FEATURE
Rethinking Water Quality Standards for Metals Toxicity A better understanding of the how natural waters inhibit metal uptake may change current regulatory limits. REBECCA
I
n the mid-1980s, EPA first set its standards for metals discharges to water for industry and municipal water treatment plants. At that time, there was limited understanding of chemical speciation in natural waters and the mechanisms of metal toxicity. Those regulations were based on toxicity test results in which aquatic organisms were exposed to metals in the laboratory. The concentrations used in these laboratory tests were reported as total metals concentrations, and this approach was adopted in the standards. However, metal toxicity in natural waters differs from water toxicity in the laboratory because of variation in water quality characteristics, such as temperature, pH, hardness, alkalinity, suspended solids, or dissolved organic carbon. In the mid-1980s, scientists knew that water quality affected bioavailability, but they did not understand how. "People knew that chemistry affected metal bioavailability, but the chemists said it was complicated, so the safe approach was to assume that it was all bioavailable," according to Dominic Di Toro, an aquatic systems modeler at HydroQual, an engineering consultant company in Mahwah, N.J. Advances in aquatic chemistry and toxicology since then have shown that these effects can be profound, according to Herbert Allen, an aquatic chemist at the University of Delaware. For example, he says, "There is a 20-fold difference in copper toxicity over the pH range that commonly exists in the natural environment." According to Allen, understanding of metals toxicity has now progressed to the point where the bioavailability of metals can be assessed on a sitespecific basis. He is among a group of researchers who, with EPA's active support, are working on models to predict metals bioavailability. The group's goal is adding such modeling to water quality regulations. Initial efforts have focused on copper, but researchers believe that a similar approach will work for other metals, including cadmium, zinc, nickel, and lead. The agency is following development of the copper model with interest and is funding work on other metals. 4 6 6 A • VOL. 31, NO. 10, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
RENNER Such efforts mark an attempt to advance the science underlying water quality regulations, but because metals are often less bioavailable in natural waters, there are concerns that revised standards would ease discharge limits and could have a negative effect on ecosystems. The Clean Water Act requires EPA to set water quality criteria—limits in fresh and marine waters—to protect diverse aquatic organisms. These criteria are based on laboratory toxicity tests relating mortality to metal concentrations for a range of sensitive organisms. In most cases, states use these criteria to set discharge permits. Using physical inform a t i o n such as river flow a n d a m b i e n t metal concentrations, the states calculate, on a site-bysite basis, how much metal can be discharged without exceeding the criteria.
Impact of site-specific water variables The national criteria are usually used without consideration of site-specific water quality. Regulations since 1983 include a mechanism to account for the site-specific effects of water quality on metals bioavailability, but it is not used in most cases. This procedure, called the water effects ratio (WER), involves determining the ratio of metal toxicity in sideby-side tests using site and laboratory water. Applying the WER can result in a fivefold increase in discharge limits. However, because the extensive toxicity testing used to determine the criteria must be duplicated, it is too expensive and time consuming for widespread use, according to Jim Pendergast, acting director of EPAs Office of Water permits division. "To work out the WER, you need to collect considerable data. This is expensive, so only the large players do it." EPA acknowledged in 1984 that because of bioavailability effects, water quality criteria, in terms of total metals concentrations, were likely to be too conservative for many natural waters (i). In 1993, the agency issued interim guidance, allowing dissolved metal concentrations to be used to set and measure compliance with water quality standards (2). By 0013-936X/97/0931-466A$14.00/0 © 1997 American Chemical Society
excluding any metals adsorbed onto large particles, the use of dissolved metals takes one of the bioavailability limits into account. However, the guidance still fails to account for other important effects, and EPA acknowledges that there are still many problems with the standards, according to Pendergast. In the past decade, scientists have made major advances in understanding metals bioavailability in water by identifying the metal's toxic form and how it is toxic to different forms of aquatic life. These advances were reviewed at a recent Society of Environmental Toxicology and Chemistry (SETAC) workshop (3). One of the most important pieces of the bioavailability puzzle came from research on algae in marine systems that identified free metal ions— for example, Cu+2 or Ni+2—as the toxic actors (4). Scientists are still trying to understand how free ions are toxic to organisms. They appear to be toxic because they can bind to key biological receptors and impair important biological processes. For example, copper toxicity to fish is related to binding free Cu2+ at the gill surface. This toxicity has been attributed to interference with two important physiological functions of the gill: transfer of gases and maintaining ionic balance at the gill membrane (5). From this mechanistic understanding comes insight into one way water chemistry affects toxicity. Other dissolved ions—for example, Ca, Fe, or Mg—can reduce metals toxicity because they, too, can bind to biological receptors and block the metal ions.
Active research on metals speciation Metals toxicity is also affected by other aspects of water chemistry, such as pH, which influences metals speciation (Figure 1). In fresh water, dissolved organic carbon has been shown to significantly reduce bioavailability by forming metal complexes, soluble compounds that remove dissolved metals from solution, and by adsorption to particulate matter. However, many aspects of metals speciation are still active research topics, particularly the role of dissolved organic carbon, according to Peter Campbell, a chemist studying the interaction between aquatic toxicity and metal speciation at the University of Quebec, Canada. "Natural organics not only complex with the metals, but they also tend to adsorb onto living surfaces so that they also have a protective effect against metals toxicity." Two approaches are being used by researchers to get a better assessment of metals toxicity in aquatic systems: direct measurement of free ions and comparison of these measurements with laboratory data, and modeling to predict metals speciation and its effect on organisms. EPA is interested in both approaches, according to Office of Water scientist Charles Delos, because it is funding some of the mod-
FIGURE 1
Natural limitations to metals bioavailability Water quality affects bioavailability by limiting the amount of the free metal ion, which is the most toxic species, that can reach important biological receptors. Other ions compete with metal ions to reach these sites; a fraction of the metal is bound to organic and inorganic compounds, and some is adsorbed onto sediment particles. (Adapted from Reference 3.)
