The contribution of roadside soil to phosphorus loading in the eutrophic Lagos Lagoon, Nigeria Akeem Abayomi, ab Malcolm Nimmo, b Claire Williams, b Kehinde O. Olayinka, a Bola Osuntogun, a Babajide Alo a and Paul J. Worsfold b Received 31st March 2011, Accepted 11th May 2011 DOI: 10.1039/c1em10266d Roadside soils were sampled from the Lagos Lagoon catchment during the wet and dry seasons over the period 2005–2009. Lagoon sediment samples were also collected within the same period. All samples were digested with aqua regia to determine total phosphorus and extracted with 0.5 M sodium bicarbonate to determine the bioavailable fraction (Olsen-P). A segmented flow analyser method was used for analysis and good accuracy was demonstrated for two reference soils (SO-2 from CCMET and SRM 2711 from NIST). The Lagos Lagoon is a hypereutrophic water body (1270  1170 mgPL 1 ), with significant areas of anoxia and water hyacinth growth. The total phosphorus concentrations in roadside soils (16 sites; mean  2 S.D.) were 285  279 mg kg 1 in the wet season and 424  629 mg kg 1 in the dry season, indicating that rainwater leaching is a major source of phosphorus in the lagoon. The bioavailable fractions were 5.17  3.47 mg kg 1 (2.1  1.5% of the total) in the wet season and 13.0  8.7 mg kg 1 (4.3  4.5% of the total) in the dry season. 1. Introduction Increasing urbanisation and agricultural activities have led to widespread discharges of nutrients, particularly phosphorus, into water bodies, including coastal waters. 1 Excessive inputs can lead to eutrophication 2,3 with consequent increases in harmful algal blooms and turbidity and expanded regions of hypoxia. 4–6 A variety of natural and anthropogenic sources input phosphorus to surface waters through different pathways and with significant temporal variability. 7 Anthropogenic inputs to receiving waters include point sources, such as wastewater discharges and industrial effluents, as well as diffuse sources, including road and track runoff, septic tank discharges and farm runoff. 8–10 The rapid urbanisation of coastal environments and the associated infrastructure, e.g. roads, housing and other built structures, has increased the imperviousness of catchments. This has exacer- bated the mobilisation and export of phosphorus into coastal receiving waters. Aquatic ecosystems become impaired when as little as 10–15% of the catchment area is occupied by impervious surfaces. 7,11 Sources of phosphorus mobilised in urban runoff include atmospheric deposition, industrial debris, animal excreta, fertilisers, particulate emissions from vehicles (which eventually accumulate along road curbs and verges as roadside dust/soil), detergents and lubricants. 7,12 In Lagos, as in most Nigerian towns and cities, many roads are unmetalled and roadside soil is therefore a permanent feature and a significant vector for pollutant transport to catchments, particularly during the wet season. Phosphorus can be trans- ported in both the dissolved and particulate phases. There is usually an increase in particulate phosphorus transport with increasing erosion and runoff induced by greater rainfall a Department of Chemistry, University of Lagos, Lagos, Nigeria b Biogeochemistry and Environmental Analytical Chemistry Group, Biogeochemistry Research Centre, University of Plymouth, Plymouth, PL4 8AA, Devon, UK. E-mail: p.worsfold@plymouth.ac.uk Environmental impact A variety of natural and anthropogenic sources input phosphorus to surface waters through different pathways and with significant temporal variability. This can lead to eutrophication and stimulate the production of harmful algal blooms. In developing countries rapid urbanisation, including extensive networks of unmetalled roads, can exacerbate the mobilisation and export of phosphorus from roadside soils into receiving waters. This study shows that Lagos metropolis is a significant source of phosphorus loading in the hypereutrophic Lagos Lagoon, particularly from sampling sites in close proximity to transport hubs. Concentrations for both total phosphorus and bioavailable phosphorus were generally higher and less variable in the dry season than the wet season, commen- surate with leaching by intense periodic rain events. 1884 | J. Environ. Monit., 2011, 13, 1884–1889 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Journal of Environmental Monitoring Cite this: J. Environ. Monit., 2011, 13, 1884 www.rsc.org/jem PAPER Downloaded by University of Strathclyde on 27 August 2011 Published on 14 June 2011 on http://pubs.rsc.org | doi:10.1039/C1EM10266D View Online