Your Answer : (Not Answered)

Correct Answer : A

The geophysical method is the Electric Resistivity method.

Geophysical exploration infers what lies underground from measurements made at the surface. In the resistivity method a current is passed into the ground between two electrodes and the potential is measured between two others. Water-bearing strata, especially with fresh water in sand or gravel, show a characteristic resistivity, so the method locates aquifers and their depth without drilling.

Test boring is direct exploration by drilling, not a geophysical method. Tracers are used to trace groundwater flow paths and velocity once water has been found, and remediation means cleaning up contaminated groundwater, not exploring for it.

Your Answer : (Not Answered)

Correct Answer : B

Steady flow occurs when the rate of recharge is equal to the rate of groundwater discharge.

Steady flow means the hydraulic head at every point stays constant in time. By continuity, the change in storage of the aquifer equals inflow minus outflow, so the storage, and with it the head, stays fixed only when recharge exactly balances discharge.

If recharge exceeds discharge, the aquifer gains storage and the piezometric surface rises; if it is less, the aquifer loses storage and the head falls. Either way the flow is unsteady. Pumping beyond the safe yield is a case of discharge exceeding recharge, which mines the aquifer and gives a continuously falling head.

Your Answer : (Not Answered)

Correct Answer : D

The incorrect statement is Darcy's velocity in unsaturated soil is always greater than that in saturated soil.

Hydraulic conductivity is highest when the pores are full of water. As a soil dries, water flows only through the smaller, still-filled pores and along thinner films, so conductivity, and with it Darcy's velocity under a given gradient, falls steeply. Flow in unsaturated soil is therefore normally slower, not faster.

The other statements are correct. Seepage velocity is Darcy's velocity divided by porosity, and porosity is less than 1, so it is always greater. Darcy's velocity depends on hydraulic conductivity and gradient, not on porosity alone. Ponding water on the surface raises the hydraulic head and so the gradient driving flow, which increases the seepage velocity.

Your Answer : (Not Answered)

Correct Answer : A

The pack-aquifer ratio is the 50% size of the gravel pack divided by the 50% size of the aquifer material.

In gravel-packed well design the pack is graded against the aquifer by comparing their median (\(D_{50}\)) grain sizes, the size at which 50% of the material is finer. The ratio of the pack’s median size to the aquifer’s median size is the pack-aquifer ratio, and design criteria keep it within a recommended range so that the pack holds back the aquifer sand while staying permeable.

The inverted ratios, aquifer over pack, would be less than 1 for any working pack. The 60% and 10% sizes are used for a different measure, the uniformity coefficient \(D_{60}/D_{10}\) of a single material, not for comparing pack with aquifer.