Ambient Water Quality Guidelines for Zinc
Prepared pursuant to Section 2(e) of the
Original signed by Don Fast
This document is one in a series that establishes ambient water quality guidelines, formerly known as criteria, for British Columbia (Table 1). This document is mainly based on a report prepared by the BC Ministry of Environment, Lands and Parks for the Canadian Council of Ministers of the Environment (CCME). It sets guidelines for zinc (Zn) to protect drinking water, recreational and aesthetics, freshwater and marine aquatic life, and agricultural water (irrigation and livestock watering) uses.
Zinc guidelines were not set for wildlife and industrial water uses, since suitable data documenting the effects of zinc for these uses were not available in the literature.
Zinc is most toxic to microscopic organisms in the aquatic environments. It is also an essential element for aquatic and terrestrial biota and its removal from the environment below certain levels can also be harmful due to its deficiency. Zinc guidelines, tabulated above, are summarized in the chapter on Recommended Guidelines. A more detailed discussion of the guidelines or criteria is presented in the main body of the report.
Zinc may bind to particulate matter. Soluble species of zinc are readily available for biological reactions and, therefore, considered as most toxic. It has been shown that zinc in water is a better predictor of fish tissue contamination than zinc in either sediment or invertebrates (i.e., food source). It is, therefore, recommended that the zinc guideline may be interpreted in terms of the dissolved metal fraction when the total zinc concentration in the environment exceeds the guideline due to particulate matter and adverse effects due to zinc are not obvious.
THE MINISTRY OF ENVIRONMENT, LANDS AND PARKS (now called Ministry of Water, Land and Air Protection) develops province-wide ambient water quality guidelines for variables that are important in the surface waters of British Columbia. This work has the following goals:
Ambient water quality objectives for specific waterbodies will be based on the guidelines and also consider present and future uses, waste discharges, hydrology/limnology/oceanography, and existing background water quality. The process for establishing water quality objectives is more fully outlined in Principles for Preparing Water Quality Objectives in British Columbia, copies of which are available from the Water Quality Section of the Water Management Branch.
Neither guidelines nor objectives which are derived from them, have any legal standing. The objectives, however, can be used to calculate allowable limits or levels for contaminants in waste discharges. These limits are set out in waste management permits and thus have legal standing. The objectives are not usually incorporated as conditions of the permit.
The definition adopted for a guideline is:
The guidelines are province-wide in application, are use-specific, and are developed for some or all of the following specific water uses:
The guidelines are set after considering the scientific literature, guidelines from other jurisdictions, and general conditions in British Columbia. The scientific literature gives information on the effects of toxicants on various life forms. This information is not always conclusive because it is usually based on laboratory work which, at best, only approximates actual field conditions. To compensate for this uncertainty, guidelines have built-in safety factors which are conservative but reflect natural background conditions in the province.
The site-specific water quality objectives are, in most cases, the same as guidelines. However, in some cases, such as when natural background levels exceed the guidelines, the objectives could be less stringent than the guidelines. In relatively rare instances, for example if the resource is unusually valuable or of special provincial significance, the safety factor could be increased by using objectives which are more stringent than the guidelines. Another approach in such special cases is to develop site-specific guidelines by carrying out toxicity experiments in the field. This approach is costly and time-consuming and therefore seldom used.
Guidelines are subject to review and revision as new information becomes available, or as other circumstances dictate.
Zinc is an essential element in trace amounts for plants and animals. In mammals, it plays a vital role in the biosynthesis of nucleic acids, RNA polymerases, and DNA polymerases and, thus, is involved in the healing processes of tissues in the body. Other physiological processes such as hormone metabolism, immune response, and stabilization of ribosome and membranes also require zinc.
Zinc toxicosis is not a common problem, but zinc poisoning in humans (e.g., from acid foods or beverages stored in galvanized containers) and animals (e.g., from ingesting or exposure to galvanized metal objects, certain paints and fertilizers, zinc-containing coins, etc.) have been documented. Several factors such as water hardness, salinity, temperature, and the presence of other contaminants influence zinc toxicity in aquatic environments. This modification in zinc toxicity is the result of an effect on zinc availability and on sorption or binding of available zinc to biological tissues. The effect of water hardness on zinc toxicity is by far the most studied factor.
Clinical manifestations of zinc deficiency in animals include growth retardation, testicular atrophy, skin changes, and poor appetite. Zinc is ubiquitous in the environment and its deficiency in humans and animals may be considered an unlikely problem. Nevertheless, zinc deficiency and related problems in humans, animals, birds, and plants have been reported in the literature.
Zinc ranks fourth among metals of the world in annual consumption, behind iron, aluminum and copper. British Columbia, Ontario, Yukon, and Northwest Territories are the major producers of zinc in Canada. Zinc uses are many:
The concentration of zinc in natural waters is generally low, but on occasion high levels have been measured in natural environments. High levels of zinc are always found in contaminated waters or waters flowing through a bedrock system containing zinc deposits.
