Soil & Aquifer Properties and Their Effect on Groundwater

Generally, soil refers to the top few feet of the land surface.  The soil acts as a natural filter to screen out many substances that mix with the water.  But water will transport some contaminants into the groundwater. The amount of groundwater recharge, storage, discharge, as well as the extent of groundwater contamination, all depend on the soil properties:

These same terms are used to describe the hydraulic properties of aquifers.


Soil Texture

Soil is a mixture of three soil separates:

  • sands (the coarsest)
  • silts
  • clays (the finest)

Classification of these separates is based on grain size.  The following table shows the soil separate and its corresponding diameter size.  

 

Grain Size

Name

Size Range (mm)
gravel > 2.0
very coarse sand 1.0-1.999
coarse sand 0.500-0.999
medium sand 0.250-0.499
fine sand 0.100-0.249
very fine sand 0.050-0.099
silt 0.002-0.049
clay < 0.002

(Loxnachar et al, 19)

 

 

Texture Classification

The relative proportion of soil separates in a particular soil determines its soil texture.  The soil texture triangle gives the texture name, which is based on the percentages of sand, silt, and clay within the soil sample. 16  

Portage County consists of three main soil areas.  The soils in the eastern and central parts of the County were mainly formed in glacial drift.  The area consists of well-drained, sandy soils dominated by irrigated agriculture.   The soils in the southern part of the County are also sandy soils, but they formed in the basin of Glacial Lake Wisconsin.  The northwest corner of the County consists of poorly-drained heavier soils of clay and silt derived from the weathering of the crystalline bedrock17 

 

Soil Series Classifications

Soil scientists evaluated the soils in the County and classified them into 38 soil series.  A soil series contains soils with similar profiles or layers.  The series are named after a town or geographic feature nearby the location of where the soil was first described.  The individual soil boundaries were then drawn onto black and white aerial photos, which appear in the Soil Survey of Portage County, Wisconsin (1978).  The book also contains information on soil descriptions, use and management of soils, formation and classification of soils, and the general nature of the County.  Copies of these books may be viewed or obtained at the NRCS (Natural Resource Conservation Service), UWSP Library, the Central Wisconsin Groundwater Center, and  County libraries. 

In addition to the aerial soil maps, a general map of eleven soil associations was created to provide a general idea of soil types in the County.  "Such a map is a useful general guide in managing a watershed, a wooded tract, or a wildlife area, or in planning engineering works, recreational facilities, and community developments. 17To view this Soil Association Map along with a legend, click here.

The map displayed here is even  simpler.  The soils have been grouped according to their textures of clay, sand, and silt.  The general soil classes are shown overlying the groundwater provinces

 

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Cubic Packing

Rhombohedral Packing

Cubic Packing with Smaller Grains Filling the Void Spaces

Porosity

The shape and arrangement of soil particles help determine porosity.  Porosity or pore space is the amount of air space or void space between soil particles.  Infiltration, groundwater movement, and storage occur in these void spaces.  The porosity of soil or geologic materials is the ratio of the volume of pore space in a unit of material to the total volume of material.  

A mathematical equation of porosity looks like this:  Porosity or n=Vvoid / Vtotal.  Porosity is often expressed as a percentage of rock or soil void of material, so multiply the answer by 100.

The arrangement or packing of the soil particles plays a role in porosity.  In the diagrams to the left, the particles stacked directly on top of each other (cubic packing) have higher porosity than the particles in a pyramid shape sitting on top of two other particles (rhombohedral packing).  Can you see the difference in pore space?  

What could happen when smaller particles are mixed with larger particles?  As the diagram shows, the smaller particles could fill in the void spaces between the larger particles, which would result in a lower porosity.  

Not all particles are spheres or round.  Particles exist in many shapes and these shapes pack in a variety of ways that may increase or decrease porosity.  Generally, a mixture of grain sizes and shapes, results in lower porosity.

One important point to remember is that the diameter size of the grain does not affect porosity.  Remember, porosity is a ratio of void space to total volume.  A room full of ping pong balls would have the same porosity as a room full of basketballs, as long as the packing or arrangement are similar.

 

Porosity Ranges for Sediments

Material

Porosity (%)

well-sorted sand or gravel 25-50
sand and gravel, mixed 20-35
glacial till 10-20
silt 35-50
clay 33-60

 (Based on Meinzer (1923a); Davis (1969); Cohen (1965); and MacCary and Lambert (1962) as quoted by C.W. Fetter 2)

 

 

Sands have large pore spaces, whereas clays have many small pore spaces.   Both sand and clay can have high porosity.  

According to Holt, the sand-plain province has a porosity value between 32% and 38%.4

 

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Specific Yield

Not all the water stored in pore spaces becomes part of flowing or moving groundwater.   Just as water clings to a glass, water also clings to soil particles due to surface tension, cohesion, or adhesion.  It forms a thin film around a particle. Thus specific yield is always less than porosity.  

Specific yield is the ratio of volume of water that drains from a saturated rock due to gravity to the total volume of rock. 

