Summary Water Quality
1 Chapter 1: Water Quality Parameters
1.1 Introduction to water quality & parameters
Water quality is not just about whether water looks clean. It describes the overall condition of water and whether that water is
suitable for a specific use. This idea is often called “fit for purpose.”
A river that is safe for fish is not automatically safe for drinking. Water used for agriculture does not need the same quality as drinking
water. Because of this, water quality is always linked to a goal or function.
Water quality combines four major aspects:
• Chemical quality → nutrients, metals, pollutants, oxygen, pH
• Physical quality → temperature, turbidity, conductivity, suspended solids
• Biological/ecological quality → microorganisms, algae, biodiversity
• Radiological quality → radioactive substances
In environmental science, water quality is usually evaluated against standards. These standards help determine whether there is a
risk for ecosystems or humans.
The main objectives: protecting ecosystem health, protecting human health (secondary poisoning), & ensuring safe drinking water.
1.1.1 European Water Quality Frameworks
In Europe, water quality is strongly regulated through several directives.
1) EU Water Framework Directive (WFD)
The WFD focuses on achieving: good ecological status + good chemical status. This means water bodies are evaluated using:
chemical parameters, biological indicators, & physical conditions.
An important concept is the Environmental Quality Standard (EQS): the maximum concentration of a contaminant allowed before
ecological damage occurs. The WFD therefore looks beyond chemistry alone. A river can chemically meet standards but still fail
ecologically if biodiversity collapses.
2) Drinking Water Directive (DWD)
The Drinking Water Directive focuses on human exposure.
Main concern: pathogens, toxic chemicals, & contaminants aLecting public health. This directive is stricter because drinking water
directly enters the human body.
3) Groundwater Directive (GWD)
Groundwater is evaluated using: threshold values for contaminants & groundwater quantity.
4) Total Maximum Daily Load (TMDL) — USA
The TMDL concept defines the maximum pollutant load a water body can receive while still meeting quality standards.
1.1.2 Why Climate Change Matters for Water Quality
Water quality and climate change are deeply connected. Climate change modifies:
- water temperature - precipitation - flooding - salinity
1) Atmospheric CO₂ and Climate Change
One of the clearest indicators of climate change is atmospheric CO₂ concentration. The famous Keeling Curve at Mauna Loa
Observatory shows a continuous increase in atmospheric CO₂ since the late 1950s.
Key trends:
• Pre-industrial CO₂ ≈ 280 ppm
• Current CO₂ ≈ 425+ ppm
• Increase mainly caused by fossil fuel combustion and deforestation
The seasonal zigzag pattern exists because plants absorb more CO₂ during the growing
season and release more during winter.
More CO₂ means a stronger greenhouse eLect, which increases global temperature.
, 2) Global Temperature Increase
Global average temperature has already increased by about 1.1–1.5°C above pre-
industrial levels.
This warming may appear small, but globally averaged temperature changes are
extremely powerful. Even a few degrees change can: melt ice sheets, shift rainfall
patterns, intensify droughts, increase floods, destabilize ecosystems.
3) Climate Tipping Points
A tipping point is a threshold after which a system changes rapidly and sometimes irreversibly.
Examples: Greenland ice sheet collapse, collapse of Atlantic ocean circulation, Amazon rainforest dieback, permafrost thaw.
These systems are dangerous because they contain positive feedback loops.
Example: Ice melts → darker surface exposed → more sunlight absorbed → more warming → more melting. Once started, the process
can continue even if emissions decrease later.
1.1.2.1 Climate Change Impacts on Water Systems
Climate change changes the entire hydrological cycle.
Northern Europe
à Expected: more precipitation, stronger rainfall events, increased flood risk.
Southern Europe
à Expected: less precipitation, stronger droughts, desertification, water scarcity.
1.1.2.2 Major Water Quality Consequences
1. Higher Water Temperature
Warmer water contains less dissolved oxygen. This stresses aquatic organisms and favors harmful microorganisms.
2. Increased Eutrophication
Warmer temperatures and nutrient pollution stimulate algal growth. à cyanobacterial blooms & oxygen depletion,
3. Increased Pollutant Concentration During Drought
Lower river discharge means less dilution. Therefore: nutrients, heavy metals, wastewater contaminants become more concentrated.
4. Salinization
Sea level rise causes saltwater intrusion into: estuaries, coastal wetlands, groundwater aquifers. This threatens freshwater supplies
and agriculture.
1.2 Organic Contaminant Cycles in Lakes
Lakes are not passive water reservoirs. They behave as chemical reactors where contaminants are continuously transported,
transformed, stored, and re-released. Organic contaminants and metals move between the atmosphere, water, sediments, and
organisms through complex biogeochemical cycles.
