Groundwater
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75 Terms
What fraction of Earth’s freshwater is groundwater?
~30%. Groundwater is the source of drinking water for about half the world’s population; ~2/3 of Wisconsin residents get drinking water from groundwater.
Soil moisture zone
definition | The top few meters of the subsurface where plant roots extract water. Water held mostly by capillary forces. Highly variable
Unsaturated zone (vadose zone, zone of aeration)
definition | Zone below the soil moisture zone but above the water table. Pores contain BOTH air AND water. Water moves downward by gravity as it percolates toward the water table.
Capillary fringe
definition | Thin transitional zone just above the water table where water is drawn upward by capillary action against gravity. Pores are saturated, but water is under negative pressure (tension).
Capillary fringe thickness
coarse vs. fine sediment | Fine-grained sediments (silt, clay): thick capillary fringe (up to a meter or more). Coarse sediments (sand, gravel): thin (just a few centimeters).
Water table
definition | The top of the saturated zone
Does the water table follow topography? |
es, but in a subdued way. higher under hills, lower under valleys. Intersects the ground surface at streams, lakes, springs. Also fluctuates seasonally and in response to rainfall and pumping.
Saturated zone (phreatic zone)
definition | Everything below the water table. All pore spaces are completely filled with water (100% saturated). Where actual groundwater lives; supplies wells and springs.
Porosity
definition and formula | Fraction of a rock or sediment volume that is empty pore space. Porosity = volume of voids / total volume. Expressed as a percentage or decimal fraction.
Does grain size determine porosity?
No! A container of marbles has the same porosity whether they’re golf balls or bb-pellets. What matters is packing and sorting.
How does sorting affect porosity?
Well-sorted sediment (uniform grain size) has HIGH porosity, nothing fills the gaps. Poorly-sorted sediment has LOW porosity
Porosity of well-sorted sand and gravel | 25–50%.
Porosity of clay | 40–70% (surprisingly high)
but the pores are tiny and disconnected, so permeability is very low.
Porosity of shale and unfractured igneous rock | Very low: shale 0–10%, unfractured igneous ~0%. Fractured igneous rock has secondary porosity up to 10%.
Porosity of karst limestone | Up to 50% due to dissolution channels (secondary porosity). Very high for a rock type.
Primary vs. secondary porosity | Primary: original pore space between grains, formed at deposition. Secondary: created after deposition
from fractures, joints, dissolution (karst), or weathering.
Example of a rock dominated by secondary porosity | Karst limestone (dissolution channels) or fractured granite (joints).
Permeability
definition | How EASILY water flows through rock or sediment. Depends on both pore SIZE and pore CONNECTIVITY.
Is porosity the same as permeability? | NO. Porosity is amount of pore space; permeability is how well pores are connected and how large they are. Clay has high porosity but nearly zero permeability.
Why is clay nearly impermeable despite high porosity? | Clay pores are tiny (<1 μm). At that scale, surface tension and electrostatic forces dominate over gravity
water sticks to clay surfaces and can’t flow. Pore space is functionally locked up.
Rank permeability from high to low: gravel, sand, silt, clay | Gravel (extremely permeable) > sand (very permeable) > silt (low) > clay (nearly impermeable).
Hydraulic conductivity (K)
what it measures | The quantitative measure of permeability. Units: length per time (m/s or ft/day). Ranges over ~13 orders of magnitude across natural materials.
Specific yield (Sy)
definition | The fraction of total volume that will DRAIN out of an aquifer under gravity.
Specific retention (Sr)
definition | The fraction of total volume that STAYS HELD by capillary forces after gravity drainage.
Relationship between porosity, specific yield, and specific retention | Porosity = Specific yield + Specific retention.
Why are clay aquifers poor water sources? | Not just because water flows slowly through them, but because most of the water stays held by capillary forces
very low specific yield (~2%) even though total porosity is high.
Aquifer
definition | A geologic unit that stores AND transmits significant water
Common aquifer materials | Sand and gravel deposits, sandstones with intergranular porosity, karst limestones, fractured igneous or metamorphic rock, basalt with cooling joints.
