Water pollution

Contamination

A chemical substance present in the environment above natural levels, whether or not there is a health effect.

Pollution

The introduction of contamination that results in or can result in adverse biological effects to resident communities.

Difference between contamination and pollution

All pollutants are contaminants, but not all contaminants are pollutants.

Natural/background/baseline level

The normal concentration of a chemical or microorganism in an environment before additional contamination occurs.

Water pollution

Any concentration of a chemical or microorganism in water above natural or background levels.

Pollutant

A contaminant that causes or can cause adverse biological effects.

Point source

A single identifiable source from which pollutants are discharged.

Non-point source

A diffuse source of pollutants that cannot be traced to one identifiable discharge point.

Groundwater

Water found in the saturated zone of soil.

Saturated zone

The zone of soil where the spaces between particles are filled with water.

Importance of groundwater

Groundwater accounts for approximately 99% of freshwater and is a significant source of drinking water.

Age of groundwater

Groundwater can range from a few years old to millions of years old.

Major use of groundwater in the United States

Approximately two-thirds of groundwater use is for irrigation.

Groundwater contamination

The presence of contaminants in groundwater above natural levels.

Contaminated aquifer

An underground water-bearing region containing pollutants or contaminants.

Organic contaminants in groundwater

A wide range of organic chemicals can be present in contaminated aquifers.

Major point sources of groundwater contamination

Municipal landfills and industrial waste disposal sites.

Leachate

Liquid containing dissolved compounds that drains from a terrestrial source such as a landfill.

Dense non-aqueous phase liquid (DNAPL)

An organic contaminant that is denser than water and does not mix with water.

Density of DNAPLs compared with water

DNAPLs are denser than water.

Miscible

Capable of mixing completely with another substance.

Immiscible

Not capable of mixing completely with another substance.

Why are DNAPLs difficult to detect and remediate

They sink to the bottom of aquifers and can remain as persistent sources of contamination.

Water-soluble contaminants

Contaminants that dissolve in water and migrate with groundwater.

DNAPL migration

DNAPLs sink through groundwater because they are denser than water.

DNAPL reservoir

A large pool of slowly migrating DNAPL that continually releases dissolved contaminant into groundwater.

Chlorinated solvents

Organic solvents containing chlorine that are common groundwater contaminants.

Common chlorinated solvents

Trichloroethylene (TCE) and perchloroethylene (PCE).

Trichloroethylene (TCE)

A chlorinated solvent commonly found in contaminated groundwater.

Perchloroethylene (PCE)

A chlorinated solvent commonly associated with contaminated groundwater.

Industrial use of chlorinated solvents

They were used for degreasing metals.

Other major use of chlorinated solvents

Dry cleaning.

Toxicity of TCE and PCE

They are classified in the presentation as “probable” carcinogens.

Why are chlorinated solvents important groundwater pollutants

They are persistent contaminants and many are DNAPLs.

Light non-aqueous phase liquid (LNAPL)

An organic contaminant that is less dense than water and does not mix with water.

Density of LNAPLs compared with water

LNAPLs are less dense than water.

Where do LNAPLs occur in an aquifer

They float on top of the groundwater.

Hydrocarbon solvents

Organic compounds commonly associated with fuels and petroleum contamination.

Most hydrocarbon solvents

LNAPLs.

Hydrocarbon contamination

The second most common form of groundwater contamination according to the presentation.

BTEX

Benzene, toluene, ethylbenzene, and xylene isomers.

Benzene

An aromatic hydrocarbon and component of gasoline that can contaminate groundwater.

Toluene

An aromatic hydrocarbon found in gasoline and included in BTEX.

Ethylbenzene

An aromatic hydrocarbon found in gasoline and included in BTEX.

Xylene

An aromatic hydrocarbon with isomers that are components of BTEX.

How can gasoline enter groundwater

Through spills, leaks from storage containers, or pipeline ruptures.

Most soluble BTEX components

BTEX compounds are among the soluble components of gasoline, allowing them to migrate into groundwater.

Alkylated benzenes

Hydrocarbon compounds that can undergo rapid aerobic microbial degradation.

Aerobic microbial degradation

Microbial breakdown of contaminants using oxygen.

LNAPL movement

LNAPLs float on top of the aquifer and move with groundwater flow.

Difference between DNAPLs and LNAPLs

DNAPLs are denser than water and sink, while LNAPLs are less dense than water and float.

Groundwater remediation

The process of treating polluted groundwater by removing pollutants.

Ex-situ remediation

Groundwater is removed and treated outside its original location.

In-situ remediation

Pollutants are removed or treated where the groundwater is currently located.

