Groundwater, Aquifers & Recharge

Groundwater is an important drinking-water source for many communities. Although groundwater is stored below the land surface, it is still connected to precipitation, surface water, land use, geology, and human activity.

Operators should understand how aquifers store and transmit water, how recharge replaces groundwater, how pumping affects water levels, and why hydrogeology influences both source quantity and water quality.

What Is Groundwater?

Groundwater is water stored below the land surface in saturated soil, sediment, and rock.

Groundwater commonly originates from precipitation that:

• infiltrates the land surface;
• moves downward through soil and geologic material;
• reaches the saturated zone.

Unsaturated Zone

The unsaturated zone lies above the water table.

Its pore spaces contain:

• air;
• water.

Water moving through this zone can eventually contribute to groundwater recharge.

Saturated Zone

The saturated zone is the subsurface region where available pore spaces are filled with water.

Groundwater occurs within this zone.

Water Table

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

The water table can rise or fall because of:

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

Aquifer

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

Aquifers may occur in:

• sand;
• gravel;
• sandstone;
• fractured rock;
• other permeable formations.

Aquitard

An aquitard is a geologic layer that transmits water much more slowly than a productive aquifer.

Materials such as clay or dense rock can act as aquitards.

Porosity

Porosity is the fraction of a material's total volume occupied by openings or pore spaces.

A simplified relationship is:

Porosity, % = Pore Volume ÷ Total Volume × 100

Porosity Example

If a soil sample has a total volume of 100 cubic centimeters and 30 cubic centimeters consist of pore space:

Porosity = 30 ÷ 100 × 100

Porosity = 30%

High Porosity Does Not Always Mean High Water Yield

A material can have many pore spaces but still transmit water poorly.

Clay is a common example because its pores can be very small and poorly connected for rapid groundwater movement.

Permeability

Permeability describes how easily water can move through connected openings in soil or rock.

High permeability generally means water can move more readily.

Porosity Versus Permeability

Porosity describes how much open space exists.

Permeability describes how well those spaces are connected for water movement.

They are related but not the same.

Examples of Aquifer Materials

Sand and gravel often have:

• good pore connection;
• relatively high permeability;
• good groundwater transmission.

Clay may have high porosity but much lower permeability.

Fractured Rock Aquifers

In fractured rock, groundwater movement may depend primarily on:

• fracture size;
• fracture connection;
• fracture orientation.

A well can have high yield if it intersects productive fractures even when the solid rock itself has low primary porosity.

Unconfined Aquifer

An unconfined aquifer has the water table as its upper boundary.

Recharge can enter relatively directly from the land surface.

Unconfined Aquifer Vulnerability

Because recharge may move more directly from the surface, unconfined aquifers can be vulnerable to contamination from:

• spills;
• septic systems;
• agriculture;
• surface runoff;
• chemical storage.

Confined Aquifer

A confined aquifer is bounded above by a less permeable geologic layer that restricts vertical water movement.

Groundwater in a confined aquifer may be under pressure.

Confining Layer

A confining layer can reduce direct vertical movement from the land surface.

However, a confined aquifer should not automatically be assumed to be free from contamination.

Contaminants may enter through:

• recharge areas;
• fractures;
• poorly constructed wells;
• abandoned wells.

Artesian Conditions

When a confined aquifer is under pressure, water in a well may rise above the top of the aquifer.

This is called an artesian condition.

Flowing Artesian Well

If pressure causes water to rise above the land surface without pumping, the well is called a flowing artesian well.

Artesian Does Not Mean Unlimited Water

An artesian well still depends on:

• aquifer recharge;
• aquifer storage;
• hydraulic pressure;
• pumping from nearby wells.

Recharge

Groundwater recharge is the addition of water to an aquifer.

Recharge may come from:

• precipitation;
• snowmelt;
• streams;
• lakes;
• other surface-water bodies.

Recharge Areas

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

Protecting recharge areas can be important because contaminants entering these areas may eventually reach groundwater wells.

Factors Affecting Recharge

Recharge can be influenced by:

• soil permeability;
• geology;
• vegetation;
• land slope;
• precipitation;
• impervious surfaces.

Impervious Surfaces and Recharge

Roads, roofs, and parking areas generally reduce infiltration.

This can:

• increase surface runoff;
• reduce local groundwater recharge.

Drought and Recharge

Long dry periods can reduce aquifer recharge.

Groundwater levels may then decline, especially where pumping continues.

Groundwater Discharge

Groundwater discharge occurs where groundwater leaves the aquifer.

Discharge can occur through:

• springs;
• streams;
• wetlands;
• wells;
• seepage.

Groundwater Flow

Groundwater generally moves from areas of higher hydraulic head toward areas of lower hydraulic head.

The direction and rate of movement depend on:

• hydraulic gradient;
• permeability;
• aquifer geometry.

Hydraulic Head

Hydraulic head represents the energy or potential driving groundwater movement.

