Chlorine is a commonly used disinfectant added to drinking water to kill disease-causing microorganisms. While chlorination is effective at protecting human health, there is concern that chlorinated water discharged into the environment may be harmful to wildlife.
How is chlorine added to drinking water?
Chlorine is added to drinking water supplies through a process called chlorination. There are different chlorine compounds that can be used, such as chlorine gas, sodium hypochlorite, and calcium hypochlorite. When added to water, these compounds form hypochlorous acid, which kills microorganisms. The amount of chlorine added is carefully controlled to provide enough disinfection while keeping by-product formation low.
Water facilities routinely monitor chlorine residuals to ensure adequate disinfection. Often, the chlorine level leaving the facility is around 0.5-1 mg/L. Additional chlorine is sometimes added to maintain a residual in the distribution system.
What happens when chlorinated water is discharged into the environment?
When chlorinated wastewater or drinking water is discharged into rivers, lakes, estuaries, and oceans, the chlorine immediately starts reacting. The reactivity and fate of the chlorine depends on the characteristics of the receiving water.
In general, when chlorinated water encounters organic matter, the chlorine reacts to form chlorinated organic compounds such as trihalomethanes. Sunlight causes additional reactions that break down the chlorine. Other chemical reactions occur with bromide, nitrogen-containing compounds, and inorganic compounds in the water. The chlorine gets depleted through these side reactions.
The chlorine residual tends to dissipate more quickly in warmer water temperatures and when more organic matter is present. Factors like pH, salinity, and depth of the receiving water also impact how quickly the chlorine dissipates.
What are the effects of chlorinated water on wildlife?
Research shows that chlorine and chlorinated byproducts can negatively impact wildlife in certain situations. However, the effects depend on the chlorine concentration, length of exposure, and sensitivity of the organism. Some key considerations include:
- Short-term exposure to low levels of chlorine residual is unlikely to have major adverse impacts.
- Longer-term exposure to slightly elevated chlorine levels may cause chronic toxicity.
- High chlorine concentrations can cause acute toxicity and mortality if wildlife is exposed.
- Early life stages of fish and amphibians are most vulnerable to chlorine impacts.
- Plants, insects, and invertebrates can also be affected by chlorinated water discharges.
Effects on Fish
Fish are especially sensitive to chlorine exposure. Effects reported in studies include:
- Damage to gills, skin, and fins
- Problems with respiration, osmoregulation, and excretion
- Inhibited growth
- Reduced swimming speed and predator avoidance
- Interference with reproduction and egg hatching success
- Altered developmental and immunological functions
- Increased susceptibility to disease
- Behavioral changes
- DNA damage
The level of chlorine that starts causing lethal effects depends on the species, but ranges between 0.03-0.5 mg/L for most fish. Salmonids seem to be particularly sensitive.
Effects on Amphibians
Amphibians in their early aquatic life stages are also sensitive to chlorinated water. Impacts include:
- Mortality of eggs and larvae
- Developmental issues
- Impaired growth
- Malformations
- Behavioral changes
- Altered swimming ability
Studies found increased mortality starting around 0.5 mg/L chlorine for most tadpoles. Salamanders appear a bit less sensitive than frogs and toads.
Effects on Aquatic Insects and Crustaceans
Chlorine can negatively impact many aquatic invertebrates, including:
- Mayflies
- Caddisflies
- Amphipods
- Copepods
- Shrimp
- Crabs
Reported effects include mortality, reduced growth, changes in behavior and physiology, and alterations to community structure. Insects, shrimp, and zooplankton seem to be affected at chlorine levels above 0.01 mg/L.
Effects on Algae and Aquatic Plants
Algae and aquatic plants can experience bleaching and tissue damage when exposed to chlorinated water. Photosynthesis may be disrupted, and chlorine can be toxic at levels as low as 0.1 mg/L for sensitive species. This can impact primary production in the ecosystem.
Effects on Birds and Mammals
There is less information available about chlorine impacts on birds and mammals compared to fish and amphibians. Based on toxicity data for terrestrial species, adverse effects may occur if wildlife is exposed to chlorine levels above 1-10 mg/L.