eling work. At the research level, direct measurements using electrochemical methods have achieved good results at low concentrations for copper, according to Campbell, because of the development of a sensitive electrode. For other metals, however, the prospect of direct measurements is far off. One of the most advanced modeling efforts using this science to account for metals bioavailability in aquatic systems is coordinated by environmental engineers at HydroQual and includes scientists from the University of Delaware, die University of Wyoming, and Manhattan College in New York. The group is developing a predictive model, with funding from the International Copper Association (ICA), an industry organization. Many of the same investigators are also working for EPA's Office of Water on a similar model for other metals. An essential part of this effort is modeling, which links metals speciation and toxicity. This is a "chemical competition" issue, according to Allen. In addition to the equilibrium metal speciation, competing chemical processes include the formation of organic and inorganic complexes that decrease toxicity. Free metal ions also compete with other cations, such as Ca+2, to bind to biological surfaces (Figure 2). VOL. 31, NO. 10, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 4 6 7 A
FIGURE 2
Effect of pH on copper toxicity One characteristic of water that affects bioavailability and hence metal toxicity is pH. The amount of total copper required to kill 50% of fathead minnows increases with pH, indicating that a smaller fraction of copper is bioavailable as pH increases. For free copper ion, which is more toxic than total copper, the toxicity level is less dependent on pH.
Source: Erickson, R. J. et al. Environ. Toxicol. Chem. 1996, 15,181-93.
Challenges in predicting bioavailability Because dissolved organic carbon has an important effect in fresh waters and because this phenomenon is still the topic of active research, predicting this effect is a significant hurdle for modeling. For the HydroQual effort, this task is accomplished by a modified version of the Windermere Humic Aqueous Model (WHAM) developed by Edward Tipping at the Institute of Freshwater Ecology in Cumbria, United Kingdom, and acknowledged to be the best model for predicting the effects of dissolved organic material (6). WHAM predicts the affinity of metals for organic matter in different aquatic conditions. The model accounts for published laboratory results fairly well, according to Tipping, who has just started comparing model results with natural waters. Validation of the model may take up to three years, he estimates. The modeling group expects to have a prototype by the end of this year, according to Allen. At that point EPA may take a more active role, said Delos. "We will be able to predict effects for one species to show how the model works," Allen said. "Then we need more organisms and validation." ICA funds aquatic toxicity, partitioning, and sediment-fate studies, obtaining information to fully develop this model and hoping that it might be incorporated as an addition to water quality regulations. However, some researchers familar with current 4 6 8 A • VOL. 31, NO. 10, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
water quality regulations fear that widespread use of modeling would increase the overall amount of metals discharged into the aquatic environment, causing long-term ecological harm. "The best way to manage total loads is to limit the amount of metals that are discharged and nothing else," according to Ken Dickson, an aquatic biologist at the University of North Texas. In the case of copper, input to the coastal ocean has increased by 30% in the past eight years, according to James Moffett, a chemist at Woods Hole Oceanographic Institute. Specific ecosystem effects can be predicted at copper levels corresponding to current criteria, Moffett argues, and researchers have observed effects on species composition and physiology that are consistent with these predictions. Copper also concerned scientists at the SETAC workshop, but they agreed that the copper criterion is overly protective in most cases where it is applied. The workshop proceedings describe sewage treatment plants and state and federal pollution control agencies that are expending resources to operate a complex permitting system despite the fact that such discharges almost never cause a toxicity problem. To improve the metals-permitting situation, the workshop recommended several changes in regulatory practice that could be adopted quickly. EPA is considering the short-term recommendations made by the SETAC workshop, according to Pendergast. The agency may allow states to use toxicity tests, such as whole effluent testing, instead of the current regime that relies solely on numerical limits. However, according to Pendergast, "We must be careful because not all toxicity tests are the same. We need to make sure we're talking about the right species, and we need to assess how well the tests replicate what happens in the real world." To increase the use of sitespecific information in making decisions about the need to regulate metal discharges from a particular site, the agency is looking closely at using screening methods to gather only as much information as needed. "To do this, we need to decide how much data are necessary to make a good decision," Pendergast said. Many states would welcome this flexibility, and some would prefer it to the rigors of a sophisticated model. "You simply cannot turn every permit into a science project," said one northeastern state regulator. "We need to get over endless arguments of what the final perfect number is and focus more on the total effect on the environment."
References (1) U.S. Environmental Protection Agency. Guidelines for Deriving Numerical Aquatic Site-Specific Water Quality Criteria for Modifying National Criteria; Environmental Research Laboratory: Duluth, MN, 1984; EPA 600/3-84/099. (2) Prothro, M. G. "EPA Office of Water Policy and Technical Guidance on Interpretation and Implementation of Aquatic Metals Criteria"; memorandum from acting assistant administrator for water; U.S. EPA Office of Water: Washington, DC, 1993. (3) Bergman, H. L.; Dorward-King, E. J. Reassessment ofMetals Criteria for Aquatic Life Protection; Society of Environmental Toxicology and Chemistry (SETAC) Press: Pensacola, FL, 1997. (4) Sunda, W. G.; Guilard, R. R. L. Journal of Marine Research 1976,34, 511-29. (5) Playle, R. C. et al. Can. J. Fish.Aquat. Sci. 1993, 50, 2678-87. (6) Tipping, E. Computers & Geoscience 1994, 6, 973-1023. Rebecca Rentier is a contributing editor to ES&T.