Two important examples are: the Mercury (Hg) cycle & the PCB/Dioxin cycle.
1.2.1 Mercury (Hg) Cycle in Lakes
Mercury is a global pollutant that cycles continuously between air, water, sediments, and organisms. The most important concept is
that mercury becomes far more dangerous after it is transformed into methylmercury (MeHg).
Forms of Mercury: The toxicological importance increases strongly from Hg(0) Form Characteristics
→ Hg(II) → MeHg.
Bioaccumulation and Biomagnification Hg(0) Elemental mercury, volatile gas
Methylmercury enters the food chain through plankton and small organisms. Hg(II) Oxidized inorganic mercury
Concentrations increase at every trophic level: water → plankton → small fish →
predatory fish → birds/humans MeHg Methylmercury, highly toxic organic form
à This is called biomagnification.
Secondary poisoning occurs in top predators because they consume many contaminated prey organisms.
,1.2.2 PCB and Dioxin Cycle in Lakes
Polychlorinated biphenyls (PCBs) and chlorinated dioxins are classic examples of Persistent Organic Pollutants (POPs).
à They are: semi-volatile, hydrophobic, highly toxic, extremely persistent.
à These compounds are often grouped as PBTs: Persistent Bioaccumulative Toxic
Why PCBs and Dioxins Are Dangerous
The key property is hydrophobicity.
à They do not dissolve well in water but strongly bind to: organic matter, sediments, fatty tissues.
à Because of this: they remain in ecosystems for decades, they accumulate in organisms, they biomagnify through food chains.
Lakes as “Chemical Reactors” à Both examples show that lakes actively transform contaminants.
1.3 Hydrologic Cycle and Water Quality
Water quality is tightly connected to the hydrologic cycle because water continuously moves through:
- Atmosphere - soils - rivers - groundwater - lakes - oceans.
During this movement, water dissolves and transports chemicals. A watershed therefore strongly determines water chemistry.
Where Does Surface Water and Groundwater Get Their Chemical Composition?
This is one of the most important hydrogeochemical concepts. Water chemistry mainly originates from interaction with:
1) Atmospheric Input
Rainwater already contains dissolved substances: CO₂, sulfate, nitrate, sea salts, pollutants.
à CO₂ is especially important because it forms weak carbonic acid: CO2 + H2O à H2CO3
à This acid enhances mineral weathering.
2) Soil Interaction
As water infiltrates soils, it dissolves: nutrients, organic matter, ions.
à Microbial processes strongly modify water composition here.
3) Bedrock Weathering Bedrock Main Ions Released
The bedrock largely controls the dominant
Limestone Ca²⁺, HCO₃⁻
ions in groundwater.
à Examples: Gypsum Ca²⁺, SO₄²⁻
This is why groundwater chemistry diLers
Halite Na⁺, Cl⁻
strongly between regions.
1.4 Major Rivers
Europe contains several large river systems such as: the Danube, Rhine, Rhône.
These rivers connect entire watersheds to coastal systems and transport enormous quantities of water and contaminants across
countries. A river is therefore not an isolated ecosystem. Its quality reflects everything happening upstream in the watershed.
1.4.1 River Type Controls River Behavior
Not all rivers behave the same way. River morphology strongly influences water quality processes.
1. Large High-Flow Rivers à Examples: Mississippi, lower Danube.
These rivers have: - very large discharge - strong currents
- high sediment transport - strong mixing.
à Characteristics: - contaminants dilute more easily,
- transport distances are large,
- suspended sediments dominate,
- residence time is relatively short.
Because turbulence is high, oxygen exchange with the atmosphere is often eLicient. However,
high-flow rivers can transport huge pollutant loads downstream very quickly.
, 2. Meandering Low-Flow Rivers à Example: Amazon
These rivers typically occur in: flat terrain, floodplains, low-gradient landscapes.
à Characteristics: - slower flow - stronger sediment deposition,
- more stagnant zones - longer residence times.
These systems are more vulnerable to: eutrophication, oxygen depletion, sediment contamination.
Because water remains longer in the system, biological and chemical transformations become more important.
1.4.2 Flow Velocity Patterns Inside a River
River flow is not uniform. Velocity changes throughout the river cross-section.
General Pattern
à Fastest water: near the center + close to the surface. This occurs because friction is lowest there.
à Slowest water: near the riverbed + near the banks. Friction with sediments & riverbanks slows water.
