Source Water & Hydrologic Cycle Fundamentals

Every drinking-water treatment plant begins with a source. The quality, reliability, and vulnerability of that source strongly influence treatment requirements, operating challenges, and the risks operators must manage.

Source water is part of the hydrologic cycle. Water continually moves among the atmosphere, land surface, rivers, lakes, soil, groundwater, and oceans. Understanding that movement helps operators understand why source-water quantity and quality change.

The Hydrologic Cycle

The hydrologic cycle, also called the water cycle, is the continuous movement of water through the environment.

Major processes include:

• evaporation;
• transpiration;
• condensation;
• precipitation;
• runoff;
• infiltration;
• percolation;
• groundwater recharge.

Evaporation

Evaporation occurs when liquid water changes to water vapor and enters the atmosphere.

Water can evaporate from:

• oceans;
• lakes;
• rivers;
• reservoirs;
• soil.

Transpiration

Transpiration is the release of water vapor from plants.

Evaporation and transpiration together return large quantities of water to the atmosphere.

Condensation

Condensation occurs when water vapor cools and forms liquid droplets.

Cloud formation is an important example.

Precipitation

Precipitation returns atmospheric water to the earth as:

• rain;
• snow;
• sleet;
• other forms of precipitation.

What Happens After Precipitation?

Water reaching the land surface can:

• run across the surface;
• enter streams and lakes;
• infiltrate soil;
• recharge groundwater;
• evaporate;
• be used by vegetation.

Runoff

Runoff is water that moves across the land surface toward:

• streams;
• rivers;
• lakes;
• reservoirs.

Runoff can transport contaminants from the land surface into source water.

Factors Affecting Runoff

Runoff generally increases with:

• heavy precipitation;
• saturated soil;
• steep slopes;
• impervious surfaces;
• frozen ground.

Impervious Surfaces

Roads, parking lots, roofs, and other impervious surfaces reduce infiltration and increase rapid surface runoff.

This can produce faster changes in:

• stream flow;
• turbidity;
• contaminant loading.

Infiltration

Infiltration is the movement of water from the land surface into soil.

Infiltration is affected by:

• soil type;
• soil saturation;
• vegetation;
• land use;
• precipitation intensity.

Percolation

Percolation is the downward movement of water through soil and geologic material.

Water that moves deep enough can contribute to groundwater recharge.

Groundwater Recharge

Recharge is the addition of water to a groundwater system.

Recharge can occur when precipitation or surface water infiltrates through soil and reaches an aquifer.

Recharge Areas

A recharge area is a location where water enters an aquifer.

Activities in recharge areas can affect groundwater quality.

Groundwater Discharge

Groundwater can return to the surface through:

• springs;
• seeps;
• streambeds;
• wetlands.

Groundwater and surface water are therefore often connected.

Watershed

A watershed is the land area that drains to a common surface-water location.

That location may be:

• a stream;
• a river;
• a lake;
• a reservoir;
• an intake.

Watershed Boundaries

Watershed boundaries are generally controlled by topography.

Water falling on opposite sides of a drainage divide may flow to different streams or reservoirs.

Why Watersheds Matter to Operators

Activities throughout a watershed can affect source-water quality.

Examples include:

• agriculture;
• construction;
• urban runoff;
• industry;
• wastewater discharges;
• forestry;
• transportation corridors.

Surface Water Sources

Common surface-water sources include:

• rivers;
• streams;
• lakes;
• reservoirs.

Surface Water Can Change Quickly

Surface-water quality can respond rapidly to:

• storms;
• snowmelt;
• upstream spills;
• algal growth;
• temperature changes;
• watershed activity.

Groundwater Sources

Groundwater is water stored below the land surface in saturated geologic formations.

It may be withdrawn through:

• wells;
• springs;
• other groundwater collection systems.

Groundwater Often Changes More Slowly

Groundwater can be less affected by short-term weather than surface water because soil and geologic material provide storage and some natural filtration.

However, groundwater can still become contaminated and may remain contaminated for long periods.

Aquifer

An aquifer is a geologic formation capable of storing and transmitting useful quantities of groundwater.

Aquifer Materials

Aquifers may occur in:

• sand;
• gravel;
• fractured rock;
• porous rock formations.

Water Table

The water table is the upper surface of the saturated zone in an unconfined groundwater system.

Its elevation can change because of:

• recharge;
• drought;
• pumping;
• seasonal conditions.

Unsaturated and Saturated Zones

Above the water table is the unsaturated zone, where pore spaces contain both air and water.

Below the water table is the saturated zone, where available pore spaces are filled with water.

Confined Aquifer

A confined aquifer is bounded by geologic material that restricts vertical water movement.

Water in a confined aquifer may be under pressure.

Unconfined Aquifer

An unconfined aquifer has the water table as its upper boundary and can receive recharge more directly from the surface.

