NAEP Science Grade 12

Category - Earth Structures

The hydraulic conductivity of a sediment is a measure of the rate at which the sediment can transmit water. Which of the following sediments is likely to have the greatest hydraulic conductivity?
  1. A poorly sorted mixture of fine- and medium-sized sand grains
  2. A well-sorted silt deposit
  3. A poorly sorted mixture of clay and small gravel
  4. A well-sorted coarse-sand deposit
Explanation
Correct Response: D. A well-sorted coarse-sand deposit will have the largest pore spaces between grains and the pore spaces will be better connected because of the uniform grain size in a well-sorted sand. This combination means both the porosity and hydraulic conductivity of the sand will be high, allowing for relatively rapid transmission of water through the sediment. A poorly sorted mixture of fine- and medium-grain sediments (A) will have a relatively low porosity as many spaces between the coarser grains will be filled with fine-grain sediments. This also blocks water transmission and leads to relatively low hydraulic conductivity. Silt deposits (B) are too fine-grained to have a high hydraulic conductivity and the tiny particles of clay in a clay-and-gravel deposit (C) will effectively fill the pore spaces and greatly reduce transmission of water through the sediment.
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