Why Velocity Matters for Water Quality
à Fast-flowing zones: keep particles suspended, increase oxygenation, reduce settling.
à Slow-flowing zones: favor sediment deposition, allow contaminant accumulation, become oxygen-poor.
1.4.2.1 The Hydrograph (Chemograph = red)
A hydrograph shows how river discharge changes over time after rainfall. It links
precipitation to river response.
Lag Time = the delay between: maximum rainfall & maximum river discharge.
à Short lag time means: rapid runoL, flashy river response, higher flood risk.
à Long lag time means: slower infiltration, stronger groundwater contribution, more
buLered discharge.
Urban areas usually have short lag times because impermeable surfaces increase runoL. Forested or natural catchments generally
have longer lag times.
Sources of River Water During Storms: River discharge during rainfall events comes from several flow pathways.
1) Surface Runoi à Fastest pathway. Water flows directly over land into the river. This pathway is very important for
contaminant pulses during storms.
2) Interflow à Water moves laterally through shallow soils. Intermediate speed. Transports: dissolved nutrients,
3) Groundwater Flow (Baseflow) à Slowest pathway. Provides long-term stable discharge between storms.
Chemograph vs Hydrograph
Chemical peaks often occur BEFORE peak river discharge.
Why? Because the first runoL rapidly mobilizes: pollutants on surfaces, sediments, organic matter, nutrients.
à This phenomenon is called: the “first flush eiect.”
Processes That Increase Chemical Concentrations During Storms (concentration spikes)
1) Mobilization of In-Stream Sediments = Increased turbulence resuspends bottom sediments.
2) Rapid Surface Runoi
3) Riverbank Erosion = High flow erodes banks and releases stored contaminants and sediments.
4) Direct Precipitation Inputs
Monitoring Challenges à River monitoring is diLicult because rivers change continuously. A single sample may therefore not
represent true river conditions. Storm events are especially important because a large fraction of annual pollutant transport can occur
during only a few extreme events.
1.5 Lake Basins
A lake begins with its basin (a bowl). The basin is the physical depression that stores water and controls how water, sediments,
nutrients, and pollutants behave inside the system.
How Lake Basins Form
Lake basins form through several geological and human processes.
1) Glacial Activity: Many lakes were carved by glaciers during ice ages. Glaciers excavate deep depressions and leave behind
large basins after melting. (North American Great Lakes)
1 Chapter 1: Water Quality Parameters
1.1 Introduction to water quality & parameters
Water quality is not just about whether water looks clean. It describes the overall condition of water and whether that water is
suitable for a specific use. This idea is often called “fit for purpose.”
A river that is safe for fish is not automatically safe for drinking. Water used for agriculture does not need the same quality as drinking
water. Because of this, water quality is always linked to a goal or function.
Water quality combines four major aspects:
• Chemical quality → nutrients, metals, pollutants, oxygen, pH
• Physical quality → temperature, turbidity, conductivity, suspended solids
• Biological/ecological quality → microorganisms, algae, biodiversity
• Radiological quality → radioactive substances
In environmental science, water quality is usually evaluated against standards. These standards help determine whether there is a
risk for ecosystems or humans.
The main objectives: protecting ecosystem health, protecting human health (secondary poisoning), & ensuring safe drinking water.
1.1.1 European Water Quality Frameworks
In Europe, water quality is strongly regulated through several directives.
1) EU Water Framework Directive (WFD)
The WFD focuses on achieving: good ecological status + good chemical status. This means water bodies are evaluated using:
chemical parameters, biological indicators, & physical conditions.
An important concept is the Environmental Quality Standard (EQS): the maximum concentration of a contaminant allowed before
ecological damage occurs. The WFD therefore looks beyond chemistry alone. A river can chemically meet standards but still fail
ecologically if biodiversity collapses.
2) Drinking Water Directive (DWD)
The Drinking Water Directive focuses on human exposure.
Main concern: pathogens, toxic chemicals, & contaminants aLecting public health. This directive is stricter because drinking water
directly enters the human body.
3) Groundwater Directive (GWD)
Groundwater is evaluated using: threshold values for contaminants & groundwater quantity.
4) Total Maximum Daily Load (TMDL) — USA
The TMDL concept defines the maximum pollutant load a water body can receive while still meeting quality standards.
1.1.2 Why Climate Change Matters for Water Quality
Water quality and climate change are deeply connected. Climate change modifies:
- water temperature - precipitation - flooding - salinity
1) Atmospheric CO₂ and Climate Change
One of the clearest indicators of climate change is atmospheric CO₂ concentration. The famous Keeling Curve at Mauna Loa
Observatory shows a continuous increase in atmospheric CO₂ since the late 1950s.