Groundwater and Surface Water Interaction

A stream can gain water from groundwater or lose water to groundwater depending on local hydraulic conditions.

This means contamination in one system can sometimes influence the other.

Springs

A spring occurs where groundwater naturally reaches the land surface.

Spring water should not automatically be assumed to be free of surface influence or contamination.

Source-Water Quality

Source-water quality can include physical, chemical, and biological characteristics.

Important parameters may include:

• turbidity;
• temperature;
• pH;
• alkalinity;
• hardness;
• organic matter;
• microorganisms;
• iron and manganese;
• nutrients;
• salts.

Physical Source-Water Characteristics

Physical characteristics include:

• temperature;
• turbidity;
• color;
• odor;
• suspended solids.

Chemical Source-Water Characteristics

Chemical characteristics may include:

• pH;
• alkalinity;
• hardness;
• dissolved minerals;
• organic compounds;
• nutrients;
• metals.

Biological Source-Water Characteristics

Biological characteristics may include:

• bacteria;
• viruses;
• protozoa;
• algae;
• other microorganisms.

Surface Water and Turbidity

Storm runoff can carry:

• soil;
• organic matter;
• microorganisms;
• nutrients

into rivers and reservoirs.

This can cause rapid turbidity increases.

Storm Response

After heavy rainfall, operators may observe changes in:

• raw-water turbidity;
• color;
• organic matter;
• temperature;
• coagulant demand;
• disinfectant demand.

Source Water Can Change Before the Plant Sees It

There may be a travel-time delay between:

• rainfall;
• watershed runoff;
• arrival of changed water at the intake.

Operators should understand local watershed response.

Reservoir Effects

Reservoirs provide storage but can also develop water-quality stratification and biological activity.

Possible issues include:

• temperature layering;
• low dissolved oxygen at depth;
• iron and manganese release;
• algal growth;
• taste and odor compounds.

Seasonal Turnover

Changes in reservoir density structure can mix water from different depths.

This can rapidly change the quality of water reaching an intake.

Groundwater Mineral Quality

Groundwater spends time in contact with soil and rock.

This can increase dissolved concentrations of:

• hardness minerals;
• iron;
• manganese;
• alkalinity;
• other dissolved constituents.

Groundwater Turbidity

Groundwater often has lower turbidity than surface water, but turbidity can increase because of:

• well construction problems;
• sand pumping;
• rapid pumping changes;
• surface influence.

Source Quantity Matters Too

Operators must consider both:

• water quality;
• available quantity.

A source with excellent quality but insufficient yield may not reliably meet system demand.

Drought

Drought can affect sources by:

• lowering streamflow;
• reducing reservoir storage;
• lowering groundwater levels;
• increasing concentration of some contaminants.

Flooding

Flooding can create:

• high turbidity;
• debris;
• microbial contamination;
• chemical contamination;
• intake damage.

Source-Water Contamination

Contamination can come from:

• point sources;
• nonpoint sources;
• natural geologic sources.

Point Sources

A point source has an identifiable discharge location.

Examples can include:

• industrial discharge;
• wastewater outfall;
• pipe discharge.

Nonpoint Sources

Nonpoint-source pollution is distributed across a broader area.

Examples include:

• agricultural runoff;
• urban stormwater;
• soil erosion;
• fertilizer runoff.

Natural Contaminants

Some source-water constituents come naturally from geologic materials.

Examples may include:

• iron;
• manganese;
• hardness minerals;
• other naturally occurring substances.

Source Vulnerability

Source vulnerability describes how likely a water source is to be affected by contamination.

Vulnerability depends on:

• source type;
• hydrogeology;
• watershed land use;
• nearby contaminant sources;
• well construction;
• travel time.

Surface-Water Vulnerability

Surface waters can be especially vulnerable to:

• spills;
• storm runoff;
• microbial contamination;
• rapid upstream changes.

Groundwater Vulnerability

Groundwater vulnerability can increase with:

• shallow water table;
• highly permeable soils;
• fractured rock;
• poor well construction;
• contaminant sources near recharge areas.

Source Protection

Protecting the source can reduce treatment risk.

Source protection may include:

• watershed management;
• wellhead protection;
• land-use controls;
• spill planning;
• monitoring.

Wellhead Protection

Wellhead protection focuses on protecting the area contributing water to a public-supply well.

Activities near the well or recharge area can influence groundwater quality.

Intake Protection

Surface-water intake protection can include awareness of:

• upstream discharges;
• transportation routes;
• industrial facilities;
• spill risks;
• storm runoff.

Source Monitoring

Routine raw-water monitoring helps operators identify:

• normal seasonal patterns;
• rapid changes;
• developing contamination events;
• treatment-demand changes.

Useful Raw-Water Trends

Depending on the source, useful trends can include:

• turbidity;
• temperature;
• pH;
• alkalinity;
• conductivity;
• organic matter;
• microbiological indicators.

