various geologic factors influencing the ground water movement and storage

Ground water is the wter that occupies pores,cavities,cracks and other spaces in the crustal rocks and includes the water precipitated from the atmosphere which has percolated through the soil,water that has risen from deep magamtic sources or liberated during the igneous activity or that which is stored in sedimentary rock layers.

Most ground water originates as recharge in upland areas,through infiltration.However,it discharges from the saturated zones back to ground surface in low lying areas,usually at springs/bottom of surface water bodies.Since,ground water always moves towards lower heads,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 and permeability to decrease.Pore fluids become hotter and more concentrated with the dissoved minerals as depth increases.Deeper than about 10 Km. ,the crust has low intrinsic permeability due to high confining pressures and ductile deformation of rocks.

Depth shallower than about 6 Km. have high rock permeabilities and pore fluids can traverse flow paths of continental scale length.Ex:-In Australia,sandstone aquifers convey water for 1100Km. from Great Dividing Range spread across Great Artesian Basin.

Compaction caused in active and 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 from the sediment.This process of compaction is most important in basins with rapid sedimentation rates.

Where magma rises 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 and there is accelerated chemical reactions that can generate fluids to induce flow.

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

In Sedimentary rocks ground water flows in aquifers in 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 and metamorphic rocks ground water flow is least predictable phenomenon.this is because the porosity of these rocks is very low and permeability is controlled by an irregular network of small fractures.However,permeability will be anisotropic with higher conductivity parallel to prominent fracture sets.