Key trends:
• Pre-industrial CO₂ ≈ 280 ppm
• Current CO₂ ≈ 425+ ppm
• Increase mainly caused by fossil fuel combustion and deforestation
The seasonal zigzag pattern exists because plants absorb more CO₂ during the growing
season and release more during winter.
More CO₂ means a stronger greenhouse eLect, which increases global temperature.
, 2) Global Temperature Increase
Global average temperature has already increased by about 1.1–1.5°C above pre-
industrial levels.
This warming may appear small, but globally averaged temperature changes are
extremely powerful. Even a few degrees change can: melt ice sheets, shift rainfall
patterns, intensify droughts, increase floods, destabilize ecosystems.
3) Climate Tipping Points
A tipping point is a threshold after which a system changes rapidly and sometimes irreversibly.
Examples: Greenland ice sheet collapse, collapse of Atlantic ocean circulation, Amazon rainforest dieback, permafrost thaw.
These systems are dangerous because they contain positive feedback loops.
Example: Ice melts → darker surface exposed → more sunlight absorbed → more warming → more melting. Once started, the process
can continue even if emissions decrease later.
1.1.2.1 Climate Change Impacts on Water Systems
Climate change changes the entire hydrological cycle.
Northern Europe
à Expected: more precipitation, stronger rainfall events, increased flood risk.
Southern Europe
à Expected: less precipitation, stronger droughts, desertification, water scarcity.
1.1.2.2 Major Water Quality Consequences
1. Higher Water Temperature
Warmer water contains less dissolved oxygen. This stresses aquatic organisms and favors harmful microorganisms.
2. Increased Eutrophication
Warmer temperatures and nutrient pollution stimulate algal growth. à cyanobacterial blooms & oxygen depletion,
3. Increased Pollutant Concentration During Drought
Lower river discharge means less dilution. Therefore: nutrients, heavy metals, wastewater contaminants become more concentrated.
4. Salinization
Sea level rise causes saltwater intrusion into: estuaries, coastal wetlands, groundwater aquifers. This threatens freshwater supplies
and agriculture.
1.2 Organic Contaminant Cycles in Lakes
Lakes are not passive water reservoirs. They behave as chemical reactors where contaminants are continuously transported,
transformed, stored, and re-released. Organic contaminants and metals move between the atmosphere, water, sediments, and
organisms through complex biogeochemical cycles.
Two important examples are: the Mercury (Hg) cycle & the PCB/Dioxin cycle.
1.2.1 Mercury (Hg) Cycle in Lakes
Mercury is a global pollutant that cycles continuously between air, water, sediments, and organisms. The most important concept is
that mercury becomes far more dangerous after it is transformed into methylmercury (MeHg).
Forms of Mercury: The toxicological importance increases strongly from Hg(0) Form Characteristics
→ Hg(II) → MeHg.
Bioaccumulation and Biomagnification Hg(0) Elemental mercury, volatile gas
Methylmercury enters the food chain through plankton and small organisms. Hg(II) Oxidized inorganic mercury
Concentrations increase at every trophic level: water → plankton → small fish →
predatory fish → birds/humans MeHg Methylmercury, highly toxic organic form
à This is called biomagnification.
Secondary poisoning occurs in top predators because they consume many contaminated prey organisms.
,1.2.2 PCB and Dioxin Cycle in Lakes
Polychlorinated biphenyls (PCBs) and chlorinated dioxins are classic examples of Persistent Organic Pollutants (POPs).
à They are: semi-volatile, hydrophobic, highly toxic, extremely persistent.
à These compounds are often grouped as PBTs: Persistent Bioaccumulative Toxic
Why PCBs and Dioxins Are Dangerous
The key property is hydrophobicity.
à They do not dissolve well in water but strongly bind to: organic matter, sediments, fatty tissues.
à Because of this: they remain in ecosystems for decades, they accumulate in organisms, they biomagnify through food chains.
Lakes as “Chemical Reactors” à Both examples show that lakes actively transform contaminants.
1.3 Hydrologic Cycle and Water Quality
Water quality is tightly connected to the hydrologic cycle because water continuously moves through:
- Atmosphere - soils - rivers - groundwater - lakes - oceans.
During this movement, water dissolves and transports chemicals. A watershed therefore strongly determines water chemistry.
Where Does Surface Water and Groundwater Get Their Chemical Composition?
This is one of the most important hydrogeochemical concepts. Water chemistry mainly originates from interaction with:
1) Atmospheric Input
Rainwater already contains dissolved substances: CO₂, sulfate, nitrate, sea salts, pollutants.