In wells, water-level elevation is commonly used as an indicator of hydraulic head.

Hydraulic Gradient

Hydraulic gradient is the change in hydraulic head over distance.

A steeper gradient generally provides a stronger driving force for groundwater movement.

Groundwater Moves Slowly Compared with Surface Water

Groundwater movement is often much slower than river or stream flow.

This means groundwater contamination can:

• take a long time to reach a well;
• remain in an aquifer for long periods.

Groundwater and Streams

Groundwater and surface water frequently interact.

A stream may:

• receive groundwater;
• lose water to an aquifer.

Gaining Stream

A gaining stream receives groundwater discharge.

Groundwater can help maintain streamflow during dry weather.

Losing Stream

A losing stream loses water through the streambed into groundwater.

Contaminated surface water can potentially influence groundwater under suitable conditions.

Groundwater Storage

Aquifers can store very large amounts of water.

However, not all water stored in pore space is easily released to a well.

Specific Yield

Specific yield is the portion of groundwater that a saturated material can release by gravity.

It is generally less than total porosity because some water remains held within the material.

Well Pumping

Pumping removes water from an aquifer and lowers hydraulic head near the well.

This creates a localized decline in water level.

Static Water Level

Static water level is the water level in a well when the well is not being pumped and has had sufficient time to recover.

Pumping Water Level

Pumping water level is the water level while the well is operating.

Drawdown

Drawdown is the difference between static water level and pumping water level.

A basic relationship is:

Drawdown = Static Water Level - Pumping Water Level

When depth below ground is used, operators should apply the signs consistently.

Drawdown Example

If static water level is 40 feet below ground and pumping water level is 65 feet below ground:

Drawdown = 65 - 40 = 25 feet

The water level has declined 25 feet during pumping.

Cone of Depression

Pumping creates a cone of depression around the well.

The water level is lowest near the pumping well and gradually rises with distance from it.

Nearby Wells Can Interfere

If cones of depression overlap, wells can influence each other.

This can cause:

• greater drawdown;
• reduced available yield;
• changes in groundwater flow direction.

Overpumping

Excessive pumping can cause:

• large drawdown;
• reduced well yield;
• pump problems;
• changes in water quality;
• aquifer depletion.

Groundwater Level Trends

Operators should trend:

• static water level;
• pumping water level;
• well flow;
• pump operating time.

Long-term changes can reveal declining aquifer performance or well problems.

Recharge Versus Withdrawal

Groundwater supply is more sustainable when long-term withdrawals remain compatible with available recharge and aquifer storage.

If withdrawals repeatedly exceed replenishment, groundwater levels may decline over time.

Groundwater Quality and Geology

Groundwater spends extended time in contact with soil and rock.

This can dissolve minerals and affect:

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

Hardness

Groundwater flowing through mineral-rich formations may contain elevated calcium and magnesium.

This can increase hardness.

Iron and Manganese

Groundwater with low dissolved oxygen can contain soluble iron and manganese.

After oxidation, these metals can form visible precipitates that may require removal.

Groundwater Can Have Low Turbidity but High Dissolved Minerals

Clear appearance does not mean groundwater requires no treatment.

Dissolved contaminants may not be visible.

Groundwater Temperature

Groundwater temperature often changes more slowly than surface-water temperature.

This can produce relatively stable seasonal treatment conditions.

Groundwater Contamination

Possible contamination sources include:

• septic systems;
• fuel storage;
• industrial chemicals;
• agricultural chemicals;
• landfills;
• road salt;
• surface spills.

Contamination Pathways

Contaminants can reach an aquifer through:

• infiltration;
• fractures;
• recharge areas;
• poorly sealed wells;
• abandoned wells.

Fractured-Rock Vulnerability

Fractures can transmit water and contaminants rapidly compared with movement through fine-grained material.

This can make groundwater response less predictable.

Karst Conditions

In soluble rock such as limestone, groundwater may move through enlarged fractures, channels, or cavities.

These systems can allow relatively rapid movement from the surface to groundwater.

Natural Filtration Has Limits

Soil and geologic material can remove or reduce some particles and microorganisms, but this should not be assumed to eliminate all contaminants.

Well Construction Protects Water Quality

A properly constructed well helps reduce contamination pathways.

Important components can include:

• well casing;
• sanitary seal;
• grout;
• protected wellhead;
• proper drainage.

Well Casing

Well casing provides structural support and helps isolate the well from undesirable shallow formations.

Grouting

Grout seals the space around the well casing and helps prevent surface water from moving downward along the outside of the casing.

Wellhead Protection

The wellhead should be protected from:

• surface runoff;
• flooding;
• animals;
• vehicle damage;
• chemical spills.

Drainage Around the Well

Ground around a well should generally direct surface water away rather than allowing ponding near the casing.

Abandoned Wells

Improperly abandoned wells can provide direct pathways for contamination into groundwater.

