Groundwater Hydrology

Definition of Groundwater

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

Relationship Between Groundwater and Surface Water

• Groundwater and surface water systems are interconnected
• Groundwater can replenish bodies of surface water sources
• Surface water can seep into groundwater reservoirs
• Base flow is streamflow derived from groundwater discharge into rivers or streams

Groundwater Recharge

• Recharge happens when infiltrated water reaches the water table
• Recharge occurs through direct infiltration from precipitation
• Recharge happens via horizontal or vertical seepage from surface water bodies

Factors Influencing Recharge

• The amount and rate of groundwater recharge depends on multiple factors
• Soil permeability affects the potential for recharge
• Forests and grasslands increase infiltration and recharge by minimizing surface runoff
• Urbanization, with paved roads and buildings, decreases recharge due to impervious surfaces
• Agricultural practices impact recharge, depending on irrigation and soil management
• Recharge depends on the amount, intensity, and timing of precipitation

Artificial Recharge Methods (Managed Aquifer Recharge - MAR)

• MAR is intentional human intervention to increase groundwater storage by artificially recharging aquifers
• MAR enhances water availability and security
• MAR combats aquifer depletion from over-extraction
• Recharge basins and infiltration ponds are types of ART
• Injection wells, riverbank filtration, and stormwater capture also act as ART

Groundwater Discharge

• Under natural conditions, groundwater discharges into rivers and lakes
• Groundwater discharges into coastal areas and directly into the ocean
• Groundwater may move upward into the capillary fringe, evaporating or being used by plants (evapotranspiration)

Significance of Groundwater in Freshwater Resources

• 30% of the world's total freshwater is groundwater
• 99% of the world's liquid freshwater is stored underground

Groundwater Dynamics

• Groundwater is always moving, albeit slowly
• Typical groundwater movement is less than one meter per day
• Groundwater residence time ranges from a few years to over 1,000 years
• Average global groundwater residence time is roughly 235 years
• Regional variations exist; some groundwater flows out in only a few years, while others remain underground for over 1,000 years

Groundwater and the Hydrologic Cycle

• Groundwater feeds rivers, lakes, and wetlands
• It forms a vital source of base flow during dry seasons
• Surface water interaction is dynamic and continuous

Human Dependence on Groundwater

• Groundwater is used for drinking water, irrigation, and industry
• In the United States, 25% of all water use is supplied by groundwater
• Some regions depend almost entirely on groundwater

Challenges in Groundwater Management

• Over-extraction decreases water tables and dries up wells
• Pollution from agriculture, industry, and waste disposal affect groundwater supplies
• Climate change impacts recharge rates and water availability
• Strategies include water conservation in agriculture and households
• Strategies include managed aquifer recharge (MAR) to improve groundwater levels
• Groundwater management must include pollution control via regulation and monitoring

Defining Aquifers

• An aquifer is a geological formation that stores and transmits groundwater at high rates
• Key characteristics of an aquifer include porosity
• Porosity refers to a rock or sediment's ability to hold water, indicated by the empty spaces or pores
• Permeability refers to a rock or sediment's ability to let water flow through it
• Permeability depends on pore connectivity and size

Unconfined and Confined Aquifers

• Unconfined aquifers have a water table that fluctuates with recharge and discharge
• Confined aquifers are trapped between low-permeability layers, meaning they are pressurized
• Understanding aquifer types is essential to groundwater management and hydrology

Groundwater Flow in Aquifers

• Groundwater moves based on potential energy gradients
• Flow direction depends on the hydraulic head
• Faster movement occurs in permeable materials and is slower in less permeable material

Defining Aquitards

• An aquitard is a semi-permeable underground barrier
• Aquitards restrict water movement, due to low permeability, unlike aquifers
• Aquitards are not completely impermeable, so some water can leak through slowly
• Key characteristics are low permeability
• Water cannot move easily through the material, but it is not completely impermeable
• Some leakage is possible, and while an aquitard slows down water movement, groundwater can pass through over long periods
• They are often above or below aquifers and act as a barrier that prevents rapid water flow between layers of the subsurface

Aquiclude

• Aquiclude refers to an outdated term that historically described "impermeable" formations
• Most geological materials allow some water movement, therefore the term is rarely used in modern hydrology

Relative Nature of Aquifers and Aquitards

• The classification of a layer as an aquifer or aquitard is relative to surrounding formations
• In sand-silt layers, sand is the aquifer, and silt is the aquitard
• In silt-clay layers, silt is the aquifer, and clay is the aquitard

Leaky Aquitards

• Some aquitards allow water transmission slowly
• Leaky aquitards influence confined aquifers by enabling limited recharge and pressure maintenance
• Leaky aquitards play a large role in groundwater recharge and pressure maintenance in confined systems

