ASBOG - Association of State Boards of Geology Hydrogeology Questions and Answers 2

Question 1

What is the difference between specific yield and specific retention of an aquifer material?

Accepted Answer

Specific yield is the water released under gravity; specific retention is the water held against gravity by capillary forces; they sum to total porosity

Answer

Specific yield is total porosity; specific retention is effective porosity minus total porosity

Answer

Specific yield is the rate of water release per unit time; specific retention is the storage at full saturation

Answer

Specific yield applies to confined aquifers; specific retention to unconfined aquifers

Question 2

What is the cone of depression in a pumping well, and what factors control its size and shape?

Accepted Answer

A funnel-shaped zone of decreased hydraulic head that develops around a pumping well, controlled by pumping rate, hydraulic conductivity, and storativity

Answer

An increase in water table elevation surrounding a recharge well, controlled by injection pressure

Answer

A conical depression in the land surface caused by subsidence over a depleted confined aquifer

Answer

A zone of increased salinity around a coastal well due to saltwater intrusion

Question 3

In a slug test, what is being measured, and what aquifer parameter is typically derived?

Accepted Answer

The water level response to a sudden change in water level in a well; aquifer hydraulic conductivity (K) using Hvorslev or Bouwer-Rice methods

Answer

The total dissolved solids in groundwater; aquifer transmissivity via water quality ratios

Answer

The rate of contaminant transport from a nearby source; aquifer dispersivity via tracer breakthrough

Answer

The temperature profile of groundwater with depth; geothermal gradient of the aquifer

Question 4

What is the safe yield (sustainable yield) of an aquifer, and why is it a controversial concept?

Accepted Answer

The maximum long-term extraction rate at which the aquifer can produce water without causing unacceptable negative impacts, including depletion, land subsidence, and reduced baseflow to streams

Answer

The maximum pumping rate that will never cause any drawdown in the aquifer

Answer

The volume of recharge entering an aquifer per year, calculated from rainfall minus evapotranspiration

Answer

The rate at which a well can pump without exceeding the gravel pack capacity of the well screen

Question 5

What is the primary difference between a gaining stream and a losing stream with respect to groundwater?

Accepted Answer

In a gaining stream, groundwater discharges into the stream channel; in a losing stream, streamflow recharges the groundwater system

Answer

Gaining streams have higher discharge than losing streams due to runoff; no groundwater interaction

Answer

Gaining streams are in humid climates; losing streams are in arid climates

Answer

Gaining streams have steeper gradients; losing streams have flatter gradients

Question 6

What does a Piper diagram (trilinear diagram) display, and what is its primary use in hydrogeology?

Accepted Answer

The major ion chemistry (cation and anion percentages) of groundwater samples, used to classify water types and identify hydrochemical facies and mixing

Answer

The distribution of hydraulic head values across a wellfield, used for aquifer delineation

Answer

The spatial distribution of dissolved contaminants in an aquifer, used for plume mapping

Answer

The relationship between specific conductance and depth in a monitoring well, used for detecting saltwater intrusion