As water professionals, we share a common goal: to preserve water quality. We work hard to understand hazards and assess risks using pollution guidelines, in order to prevent harm to humans and the environment. This raises an important question: how are these guidelines derived, and are they truly protective?
As Young Water Professionals, we learn to monitor environmental contaminants in a variety of effluents (predominantly at wastewater outflows). We measure the mass of pollutants from point sources like wastewater treatment plants and sample streams for nutrient levels downstream of agricultural fields to ensure compliance. We consider any exceedance of these guidelines a hazard. Do we question often enough how these guidelines were developed in the first place? This blog post will explore some of these foundational questions about how we establish chemical guidelines, and my research on the adequacy of existing guidelines for major ions in freshwater ecosystems.
The Limitations of Current Pollution Guidelines
Developing pollution guidelines is a question of ecotoxicology—the study of the source, transport, and fate of contaminants in the natural environment. To create these guidelines, we build risk profiles for contaminants. Ecotoxicologists expose a species to a range of contaminant concentrations in a lab or in the field, and identify a threshold at which there is minimal or no adverse effect. This is known as a No-Observable Effect Concentration (NOEC) or a Lowest-Observable Effect Concentration (LOEC).
In these tests, species are exposed to a contaminant for a set duration, and at a fixed concentration, and the impact is measured as the proportion of individuals experiencing a sublethal, chronic, and or acute response to exposure: these include impacts like behaviour, changes in reproduction, or death.
We use this information to inform toxicity thresholds that are in line with the precautionary principle. That being, concentrations should remain below a level that is known to cause toxic impact, where environmental impacts are possible. When under a perceived ‘conservative’ level, pollutants are assumed to not pose a threat of serious or irreversible damage to humans and nature.
The decision to test this way is pragmatic. We have nearly 84,000 substances listed on the U.S. Environmental Protection Agency (USEPA) Toxcast as of 2016; in Canada, nearly 28,000 substances on the Domestic substances list and ~50,000 on the Non-domestic substances list.For this extent of contaminants, we simply do not have the resources, nor scientific capacity, to continually monitor the dynamics of all contaminants in the environment. It is a task to test thousands of chemicals against hundreds of species with thousands of endpoints, let alone the interactive effects of chemicals. Consequently, environmental risk assessment still revolves around evaluating chemical X of interest, onto species Y, examining endpoint Z in a continuous exposure standardized assay: the approach is unyieldly and lags behind the rate at which new chemicals are emerging
This standard assumes that toxicity is a product of both the concentration of a contaminant and the duration of exposure—a first-order toxicokinetic model. In this model, toxicity is driven solely by the total cumulative exposure or assimilation rate of a contaminant. While this may be true for some pollutants, it is not always the case. The continuous exposure model does not account for the impact when exposures may vary over time. We need to remind ourselves of the fundamental ecology in which we operate. We know organisms in nature are rarely exposed to one constant concentration, as they are in laboratory tests. On a given day, humans are exposed to a range of temperatures moving between their homes and workplaces, similarly as rivers flow and lakes circulate, causing chemical concentrations to vary. There are numerous environmental, physical, geological, and even economic and social factors which influence what concentration of a contaminant is where and when. For instance, farmers may balance fertilizing and pesticide application by timing to ensure their crop is the most economically viable; or when a snowplow goes through a city, there are social and political decisions which frame which neighborhoods are plowed and salted prior to others.
While we can likely continue to appeal to the standard practice, I encourage us to think more about the dynamics of exposure, their ecological consequences, and in particular the cellular to community-level implications for dynamics and ask whether they are important to consider, beyond just the cumulative dose of the contaminant.
When we use mean daily or weekly contaminant loading estimates, it is suspect and not proven whether we are ‘adequately’, ‘conservatively’, or ‘precautionarily’ protecting ecological life. In line with the precautionary principle, it should be our responsibility to prove that greater toxic impacts do not occur when concentrations fluctuate. Particularly when we claim dilution events as ameliorating toxic impacts, or short-term exceedances as not significant to the overall toxic load.
Major Ions and Dynamic Exposure
Major ions are a group of contaminants of global concern in freshwater ecosystems, sparking increased interest in desalination and ecological monitoring projects. Recent evidence suggests that established guidelines may be too lenient for major ions, such as chloride—a key component of road de-icers in temperate climates. Species are still exhibiting adverse reactions in locations where guidelines are not exceeded. It may be that guidelines were derived from continuous exposure tests that don’t reflect the dynamic loading of major ions that aquatic species experience, and do not consider the cellular mechanisms responsible for regulating dynamics.
