Simple equations for the calculation of free metal ion activities in natural surface waters

STOWA, the Dutch foundation for applied research and water management, introduced the concept of ecological key factors to determine the ecological status of a surface water system. One of these key factors is toxicity. A tool was developed that translates concentrations of multiple chemicals into a single value for toxic pressure. This tool needs free metal ion activities as an input variable, instead of total dissolved concentrations. This report presents easy-to-use equations in order to predict free metal ion activities in surface waters. Data from Dutch monitoring programmes were used to derive these equations. The chemical speciation model WHAM7 (Windermere Humic Aqueous Model) was used to calculate free metal ion activities from total dissolved concentrations and common water characteristics, such as DOC, pH, and major cations and anions. Statistical multivariate regression was performed to derive equations that relate the free metal ion activity to commonly measured water characteristics. For 17 metals (Ag, Ba, Be, Cd, Co, Cr, Cu, Hg, La, Mn, Ni, Pb, Sn, Sr, U, V, Zn) relationships were derived using the thermodynamic constants provided by WHAM7 databases. The predictive value of the equations is large (r2>0,9 in all cases) and agrees well with the outcome of numerical speciation modelling performed with WHAM7. Therefore, the presented equations can be used for the reliable prediction of free ion concentrations, in order to predict toxic pressure in surface waters.