à CO₂ is especially important because it forms weak carbonic acid: CO2 + H2O à H2CO3
à This acid enhances mineral weathering.
2) Soil Interaction
As water infiltrates soils, it dissolves: nutrients, organic matter, ions.
à Microbial processes strongly modify water composition here.
3) Bedrock Weathering Bedrock Main Ions Released
The bedrock largely controls the dominant
Limestone Ca²⁺, HCO₃⁻
ions in groundwater.
à Examples: Gypsum Ca²⁺, SO₄²⁻
This is why groundwater chemistry diLers
Halite Na⁺, Cl⁻
strongly between regions.
1.4 Major Rivers
Europe contains several large river systems such as: the Danube, Rhine, Rhône.
These rivers connect entire watersheds to coastal systems and transport enormous quantities of water and contaminants across
countries. A river is therefore not an isolated ecosystem. Its quality reflects everything happening upstream in the watershed.
1.4.1 River Type Controls River Behavior
Not all rivers behave the same way. River morphology strongly influences water quality processes.
1. Large High-Flow Rivers à Examples: Mississippi, lower Danube.
These rivers have: - very large discharge - strong currents
- high sediment transport - strong mixing.
à Characteristics: - contaminants dilute more easily,
- transport distances are large,
- suspended sediments dominate,
- residence time is relatively short.
Because turbulence is high, oxygen exchange with the atmosphere is often eLicient. However,
high-flow rivers can transport huge pollutant loads downstream very quickly.
, 2. Meandering Low-Flow Rivers à Example: Amazon
These rivers typically occur in: flat terrain, floodplains, low-gradient landscapes.
à Characteristics: - slower flow - stronger sediment deposition,
- more stagnant zones - longer residence times.
These systems are more vulnerable to: eutrophication, oxygen depletion, sediment contamination.
Because water remains longer in the system, biological and chemical transformations become more important.
1.4.2 Flow Velocity Patterns Inside a River
River flow is not uniform. Velocity changes throughout the river cross-section.
General Pattern
à Fastest water: near the center + close to the surface. This occurs because friction is lowest there.
à Slowest water: near the riverbed + near the banks. Friction with sediments & riverbanks slows water.
Why Velocity Matters for Water Quality
à Fast-flowing zones: keep particles suspended, increase oxygenation, reduce settling.
à Slow-flowing zones: favor sediment deposition, allow contaminant accumulation, become oxygen-poor.
1.4.2.1 The Hydrograph (Chemograph = red)
A hydrograph shows how river discharge changes over time after rainfall. It links
precipitation to river response.
Lag Time = the delay between: maximum rainfall & maximum river discharge.
à Short lag time means: rapid runoL, flashy river response, higher flood risk.
à Long lag time means: slower infiltration, stronger groundwater contribution, more
buLered discharge.
Urban areas usually have short lag times because impermeable surfaces increase runoL. Forested or natural catchments generally
have longer lag times.
Sources of River Water During Storms: River discharge during rainfall events comes from several flow pathways.
1) Surface Runoi à Fastest pathway. Water flows directly over land into the river. This pathway is very important for
contaminant pulses during storms.
2) Interflow à Water moves laterally through shallow soils. Intermediate speed. Transports: dissolved nutrients,
3) Groundwater Flow (Baseflow) à Slowest pathway. Provides long-term stable discharge between storms.
Chemograph vs Hydrograph
Chemical peaks often occur BEFORE peak river discharge.
Why? Because the first runoL rapidly mobilizes: pollutants on surfaces, sediments, organic matter, nutrients.
à This phenomenon is called: the “first flush eiect.”
Processes That Increase Chemical Concentrations During Storms (concentration spikes)
1) Mobilization of In-Stream Sediments = Increased turbulence resuspends bottom sediments.
2) Rapid Surface Runoi
3) Riverbank Erosion = High flow erodes banks and releases stored contaminants and sediments.
4) Direct Precipitation Inputs
Monitoring Challenges à River monitoring is diLicult because rivers change continuously. A single sample may therefore not
represent true river conditions. Storm events are especially important because a large fraction of annual pollutant transport can occur
during only a few extreme events.
1.5 Lake Basins
A lake begins with its basin (a bowl). The basin is the physical depression that stores water and controls how water, sediments,
nutrients, and pollutants behave inside the system.
How Lake Basins Form
Lake basins form through several geological and human processes.
1) Glacial Activity: Many lakes were carved by glaciers during ice ages. Glaciers excavate deep depressions and leave behind
large basins after melting. (North American Great Lakes)