Conductivity as a Source Indicator

A sudden conductivity change can indicate a change in dissolved ionic material.

It may support investigation of:

• runoff;
• industrial influence;
• source blending;
• salt intrusion.

Raw-Water Turbidity Trend

Increasing raw-water turbidity can signal:

• storm runoff;
• erosion;
• intake disturbance;
• changing reservoir conditions.

Source Water Determines Treatment Challenge

Treatment requirements depend strongly on source characteristics.

For example:

• high turbidity increases particle-removal demand;
• high organic matter can increase coagulant and disinfectant demand;
• high iron or manganese may require oxidation and removal;
• microbial risk affects disinfection needs.

Surface Water Often Requires More Particle Treatment

Surface-water plants commonly need strong particle-removal barriers because raw-water turbidity and microbial conditions can change quickly.

Groundwater Treatment Can Be Different

Groundwater treatment may focus more heavily on dissolved constituents such as:

• iron;
• manganese;
• hardness;
• other minerals.

The exact treatment depends on actual source quality.

Source Blending

Some systems blend water from multiple sources.

Blending can change:

• hardness;
• alkalinity;
• pH;
• conductivity;
• organic matter;
• disinfectant demand.

Do Not Assume Two Sources Behave the Same

Changing from one source to another can require operating adjustments even when total flow remains unchanged.

Source Change Example

If a plant begins using a source with higher organic matter, operators may observe:

• higher coagulant demand;
• higher chlorine demand;
• greater disinfection byproduct precursor load.

Source Water and Chemical Feed

Chemical dose should respond to source-water conditions rather than remain fixed simply because yesterday's setting worked.

Source Water and Laboratory Data

Laboratory and field results help distinguish:

• source change;
• treatment problem;
• instrument problem.

Example: Sudden Raw-Water Turbidity Increase

Review:

• recent rainfall;
• watershed conditions;
• intake location;
• coagulant demand;
• filter performance.

Example: Groundwater Iron Increase

Review:

• well pumping rate;
• well condition;
• source blending;
• oxidation and filtration performance.

Example: Conductivity Increase

Review:

• source change;
• road salt or runoff influence;
• industrial activity;
• instrument verification.

Common Source-Water Mistakes

• Assuming source-water quality is constant.
• Ignoring watershed events.
• Assuming groundwater cannot be contaminated.
• Assuming spring water is automatically protected from surface influence.
• Ignoring source-water travel time after storms or spills.
• Changing treatment without verifying the source condition.
• Ignoring source blending effects.
• Looking only at source quality and not source quantity.

A Practical Surface-Water Review

1. Review recent weather and watershed conditions.
2. Review raw-water turbidity.
3. Review temperature and pH.
4. Review organic-matter indicators where available.
5. Review intake conditions.
6. Compare with historical seasonal trends.
7. Adjust treatment according to actual source response.

A Practical Groundwater Review

1. Review well level and pumping conditions.
2. Review turbidity.
3. Review iron and manganese.
4. Review hardness and alkalinity.
5. Review conductivity.
6. Review microbiological monitoring.
7. Compare with historical well performance.

A Practical Source-Change Review

1. Confirm which source or source blend is in use.
2. Compare current source-water quality with baseline conditions.
3. Review treatment chemical demand.
4. Review filtration or solids-removal performance.
5. Review disinfectant demand.
6. Trend finished-water response.
7. Document operational changes.

What to Remember for the Exam

• The hydrologic cycle moves water through evaporation, transpiration, condensation, precipitation, runoff, infiltration, percolation, and recharge.
• Runoff moves water across the land surface and can carry contaminants into surface-water sources.
• Infiltration is movement of water into soil, while percolation is downward movement through soil and geologic material.
• Recharge adds water to a groundwater system.
• A watershed is the land area draining to a common surface-water location.
• Activities throughout a watershed can affect source-water quality.
• An aquifer stores and transmits groundwater.
• The water table is the upper surface of the saturated zone in an unconfined groundwater system.
• Groundwater and surface water can interact.
• Surface-water quality can change rapidly after storms, spills, and seasonal events.
• Groundwater often changes more slowly but can still become contaminated.
• Impervious surfaces generally increase runoff and reduce infiltration.
• Source-water vulnerability depends on source type, geology, land use, nearby contamination sources, and travel pathways.
• Source protection can reduce treatment risk.
• Source-water quality affects chemical demand, filtration, disinfection, and other treatment requirements.
• Surface water often presents greater particle and rapid-variability challenges.
• Groundwater may contain higher concentrations of dissolved minerals such as iron, manganese, hardness, and alkalinity.
• Source quantity is important as well as source quality.
• Changing or blending sources can change treatment requirements.
• Good operators trend source-water conditions and connect source changes to treatment response.