View
10
Download
0
Category
Preview:
Citation preview
Instrumentation Protocol for Monitoring Hydric soils
By: Wade Hurt, USDA, NRCS, NSSC
Minor contributions & meddling: Willie Harris, UF
Instrumentation
• The hydric soil technical standard requires proof of anaerobic & saturated conditions.
• This lecture provides information on instrumentation needed to document hydric/nonhydric conditions.
• Data can be collected by automated systems such as shown in the previous slide. We will demonstrate manual methods.
Site Selection • Site selection is critical. A
clear purpose is necessary for installation is costly & time consuming & years of data might be needed.
• The site selected for this demonstration (right) is at a convex to concave slope break & at a hydric to nonhydric soil change.
Soil • A thorough soil description
should be performed for profiles on both sides of the delineation boundary.
• Note if contrasting areas are formed from redox processes or not & indicating which, if any, hydric soil indicator is met.
• The soil to the right is hydric by meeting HS Indicator S6.
Wells & Piezometers
• Wells & Piezometers are available commercially.
• These are much more expensive than the PVC pipe we will use to demonstrate proper instrumentation.
PVC Pipe • PVC pipe are cut to the proper length: • One open well: 2 m plus a minimum of 20 cm for elevation
above ground surface (which could be as much a 1 m or more if inundation is expected).
• Two 1 m piezometers: 1 m plus a minimum of 20 for elevation above ground plus 15 cm to center the perforated area (the reason for the extra length will become clear later) for a total length (for our installation) of 1.35 m.
• Two 25 cm piezometers: 25 cm plus a minimum of 20 for elevation above ground plus 15 cm to center the perforated area for a total length of 60 cm.
Open Well
• Holes, approximately 1 to 1.5 cm apart are bored throughout the length of the open well.
Piezometer • Holes, approximately 1 to 1.5 cm
apart, are bored for a distance of approximately 15 cm (starting at 10 from bottom).
• A saw could also be used to slot the pipe.
• Dimensions of holed (or slotted) area dictated by objective; i.e., if one were interested in learning if a water table existed above a spodic horizon the holed area could be 10 cm long.
Protection Fabric
• Geofabric from well supply stores or weed guard from building supply stores (right) is used to keep soil material from entering well and piezometers.
• Weed guard is much cheaper than geofabric. Well supply stores also sell well socks which are even more expensive.
Fabric Wrap
• Seven to eight layers of the protective fabric are wrapped around the holed area of the piezometers.
• Duct tape secures the fabric to the piezometer. The fabric keeps out soil particles.
Bottom Caps
• Tightly fitting caps are placed on the bottom of the open well & piezometers with PVC glue.
• Note seepage of blue glue on image to the right.
25 cm Piezometer • On right is one of the 25 cm
piezometers ready for installation. Holes have been bored, 7-8 layers of fabric have been wrapped around the bored area & duct taped to the pipe, & the bottom of the piezometer has water tight cap.
• Note that the center of the piezometered area is 25 cm from top of upper piece of duct tape. Tape is placed to guide installation (top of tape will be elevation of ground surface when piezometer is installed).
1 m Piezometer • On right is 1m piezometer ready for
installation. • Holes have been bored, 7-8 layers of
fabric wrapped around bored area & duct-taped to pipe.
• Bottom of piezometer has been water tight capped. Note that center of perforated area is 1 m from top of upper piece of duct tape. Tape is placed to guide installation (top of tape will be elevation of ground surface when piezometer is installed).
Test
After preparation piezometers & wells are tested to determine if water moves through fabric.
Identification
• Marking well & piezometers helps ensure water data are recorded correctly.
Caps for Piezometers & Open Well • Caps are secured to bottom of
piezometer, preferably with PVC glue to ensure water tight seal.
• Holes are bored in bottom of open well but not in piezometer.
• Piezometers are used to determine presence of water at a specific location in soil. Wells are used to determine if there is water anywhere within 2 m of surface.
Holes (for wells only)
Finished Pipes • Holes in open well are wrapped
with fabric (7 or 8 layers) & duct-taped to pipe.
• By holing & protecting the open well, water is allowed to leave the well, confirming that water is not present within 2 m.
Fabric wrap
Ready for Installation
Photo shows completed open well, two 1m piezometers, & two 25cm piezometers. Scale is 1 meter.
Platinum Electrodes • Platinum (Pt) are actually heavy duty
insulated Cu tipped with about 1/2 cm of platinum welded or soldered to the Cu & secured with heavy duty epoxy.
• Pt electrodes are available commercially, e.g., Jensen Instruments (2021 S. 7th St., Tacoma, WA 98405) & Cypress Systems (http://www.cypresshome.com/). Most land grant institutions have soil & wetland scientists who construct Pt electrodes.
Platinum tip
Epoxy
Copper wire
Volt Meter • Volt meters are available at most
radio & electronic supply stores. One clip is connected to Pt electrode & one to reference electrode. Both are in contact with soil.
• Possible readings range from +1000 to -1000 mv. Remember to set the reading for mv, record the reading including + or - sign, & add approximately 200 to results. Correction values vary with type of electrode, electrolyte, & temperature.
Reference Electrode
There are various reference electrodes (mercury, calomel, & silver/silver chloride in varying strengths of KCl). Correction factor added to field reading are based on type of reference electrode & electrolyte (next slide).