What factors influence chlorine toxicity?
The toxicity of chlorine depends on water chemistry and characteristics of the organism:
- pH: Chlorine is more toxic at lower pH because more hypochlorous acid is present.
- Temperature: Toxicity increases at higher water temperatures.
- Life stage: Early life stages are most vulnerable.
- Body size: Smaller organisms are more quickly affected.
- Sensitivity: Some taxonomic groups are more sensitive.
- Exposure time: Longer exposure increases toxicity.
Additionally, chlorine can have indirect ecological impacts by altering habitats, reducing biodiversity, and enabling invasive species.
Do chlorinated byproducts harm wildlife?
In addition to chlorine itself, its reaction byproducts can also be problematic for wildlife. Of particular concern are:
- Trihalomethanes like chloroform – Linked to toxicity in aquatic species at elevated levels
- Haloacetic acids – Show toxicity towards phytoplankton and invertebrates
- Chloramines – Formed when chlorine reacts with nitrogen – Can be directly toxic
- Brominated compounds – More persistent and bioaccumulative than chlorine alone
The types and amounts of byproducts formed depend on characteristics of the receiving waters. Monitoring studies find chlorinated byproducts at measurable levels in surface waters near wastewater outfalls.
How quickly does chlorine dissipate in natural waters?
Water Type | Typical Chlorine Half-life |
---|---|
Rivers | 2-20 minutes |
Lakes | 2-10 hours |
Estuaries | 1-5 days |
Coastal marine waters | 4-10 days |
As shown in the table, chlorine tends to dissipate most quickly in flowing rivers and streams. It persists longer in lakes, estuaries, and seawater.
The rate of chlorine decay is fastest in warm water with lots of organic matter. Sunlight, mixing, salinity, and depth also influence dissipation rates.
How far from the discharge point is wildlife impacted?
The zone of impact depends on the initial chlorine concentration, characteristics of the receiving waters, and sensitivity of local species. Scientists have defined impact zones extending:
- 50-700 meters downstream in rivers
- 50-4000 meters around discharge points in estuaries
- Over 1 km around ocean outfalls
Mobile organisms like fish may avoid or swim through chlorinated zones, while less mobile species may experience more continuous exposure and effects.
What are the guidelines for chlorine discharge limits?
Environmental regulations restrict how much chlorine can be discharged to natural waters. Guidelines and standards aim to limit chlorine to levels that are protective of aquatic life.
Some example discharge limits for total residual chlorine are:
- EPA National Recommended Water Quality Criteria – 0.011 mg/L (4-day average)
- California Toxics Rule – 0.019 mg/L (4-day average)
- EU Environmental Quality Standard – 0.004 mg/L (long-term)
Stricter limits down to 0.001 mg/L may be set for special water bodies. Waters with endangered species often require more stringent chlorine discharge restrictions and monitoring.
How can facilities reduce chlorine discharge impacts?
Treatment plants can implement methods to reduce chlorine release and minimize ecological impacts. Strategies include:
- Dechlorination – Chemically neutralizing residual chlorine before discharge
- Improved disinfection techniques – Minimizing required chlorine doses
- Better mixing – Promoting rapid chlorine dissipation
- Relocation of discharge points – Discharging away from sensitive habitats
- Habitat creation or restoration – Improving overall ecosystem health
Conclusion
In conclusion, research shows that chlorine and its byproducts can be toxic to aquatic wildlife when discharged into surface waters. The level of chlorine causing effects is species-dependent but can range from 0.01-1 mg/L for sensitive organisms.
To protect wildlife, chlorine discharge limits are set low enough to prevent acute and chronic toxicity. Treatment facilities can implement dechlorination and other mitigation strategies to reduce ecological risks. The zone of impact depends on site-specific characteristics but may extend over 1 km in some cases.
While chlorine is beneficial for protecting human health, discharges should be managed carefully to balance effective disinfection with environmental protection. Understanding chlorine fate and effects enables appropriate discharge limits and monitoring programs to be established.