Aquitard
definition | A unit that transmits water very slowly but not zero. Water can leak through over long timescales; not a viable water source. Silts, shales, glacial tills are typical.
Aquiclude
definition | A unit that transmits essentially NO water. True aquicludes are rare; most low-permeability layers leak a little. Thick dense clay or shale is the classic example.
Confining bed / confining layer
definition | General term for a low-permeability unit above or below an aquifer that restricts water movement into or out of it. Aquitards and aquicludes serve as confining beds.
Unconfined aquifer (water table aquifer)
definition | Aquifer whose upper boundary IS the water table itself. No confining layer above. Water freely infiltrates from surface to recharge the aquifer.
Characteristics of unconfined aquifers | Water table is the top; recharge directly from precipitation; water level rises/falls with recharge and pumping; wells show water at the water table; vulnerable to surface contamination.
Confined (artesian) aquifer
definition | Aquifer sandwiched between two confining layers. The upper confining layer prevents direct recharge from above; recharge occurs where the aquifer material outcrops at the surface elsewhere.
Characteristics of confined aquifers | Water is under pressure; recharge occurs far away at outcrops; water travels long distances underground; less vulnerable to contamination; water can be thousands of years old.
Potentiometric surface (piezometric surface)
definition | The level to which water would rise in a well penetrating a confined aquifer. NOT the top of the aquifer
Flowing artesian well
definition | A well in a confined aquifer where the potentiometric surface is ABOVE the ground surface. Water flows at the surface without pumping.
Perched aquifer
definition | A small, localized zone of saturation ABOVE the regional water table, held up by a discontinuous confining layer (e.g., a clay lens). Above and below the perched zone, material is unsaturated.
Where are perched aquifers common? | Glaciated regions where clay lenses are interbedded with sand and gravel. Often small volume, may dry up seasonally, and can produce misleading well data.
Recharge
definition and main source for unconfined aquifers | Water entering an aquifer to replenish storage. Main source for unconfined aquifers: infiltration from precipitation.
Sources of recharge (list) | Infiltration from precipitation, losing streams, irrigation return flow, leakage from lakes/reservoirs, injection wells (artificial).
Where do confined aquifers recharge? | Typically where the aquifer material OUTCROPS at the surface
often many miles from where water is eventually withdrawn.
Discharge
definition and mechanisms | Water leaving an aquifer. Mechanisms: springs, gaining streams, baseflow to lakes/wetlands, riparian ET, wells, submarine groundwater discharge.
Aquifer depletion
cause | When pumping exceeds recharge over long time, causing water table decline. Major issue: Ogallala Aquifer (US Great Plains), North China Plain, parts of India.
Topic G Part 2: Water Table Dynamics, Wells, and Surface-Groundwater Interactions
Cone of depression
definition | A downward-sloping funnel in the water table centered on a pumping well. Deeper cone with more pumping; wider/shallower with higher K; extends farther with longer pumping.
Overdraft
definition | When multiple wells with overlapping cones of depression dramatically lower the regional water table over time. A form of aquifer depletion.
Hydraulic head (h)
definition | The elevation to which water would rise in a well open to that point in the aquifer. Represents total energy per unit weight. Drives all groundwater flow.
Two components of hydraulic head | Elevation head (z, height above a reference) + pressure head (P/ρg, height of water column the pressure could support). Written: h = z + P/ρg.
Direction of groundwater flow | From HIGH hydraulic head to LOW hydraulic head. NOT necessarily from high elevation to low elevation
water can flow UPWARD if pressure head is high enough (this is what happens in artesian systems).
Hydraulic gradient
definition and formula | The slope of the water table (or potentiometric surface) in the direction of flow. i = Δh / L, where Δh is head difference and L is horizontal distance.
Units of hydraulic gradient | Dimensionless (m/m or ft/ft)
similar to surface slope. Δh and L have the same units, so the ratio has none.
