Groundwater flow

Groundwater

• groundwater is the water in the saturated zone (Fig) (Fig)
  
• recharge is the water entering the saturated zone
• in many parts of the world, groundwater is the only source of fresh water
• in the US about 10% of the rainfall becomes groundwater eventually. This amount equals the annual use of water in the US, about 3 inch per year
• Water use in the US (Fig)
• water may stay in the groundwater reservoir between several days and thousands of years. We will discuss tracer techniques that may be used to derive residence times later in the class

Water in natural formations

• an aquifer is a saturated geological formation that contains and transmits "significant" quantities of water under normal field conditions (=> gravel, sand, volcanic and igneous rocks, limestone) (Fig 6.6) (Fig)
  
• an aquiclude is a formation that may contain water but does not transmit significant quantities (clays and shales)
• an aquitard is a formation with relatively low permeability
• confined and unconfined (water-table) aquifers
• an unconfined aquifer has a water table (water table aquifer)
• a confined aquifer does not have a water table. If you drill a well, water will rise (in the well) above the top of the aquifer
• perched groundwater is groundwater sitting on top of a poorly permeable layer with an unconfined aquifer underneath
• the height to which water rises in a well defines the piezometric or potentiometric surface
• geology of aquifers
  
• unconsolidated sediments: loose granular deposit, particles are not cemented together (e.g.: Long Island)
• consolidated sediments, most important: sandstone, porosity varies depending on the degree of compaction (e.g. Zion, Bryce, and Grand Canyon National Parks)
• limestone: composed mainly of calcium carbonate, CO2 rich water dissolves limestone, e.g.: limestone caves, karst (e.g. Floridan aquifer)
• volcanic rock
• basalt lava, fractures (e.g.: Hawaii, Palisades)
• crystalline rocks: igneous and metamorphic rocks, e.g. Granite, have often very low porosity, flow through fractures
• porosities and hydraulic conductivities of different aquifer rocks (Fig 6.5)

Flownets

• 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
• flow in a horizontal confined aquifer  (Fig 6.7)
• lines of equal hydraulic head are called equipotentials
• flow occurs perpendicularly to those, lines indicating those are called flowlines
• together, the equipotentials and the streamlines constitute a flow net (Fig 6.8)
• generally, groundwater flow follows topography, in detail the situation can be more complicated though
• groundwater flow not only occurs near the water table, but does penetrate deep into the aquifer (Fig 6.9)
• 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 valueable 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/anisotropy (Fig 7.8)
• so far we have considered only homogeneous aquifer (the same K everywhere)
• virtually all natural materials through which groundwater flows display variations in intrinsic permeability from point to point, this is referred to as heterogeneity (Example: Fig)
• permeable zones tend to focus groundwater flow, while, conversely, flow tends to avoid less permeable zones in anisotropic media the permeability depends on the direction of measurement, in isotropic media, it does not

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 surface, 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:
• in the photo below, taken in central Mexico City, the young boy is not leaning against a utility pole, but a well casing, once completely below the land surface (Fig)
• in Mexico City the lowering of the potentiometric surface in the Mexico City Aquifer has resulted in the removal of water from the overlying clays. The land surface has subsided by some 7.5 m in the central part of Mexico City (Fig. 7.15)
• Water-level change in the High Plains Aquifer, 1980 to 1994 (Fig)
• 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?

Comprehensive statement of groundwater flow

• general flow equation
• S_o is the specific storativity, the specpfic yield divided by the thickness of the aquifer for an unconfined aquifer, or the storativity divided by the thickness of the aquifer for the confined aquifer. The dimension of S_o is [1/L].

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 hydrualic conductivity by performing a pumping tests
• shape of depression cone (Fig 7.13)
• how does this cone look like in different geol. environments?

What information can be drawn from the hydraulic head?

• where the water is flowing
• how fast it is flowing
• how much water there is
• well hydrograph shows the variation in water level in a well through time
• water level in an unconfined aquifer in VA (Fig 7.9)

Movemement of water in the unsaturated zone

• the zone between the ground surface and the water table where pore spaces of the rock or soil may be partly filled with air and partly with water is referred to as the unsaturated zone or vadose zone and water in this zone is referred to as soil moisture (Fig)
• the capillary fringe is a saturated zone above the water table where water is affected by capillary forces
• distribution of moisture in the vadose zone (Fig 8.1)
• grain sizes in soils (and thus pore sizes) vary over several orders of magnitude (Fig)
• definitions of porosity, field capacity, specific retention, specific yield (Fig)
• porosity, field capacity, and wilting points for typical soils (Fig)

• the potential in the unsaturated and saturated zone are similar: h = z + p/(rg) = z + y
• this pressure is negative in unsaturated soils and is often called pressure, suction or tension head (y), y is given as "cm water column"
• Water pressure within capillary tubes is less than atmospheric pressure
• y as a function of moisture content for a fine sand (Fig 8.4)(Fig)
• soil water status as a function of pressure (tension) (Fig)
• driving forces for flow of water in the unsaturated zone are: gravity and adhesion/cohesion
• Darcy's law describes the flow in porous media:
• q = -K * dh/dz = -K * d(z+y)/dz = -K * d(z + p/(rg))/dz
• K: hydraulic conductivity, function of saturation (Fig 8.5)
• wetting fronts

 Resources

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

Hornberger, G.M., Raffensberger, J.P., Wiberg, P.L., and Eshleman, K.N. (1998) Elements of physical hydrology. Johns Hopkins University Press,  Baltimore, 302p.