Chlorine in freshwater and marine water

​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals' of the ANZECC & ARMCANZ (2000) guidelines.

The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.

Description of chemical

Chlorine (CAS 7782-50-5) is a basic industrial chemical for manufacture of chlorinated organic and inorganic chemicals. It is a gaseous material, which is very soluble in water and highly reactive. Chlorine is also used for pulp and paper manufacture, as an industrial and household bleach, an antifoulant in cooling water, a disinfectant and water and wastewater treatment chemical (CCREM 1987).

Environmental fate

Chlorine does not persist for extended periods in water but is very reactive and its by-products persist longer. It has been common practice to maintain a residual level of chlorine in wastewater plants (CCREM 1987) but recent awareness of the environmental effects of chlorine has resulted in moves to reduce this residual (Sydney Water 1996b).

Chlorine is rapidly converted to hypochlorous acid (HOCl) and hydrochloric acid (HCl) in receiving waters (CCREM 1987). The term’ free chlorine’ refers to Cl2​, HOCl and hypochlorite ion OCl- in equilibrium. The relative amounts of the different forms in equilibrium are governed by pH, temperature and ionic strength. At extremely low pH, Cl2​ is essentially un-hydrolysed and hence it is the dominant species. Between pH 2 and 7, HOCl is the dominant form while at pH 7.4 and 20°C, there is equimolar contribution of HOCl and OCl- (CCREM 1987). In seawater, reaction with bromine results in formation of chloride ion and HOBr.

Chlorine reacts readily with nitrogenous substances (e.g. ammonia) to form N-chlorinated compounds which constitute the combined chlorine. These compounds are more persistent than the free chlorine. Among these N-chlorinated compounds is monochloramine (NH2Cl) which contributes significantly to the combined available chlorine in water. In water treatment, intentional production of N-chloramines is used to extend the effectiveness of chlorination. After water treatment, the sum of free chlorine and combined chlorine is referred to as total residual chlorine (TRC).

Aquatic toxicology

The toxicity figures were derived using measurements of total residual chlorine (measured as µg Cl per L) rather than free chlorine. The chemicals used for testing the effect of chlorine included chlorine gas (Cl2​) bubbled in water, sodium hypochlorite (NaOCl) or hypochlorous acid (HOCl) and combinations (at different molar ratios at specific pH values) of ammonium sulfate or chloride and NaOCl to form monochloramine or dichloramine. Chronic toxicity levels were similar to concentrations that caused acute effects. In marine water, which contains iodide and bromide, total residual oxidants was measured as µg Cl per L.

Freshwater fish: seven species, 24 to 96-hour LC50, 70 to 840 µg/L. Two figures for Oncorhynchus mykiss were 14 and 29 µg/L (Basch et al. 1971).

Freshwater crustaceans: three species cladocerans, 24 to 48-hour LC50, 12 to 160 µg/L. Two 48-h LC50 values were 5 and 6 µg/L, measured under continuous flow of test solution (Taylor 1993). Crayfish Orconectes nais (96-hour LC50, 760 to 960 µg/L), Mesocyclops aspericomis and M. longisetus (24-hour LC50 470 and 1010 µg/L respectively) and Asellus aquaticus (24-hour EC/LC50 315 to 754 µg/L) were less sensitive. Chronic NOEC, 10-day immobilisation Ceriodaphnia dubia, 48 µg/L (same as acute figures).

Freshwater mollusc: one species, Nitocris sp. 24 to 48-hour LC50, 7700 to 15,600 µg/L. Chronic 168-h LC50 of 32 µg/L for a periphyton.

Freshwater annelid: one species, 24 to 48-hour LC50 Aelosoma headleyi, 1680 to 3200 µg/L.

Freshwater insects: three species, 24-hour LC50 710 to 1350 µg/L.

Freshwater rotifer: one species, Philodina acuticornis, 48-hour LC50, 50 to 100 µg/L.

Marine fish: two species, 48 to 96-hour LC50 128 to 250 µg/L (2 to 8 hours/day intermittent to continuous dosing). Chronic NOEC (7-day growth), Menidia beryllina, 87 to 186 µg/L.

Marine crustacean: one species, Mysidiopsis bahia, 96-hour LC50, 73 to 268 µg/L (2 to 8 hours/day intermittent to continuous dosing). Chronic NOEC (7-day reproduction), M. bahia, 20-87 µg/L.

Australian and New Zealand data

Manning et al. (1996) assessed the toxicity of chlorine and N-chloramines to Australian aquatic crustaceans under flow-through conditions at pH range 7.5 to 8.3. The 1-hour LC50 for the freshwater C. dubia was 590 µg/L (sodium hypochlorite) or 280 µg Cl/L. The corresponding 24-hour figures were 260 µg Cl/L. A 10-day reproductive impairment test with C. dubia,the LOEC was 66 µg Cl/L and the NOEC was 48 µg/L.

The 24-hour LC50 for the marine prawn, Penaeus plebejus, was 180 µg/L (Manning et al. 1996).

Factors that change toxicity

Cairns et al. (1978) studied the effect of temperature on toxicity of chlorine. Higher temperatures, around 25°C, resulted in complete loss of measurable residual chlorine and chloramines from test vessels within 24 hours, and this was reflected in a slight decrease in toxicity of chlorine at higher temperatures and more rapid recovery of algal growth.


A freshwater moderate reliability trigger value of 3 µg Cl/L measured as total residual chlorine was derived using the statistical distribution method with 95% protection. This figure was obtained from application of the default acute-to-chronic ratio (ACR) of 10 instead of the empirical ACR of 2.7 from geometric mean of eight figures. The smaller ACR would have resulted in a value not protective of some species under continuous exposure to chlorine for at least 48 hours. This figure was adopted as a marine low reliability trigger value, to be used only as an indicative interim working level.

Although the chlorine figure at 95% protection is 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.


ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.

Basch RE, Newton ME, Truchan JC & Fetterolf CM 1971. Chlorinated municipal waste toxicities to rainbow trout and fathead minnows. EPA 18050-GZZ-10/71, Water Pollution Control Research Series, US EPA (US NTIS PB-209890).

Cairns J Jr, Buikema AL Jr, Heath AG & Parker BC 1978. The effects of temperature on aquatic organism sensitivity to selected chemicals. Virginia Water Resources Research Center Bulletin 106, Blacksburg, Va.

CCREM 1987. Canadian water quality guidelines. Canadian Council of Resource and Environment Ministers, Ontario.

Manning TM, Wilson SP & Chapman JC 1996. Toxicity of chlorine and other chlorinated compounds to some Australian aquatic organisms. Bulletin of Environmental Contamination and Toxicology 56, 971–976.

Sydney Water 1996b. Ecological and human health risk assessment of chemicals in sewage treatment plant discharges to the Hawkesbury-Nepean river system. Sydney Water Corporation Ltd, Sydney South.

Taylor PA 1993. An evaluation of the toxicity of various forms of chlorine to Ceriodaphnia dubia. Environmental Toxicology and Chemistry 12, 925–930.