Historical zinc concentrations should be viewed with caution. Results from cleaner laboratory analytical methods with lower detection limits show that background zinc concentrations are lower than previously thought. Older high values may be the artifacts of high detection limits and artificial contamination during measurement.
1. DRINKING WATER
This guideline is consistent with the recent Health and Welfare Canada drinking water guideline.
The Health and Welfare Canada guideline is based on aesthetic considerations for three reasons: (a) food, not the drinking water, is by far the largest dietary source of zinc; (b) zinc toxicity to humans is unlikely because of efficient homeostatic control mechanism, and (c) water containing zinc at concentrations in excess of 5000 µg/L has an undesirable astringent taste and may be opalescent and develop a greasy film on boiling.
This guideline is consistent with the Health and Welfare Canada and the 1987 CCME (formerly known as CCREM) guidelines.
It is unlikely that zinc concentrations found in ambient waters will impair the use of recreational activities such as swimming. The aesthetic considerations discussed for drinking water are also valid for recreation.
The guidelines for maximum concentration are based on 96-h LC50 of 66 mg/L at 9.5 mg/L CaCO3 for rainbow trout. The slope and the start of the relationship between zinc toxicity and water hardness was assumed to be the same as that for the chronic toxicity.
Acute LOELs lower than 66 µg/L were reported in literature. However, they were not used in development of the guidelines, because the data were dated, original articles were not available for confirmation of data quality, or data were incomplete (e.g., water hardness was not stated). Such rejection of suspect or incomplete data is consistent with the CCME and the Ministry of Environment, Lands and Parks protocols for the development of guidelines.
The recommended guideline is based on lowest observed effect (chronic) levels of 19-19.6 µg/L zinc for the marine algae S. schroederi and S. constatum. A safety factor of 0.5 was used.
The recommended guideline is based on lowest observed acute values of 112-168 µg/L (96-h LC50) for Arctic grayling and 119-310 µg/L (48-h LC50) for Pacific oyster. A safety factor of 0.5 was used.
These guidelines replace the 1987 CCME guidelines which were based on old (pre 1980) data.
This guideline replaces the 1987 CCME guideline which was based on old (pre 1980) data.
Zinc is ubiquitous in the environment. Its impact on the environment depends upon several factors related to Zn sources and environmental variability. Therefore, care must be exercised when the water quality guidelines are applied to assess environmental impacts of zinc.
Zinc shows variable behaviour in binding to particulate matter depending upon physical-chemical characteristics of the aquatic system. The literature shows that particulate zinc in rivers and lakes varied from 10-78% of the total zinc concentration. Furthermore, soluble species of zinc are readily available for biological reactions and, therefore, most toxic. It has also been shown in the literature that zinc concentration in water is a better predictor of fish tissue contamination than the concentration in either sediment or invertebrates (i.e., food). In view of these facts, it is recommended that the zinc guideline should be interpreted in terms of the dissolved metal fraction when the total zinc concentration in the environment exceeds the guideline due to particulate matter and adverse effects due to zinc are not obvious.
The water quality guidelines recommended in this document are primarily based on controlled, laboratory bioassays in which the toxic effects on organisms were measured in terms of the zinc levels in water. However, the zinc body burden of aquatic organisms in their natural environments is the result of exposure to both water and food sources. Zinc associated with the sediment fraction may also become available to the organisms under favourable environmental conditions. Thus, the zinc concentrations in water alone should not be taken as a true reflection of the potential zinc problem in a given waterbody. Other assessment techniques may be required to address issues related to zinc, including measurement of zinc concentrations in fish and/or sediment and long-term bioassays with resident species using local water. If available, guidelines for maximum and average zinc concentrations in fish tissue and sediment should also be used to assess existing water quality. Long-term bioassays are complex and costly; they are likely to be undertaken for waterbodies with high resource values and which are threatened by a controllable point-source of zinc pollution.
In most cases, water quality objectives will be the same as the guidelines. When concentrations of zinc in undeveloped waterbodies are less than the recommended guidelines, then more stringent values, if justified, could apply. In some cases, socioeconomic or other factors (e.g., higher background levels) may justify objectives which are less stringent than the guidelines. Site-specific impact studies would be required in such cases.
Zinc availability, and hence its toxicity, in the aquatic environment can be influenced by many factors, including water hardness. Although the literature alludes to this fact, there is a general lack of available research in this area. However, methods (e.g., water effects ratio, resident species toxicity in the field, etc.) are available to adapt the recommended guidelines to a given site by considering these factors (other than hardness which has been considered in this document). Where necessary, these methods can be employed to set site-specific water quality objectives. Because these approaches are costly and time consuming, they are seldom used.
In some instances, the ambient or existing concentrations of zinc in the environment may exceed the recommended guidelines. This may be true especially in the soft water (hardness less than or equal to 90 mg/L CaCO3) environments. To protect aquatic life in such environments, it is recommended that degradation of the existing water quality should be avoided.