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Groundwater flow

What drives groundwater flow?

• gravity is the dominating driving force
• water flows from high elevation to low elevation and from high pressure to low pressure, gradients in potential energy (hydraulic head) drive groundwater flow
• 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 the Great Hungarian Plain and leaves the dissolved salts behind -> reduction of soil quality -> bad conditions for agriculture
• example: evaporation in the Sahara, loss of valuable groundwater resources that were recharged in the last ice age (loss may be up to a few 10's inches per year)
• example: springs, e.g. at Grand Canyon
• we can draw flownets in a qualitative way if we know  geology and topography, flow lines have to be parallel to no-flow boundaries
• the hydraulic head along any equipotential is equal to the elevation of its intersection with the water table (Fig 7.3)

Regional groundwater flow

• effect of basin aspect ratio (length to depth) (Fig 7.4)
• basin yield higher in the deeper basin
• effect of water-table topography (Fig 7.5)(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, only local flow systems will develop.
• if both local and regional topography exists in a basin, all three types of flow systems (local, intermediate, and regional) will develop
• effect of heterogeneity
• (Fig 7.8)
• Patterns of Groundwater Flow in Heterogeneous Systems

Well hydrographs

• a well hydrograph shows the variation in water level in a well through time
• water level in an unconfined aquifer in VA (Fig 7.9)

Storage of groundwater in aquifers

• in many areas of the world the hydraulic head is declining with time because a 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 and the sediments.
• the same change in water table represents a larger amount of water if taken from an unconfined aquifer as compared to a confined aquifer
• storage of water in aquifers: yield per unit area and 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 volume of water produced per unit aquifer area is the specific yield, Sy. (Fig 7.10)
• in confined aquifer much smaller ~10^-6
• for a 1-m decline in the potentiometric head, the volume of water produced per unit aquifer area is the storativity, S. The aquifer material is not drained and remains saturated.(Fig 7.11)
• 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 head above 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 about 15000 wells. Most of them do not flow anymore
• New Mexico, where an old school well was still flowing when visited, why did it break?

How to measure hydraulic head and hydraulic conductivity?

• hydraulic head: install a well open to the aquifer only over a small distance (short screen), measure the level of the water in the well relative to a reference surface, for example sea level
• hydraulic conductivity or transmissivity:
• the change in water level in the pumping well, or in observation wells nearby, is referred to as a drawdown
• the amount of this drawdown will decrease as one moves away from the pumping well, and the pattern that is produced is called a cone of depression
• we can measure the hydraulic conductivity by performing a pumping tests
• shape of depression cone (Fig 7.13)
• how does this cone look like in different geol. environments?
• quantification (Theis equation)

What information can be drawn from the hydraulic head?

• 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.