Peter Dahlhaus SCGEO 2106 Week 4

PrecipitationEvapotranspirationPond Storage Overland FlowThroughfall InterceptionInterception StorageInfiltrationSoil moisture storageInterflowThroughflow Groundwater recharge Groundwater storage Baseflow Return flow Channel storage Runoff Groundwater discharge

Unsaturated zone (Vadose zone) Saturated zone (Phreatic zone) When does water become groundwater? Soil moisture

pressure +’ve -’ve (suction) (pressure) Hydrostatic increase

Water rises in a column of soil due to capillarity. The ‘suction’ is due to the surface tension between the water molecules and the soil particle surfaces. Small diameter Greatest height Surface tension

Capillary Fringe: Silty Clay ~ 1 metre Fine Sand ~ 0.1 metre Gravel ~ metre

Groundwater is stored in the spaces and voids in the rock mass, such as the pore spaces between the grains and particles, or in fractures or in cavities. For groundwater to move, the spaces or voids need to be interconnected. The pathways can be very torturous and complex, like a three-dimensional maze. Groundwater moves at varying speed but is usually very slow. Velocity ranges from a few microns per year (in a clay) to hundreds of metres per day (in a very open-fractured rock). “Underground rivers” don’t really exist. (Rivers might disappear underground into a cave, but that’s not the same as groundwater. Deep Leads are buried rivers, but the surrounding rocks are saturated with groundwater as well).

Groundwater storage Volume of voids (V v ) Total volume (V t ) Porosity (n) = Effective porosity (permeability) enables an aquifer/rock unit to store, transmit and release water

Primary porosity is made at the same time as the rock – sands, gravels, sandstone, limestone Calcarenite (dune limestone) Barwon Heads Scoria Mt Buninyong

Secondary porosity is made when rocks are fractured or “dissolved” by later processes Limestone cave Port Campbell Fractured rhyolite Wannon Fractured basalt Dunnstown

Specific Yield is the ratio of the volume of water drained under gravity to the volume of saturated rock. Groundwater storage Specific yield (S y ) = Volume drained (V d ) Total volume (V t ) Specific Retention is the ratio of the volume of water retained after gravity drainage to the volume of saturated rock. Specific retention (S r ) = Volume retained (V r ) Total volume (V t ) Specific yield + specific retention = porosity S y + S r = n

A core one metre long and 10cm diameter is extracted from an aquifer. Saturated weight is 19.65kg It is left to drain (by gravity) and then weighed as 17.29kg It is then oven dried (105 o C) to a constant weight of 16.90kg. Calculate specific yield and specific retention and porosity. 1 m 0.1 m Groundwater storage

1 m 0.1 m A core one metre long and 10cm diameter is extracted from an aquifer. Saturated weight is 19.65kg It is left to drain (by gravity) and then weighed as 17.29kg It is then oven dried (105 o C) to a constant weight of 16.90kg Total Volume (V t ) = m 3 Weight of water drained = 2.36kg Volume of water drained (V d ) = 2.36L = m 3 Specific yield (S y ) = / = 0.3 = 30% Volume of water retained (V r ) = 0.39L Specific retention (S r ) = / = 5% Porosity (n) = S y + S r = 35% Groundwater storage

The Saturated Zone The watertable is usually a subdued replica of the land surface Springs, seeps, swamps, rivers & lakes occur where the groundwater intersects with the land surface Unsaturated zone (Vadose zone) Saturated zone (Phreatic zone)

Water tables fluctuate with seasonal input (recharge) The amount of groundwater in storage changes with the seasons

Movement of water Groundwater flows from higher elevations to lower elevations. It travels from where it enters the system (recharge) to where it leaves the system (discharge)

Unconfined aquifer - Open to the surface - Broad recharge area - Includes most aquifers

Aquifer conditions Unconfined – open to the surface. Confined – sandwiched between less permeable beds. Fractured rock – water stored in fractures.

Confined aquifer - “Sandwiched” between less permeable beds - Recharge area is limited to aquifer outcrop - Source of artesian water

Covering layerAquifer type ImperviousConfined Semi-pervious, negligible horizontal flow Semi- confined Less pervious than the main aquifer, significant horizontal flow Semi- unconfined Aquifer – carries water in useable quantity No covering layer = Unconfined

Confining beds make up the non-aquifers and may be referred to as: aquifuge - an absolutely impermeable unit that will not transmit any water, aquitard - a low permeability unit that can store groundwater and transmit is very slowly, and aquiclude - a unit of low permeability located adjacent to a high permeability layer.

An aquifer system is the complete 3-d package of aquifers and confining beds

Total head  Static head Pressure Head Elevation Head Groundwater Head Australian Height Datum (AHD) ground level groundwater bore

Hydraulic Gradient (i) = (h 1 – h 2 )/L Hydraulic gradient shows flow direction

Three point Problems Three points are needed to fix a plane in space N Scale 0 100m Bore C RLgw = 39m Bore A Elevation = 52m SWL = 10m  RLgw = 42m RLgw 50m RLgw 45m RLgw 40m Bore B RLgw = 49m Flow direction ΔL = 100m ΔH = 5m Hydraulic gradient = 0.05

Vertical Gradients Groundwater flow is three-dimensional

DARCY’S EXPERIMENT Q  Cross sectional area (A) Q  to the head loss over a distance (i)

Darcy’s Law Q = kiA k = Hydraulic Conductivity

Transmissivity (T) Hydraulic Conductivity (k)

mm mm m km

Constant Head PermeameterFalling Head Permeameter

Hydraulic conductivity is often varied in a single aquifer

Homogeneity / Heterogeneity

Deltas, alluvial plains, lacustrine deposits, paludal deposits and glacial sediments are examples of heterogeneous aquifers.

Isotropy / Anisotropy

Isotropic Homogeneous Isotropic Heterogeneous Anisotropic Homogeneous Anisotropic Heterogeneous

Reality: Most aquifers are heterogeneous and anisotropic in three dimensions. The degree of variation depends on the scale of the investigation. As you zoom out the variations become less important.