geologic factors influencing the ground water movement & storage

Most ground water originates as recharge in upland areas,through infiltration.however,it discharges from saturated zones back to surface in low lying areas,usually at the springs/bottom of surface waters.Since,ground water always move towards usually at springs/bottom of surface waters.

Since,ground water moves towards lower head,these exist points are always at a lower elevation than the water table where ground water enters the system as recharge.

Increasing depth of the earth's crust causes rock porosity & permeability to decrease.pore fluids become hotter & more concentrated with dissolved minerals as depth increases.Deeper than about 10 Km.,the crust has low intrinsic permeability due to high confining pressures & ductile deformation of rocks.

Depth shallower than about 6 Km. have high rock permeability & pore fluids can traverse flow paths of continental length scales.For Example:-In Australia,sandstone aquifers convey water for 1100 Km from great dividing range spread across great artesian basin.

Compaction caused in active & subsiding sedimentary basins or caused by tectonic forces causes ground water flow.if the sedimentary zone is thick then the load will squeeze pore water fom the sediment.this process of compaction is most important in basins with rapid sedimentation rates.

 

Where magmas rise into shallow crust it sets up convection currents by virtue of differential temperature of pore fluids near the magma than away from it causing flow.Magma itself is a source of fluids & there is accelerated chemical reactions rhat can generate fluids to induce flow.

Unconsolidated deposits like sand,silt, & clay usually have their geologic origin as alluvial,marine or glacial deposits.The coarser deposits,sands & gravels are among the porous & permeable of the earth material.Most shallow unconsolidated deposits have little cement in them & their porosity is governed mostly by the grain size distribution.the more uniform (well sorted) the grain size are,the higher the porosity.the coarser the material,the higher it's permeability & hydraulic conductivity.water laid sediment is stratified,which causes anisotrophy in the large scale average hydraulic conductivity.

In sedimentary rocks ground water flow in aquifers is mostly parallel to the dip of the layers ,but flow in aquitards is mostly normal to the layering creating leakage between separate aquifers.

In Igneous & metamorphic rocks ground water flow in the least predictable phenomena.this is because the porosity of these rocks is very low & permeability is controlled by an irregular network of small fractures.however,permeability will be anisotrophic with higher conductivity parallel to prominent fracture sets.Moreover,permeability of a fracture is controlled by the it's aperture & smoothness.

Fracture permeability decreases with depth due to decrease in average fracture aperture & increasing weight of overlying rocks.

Lava tubes running down slope can be significant conduits of ground water flow.

 

Permafrost causes ground water which acts like low conductivity aquitard.the unfrozen ground water beneath it can serve as a confined aquifer.Most pore water in the permafrost region remains frozen & there is little ground water flow.

In a confined aquifer ground water storage changes because:-

a)Since water is slightly compressible hence it can expand or contract with changes in water pressure.

b)The solid matrix could expand or contact allowing more or less quantity of water storage.

 

Both these processes occurs when the & pressure fluctuates during transient flow.the combined effect of the aforesaid to storage possesses in a confined aquifer is called elastic storage.

in the water table storage/phreatic storage,storage is determined by the raising/lowering boundary between saturated & unsaturated zones within the volume.in this type of storage storivity in an unconfined aquifer called specific yeild which is conceptually equal to the storivity 'S' in an aquifer.

 

s=Sy(unconfined aquifer)

Unconfined aquifers have elastic storage just like confined aquifers but their elastic storage is usually insignificant compared to the storage associated with the drainage of water at the water table.