Geochemical modelling of the surface water environment around active mineral operations

Abstract

In recent years, the input of trace metals into surface water systems has increased significantly, raising interest among scientists to better define water quality standards (WQS) at the catchment scale. Active mineral operations and other land use practices add to the naturally high metal concentrations of the surface waters in many parts of the world, and spatially and temporally impact the physico-chemical quality of water bodies. Regulatory authorities are well aware that it is important to consider chemical and toxicological principles, such as bioavailability, in the development of WQS; however, current metal WQS thresholds for surface waters are based on the dissolved or total element concentration alone. Furthermore, it is important to understand how the metal levels and bioavailable fractions change spatially and temporally, and how the natural and anthropogenic sources control the content and bioavailability of metals in the surface water of a catchment. This PhD research aimed to address this issue and improve the definition of water quality standards taking into account both water chemistry and metal speciation principles. The specific objectives of the project were to: identify natural and anthropogenic sources of metals in surface water systems, taking into account the natural environment (geology, mineralization, climate, etc) and past and present human activities; determine seasonal variations and their influence on the chemical speciation; and develop a methodology to characterise and quantify the spatial distribution of the metal species in the stream water and sedimentary environment. The Rapel River Basin in Central Chile was selected as field study region since it offers a diverse geology, hosts intense mineral exploitation and agriculture, and exhibits marked seasonal variations in the surface flow regime. One hundred surface water and sixty sediment samples were collected during the low (April-May, 2006) and high flow season (December, 2006–January, 2007; repeat water samples only). Statistical analysis methods were used to assess the statistical properties of the data and investigate the relationships between the parameters. Together with these, profile analysis and Piper diagrams were used to describe the water chemistry spatially and temporally along the rivers, and assess the water quality in relation to the chemistry of the sediment. Multivariate analyses, including cluster and principal factor analysis methods, were used to: study the complex associations among the water quality parameters and their relationship with the land use and geology; to determine the underlying geochemical processes; and quantify the relative contribution of natural and anthropogenic sources to the metal loads. Finally, chemical equilibrium and biotic ligand models (WHAM and PHREEQC, and BLM, respectively) were used to estimate quantitatively how the metals were partitioned in different species in the two hydrological regimes, and to identify the principal factors that control their bioavailability and toxicity. The approach developed in this research can be used to identify areas vulnerable to metal toxicity and suggest appropriate management strategies to protect water quality at the catchment scale. Areas with similar geochemical characteristics are distinguished, and effective water quality monitoring procedures can be designed given information on background geology and existing land use practices. In addition, the potential effect of metal releases to aquatic environment can be determined and the uncertainty estimated by the QA/QC can be integrated to the Cu risk assessment yielding realistic results and protective WQS