Groundwater Formation Analysis
This question requires identifying the correct statement regarding the geological formations that store and transmit groundwater. The key terms are sandstone, sandy shale, aquifers, and aquitards.
Sandstone Role in Groundwater
Sandstone is a sedimentary rock composed primarily of sand-sized grains (like quartz). Due to the spaces (pores) between these grains, sandstone typically exhibits high porosity and permeability. This allows it to:
- Store significant amounts of groundwater.
- Allow groundwater to flow through it readily.
Rocks that perform these functions are called aquifers. Therefore, sandstone commonly forms aquifers.
Sandy Shale Role in Groundwater
Sandy shale is a sedimentary rock containing sand grains mixed with significant amounts of finer materials like silt and clay (shale). The presence of these finer particles reduces the size and connectivity of pore spaces compared to pure sandstone. This results in lower permeability. Consequently, sandy shale:
- Restricts the flow of groundwater.
- Can store some water but transmits it much more slowly than an aquifer.
Geological layers that significantly impede but do not completely stop groundwater flow are known as aquitards.
Conclusion
Based on these definitions:
- Sandstone → Aquifer
- Sandy Shale → Aquitard
The statement reflecting this relationship is the correct one.
Important Questions from Aquifer Properties
- An aquifer has a surface area of 100 km$^2$. With no recharge, assume that pumping out 2 $\times$ 10$^7$ m$^3$ of ground water resulted in a drop of the water table by 6 m. If the aquifer has a specific retention of 15 %, then the porosity of the aquifer is _____ % (rounded off to one decimal place).
- A well penetrates an unconfined aquifer. The water level (i.e., head) in the well prior to pumping is 25 m. After a long period of pumping at a constant rate of 0.05 m$^3$/s, the drawdowns at distances of 50 m and 150 m from the well are observed to be 3 m and 1.2 m, respectively.
The hydraulic conductivity of the unconfined aquifer is ______ $\times 10^{-4}$ m/s (rounded off to two decimal places).
Consider $\pi$ = 3.14. - A confined aquifer with a uniform saturated thickness of 10 m has hydraulic conductivity of $10^{-2}$ cm/s. Considering a steady flow, the transmissivity of the aquifer in $m^2/day$ is _______ (rounded off to one decimal place).
- A stratified confined aquifer consists of three parallel homogeneous and isotropic horizontal layers with thickness of 10 m, 5 m and 5 m. The layers have the same width. The hydraulic conductivities of the strata are 15 m/day, 20 m/day and 30 m/day, respectively. The water flow follows Darcy's law and is parallel to the strata. Considering the same hydraulic gradient for all the layers, the effective hydraulic conductivity of the aquifer is ________ m/day. [in integer]
If $d$ is the depth of an aquifer through which water is flowing, then the relationship between permeability $K$ and transmissibility (also known as transmissivity) $T$ is given by ________.
