Groundwater Recharge

• Groundwater exists under positive pressure in the saturated zone of earth materials and beneath the surface where pore spaces are fully saturated

Groundwater and Surface Water

• Groundwater and surface water are interconnected, with groundwater recharging rivers, lakes, and oceans
• Surface water seeps into groundwater reservoirs
• Base flow is streamflow from groundwater discharge into a river or stream

Groundwater Recharge

• Recharge occurs when infiltrated water reaches the water table, facilitated by direct infiltration from precipitation and horizontal or vertical seepage from surface water bodies

Factors Influencing Recharge

• The amount and rate of recharge depends on soil permeability, land use, vegetation, climate, and precipitation patterns
• Forests and grasslands promote infiltration and recharge by reducing surface runoff
• Urbanization reduces recharge due to impervious surfaces preventing water soaking into the ground
• Agricultural practices influence recharge based on irrigation methods and soil management

Artificial Recharge

• Managed Aquifer Recharge (MAR) enhances groundwater storage through intentional human intervention to increase water availability, security, and combat aquifer depletion caused by over-extraction
• This is achieved via recharge basins and infiltration ponds, injection wells, riverbank filtration, and stormwater capture and recharge

Groundwater Discharge

• Under natural conditions, groundwater discharges into rivers, lakes, and coastal areas
• Water moves upward into the capillary fringe and the unsaturated zone used by plants (evapotranspiration)

Significance of Groundwater

• Groundwater comprises 30% of the world's total freshwater
• 99% of the world's liquid freshwater is stored underground
• Groundwater is constantly moving, but slowly, with typical movement rates of less than 1 meter per day
• The global groundwater residence time is estimated at 235 years, with regional variations
• Groundwater feeds rivers, lakes, and wetlands, serving as a vital source of base flow in dry seasons
• Interaction with surface water is dynamic and continuous with 25% of all water use in the United States coming from groundwater, and some regions rely almost entirely on it

Challenges and Management

• Over-extraction lowers water tables and dries up wells
• Pollution from agriculture, industry, and waste affects groundwater
• Climate change impacts recharge rates and availability
• Managment strategies include, water conservation, managed aquifer recharge, and pollution control

Aquifers

• An aquifer is a geologic formation storing and transmitting groundwater at hydrologically significant rates

Aquifer Key Characteristics

• Porosity signifies a rock/sediment's ability to store water, referring to empty spaces or pores
• Permeability signifies a rock/sediment's ability for water flow, dependent on pore connectivity and size

Aquifer Types

• Unconfined aquifers have a fluctuating water table with recharge and discharge
• Confined aquifers are trapped between low-permeability layers, causing pressurized conditions.
• Understanding unconfined and confined aquifers is essential for groundwater management and hydrology

Aquitard

• An aquitard is a semi-permeable barrier restricting water movement
• Water cannot move easily through the material, even though it is not totally impermeable
• Aquitards can be found above or below aquifers, acting as a barrier and preventing rapid water flow

Aquiclude

• An aquiclude historically described ‘impermeable’ formations, but is rarely used in modern hydrology because most geologic material allow some water movement
• Whether a layer is an aquifer or an aquitard depends on the surrounding formations, for example in sand-silt layers, sand is the aquifer, and silt is an aquitard

Leaky Aquitards

• Leaky aquitards allow slow water transmission, influencing confined aquifers and allowing limited recharge and pressure maintenance

Potentiometric Surface

• This is the level to which groundwater in a confined aquifer rises if unobstructed
• In artesian wells, water flows naturally whenever it is above ground level, no pumping required

Recharge and Discharge

• Recharge in confined aquifers occurs where the aquifer is exposed, allowing infiltration
• Discharge occurs through springs, leakage, or wells
• Confined aquifers are more vulnerable to depletion

Discharge Mechanisms

• Water exits a confined aquifer through springs, leakage, and wells

Managing Aquifers

• Sustainable groundwater management includes artificial recharge, conservation practices, regulatory policies, monitoring, and research
• Without sustainable management, groundwater depletion causes water shortages, ecological harm, and economic losses

Storage Properties of Aquifers

• Aquifers store and release water due to changes in hydraulic head
• Storage properties determine how much water an aquifer can hold and release
• Key parameters include Specific Storage (Ss) and Storage Coefficient (S)
• Hydraulic head represents the energy level of groundwater, determining flow direction and movement
• When hydraulic head increases, it causes more water storage in the aquifer
• When hydraulic head decreases, water is released naturally or through pumping

Aquifer Types: Storage Differences

• Unconfined aquifers water table moves up or down through recharge or discharge
• Confined aquifers have water stored under pressure with storage changes due to compression/expansion of materials

Specific Storage Definition

• Volume of water released / unit volume of aquifer per unit drop in hydraulic head measured in [L-1]
• Accounts for water released due to compressibility of water and the aquifer matrix

Storage Coefficient

• Volume of water released or taken into storage per unit of aquifer storage area per unit change in head
• This is referred to as storativity in confined aquifer; S = Ss * b

Storage in Storage in Confined Aquifers

• Water is released during compression of aquifer materials and expansion of water.
• Storage occurs without a water table change; Equation: S = b × Ss
• Storage coefficient (S) is also called the storativity
• Storativity values are much lower than in unconfined aquifers

Storage Coefficient in Unconfined Aquifers

• Water is released from specific yield (Sy), and changes in head affect the water table
• Specific Yield, Sy: Volume of water released/ the volume of saturated material
• some water remains in the pores due to specific retention (Sr)
• total = Sy + S,

Real World Examples

• Ogallala Aquifer (USA) is one of the largest unconfined aquifers; heavy irrigation has significantly lowered water levels
• Edwards Aquifer (Texas, USA) is a confined system with artesian pressure that is springs dependent on aquifer pressure for flow
• Oak Ridges Moraine Aquifer (Ontario, Canada) is the unconfined aquifer where water is stored in sand and gravel with fluctuations in the water table
• Bassano Aquifer (Alberta, Canada) is the confined aquifer that supports municipal and agricultural water use with stable water levels when managed correctly

Key Takeaways

• Unconfined aquifers fluctuate and store water by fluctuating the water table
• Confined aquifers store water through compression and expansion
• Storage coefficient (S) is higher in unconfined aquifers