Wellhead Protection Area

A wellhead protection area is an area around or contributing to a public water-supply well where activities may affect groundwater quality.

Potential Contaminant Sources

Operators should be aware of nearby:

• fuel tanks;
• chemical storage;
• septic systems;
• agricultural operations;
• industrial facilities;
• transportation routes.

Source-Water Monitoring

Useful groundwater monitoring can include:

• static water level;
• pumping level;
• well flow;
• turbidity;
• pH;
• conductivity;
• iron;
• manganese;
• microbiological indicators.

Sudden Groundwater Turbidity

A sudden turbidity increase may indicate:

• well construction problems;
• sand production;
• rapid pumping change;
• surface influence;
• sample or instrument error.

Changing Conductivity

A conductivity change can indicate changing dissolved mineral content or source influence.

Review:

• well operation;
• source blending;
• seasonal conditions;
• nearby contamination events.

Microbiological Change

Unexpected microbial results in groundwater should prompt review of:

• sampling technique;
• wellhead condition;
• flooding;
• surface drainage;
• well integrity;
• treatment performance.

Flooding and Wells

Floodwater around a well can increase contamination risk.

Operators should follow applicable procedures when a well is affected by flooding or possible surface-water intrusion.

Drought and Wells

During drought, operators may observe:

• lower static water levels;
• greater drawdown;
• reduced well yield;
• changes in water quality.

Pumping Rate Can Affect Water Quality

Changing pumping rate can alter:

• which fractures contribute water;
• aquifer flow direction;
• sand movement;
• water chemistry.

Multiple Wells

Systems with multiple wells should compare:

• yield;
• drawdown;
• water quality;
• energy use;
• recovery.

Blending Groundwater Sources

Combining wells can change finished raw-water characteristics such as:

• hardness;
• iron;
• manganese;
• alkalinity;
• conductivity.

Do Not Assume Every Well Is Chemically Identical

Two wells in the same system can produce significantly different water because they may:

• draw from different aquifers;
• intersect different fractures;
• have different depths;
• receive different recharge.

Groundwater Travel Time

Groundwater contamination may take significant time to travel from the source area to a well.

This means current well quality may reflect land-use or contamination events that occurred much earlier.

Groundwater Recovery Can Also Be Slow

Once contamination enters an aquifer, removal can be difficult because groundwater moves slowly and large volumes may be stored underground.

Common Groundwater Mistakes

• Confusing porosity with permeability.
• Assuming high porosity always means high aquifer yield.
• Assuming confined groundwater cannot be contaminated.
• Assuming clear groundwater requires no treatment.
• Ignoring recharge areas.
• Ignoring interaction between groundwater and surface water.
• Ignoring well interference.
• Ignoring long-term water-level trends.
• Assuming every well in a system has the same water quality.
• Ignoring wellhead drainage and sanitary protection.

A Practical Groundwater Source Review

1. Review static water level.
2. Review pumping water level and drawdown.
3. Review well flow.
4. Review recent precipitation and drought conditions.
5. Review turbidity and conductivity.
6. Review iron, manganese, hardness, and alkalinity.
7. Review microbiological monitoring.
8. Inspect wellhead and surrounding drainage.
9. Compare current results with historical well trends.

A Practical Unexpected-Water-Quality Review

1. Verify the analytical result.
2. Confirm which well or source blend is operating.
3. Review pumping rate and drawdown.
4. Review recent weather and groundwater-level changes.
5. Inspect wellhead condition.
6. Review nearby potential contamination events.
7. Compare other wells and historical data.
8. Adjust operations according to approved procedures.

What to Remember for the Exam

• Groundwater occurs in the saturated zone below the land surface.
• The water table is the upper surface of the saturated zone in an unconfined aquifer.
• An aquifer stores and transmits useful quantities of groundwater.
• Porosity describes the amount of pore space, while permeability describes how easily water moves through connected openings.
• High porosity does not automatically mean high permeability.
• An unconfined aquifer has the water table as its upper boundary.
• A confined aquifer is separated from the surface by lower-permeability material and may be under pressure.
• Artesian conditions occur when groundwater in a confined aquifer rises in a well because of pressure.
• Recharge adds water to an aquifer, while discharge removes water from the groundwater system.
• Recharge areas are important locations for groundwater protection.
• Groundwater generally moves from higher hydraulic head toward lower hydraulic head.
• Groundwater and surface water can exchange water.
• Static water level is measured when a well is not pumping, while pumping water level is measured during operation.
• Drawdown is the decline in water level caused by pumping.
• Pumping creates a cone of depression around a well.
• Nearby pumping wells can interfere with one another.
• Groundwater quality is strongly influenced by geology and may contain dissolved hardness, iron, manganese, alkalinity, and other minerals.
• Well construction, grouting, drainage, and wellhead protection help reduce contamination pathways.
• Groundwater contamination can persist for long periods because groundwater movement is often slow.
• Good operators trend groundwater levels, well yield, water quality, and wellhead condition together.