What does a steeper hydraulic gradient mean for groundwater flow? | Faster flow, all else equal. Steeper gradient is the analog of steeper surface slope for streams.
Water table contour map
what do contour lines represent? | Points of equal water table elevation (equal hydraulic head).
Water table contour map
direction of flow lines | Flow lines run PERPENDICULAR to contour lines, from HIGH head (higher contour value) to LOW head (lower contour value).
Water table contours
what does closely spaced vs. widely spaced mean? | Closely spaced contours = steep hydraulic gradient = fast flow. Widely spaced = gentle gradient = slow flow.
Gaining stream
definition | A stream where the water table is HIGHER than the stream surface. Groundwater flows INTO the stream, adding to its discharge. Provides baseflow. Common in humid regions.
Losing stream
definition | A stream where the water table is LOWER than the stream surface. Stream water seeps DOWNWARD into the ground, recharging the aquifer. Common in arid regions or over-pumped areas.
Disconnected losing stream
definition | A losing stream where the water table has dropped below the streambed with an unsaturated zone in between. Stream and aquifer no longer directly connected.
Can streams switch between gaining and losing? | Yes
seasonally, along their length, or in response to pumping. A stream can be gaining in wet season and losing in dry; can gain in one reach and lose in another. Pumping can convert gaining to losing.
Why is the perennial/intermittent/ephemeral classification connected to groundwater? | Perennial: water table above streambed year-round (sustained by baseflow). Intermittent: water table rises above streambed seasonally. Ephemeral: bed always above the water table
flows only from surface runoff, no groundwater support.
Capillarity
definition | The ability of water to move against gravity through narrow pore spaces, driven by surface tension and adhesion to pore walls.
Capillarity mechanism | Water molecules stick to soil particles (adhesion) and to each other (cohesion). In narrow pores, adhesion pulls water up along walls, cohesion drags the rest of the water column. Narrower pores = higher rise.
Which material has higher capillary rise: gravel or clay? | Clay
MUCH higher (up to several meters vs. a few cm in gravel). Narrower pores generate stronger capillary forces.
Capillary rise in coarse sand vs. silt vs. clay | Coarse sand: 2–5 cm. Silt: 70–150 cm. Clay: up to 3+ m (but very slowly).
Freshwater-saltwater interface
definition | The boundary in coastal aquifers between fresh groundwater (above/inland) and saltwater (below/seaward). Saltwater is denser (~1025 kg/m³ vs. 1000 kg/m³ for fresh).
Ghyben-Herzberg principle | Under natural equilibrium in a coastal aquifer, for every 1 m the water table sits above sea level, the freshwater-saltwater interface sits ~40 m BELOW sea level. Ratio is 40:1 from the density contrast.
Saltwater intrusion
cause and effect | Heavy pumping lowers freshwater head, causing the freshwater-saltwater interface to move INLAND and UPWARD. Saltwater eventually reaches wells, contaminating them with chloride. Very hard to reverse once it happens.
Where is saltwater intrusion a big problem? | Coastal Florida, Long Island NY, coastal California, coastal Mediterranean regions, low-lying Pacific islands. Any coastal area with heavy groundwater use.
Contact spring
definition | Spring that forms where a permeable layer overlies an impermeable one; water flows along the top of the impermeable layer and emerges at the exposed contact.
Depression spring
definition | Spring that forms where the water table intersects a topographic low (a dip in the ground surface).
Fracture spring
definition | Spring where water follows fractures or joints in bedrock to reach the surface.
Karst spring
definition | Spring where water from underground caverns emerges through dissolution channels. Can be extremely high-discharge (Silver Springs, Florida ~800 million gallons/day).
Artesian spring
definition | Spring where fractures or faults allow confined aquifer water to reach the surface under natural pressure (potentiometric surface above ground level).
Why are surface water and groundwater the same water? | They’re different phases of the hydrologic cycle. Withdrawing groundwater ultimately reduces surface water (cuts baseflow); pumping surface water ultimately reduces groundwater. Not separate resources.