Groundwater flow (2024)

Climate and water

Take away ideas and understandings

• In recharge areas water is added to groundwater, in discharge areas wateris lost from groundwater.
• Topography and geology are the dominant factors controlling groundwaterflow.
• Storativity describes the property of an aquifer to store water.
• Hydraulic conductivity is measured by performing a pumping test, i.e. bypumping one well and observing the changes in hydraulic head in neighboringwells.

What drives groundwater flow?

• gravity is the dominating driving force
• water flows from high elevation to low elevation and from high pressureto low pressure, gradients in potential energy (hydraulic head) drive groundwaterflow
• recharge and discharge (Fig 7.2)
• in recharge areas water is added to groundwater
• in discharge areas water is lost from groundwater
• in recharge (discharge) areas, the hydraulic head decreases (increases)with depth
• recharge occurs from the unsaturated zone or from surface waters
• groundwater discharge occurs into rivers, lakes, springs, or by evapotranspiration
• examples:
• "Puszta" in Hungary: groundwater is discharging in the low lands of theGreat Hungarian Plain and leaves the dissolved salts behind -> reductionof soil quality -> bad conditions for agriculture
• example: evaporation in the Sahara, loss of valueable groundwater resourcesthat were recharged in the last ice age (loss may be up to a few 10's inchesper year)
• example: springs, e.g. at Grand Canyon
• we can draw flownets in a qualitative way if we know geology andtopography, flow lines have to be parallel to no-flow boundaries
• the hydraulic head along any equipotential is equal to the elevation ofits intersection with the water table (Fig7.3)

Regional groundwater flow

• effect of basin aspect ratio (length to depth) (Fig7.4)
• basin yield higher in the deeper basin
• effect of water-table topography (Fig7.5)(Fig 7.6)
• local, intermediate, and regional flow systems
• if local relief is negligible, but a regional water-table slope exists,only a regional flow system will develop
• if local hill-and-valley topography exists, but no regional slope, onlylocal flow systems will develop.
• if both local and regional topography exists in a basin, all three typesof flow systems (local, intermediate, and regional) will develop
• Explore 7
• effect of heterogeneity
• (Fig 7.8)
• Patterns of Groundwater Flowin Heterogeneous Systems

Well hydrographs

• a well hydrograph shows the variation in water levelin a well through time
• water level in an unconfined aquifer in VA (Fig7.9)

Storage of groundwater in aquifers

• in many areas of the world the hydraulic head is declining with time becausea lot of water is pumped out of the aquifer
• storage in unconfined and confined aquifers is different
• in unconfined aquifers the water pumped stems from drained void space
• in confined aquifers the water stems from decompression of the water andthe sediments.
• the same change in water table represents a larger amount of water if takenfrom an unconfined aquifer as compared to a confined aquifer
• storage of water in aquifers: yield per unit areaand unit change in hydraulic head
• unit: m³/m/m² (=> dimensionless)
• in unconfined aquifers the storage coeff. is high,somewhat smaller than the porosity
• for a 1-m decline in the water table, the volumeof water produced per unit aquifer area is the specific yield, Sy.(Fig7.10)
• in confined aquifer much smaller ~10^-6
• for a 1-m decline in the potentiometric surface, the volume of water producedper unit aquifer area is the storativity, S. The aquifer material is notdrained and remains saturated.(Fig7.11)
• where is water being stored in confined aquifers?=> compressibility of water and change in aquifer structure
• land subsidence as a result of overpumping
• examples:
• Mexico City aquifer/Ogallala aquifer
• the Dakota artesian basin: flowing artesian wells (hydraulic headabove surface) are wells in which the water level is higher than the surface.A lot of wells were drilled into the Dakota basin, in South Dakota about15000 wells. Most of them do not flow anymore
• New Mexico, where an old school well was still flowing when visited, whydid it break?

How to measure hydraulic head and hydraulic conductivity?

• hydraulic head: install a well open to theaquifer only over a small distance (short screen), measure the levelof the water in the well relative to a reference surface, for example sealevel
• hydraulic conductivity or transmissivity:
• the change in water level in the pumping well, orin observation wells nearby, is referred to as a drawdown
• the amount of this drawdown will decrease as onemoves away from the pumping well, and the pattern that is produced is calleda cone of

depression
• we can measure the hydrualic conductivity by performinga pumping tests
• shape of depression cone (Fig7.13)
• how does this cone look like in different geol. environments?
• quantification (Theis equation)

What information can be drawn from the hydraulichead?

• where the water is flowing
• how fast it is flowing
• how much water there is

Resources

Freeze, R.A. and Cherry, J.A. (1979) Groundwater.Prentice Hall, 604p.