Dugout Site Testing

by Ron Lien, PFRA, Regina, Saskatchewan

Lined Dugout at RiverEdge Tree Nursery Ltd., Saskatoon

Before digging, an assessment of the general landforms should be carried out. Areas of sand dunes or thick wind deposited soil layers will be unsuitable. Areas of layered water deposited material are doubtful; however, an area of clay till or uniform lake bottom deposits will likely be satisfactory.

Storage Dugouts
Soil testing for a water storage dugout must check for the following:

• Soils to a depth of one metre deeper than the dugout should be an impervious
  -Heavy Clay
  -Clay
  -Silty Clay
  -Sandy Clay
  -Clay Loam
  
• Any evidence of fractures or cracking at depth in clay type soils indicates a marginal or unsuitable soil. In this case or in the case of light loam soils, local experience should be relied on in determining water retention on a long-term basis.
• If soil testing encounters a layer of sand, silt or sandy loam soil, an unsuitable area for a storage dugout is indicated.
• Should a testhole indicate silt, sand or gravel less than 0.9 m thick, a suitable site is indicated provided that
  -Other testholes do not show the unsuitable material, thus indicating that they are a limited size pocket and not an extensive layer.
  -During construction the silt, sand or gravel layer should be over excavated 0.6 m and replaced with compacted clay to isolate the pocket from the dugout.

Lined Dugouts
Where a water storage dugout is required and a site composed of impervious soil is not available, a lined dugout may be required. Liners of compacted clay, bentonite and clay, straw gleization, polyethylene or PVC have been used successfully. All liners must have a layer of about 0.3 m of soil covering them to protect from:

• Ultraviolet light damage to plastic liners
• Cracking of clay liners due to drying
• Accidental damage (especially cattle)
• Hydraulic uplift from local soil moisture

For a lined dugout it is critical to check for depth to water table. It must be at least one metre below the proposed dugout bottom./P>

Seepage Dugouts
Should soil testing encounter an aquifer with water table near enough to the surface to be exposed with an open hole, the following should be checked by soil testing:

• The aquifer should be an extensive layer, not an isolated pocket.
• The aquifer should be at least 0. 15 m thick for a gravel layer or at least 0.9 m thick for fine sand.
• For livestock use, the groundwater salinity should be less than 3000 mg/L although some use may be made of water as high as 7000 mg/L (depending on what a laboratory analysis recommends).
• Any excavation penetrating through an aquifer must not come within one metre of fractured bedrock or another aquifer that could be contaminated by seepage from the surface.
• For household domestic use an open excavation is not recommended as the aquifer should be closed off from contamination from such things as livestock, wildlife or airborne pesticides.

A seepage dugout need not be very large as the amount of water stored will not improve its ability to supply water through a drought.

PFRA provides technical assistance to farmers for planning the most suitable water supply for their farm.