PAH 1.64-421.23 mg.g-1 (mean 47.99 mg.g-1) and Σ7PCB 2.56-148.72 µg.kg-1 (mean 20.82 µg.kg-1). Surface soils were less polluted than sub-surface soils due to a decline in industry, power generation, coal burning and traffic. PAH and PCB showed a stronger affinity for BC than TOC and were higher than many other international cities. South east London (Greenwich, Woolwich, Deptford) had the highest PAH pollution. Source PAH ratios confirmed a combustion/urban road run-off origin with minor petroleum inputs. Random Forest spatial modelling (machine learning) revealed large scale pollution trends across London soils. Normal background concentrations (NBC) were calculated and compared to risk-based human health generic assessment criteria (GAC). Benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, and dibenzo[a]anthracene exceeded the Land Quality Management GACs for three land uses (residential, allotments and public open space near residential housing). The NBC determined for ∑7PCBs (110 µg.kg-1) and dioxin-like PCB 118 (59 µg.kg-1) exceeded the residential and allotment soil guideline values.
Highlights
The metropolitan area of London has a population of some 9 M people and is one of the world’s preeminent cities forming the cultural, economic and governmental hub of the UK
This study aims to fill these knowledge gaps: (1) provide a baseline geochemical survey of Total organic carbon (TOC), black carbon (BC), total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) across Central London; (2) compare the concentrations found at two depth intervals and with other major cities; (3) discuss the concentrations with respect to published human health generic assessment criteria (GAC) so this dataset can provide a baseline for long-term environmental monitoring and help to quantify the impact of future pollution changes and/or events; (4) identify, where possible ratios of PAHs and PCBs to describe potential sources of pollution and characterise the likely contribution of emissions; (5) Explore machine learning Random Forest modelling approach to mapping organic pollution trends across Central London
A similar near surface, rise in TOC ascribed to greater amounts of fine roots and decomposed plant remains has been previously observed in upper horizons of urban soils and sediments from Staten Island, New York, USA and vegetated mud islands on the River Thames, London, UK [3, 8, 11]
Summary
The metropolitan area of London has a population of some 9 M people and is one of the world’s preeminent cities forming the cultural, economic and governmental hub of the UK. Petroleum hydrocarbons in urban soils originate from a variety of sources including road runoff, crude and refined oils, refined petroleum as well as dusts and atmospheric particles from factories, incinerators and hydrocarbon fuelled power stations [6,7,8]. These inputs generate a continuum of simple to complex (monomers-molecular aggregates) that may include aliphatic, monoaromatic, polyaromatic hydrocarbons as well as heteroatom containing resins and higher molecular weight asphaltenes [9].
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