Pump-and-treat

A remediation system that pumps contaminated groundwater to the surface for treatment before returning or discharging the treated water.

Purpose of pump-and-treat systems

To remove contaminants from groundwater by extracting and treating contaminated water.

What happens to treated groundwater in pump-and-treat systems

It may be returned to the aquifer or another water body.

Why can pump-and-treat be difficult for low-solubility contaminants

Treatment may need to continue indefinitely because contaminants can remain as reservoirs.

In-situ treatment barrier

A barrier placed in the groundwater flow path to treat contaminants as groundwater passes through.

Iron-filled treatment wall

An in-situ barrier containing sand and small iron (Fe⁰) fillings used to treat contaminated groundwater.

Purpose of Fe⁰ in groundwater remediation

Iron reacts with contaminants and promotes reductive dechlorination.

Oxidation of elemental iron

Fe⁰ → Fe²⁺(aq) + 2e⁻.

Reductive dechlorination

A process in which chlorine atoms are removed from a chlorinated contaminant through reduction.

TCE reductive dechlorination reaction

C₂HCl₃ → C₂H₄ + 3Cl⁻.

Net iron/TCE reaction

Fe(s) + C₂HCl₃ + 3H₂O → 3Fe²⁺(aq) + C₂H₄ + 3Cl⁻ + 3OH⁻.

In-situ remediation advantage

Contaminants can be treated directly within the contaminated groundwater.

Bioremediation

Use of biochemical or microbial processes rather than chemical or physical processes to decontaminate soil and water.

Natural attenuation

Reduction of contaminant concentrations through naturally occurring physical, chemical, or biological processes.

Aerobic biodegradation

Microbial degradation that involves oxygen.

What happens during aerobic biodegradation

Oxygen atoms are inserted into the contaminant molecule.

Role of enzymes in aerobic biodegradation

Enzymes catalyze the biochemical reactions involved in degradation.

Major degradation pathway for hydrocarbons

Aerobic biodegradation is a predominant route.

Effect of increasing chlorination on aerobic biodegradation

Aerobic biodegradation becomes less effective as chlorination increases.

PCE and aerobic biodegradation

Aerobic biodegradation is ineffective for PCE according to the presentation.

Anaerobic biodegradation

Microbial degradation that does not use oxygen.

Anaerobic reductive dechlorination

A series of reactions in which highly chlorinated molecules lose chlorine atoms under anaerobic conditions.

Highly chlorinated molecules

Molecules containing a relatively large number of chlorine atoms.

What affects anaerobic dechlorination half-life

Molecular structure, degree of chlorination, and the microbial species present.

Half-life (t₁/₂)

Time required for the concentration of a contaminant to decrease by half.

Aerobic vs. anaerobic biodegradation

Aerobic biodegradation uses oxygen, whereas anaerobic biodegradation does not.

Chemicals of Emerging Concern (CEC)

Compounds not currently monitored in the environment that might enter the environment and potentially cause adverse effects to ecosystems or human health.

Why are CECs important

They may enter the environment without being routinely monitored and may cause ecological or human-health effects.

Wastewater

Water that has been adversely affected by anthropogenic inputs.

Anthropogenic

Originating from human activities.

Examples of wastewater

Sewage and washing water from sources such as sinks and washing machines.

Wastewater treatment

A multistep process used to treat wastewater before it is returned to the environment.

Main goal of wastewater treatment

Reduce particulates, BOD, and diseases before water is returned to the environment.

WWTP

Wastewater Treatment Plant.

Gold Bar WWTP

A wastewater treatment plant identified in the presentation.

Particulates

Solid particles present in wastewater that treatment processes aim to remove.

BOD in wastewater treatment

Biological oxygen demand that treatment aims to reduce before wastewater is released.

Why is BOD reduced during wastewater treatment

To reduce oxygen consumption in receiving water bodies.

Pharmaceuticals in wastewater

Pharmaceutical compounds can remain in wastewater treatment plant effluent.

Pharmaceuticals in WWTP effluent

Pharmaceutical compounds can be present in treated wastewater leaving a wastewater treatment plant.

Where have pharmaceuticals from WWTPs been detected

In rivers.

Why are pharmaceuticals considered emerging contaminants

They can pass through wastewater treatment and enter natural waters.

Wastewater treatment and emerging contaminants

Conventional wastewater treatment may not completely remove some emerging contaminants such as pharmaceuticals.

Environmental concern with pharmaceuticals

Their presence in aquatic environments may pose risks to ecosystems and human health.

Groundwater pollution challenge

Persistent organic contaminants can remain in groundwater and are difficult to remove.

Why are DNAPLs challenging to remediate

They sink, have low water solubility, and can act as long-term contaminant reservoirs.