Can the unit world model concept be applied to hazard assessment of both organic chemicals and metal ions?

Abstract

A unit world model that has the potential to be used for the hazard assessment of both metal ions and organic chemicals is described and discussed, with an emphasis on the problems that arise when treating metal ions. It is based on the steady-state equilibrium criterion model that is designed to simulate the fate of organic chemicals in a 100,000-km(2) region and comprises four well-mixed compartments: Air, water, soil, and sediment. To be applicable to metal ions, modifications are required. The single soil and sediment layers should be replaced by two layers to accommodate aerobic and anaerobic conditions. The more complex and variable partitioning of metals resulting from dependence on pH, redox conditions, ionic oxidation state, and presence of sulfide also must be addressed, but preferably in a separate geochemical model, because these factors can result in nonlinearity. For metals, a dynamic as well as a steady-state model is desirable. It is shown that the resulting model can be applied to both organics and metals. Rather than seeking to apply the hazard criterion of persistence to metal ions, the model can be used to deduce a critical loading that results in a defined toxic end point, thus integrating the hazard criteria of persistence, toxicity, and possibly, bioaccumulation. This approach is applied illustratively to naphthalene as a typical organic substance and to four environmentally relevant metal ions. Results are discussed and recommendations made for further development. Specifically, the absence of metal degradation can result in large, steady-state quantities in soils and sediments corresponding to residence times of many centuries. Consequently, the dynamic calculations are more relevant for fate assessments of metals over a period of years, and more focus on the aquatic environment is justified.

Critical load analysis in hazard assessment of metals using a Unit World Model

“…Finding a scientifically sound and tractable approach to evaluate ecotoxicological impacts of metals released to the environment requires a modification in use of the criteria of persistence (P), bioaccumulation (B), and toxicity (T) as applied to assess the hazard of organic chemicals in several jurisdictions (e.g., Adams and Chapman [1]). Harvey et al [2] recommended calculating a critical load (CL) for metal hazard or risk analysis, because CL is a more informative criterion than persistence, which is infinite for metals. A CL is the load that can be added to a system that could cause or prevent a defined effect in a receptor, in this case, an aquatic organism.…”

“…Harvey et al [2] first developed the UWM that is applicable to both metals and organic substances and that would allow for comparison of the hazards posed by both classes of substances. However, their model used a fixed value of metal particle-tototal dissolved partition coefficient (K d ) from the literature that reduces the transparency of the assessment and potentially introduces biases when using a UWM to compare metal CLs and CLs of metals with organics.…”

Factors affecting metal exchange between sediment and water in an estuarine reservoir: A spatial and seasonal observation

“…However, despite their obvious macro-and micro-scale influence on metal partitioning, the spatial and temporal shifts of metals in an estuarine reservoir are often complex (not simply predictable), requiring further in-depth investigation with consideration of environmental implications. [12][13][14] As a result, this paper employs a statistical approach for describing the exchange of five metallic elements (Fe, Mn, Zn, Ni, and Cu), with special reference to spatial and seasonal conditions. For an extensive evaluation, a large suite of data from 2005 to 2006 was collected from the surface sediment and overlying water at 48 sites densely distributed along the Yeongsan Reservoir (YSR), Korea, which had undergone widespread deterioration in water quality over the past two decades due to continuous growth of anthropogenic sources, bed elevation, seawater intrusion, and stratification.…”