A baseline geochemical survey of Gibraltar to aid socio-economic development : a method using a Geographical Information System

Abstract

The urban environment is affected by a wide variety of anthropogenic activities which tend to increase metal concentrations in soils. Heavy metal concentrations in urban soils are influenced by a combination of factors including geology, historical and current industrial land use, transportation routes and urban regeneration. These factors must be taken into account in future planning development. Gibraltar is situated at the southern tip of Spain, and the extreme density of the population exerts considerable pressure on land use. Therefore future planning is of the utmost importance as the population of about 33,000 is concentrated on an area of 6.4 km2. A geochemical survey of Gibraltar has been carried out to establish current baseline data for a wide range of elements in surface soils and dusts to identify areas with metal concentrations which exceed soil guideline values and world means. Soil samples (0-15cm) have been taken from as wide a range of exposed sites as possible, from both bare and vegetated soils, in order to provide optimum geographical distribution. The main aims were to investigate geochemical associations and the spatial distribution of heavy metals within the urban environment and the nature reserve so as to make recommendations to the Government concerning sustainable development. House dusts and street dust samples were also taken, to allow the investigation of any possible migration of metals between different environmental media, in order to assess any potential risks to environmentally sensitive receptors including the residential population. The “total” elemental concentrations of 26 elements were determined by ashing in a nitric perchloric acid mixture followed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) analysis. A chemical sequential extraction technique was used to assess partitioning of elements within the soil phase, thereby giving an indication into any potential bio availability. This was also applied to dusts samples. Although it is understood that the phases applicable to soils and sediments are unlikely to be found as such in dust samples, a series of sequential extractions can give valuable information concerning the extractability, and thus mobility, of metals in the dusts. Geochemical associations in soils and dusts have been identified using factor analysis. Geostatistical models and geographic information systems (GIS) have been used to assess the spatial distributions of elements in relation to geographical factors, such as the underlying geology, urban topography, current and former land use. The GIS database allows spatial operations as well as statistical analysis to be made on the dataset whilst delivering the results in an “easy to use" graphical user interface. Areas with metal concentrations exceeding soils guideline values were identified in locations close to former and/or current industrial sites. The concentration of metals in soils and dusts in Gibraltar is heterogeneous and site specific, therefore a portion of the variability found between different sites is not spatially related. This enhanced the likelihood of errors when predicting concentration surfaces using GIS. Interpolation techniques are generally not suited to areas with distinctly varying topologies such as those found in Gibraltar. Various methods for calculating current baseline concentrations for Gibraltar (defined as 95% of the expected range of background concentrations) have been investigated. Their use as an aid in the identification of areas with increased metal concentrations enhances the use of risk assessments and allows future policy regulations to be implemented. The implications of this research are discussed in terms of its effects on land use management and the future socioeconomic development of Gibraltar within the EU.