At Urban Water Toronto Metropolitan University, in Toronto, Canada, my PhD looks at the dynamic impacts of major ions on aquatic communities. In continental temperate climates, road salting is a seasonal practice, and its concentrations are variable across space and time. Some roads we salt, some we don’t. Some private assets like parking lots receive more salt than other properties, as concerns over liability drive application rates, whereas a homeowner might not salt their driveway, and just waddle down a slippery driveway to their car. When the snow melts in the spring, watercourses and waterbodies surge in volume, and can cause a short-term spike in salt concentrations. While chloride can remain elevated even in the non-salting season, during the summer, chronic levels can be temporarily diluted after heavy rainfall.
My research with zooplankton—small, sensitive primary consumers in lake environments—shows that these dynamic exposures can increase toxic responses, even when they remain below existing guidelines. We have been evaluating two scenarios based off of high-frequency field data from the Toronto and Region Area of Concern: one of 43 designated zones in the Great Lakes with adverse environmental pollution. From field data we have reconstructed these two exposure scenarios – spring acute guideline exceedance and chronic guideline intermittent dilution – in the laboratory environment. In the former, a spring melt scenario, we simulate a short-term spike in concentrations. In the latter, a summer dilution scenario where chronically elevated levels are temporarily lowered following heavy wet weather.
Research Findings and Future Implications
Our results suggest that in a spring exposure scenario, simulating a short-term period of elevated ion concentrations, organisms experience more acute impacts. We evaluated three different nearshore zooplankton species, in a variety of different durations, frequencies, and intensities of pulsing. Compared to the conservative approach of continuous exposure, and toxicity determined as a net exposure, we are seeing organisms exhibit greater lethal responses with smaller total ion loads. This directly violates the assumption of a first-order model (where toxicity is proportionate to toxic load), as shorter net exposures with variable concentrations lead to worse outcomes.
In the second scenario, where storms intermittently diluted chronically elevated ion concentrations, we found that this dilution did not actually mitigate toxicity. Instead, periodic dilution causes greater chronic impairment, meaning lower reproductive output and adverse impacts on population structure: by different dilutions, we are seeing different resulting population structures and relative abundance of different life-stages of zooplankton that have direct ecological consequences. With less adults and a higher number of youth, the utilized niche among the species is also different. Juvenile zooplankton occupy a higher position in the lake, compared to adults which have larger daily migrations between the surface and the sediment.
While major ions are just one group of contaminants, it ultimately brings into question the environmental relevance of existing pollution guidelines. As it is for major ions, do the dynamics of exposure for different contaminants change the toxic response?
A Call to Action for Water Professionals
As practitioners, we should be putting more focus on considering the dynamic ecologies of the systems in which we operate, and ultimately thinking about whether dynamics pose additional, ameliorative, or interactive impacts on the environment. Many current guidelines that rely on constant contaminant exposure may not be protective, and future work should be considering this paradigm and embrace a more dynamic approach to water quality management and assessment.
Two main levers are needed, including:
- Prioritize Dynamic Monitoring: We need to move beyond single-point sampling and mean-daily estimates. High-frequency, real-time monitoring is important to capture the true variability of contaminants in our waterways. This data is critical for understanding actual exposure of contaminants, and thus prerequisite to evaluating risk in ecologically-meaningful ways.
- Clarify Risk Assessment: Our current risk assessment models are incomplete and simplified, and no standardized methods exist for time-variable exposure. More collaboration to develop standards is needed, drawing on the diverse collective expertise of water scientists, engineers, ecotoxicologists, and government agencies to develop new standards that reflect real-world conditions.
The science is clear: the dose isn’t the sole driver of environmental impact—the dynamics of exposure matter and our risk assessment practices need to catch up with a more holistic understanding of the environment. In an increasingly complex world of human systems and contaminants, we need to work to strike a balance between our practical need for simplified, actionable guidelines with the scientific reality of dynamic exposures being more relevant but complex to conduct and inform guidelines.
Written by: Bryant Serre
Bryant is a PhD candidate in Environmental and agricultural toxicology, and well-versed in a diversity-informed perspective in assessment and innovation. Bryant has collaborated on research with farmers, engineers, agricultural industry, multi-jurisdictional partners, Indigenous Communities and Band Councils, and chemical regulators. His technical expertise includes advancing methodologies of risk assessment, risk assessment in complex natural and regulatory environments, environmental consulting, and evaluating chemical mixtures impacts using industry-standard and novel toxicology methods. He is a steering committee member on SETAC’s Salinization Committee, and a member of the Remedial Action Plan for Toronto Harbour.
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