Environmental Geology & Hydrogeology Flashcards

The Cooper-Jacob straight-line approximation of the Theis equation is generally considered valid when the well function argument u is:

Answer: Less than about 0.01

Cooper-Jacob truncates the Theis series, which is accurate only when u = r²S/4Tt is small (commonly < 0.01).

Which range of storativity values is most typical of a confined aquifer?

Answer: 0.00001 to 0.001

Confined aquifers release water by elastic compression and expansion, giving storativity on the order of 10^-5 to 10^-3.

In a monitoring well screened across the water table, the measured LNAPL thickness is typically:

Answer: Greater than the actual mobile LNAPL thickness in the formation

LNAPL depresses the water level in the well, so apparent well thickness usually exceeds true formation thickness, especially in fine-grained soils.

A solute has Kd = 0.5 mL/g in an aquifer with bulk density 1.6 g/cm³ and porosity 0.30. What is the approximate retardation factor?

Answer: 3.7

R = 1 + (ρb/n)Kd = 1 + (1.6/0.30)(0.5) ≈ 3.7.

On a coastal island, the fresh groundwater head is 2 m above sea level. Using the Ghyben-Herzberg relation, the freshwater-saltwater interface is about how deep below sea level?

Answer: 80 m

Ghyben-Herzberg gives an interface depth about 40 times the freshwater head above sea level, so 40 × 2 = 80 m.

Which method is most commonly used to analyze slug-test data from a partially penetrating well in an unconfined aquifer?

Answer: Bouwer and Rice

The Bouwer and Rice method was developed for slug tests in unconfined aquifers with fully or partially penetrating wells.

Which investigation technique is generally best for defining groundwater flow paths and velocities in a mature karst aquifer?

Answer: Dye tracer testing

Karst flow follows discrete conduits that potentiometric maps miss, so dye tracing directly links input points to springs and wells.