Hydrogeological Properties of Formations Questions and Answers

Answer: B

An aquifer bounded by aquitards is a Leaky aquifer.

An aquitard is a poorly permeable layer that slows but does not stop the movement of water. An aquifer overlain or underlain by such a layer receives or loses water through it when pumped, so it is called leaky or semi-confined.

An artesian (confined) aquifer is bounded by impermeable aquicludes, so it does not leak. An unconfined aquifer has the water table as its upper surface, with no confining layer above. A perched aquifer is a local body of groundwater held above the main water table by a small impermeable lens.

Answer: B

Such a layer is a Confined aquifer.

A pervious, water-bearing layer sandwiched between two clay layers is confined, because clay acts as an aquiclude that holds the water under pressure. The water in a well drilled into it rises above the top of the aquifer, and where the pressure is high enough it flows freely as an artesian well.

An unconfined aquifer has no confining layer above it. Semi-confined (leaky) aquifers are bounded by aquitards that let some water through, whereas the paper describes clay layers as full confining beds. 'Semi-unconfined' is not a standard category.

Answer: D

The formation is an Aquifer.

An aquifer is a saturated formation that is both porous and permeable enough to store, transmit and yield useful quantities of water to wells and springs. Sands, gravels, sandstones and fractured or karstic limestones are typical aquifers.

An aquiclude can hold water but cannot transmit it in useful amounts, as with clay. An aquitard transmits water only slowly and does not yield it freely. An aquifuge, such as massive unfractured granite, neither holds nor transmits water.

Answer: A

One darcy is about \(0.98 \times 10^{-12}\ m^{2}\).

The darcy is a unit of permeability, which has the dimensions of area. A medium has a permeability of one darcy when a fluid of viscosity 1 centipoise flows through it at 1 cm³/s across 1 cm² under a pressure gradient of 1 atm per cm. Converted to SI units this is

\[ \begin{aligned} 1\ \text{D} &\approx 9.87 \times 10^{-13}\ \text{m}^{2} \\ &\approx 0.987\ \mu\text{m}^{2} \end{aligned} \]

which is the first option. The options in m and km have the dimension of length, not area, so they cannot be a permeability, and \(2.3 \times 10^{-15}\ m^{2}\) is a few millidarcies.

Answer: B

The porosity is about 17.3%, so the answer is 17.30%.

The gain in weight is the brine that fills the pores. Dividing it by the brine's density gives the pore volume, and the core's bulk volume is that of a cylinder of radius 2 cm and length 12 cm.

\[ \begin{aligned} V_p &= \frac{453 - 425}{1.07} = \frac{28}{1.07} \\ &= 26.17\ \text{cm}^3 \end{aligned} \]

\[ \begin{aligned} V_b &= \pi r^2 L = \pi (2)^2 (12) \\ &= 150.8\ \text{cm}^3 \end{aligned} \]

\[ \phi = \frac{26.17}{150.8} \times 100 = 17.35\% \]

That rounds to the printed 17.30%. Using the 4 cm diameter as the radius would make the bulk volume four times larger and the porosity about 4%, and forgetting to divide by the specific gravity gives 18.6%; neither matches 7%, 27.3% or 25%.

Answer: C

The infiltration rate depends on both (A) and (B).

Infiltration is fastest when a dry soil first receives water and declines as the soil wets up, so initial soil moisture matters (A). The physical properties of the soil, its texture, structure, porosity, compaction, organic matter and surface crusting, set how quickly water can enter and move down at all (B). Horton's infiltration equation captures this decay from an initial rate to a steady rate fixed by the soil.

Naming only one of the two leaves out a controlling factor, and 'Neither' denies both.

Answer: A

Hydraulic conductivity is high in coarse grained and fractured material.

Hydraulic conductivity measures how easily water passes through a material. In coarse sands and gravels the pores are large and well connected, so water flows readily, and open fractures provide channels even through otherwise tight rock. Values for gravel can exceed 100 m/day.

Fine-grained, unfractured material such as clay has small pores in which water is held by surface forces, so its conductivity is very low, often below 0.0001 m/day, even if its porosity is high. So 'both' is wrong, and so is 'neither'.