EPRI Models Inform a More Site-Specific Approach to Water Quality Protection

By Chris Warren

Two new EPRI models aim to enhance the effectiveness of water quality protection at power plants.

Wastewater produced by steam electric power plants can contain pollutants such as mercury, arsenic, and selenium. The U.S. Environmental Protection Agency (EPA) and state regulators both play a role in developing rules that limit discharge of these pollutants into bodies of water.

“To protect aquatic life and human health, the EPA develops water quality criteria, which define the maximum amount of a pollutant that can be present in water,” said Jeff Thomas, a water quality expert at EPRI. “Then states take those criteria and turn them into standards that can be applied statewide. For example, a statewide standard could say that no body of water can have more than 12 parts per trillion of mercury.”

States use these standards to grant power plant permits that define allowable levels of pollutant discharge into bodies of water. Utility compliance personnel typically sample and test wastewater discharges every quarter (or sometimes more frequently) to ensure permit limits are not exceeded. Permits generally are up for renewal every five years.

From Conductivity to the Multi-Ion Toxicity Model

The health of aquatic life like mayflies and fish can be negatively affected when the concentration of total dissolved solids in fresh water is high, particularly when salts separate into ions. The cooling of power plant turbines involves a recirculation process that concentrates salt ions through water evaporation. In addition, flue gas desulfurization systems that remove sulfur dioxide from combustion gases discharge wastewater with salt byproducts.

Recognizing the link between high ion concentrations and adverse impacts on aquatic life, regulators historically have considered establishing water quality criteria that include limits on conductivity levels. “The more salt and ions like potassium, calcium, magnesium, and chloride you have in the water, the higher the conductivity level is going to be,” said Thomas. “There is a correlation between conductivity levels and the health of aquatic organisms.”

However, the latest science indicates that the prevalence of seven specific ions—including calcium, sodium, and potassium—is more predictive of the potential harm to aquatic life than conductivity levels. This means that two water samples with the same conductivity levels could have very different toxicity levels, depending on the combination of ions present.

Drawing on this science, EPRI and the environmental services company HDR, Inc. have developed the multi-ion toxicity (MIT) model, a tool that can predict ion toxicity to aquatic life more accurately than traditional conductivity measures. In addition to measuring the conductivity of their quarterly water samples, utility compliance personnel can analyze their ion composition and feed that information into the MIT model, which predicts the impacts on various aquatic species. The model can enable regulators to develop site-specific water permit limits based on the unique chemistry of wastewater discharges and the water bodies that the discharges enter.

“Site-specific criteria allow regulators to move away from a uniform, statewide measure of conductivity,” said Thomas. “This process creates discharge permit limits that are more tailored to local conditions rather than applying a one-size-fits-all approach.”

The model’s beta version currently is available for testing by utilities that funded its development, and EPRI will incorporate their feedback. EPRI is also collaborating with the EPA to strengthen the model. If the EPA accepts it as a tool for utility use, it could improve the accuracy of water quality permits, enhance environmental protection, and allow utilities to direct limited resources to other important environmental issues.

“The potential benefits of this new approach for utilities, customers, and the environment are enormous. If utilities can’t meet a conductivity limit, they may have to install wastewater treatment technologies or transport wastewater for discharge in a different body of water—both of which could cost hundreds of thousands or millions of dollars,” said Thomas. “A site-specific permit limit based on ion composition rather than conductivity levels could result in additional protection of aquatic organisms. In other cases, it could loosen permit limits that don’t provide any additional environmental protection, allowing companies to focus their resources on genuine environmental risks.”

The Biotic Ligand Episodic Exposure Model

EPA water quality criteria—and associated state permits for allowable power plant discharges of contaminants—water are based on tests in which aquatic organisms are continuously exposed to a particular contaminant for a defined period, such as 24 hours or 96 hours. But in many cases, power plants do not discharge contaminants into bodies of water for extended periods of time. For example, power plants discharge water accumulated during rainstorms, which can often contain zinc and copper.

EPRI partnered with the consulting company Windward Environmental and University of Florida researchers to better understand the risks of episodic copper and zinc exposure to the fathead minnow and a certain type of water flea. They consistently found that shorter exposure times led to lower mortality rates. “The results of this research suggest that water quality permits based on an assumption of long-term, continuous exposure to pollutants will overestimate the toxicity to aquatic life,” said Thomas.

The findings prompted the question, could water quality standards and permits be made more effective by accounting for the intermittent nature of discharges? To answer this question, EPRI developed the biotic ligand episodic exposure model, which is expected to be available this year for testing by utilities.

To assess exposure of aquatic organisms to zinc and copper, users input into the model information on a discharge’s zinc, copper, and dissolved ion concentrations as well as its organic content, pH , and temperature. Another key input is the amount of time the organisms are exposed to each of the measured parameters. The output of the model is the likelihood of toxicity of a given discharge to various aquatic organisms. In the short term, this model offers value in characterizing the potential toxicity of stormwater and other intermittent discharges. In the long term, if the model is adopted by the EPA and state regulators, it could be used to determine site-specific permit limits.

“This model can enable utilities to conduct site-specific analyses that take into account the actual exposure of aquatic life to zinc and copper,” said Thomas. “It can inform permit limits based on a more accurate understanding of the impacts of pollutants while minimizing the use of assumptions about how long aquatic life is exposed.”

Artwork by Edge Design

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