Potentiometric Surface

• A potentiometric surface is the level groundwater in a confined aquifer will rise to if unobstructed
• This differs from the water table and indicates pressure in a confined system
• In artesian wells, water flows with no pumping if the potentiometric surface is above ground level

Recharge and Discharge in Confined Aquifers

• In confined aquifers, recharge happens where the aquifer is exposed, and allows infiltration
• Discharge is through springs, leakage, or wells
• Since recharge zones are limited, confined aquifers are susceptible to depletion

Discharge Mechanisms in Confined Aquifers

• Water exits a confined aquifer via several pathways
• Springs create natural points where pressurized groundwater emerges
• Leakage moves water slowly through semi-permeable layers into adjacent formations
• Wells use human extraction via pumping to alter natural flow patterns

Managing Aquifers

• Sustainable groundwater management methods such as artificial recharge replenish groundwater
• Spreading basins are shallow and allow water to percolate into the ground naturally
• Injection wells directly inject treated surface water or stormwater into the aquifer
• Recharge trenches use channels designed to enhance infiltration in areas where water naturally seeps into the subsurface
• Promote efficient water use and reduce unnecessary waste with conservation practices
• Impose extraction limits and conservation plans with regulatory policies
• Sustainable management includes continuous observation to improve management with monitoring and research
• Lack of sustainable management leads to water shortages, ecological harm, and economic losses

Storage Properties of Aquifers

• Aquifers store and release water as hydraulic head changes
• Storage properties dictatate how much water an aquifer can hold or release
• Key parameters relating to the storage properties of aquifers are Specific Storage (Ss) and Storage Coefficient (S)
• Storage properties vary between confined and unconfined aquifers
• Hydraulic head indicates the energy level of groundwater, which determines the flow's direction and movement
• Increases in hydraulic head mean more water is stored in the aquifer
• Decreases in hydraulic mean water released, either naturally (through springs or seepage) or via pumping

Unconfined vs. Confined Aquifer Storage

• In unconfined aquifers, the water table moves depending on recharge/discharge
• Storage change mainly results from water table fluctuation
• In confined aquifers, water is stored under pressure
• Storage changes due to the compression/expansion of water within the aquifer materials

Specific Storage (Ss)

• Specific Storage (Ss) refers to the volume of water released per unit volume of aquifer per unit drop in hydraulic head
• Expressed as Ss [L⁻¹]
• Accounts for water released because of compressibility of water and the aquifer matrix

Storage Coefficient (S)

• S refers to the volume of water released or taken into storativity per unit of aquifer storage
• S = Ssb (unitless equation)
• S = volume of water released per 1 m3 of aquifer volume per 1m change in head
• Example: S = 0.01 m-1 (0.01 m3 released per 1m3 box)
• Aquifer volume per 1 m2 of aquifer area= aquifer thickness = b
• Example: = 3m x 1m2 / 1m2 = 3m = b
• Range of S = 0.005 to 0.00005 (10-3 to 10-5)

Storage in Confined Aquifers

• Water is released due to compression of aquifer materials and expansion of water
• Storage occurs without water table change
• The equation for this is S = b × Ss
• A confined aquifer's dimensionless storage coeffication, S, often refers to storativity
• Storativity values are lower than in unconfined aquifers

Storage Coefficient in Unconfined Aquifers

• Changes in head affect the water table
• Water is released from specific yield (Sy)
• Specific yield or (Sy) defines the volume of water released per unit surface area, per unit decline of water table
• S = Sy
• Some water stays in the pores because of specific retention (Sr)
• The ratio of water retained in the portion of the aquifer experiencing the a head decline is the specific retention (Sr)

Real-World Examples

• The Ogallala Aquifer in the USA is one of the largest unconfined aquifers
• Heavy irrigation has significantly lowered water levels
• The Edwards Aquifer in Texas is a confined system with artesian pressure
• Springs are dependent on aquifer pressure for flow
• The Oak Ridges Moraine Aquifer in Ontario, Canada is an unconfined aquifer
• The aquifer is a system that provides drinking water to over 250,000 people in southern Ontario
• Water is stored in a sand and gravel matrix, with rises and falls in the local water table for storage behavior
• Urbanization and groundwater extraction threaten the aquifer, which have led to reduced recharge rates
• The Bassano Aquifer in Alberta, Canada is confined
• It supports municipal and agricultural water use
• Water levels remain relatively stable to mitigate excessive pumping reduces pressure in the system
• The major challenge to the aquifer is carefull managing over-extraction and loss of artesian pressure

Key Takeaways

• Unconfined aquifers store water by water table fluctuations (Sy factor)
• Confined aquifers store water through compression and expansion (governed by Ss)
• S is higher in unconfined aquifers