A geochemical analytical approach for the evaluation of heavy metal distribution in lagoon sediments

Background, Aim, and Scope Italian lagoon environments are of great importance due either to their frequency and distribution along the coasts or to their management. Agriculture, urban and industrial activities in lagoon catchments can be sources of heavy metal (HM) pollution by direct waste dumping, atmospheric deposition of fumes or, simply, as a consequence of a lack of natural water recharge. HM concentration in lagoon sediments is a tool of HM monitoring in the surrounding environment. Application of sequential extraction procedures (SEP) to sediments makes it possible to study the HM distribution among the main geochemical phases and to assess their potential mobilization as a consequence of environmental condition variations. In the present study, the three-step SEP (0.11 m HOAc; 0.1 m NH₂OH·HCl; 8.8 m H₂O₂ and then 1 m NH₄OAc), developed by the Measurement and Testing Programme of the European Commission (BCR), was combined with information on the chemical and mineralogical properties of the sediments in order to assess the level and risk of mobility of HM (Cu, Cr, Cd, Pb, Ni, Zn) in sediments from the Fusaro volcanic coastal lagoon of southern Italy. The effect of sediment air drying and physico-chemical properties on the metal distribution in various geochemical forms and fractions was considered. Materials and Methods Surface (5 cm depth) sediment samples were collected from twenty-one sites on the Fusaro lagoon. Moisture, pH, organic carbon, carbonates, particle-size distribution and HM total content were determined on the dried sediment samples. The mineralogy of the clay fraction was determined at room temperature on random and oriented samples. The different forms of Al, Si and Fe in the bulk sediment (< 2 mm) were analyzed after oxalate, dithionite-citrate and pyrophosphate extractions. The BCR-SEP was used to fractionate trace element chemical forms in nine selected sediment samples. Effects of sediment air-drying on HM fractionation were analyzed. Results Middle (Cu, Cr) and low (Cd, Pb, Zn) concentrations of heavy metals in sediment samples from the deeper central part of the lagoon were observed. Cadmium resulted in mainly being associated with carbonate forms (~60%), whereas Cr and Ni occurred primarily as residual mineral phases (>50%). Zn appeared uniformly distributed among the main geochemical forms. Considerable presence of Cr, Cu and Pb was found in the oxidizable fraction (~50%), indicating organic matter and/or sulphides major sinks for these metals. Differences of heavy metal distribution among sediment samples were observed, depending on the total metal content and on sediment properties. Modifications in metal speciation induced by sediment air drying mainly consist of a transfer of the actual and potentially mobile fractions towards immobile fractions (mineral residue) by oxidation and formation of Fe and Mn oxides. Discussion Both total content and sequential extraction results did not show an environmental critical situation: the only possible risk, associated with heavy element mobility, could occur in consequence of a drastic pH and redox equilibria variation at the water-sediment interface. Conclusions Despite all limitations, the BCR-SEP combined with information on the chemical and mineralogical properties of the sediment solid matrix provides a valuable approach to describe the risks related to the potentially toxic HM occurring in sediments (in this paper the case of the Fusaro lagoon is examined). HM distribution among the sequentially extracted geochemical forms is affected by sediment properties, such as pH (Zn), clay (Cd), Fe easily reducible oxides (Pb) and organic matter (Cu) content. Recommendations Political managers, usually facing limitations of time and resources, have to optimize the cognitive phases, selecting the most useful information to make 'effective and correct decisions' in the environmental field. Therefore, although conscious of the limits of the adopted quality assessment, we believe it represents a good compromise between time and resource availability (i.e. type and quality of information). Outlook The complementary use of the simple and rapid European fractionation scheme with information on the chemical and mineralogical properties of the sediments provides more awareness about the nature of the sediment components involved and offers a more reliable way for studying HM speciation in sediments.