Higher Risks of Copper Toxicity in Turbid Waters: Quantifying the Bioavailability of Particle-Bound Metals to Set Site-Specific Water Quality Criteria

Abstract

In coastal waters, particulate metals constitute a substantial fraction of the total metals; however, the prevalent water quality criteria are primarily based on dissolved metals, seemingly neglecting the contribution of particulate metals. Here we developed a method to quantify the toxicity risk of particulate metals, and proposed a way to calculate modifying factors (MFs) for setting site-specific criteria in turbid waters. Specifically, we used a side-by-side experimental design to study copper (Cu) bioaccumulation and toxicity in an estuarine clam, Potamocorbula laevis, under the exposure to “dissolved only” and “dissolved + particulate” 65Cu. A toxicokinetic-toxicodynamic model (TK-TD) was used to quantify the processes of Cu uptake, ingestion, assimilation, egestion, and elimination, and to relate mortality risk to tissue Cu. We find that particulate Cu contributes 40–67% of the Cu bioaccumulation when the suspended particulate matter (SPM) ranges from 12 to 229 mg L–1. The Cu-bearing SPM also increases the sensitivity of organisms to internalized Cu by decreasing the internal threshold concentration (C IT) from 141 to 76.8 μg g–1. MFs were derived based on the TK-TD model to consider the contribution of particulate Cu (in the studied SPM range) for increasing Cu bioaccumulation (MF = 1.3–2.2) and toxicity (MF = 2.3–3.9). Water quality criteria derived from dissolved metal exposure need to be lowered by dividing by an MF to provide adequate protection. Overall, the method we developed provides a scientifically sound framework to manage the risks of metals in turbid waters.

Turbid Waters and Clearer Standards: Refining Water Quality Criteria for Coastal Environments by Encompassing Metal Bioavailability from Suspended Particles

“…Modifying Factor. To account for the contribution of SPM-bound metals to metal bioaccumulation, a modifying factor (MF) was calculated using the equation adapted from Cao et al, 10 where a more detailed derivation and explanation of the equation is available.…”

“…10,61 SPM's contribution depends on its concentration (i.e., turbidity), metal-carrying capacity (K d ), and the bioavailability of the bound metals (AE), as described in eq 3. By integrating the physiological parameters k u and IR, 10 we can calculate SPM's relative contribution in metal bioaccumulation and hence MF. This calculation uses Monte Carlo simulations to factor in uncertainties associated with each parameter (Note S2).…”

“…10,20−22 SPM itself may also act as a physical stressor, increasing organisms' susceptibility to the toxicity of metals. 10,23 Ignoring metals associated with SPM would lead to underestimation of the risks of metals in turbid waters.…”

“…Toxicity is linked to exposure by bioaccumulation, 26,27 where contributions from both the dissolved metals and SPM can be separately quantified in a toxicokinetic model. 10,28 Within the toxicokinetic framework, assimilation efficiency (AE) is a critical parameter in quantifying metal bioavailability from particles. 29,30 The γ-radiating radioisotope tracer method 30 is preferred for determining AE; however, its use is limited by safety, licensing, waste disposal challenges, and the lack of suitable isotopes for certain environmentally important metals (e.g., Cu, Pb, and Ni).…”

“…8,9 Nonetheless, the omission of SPM-bound metals is not an indication of their insignificance. 10 In turbid coastal waters, a significant portion of waterborne metal binds to SPM, often in greater concentrations than in the dissolved phase. 7,11,12 SPM consists of resuspended sediments and biological particles.…”