All igneous and metamorphic rocks exposed near the earth's surface are in an unstable chemical and physical condition and over geologic time these rocks breakdown into finer and finer components. Destruction of rocks and the redistribution of the rock particles play a significant role in producing all the major types of aquifer systems. These particles are entrained and re-distributed by the three agents of erosion- wind, running water and glacier ice.
The factors responsible for the formation of aquifer system are:

· Weathering
· Erosion
· Meandering in river
· Glaciation and glacial deposits
· Structural changes (fracture/joints) in igneous / metamorphic rocks
· Formation of extrusive(void -containing) rocks at great depths

Groundwater flow pattern

Usually, the general direction of groundwater flow can be established on the basis of the local topography (use of topographical maps or aerial photos) and the presence of streams or rivers which act as groundwater discharge boundaries. The near-surface groundwater flow generally follow surface drainage pattern. If the flow direction can not be established, three small diameter (piezometric) wells are temporarily installed in the aquifer. Relative water level in these wells reveal the direction of flow. Other methods of delineating groundwater flow pattern include electromagnetic induction, earth resistivity survey, radioactivity and temperature logging.

Geology also offers control on the rate of groundwater movement. The size of the pores and their relative connectivity (permeability) determine the rate at which water moves into , through and out of aquifer. Groundwater moves more quickly in coarse sand sometimes as much as several meters per day while in case of clay which are generally impermeable the movement may seldom move not more than an inch per year.

Pollutants, once trapped in the saturated groundwater flow, tend to form plumes of polluted water extending down stream from the pollution source. Where groundwater has a rapid flow, plume tends to be long and thin, whereas at less flow rate the groundwater, especially in unsaturated zone, move vertically and may take several months and years to reach the saturated zone and get mixed with water table. Once in saturated zone, the pollutants spread out laterally and move in the direction of groundwater flow.

The pollutants in the saturated zone either float on top of aquifer, if pollutants are of low density and immiscible, or move into the aquifer if contaminated water is buoyant . The movement of pollutants in saturated zone can take place by 'Convection'- transfer of pollutants, or by ' Dispersion' - in the form of molecular diffusion, resulting in mixing of two adjacent miscible liquids even if there is no flow.

Groundwater Chemistry

Apart from the use of groundwater, which require specific characteristics, the role of groundwater chemistry is important in determining suitability of well. The chemistry of groundwater has an unusual characteristics in terms of its ability to dissolve a greater range of substances than any other liquid by virtue of its occurrence in different environmental conditions controlled by specific pressure and temperature different from on-land conditions. Chemical process in groundwater zone can influence the strength of geologic materials and in situations where they are not recognized, can cause failure of artificial slopes, dams, mining excavations etc.

In certain cases, the dissolution characteristics of Groundwater is influenced by the decaying humus in soil and concomitant release of carbon dioxide which forms a complex medium with Groundwater and increases its reactivity.

Most of the calcium, magnesium and bi-carbonate ions found in groundwater come from dissolution of carbonate rocks. Near surface groundwater has a nearly constant temperature however, with increasing depth mostly influenced with the geo-thermal heat the water temperature rises in the order of 1oF per 1000 ft. depth. Therefore most groundwater found at greater depth is more mineralized due to increased solubility on account of elevated temperature.

The most abundant dissolved gases present in groundwater are nitrogen, oxygen, carbon-dioxide, methane, hydrogen sulphide and nitrous oxide. While the last three gases mostly result from bio-geo-chemical process the rest are mostly contributed by atmospheric influence in their relative order of abundance.