A mathematical equation of specific yield looks like this:  Specific Yield or Sy=(Vdrained / Vtotal) x 100.  

Unlike porosity, specific yield is influenced by grain size. For example, if two soil samples have the same porosity, but different grain sizes (e.g. clay and sand), the sample with smaller grain sizes will have a lower specific yield.  Clay has a greater surface area than sand, therefore, more water will remain behind clinging to the clay particle surfaces.

According to Holt, the sand-plain province has a specific yield between 19% and 30%, which indicates that recharge from precipitation should pass easily from the surface to the zone of saturation.4

 

 
 

Specific Yield (%)

Material Maximum Minimum Average
coarse gravel 26 12 22
medium gravel 26 13 23
fine gravel 35 21 25
gravelly sand 35 20 25
coarse sand 35 20 27
medium sand 32 15 26
fine sand 28 10 21
silt 19 3 18
sandy clay 12 3 7
clay 5 0 2

(Johnson (1967) as quoted by C.W. Fetter  2)

 

 

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Permeability for Sediments

Material Permeability or Hydraulic Conductivity (cm/s)
well-sorted gravel 10-2 to 1
well-sorted sands, glacial outwash 10-3 to 10-1
silty sands, fine sands 10-5 to 10-3
silt, sandy silts, clayey sands, till 10-6 to 10-4
clay 10-9 to 10-6

(C.W. Fetter 2)

Permeability

The size of pore space and interconnectivity of the spaces help determine permeability, so shape and arrangement of grains play a role.  Permeability is a measure of a soil's or rock's ability to transmit a fluid, usually water.  Often the term hydraulic conductivity is used when discussing groundwater and aquifer properties.  Hydraulic conductivity simply assumes that water is the fluid moving through a soil or rock type. 

Water can permeate between granular void or pore spaces, and fractures between rocks.  The larger the pore space, the more permeable the material.   However, the more poorly sorted a sample (mixed grain sizes), the lower the permeability because the smaller grains fill the openings created by the larger grains. "The most rapid water and air movement is in sands and strongly aggregated soils, whose aggregates act like sand grains and pack to form many large pores" 16

On the other hand, clay has low permeability due to small grain sizes with large surface areas, which results in increased friction.  Also these pore spaces are not well connected. Clay often creates confining layers in the subsurface.

In rocks with fractures, the size of the openings, degree of interconnectedness, and the amount of open space all help determine permeability.  The crystalline rock of the northwest part of the County has low porosity because it contains very few openings, so water cannot pass through.  However, volcanic rock in the area may have high permeability if the openings are large and are well connected.

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Soil Attenuation or the Soil's Filtering Capability

Soil properties such as depth, texture, and permeability help determine the rate of groundwater recharge, as well as protection from groundwater contamination.  Land surface factors such as topography, geology, and vegetation along with soil properties determine the potential for groundwater contamination.  The soil acts as a natural filter.  In this context filtering means more than capturing solid particles.  Filtering also means retaining chemicals or dissolved substances on the soil particle surface, transforming chemicals through microbial biological processing, and retarding movement of substances.

The soil's ability to lessen the amount of or reduce the severity of groundwater contamination is called soil attenuation.  "During attenuation, the soil holds essential plant nutrients for uptake by agronomic crops, immobilizes metals that might be contained in municipal sewage sludge, or removes bacteria  contained in animal or human wastes. 18"

However, the soil's ability to filter contaminants is limited.  Contaminant attenuation in soils depends on water moving through the top two layers of soil (horizons A and B) at a rate that ensures maximum contact between the percolating water that contains contaminants and the soil particles. 18  Deep, medium and fine-textured soils are the best, whereas coarse-textured materials are the worst in terms of contaminant removal.  In coarse materials like sand, water moves through rapidly, reducing contact between the water and soil particles.  

 

 

 

 

(Data Source:  SURGO, 1998 and  Soils of Portage County and Their Ability to Attenuate Contaminants Map by Ward, 18)

Based on information from the Soil Survey of Portage County, Wisconsin17, Good and Madison used seven soil characteristics to classify each soil series according to its ability to attenuate contaminants.   The soil series were grouped into four categories of best, good, marginal, and least potential for soil attenuation. 

 

 In Portage County, the soils with the:

  • least potential for attenuation include deep sands and organic soils found in wetland areas; cover 64% of the County mainly in the central, southern, and northwestern parts
  • marginal potential for attenuation include those with 20-40 inches of loamy materials overlying till, outwash, or sandstone; cover 28% of the County mostly scattered among the moraines in the eastern part
  • good potential for attenuation occurs in the southern corner of the northwest corner where there are 30 inches of silt, and scattered among the moraines in the east where there are 40 inches of loamy materials over sand and gravel; covers 7% of the surface
  • best potential for attenuation includes no soils in the County; 0% 18

In summary, most of the County consists of soils with poor to marginal filtering capabilities.  In addition the water table is high in many areas.  These two factors suggest that land use practices, especially  irrigated agriculture, should be monitored to prevent groundwater contamination.  To learn about actions that help protect groundwater, click here.

 

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(Italicized words defined in the glossary.)

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