Alberta West News: In a saturated aquifer, what percentage of total volume is occupied by free water for sand, gravel, sandstone, silt, etc.?

Thursday, October 16, 2025

In a saturated aquifer, what percentage of total volume is occupied by free water for sand, gravel, sandstone, silt, etc.?

Saturated Aquifer Free Water Content

In a saturated aquifer, the percentage of total volume occupied by free water varies significantly depending on the material type. Understanding this requires distinguishing between total porosity (all void space) and specific yield (water that drains freely under gravity).

Total Porosity vs. Free Water

When an aquifer is fully saturated, all pore spaces are filled with water. However, not all of this water is "free" to move or drain. The total water content equals the porosity, while the freely draining water is measured by specific yield.pubs.usgs+1

The relationship is expressed as:

$\text{Porosity} = \text{Specific Yield} + \text{Specific Retention}$

where specific retention represents water held by capillary and molecular forces that cannot drain by gravity.pubs.usgs+1

Free Water Content by Material Type

Based on comprehensive USGS compilations, here are the typical values for different materials in saturated conditions:aqtesolv+1

Sand

Coarse sand: 27% specific yield (total porosity 31-46%)
Medium sand: 26% specific yield (total porosity 29-49%)
Fine sand: 21% specific yield (total porosity 25-35%)

The total porosity of sand ranges from 25-40%, but much of this water is freely drainable, resulting in relatively high specific yields.ebsco+2

Gravel

Coarse gravel: 22% specific yield
Medium gravel: 23% specific yield
Fine gravel: 25% specific yield (total porosity 24-44%)

Gravel has total porosity of 20-40%, with specific yields of 19-28%. The coarse grain size minimizes capillary retention.wikipedia+1

Sandstone

Fine-grained sandstone: 21% specific yield
Medium-grained sandstone: 27% specific yield
Total porosity: 10-30%ebsco+1

Consolidated sandstone has lower porosity than unconsolidated sand due to cementation, typically around 20% porosity with 5-15% specific yield.aqtesolv+1

Silt

Specific yield: 8% (range 3-19%)
Total porosity: 35-50%minnstate.pressbooks

Despite high porosity, silt retains most water through capillary forces, yielding only a small fraction freely.pubs.usgs+1

Clay

Specific yield: 2% (range 0-5%)
Total porosity: 45-55%wikipedia+1

Clay exemplifies the distinction between porosity and free water content—it can hold enormous amounts of water (up to 60% porosity) but yields almost none by gravity drainage due to strong molecular attraction.ebsco+3

Additional Materials

Limestone: 14% specific yield, 1-10% total porosity (can reach 50% in solution-weathered formations)aqtesolv+1

Gravel-Sand Mixtures: 25% specific yield (gravelly sand)pubs.usgs+1

Till: 6-16% specific yield depending on composition (predominantly silt: 6%; predominantly sand or gravel: 16%)aqtesolv

Key Factors Affecting Free Water Content

The amount of free water in saturated aquifers depends on several critical factors:pubs.usgs+1

Grain Size: Coarser materials like gravel have larger pores that cannot retain water against gravity, resulting in higher specific yields. Fine materials like clay have microscopic pores where surface tension dominates.wikipedia+1

Sorting: Well-sorted sediments (uniform grain size) have higher porosity and specific yield than poorly sorted sediments, where smaller grains fill spaces between larger ones.opentextbc+1

Degree of Consolidation: Unconsolidated sediments typically have porosity of 10-70%, while cemented sedimentary rocks have reduced porosity (10-30%) due to mineral cements filling pore spaces.opentextbc+1

Duration of Drainage: Specific yield values increase with drainage time. Fine-textured materials not only yield less water but yield it more slowly. Maximum specific yield may require months to years to achieve.pubs.usgs+1

Practical Implications

For your watershed hydrology research in Alberta, these values have important implications. A saturated sand aquifer with 35% porosity might contain that entire percentage as water when fully saturated, but only about 26-27% would be available as free water that could drain during drought conditions or be extracted by wells. The remaining 8-9% would be retained by capillary forces even after prolonged drainage.pubs.usgs+2

This distinction is crucial for understanding how aquifers respond to drought—the specific yield determines how much water is actually released as water tables decline, not the total porosity.pubs.usgs+1

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