Correction factors needed to convert voltages measured in the field to redox potentials (Ehs). Add the correction factor to the field reading based on the type of reference electrode & electrolyte (Light, 1973). All correction factors are in millivolts (mv).
Redox Potential Measurements
Platinum Electrode
Reference Electrode
Volt meter
Soil
Air
Installing Open Well & Piezometers
• Holes are dug with a 10cm bucket auger such as shown below & the well & piezometers are installed according to the process outlined in the next few slides. Augers are available from a variety of sources including www.forestry-suppliers.com.
Materials • Materials needed include bentonite clay
(upper right), sand (middle right), & sometime concrete (lower right).
• Sand is packed around perforated & wrapped part of piezometers.
• Bentonite is packed on top & bottom of the sand pack.
• A concrete cap is placed over the upper bentonite in cold climates to prevent frost action & for installation in very clayey soils to prevent movement of pipes from shrinking and swelling.
Install PVC pipe 4 cm in diameter in open bore hole 10 cm in diameter
B
Piezometer Installation
A = non slotted pipe wrapped with duck tape
B = slotted pipe wrapped with geofabric (7 layers)
Piezometer installed at any depth: (A) is for water occlusion (B) is for silt and clay occlusion
A
A Bentonite 10 cm
4 cm Piezometer
10 cm Bore Hole
Sand 15 cm
Bentonite 10 cm
Piezometer Placement
The center of the perforated & protected piezometer is placed in auger hole at 25 cm for piezometer that is to measure water at 25 cm (or 100 cm for piezometer that is to measure water at 100 cm). Bentonite is then packed below the perforated & protected area, sand is packed around the perforated & protected area, & more bentonite is placed above the perforated & protected area.
Bentonite pack
Sand pack
Bentonite pack
Install PVC pipe 4 cm in diameter in open bore hole 10 cm in diameter
B
Open Well Installation
A = non slotted pipe wrapped with duck tape
B = slotted pipe wrapped with geofabric (7 layers)
Open Well installed at any depth: (A) is for water occlusion (B) is for silt and clay occlusion
A Bentonite 10 cm
4 cm Open well
10 cm Hole
Sand 2 m
The illustration below shows why it is important to have piezometers & not just an open well. Piezometers are used to determine if free water exists at a certain depth in soil. Open wells tell us if there is water in the soil anywhere within the 2-m installation depth. Wells give us regional hydrology data; piezometers give us water data for a specific point or soil horizon.
Installation Completed (or is it?) Please note that the open well & one of the piezometers have loose fitting caps; installation is not complete until all have loose fitting caps.
Installation Completed • The previous slide showed the two
25 cm piezometers, the two 1 m piezometers, & the open well installed but loosely fitting caps had not been placed on three of the piezometers. Remember, the bottom of piezometers are tightly sealed & the top of the piezometers are loosely capped (right photo). The bottom of the well is perforated & wrapped & the top is loosely sealed. The red bars on the previous slide shows where the 5 electrodes would be placed in this installation example.
Measurements? • Measurements (electrodes, open wells, &
piezometers) should be measured weekly preferably the same day & hour.
• Often wells & piezometers retain water during periods of receding water tables. I recommend using a siphon (right photo) to remove water from the piezometers after recording the depth to water. This is to ensure residual water does not affect the next weekly measurements. It is not necessary to remove water from the well.
Protection
• I recommend protecting instrumented sites (including data recorder) with wire to prevent human disturbance & removal of vegetation in the herb & shrub strata to avoid animal disturbance.
Adapted from : Richardson, J.L., J.L. Arndt, and J.A. Montgomery. 2001. Hydrology of wetland and related soils. In Wetland Soils: Their Genesis, Morphology, Hydrology, Landscapes and Classification. J.L. Richardson and M.J. Vepraskas, eds. CRC Press, Boca Raton, FL.
Wells & Piezometers
A. Stagnant B. Recharge C. Discharge
W1=W2=P1=P2 No flow
W1>P1 W2>P2 P1<P2 Flow downward, Right to left
P1>W1 P2>W2 P1>P2 Flow upward, Left to right
From: Richardson, J.L., J.L. Arndt, and J.A. Montgomery. 2001. Hydrology of wetland and related soils. In Wetland Soils: Their Genesis, Morphology, Hydrology, Landscapes and Classification. J.L. Richardson and M.J. Vepraskas, eds. CRC Press, Boca Raton, FL.
Summary
• Proper preparation, instrumentation, & maintenance of instruments is essential to the collection of usable data. I call this instrumentation hygiene as well as instrumentation common sense. Instrumentation sites should be protected from vandals; both human and animal.
Literature Cited
• Light, T.S. 1973. Standard solution for redox potential measurements. Analytical Chemistry 44:1038-1039.
• Richardson, J.L., J.L. Arndt, and J.A. Montgomery. 2001. Hydrology of wetland and related soils. In Wetland Soils: Genesis, Hydrology, Landscapes, and Classification (J.L. Richardson and M.J. Vepraskas, eds. CRC Press. Boca Raton, FL.
• Sprecher, S.W. 1993. Installing monitoring wells/piezometers in wetlands. WRP Technical Note HY-1A-3.1. US Army COE, Environmental Laboratory, Vicksburg, MS
Recommended