Editing Aquifer (section)

====Porous====
[[File:Water seep from sandstone in Hanging Garden SE Utah.jpg|thumb|left|alt=Water slowly seeping from tan porous sandstone at contact with impermeable gray shale creates a refreshing growth of green vegetation in the desert. |Water in porous aquifers slowly seeps through pore spaces between sand grains]]

Porous aquifers typically occur in sand and [[sandstone]]. Porous aquifer properties depend on the [[depositional environment|depositional sedimentary environment]] and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the [[porosity]] and [[Permeability (earth sciences)|permeability]] of sandy aquifers.<ref name="SandSandstone">{{cite book|last1= Pettijohn |first1= Francis |last2=Potter |first2=Paul |last3=Siever |first3=Raymond |date=1987 |title=Sand and Sandstone |location=New York |publisher= Springer Science+Business Media |isbn= 978-0-387-96350-1 |doi=10.1007/978-1-4612-1066-5 }}</ref>{{rp|413}}

Sandy deposits formed in [[Shallow water marine environment|shallow marine environments]] and in [[aeolian processes|windblown sand dune environments]] have moderate to high permeability while sandy deposits formed in [[Fluvial processes|river environments]] have low to moderate permeability.<ref name="SandSandstone" />{{rp|418}} Rainfall and snowmelt enter the groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from [[potentiometric surface]] maps of water levels in wells and springs. [[Aquifer test]]s and [[well test]]s can be used with [[Darcy's law]] flow equations to determine the ability of a porous aquifer to convey water.<ref name="FieldMethodsGeoHydrogeo" />{{rp|177–184}}

Analyzing this type of information over an area gives an indication how much water can be pumped without [[overdrafting]] and how contamination will travel.<ref name="FieldMethodsGeoHydrogeo" />{{rp|233}} In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day (0.3 m/d) is considered to be a high rate for porous aquifers,<ref>{{cite book |title=Sustainability of ground-water resources. |publisher=U.S. Geological Survey |location=Denver, Colorado |series=Circular 1186 |url=https://archive.org/details/sustainabilityof00alle/page/8 |last1=Alley |first1=William |last2=Reilly |first2=Thomas |last3=Franke |first3=O. |page=[https://archive.org/details/sustainabilityof00alle/page/8 8] |date=1999 |isbn=978-0-607-93040-5 |doi=10.3133/cir1186 |url-access=registration }}</ref> as illustrated by the water slowly seeping from sandstone in the accompanying image to the left.

Porosity is important, but, ''alone'', it does not determine a rock's ability to act as an aquifer. Areas of the [[Deccan Traps]] (a [[basalt]]ic lava) in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly, the micro-porous (Upper [[Cretaceous]]) [[Chalk Group]] of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.