Safety of CCA Treated Wood for Vegetable Gardens

1
Safety of CCA Treated Wood for Vegetable Gardens

• Carl Rosen, Farhana Alamgir, and Deborah Allan
• Department of Soil, Water, and Climate
• University of Minnesota

2
Outline

• What is CCA and why is it used
• Health concerns associated with arsenic
• Overview of CCA treated wood study

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CCA -

• Chromated copper arsenic
• Waterborne wood preservative
• Enhances useful life of wood
• Resists insect and fungal attack

4
CCA -

• CCA is used on 98 of the wood treated in the
  U.S.
• No odor or vapor, convenient, economical
• Most common form is CCA C
• Leaching of CCA from wood has been cause of
  concern (primarily arsenic)

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Arsenic (As) - atomic number 33
Periodic Table
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Arsenic

• Arsenic is a metalloid
• Can be lethal in high doses, lower levels can
  cause skin lesions, cancer
• Arsenic compounds are white or colorless
• No specific taste or odor
• Soil background levels

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Treated Wood Study

• Numerous questions asked about safety of CCA
  treated wood for vegetable gardens
• Wood preservers vs. Organic gardening
• Texas studies with newly constructed beds showed
  some leaching of CCA, but uptake by plants low

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Treated Wood Study - Objectives

• Characterize the distribution of arsenic in soil
  from garden beds made with CCA treated wood
• Determine extent of arsenic uptake by selected
  vegetables

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Treated Wood Study - Methods

• Six sites selected - beds were at least ten years
  old
• Soil core samples were taken using a 1-inch soil
  probe, to a depth of 6 inches
• Soil was sampled inside the beds at three
  distances from the treated wood 0-1, 3-4 and
  12-13, and outside the bed - 5 ft away).
• The soil samples were analyzed for arsenic

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Study Site Characteristics
SITE DESCRIPTION
Vegetation
Bray
-
P
Vegetation
Bray
-
P
O.M.
Soil
Soil
Location
O.M.
Soil
Soil
Location
Site
Site






in bed
(ppm)
in bed
(ppm)
(L.O.I)
pH
type
in MN
(L.O.I)
pH
type
in MN
Evergreens
Loamy
Evergreens
Loamy
10.9
101.9
6.6
10.9
101.9
6.6
Marine
Site 1
Marine
Site 1
sand
sand
Stillwater
Site 2
Stillwater
Site 2
6.8
Grass
66.6
6.2
Loamy
6.8
Grass
66.6
6.2
Loamy
sand
sand
Wayzata
Site 3
Wayzata
Site 3
13.2
Flowering
365.6
6.8
Loamy
13.2
Flowering
365.6
6.8
Loamy
plants
sand
plants
sand
Crystal
Site 4
Crystal
Site 4
6.3
Vegetable
230.9
6.4
Sandy
6.3
Vegetable
230.9
6.4
Sandy
crops
loam
crops
loam
Roseville
Site 5
Roseville
Site 5
5.5
Flowering
49.6
7.3
Sandy
5.5
Flowering
49.6
7.3
Sandy
plants
loam
plants
loam
Grass
South
Site 6
South
Site 6
5.6
49.8
7.3
Sandy
5.6
49.8
7.3
Sandy
Minneapolis
Minneapolis
loam
loam
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Arsenic distribution in beds
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Arsenic Uptake Pot Study

• Two sites selected based on highest arsenic
• 100 lbs of soil was sampled from inside the bed,
  0-1 inch away from the treated wood and in the
  middle of the bed
• Four types of crops were selected for the study
  carrots, spinach, bush beans, and buckwheat
• After 8 weeks, the plants were harvested, dried
  and analyzed for arsenic

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Soil Characteristics Pot Study

• Soil Distance from wood As pH O.M.
• (inches) (ppm) ()
• A 0-1 40 7.4 6.5
• A 45-50 10 7.0 9.2
• B 0-1 50 6.7 4.2
• B 45-50

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Greenhouse study
Spinach
Beans
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Carrots
Washing carrots
Peeling carrots
17
Carrots
200
Soil A
Carrot peels
180
Carrots with peel
160
140
120
100
As concentration f.w. (ppb)
80
60
40
20
0
0-1 inch
45-50 inches
Distance from treated wood

Soil B

400
350
300
250
200
As concentration f.w. (ppb)
150
100
50
0
0-1 inch
45-50 inches
Distance from treated wood
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Spinach
Soil A
Beans
80
Buckwheat
70
60
50
40
As Concentration f.w. (ppb)
30
20
10
0
0-1 inch
45-50 inches
Distance from treated wood
Soil B
350

300


250
200
As concentration f.w. (ppb)
150
100
50
0
0-1 inch
45-50 inches
Distance from treated wood
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Implications

• Plants grown in soil close to the treated wood
  accumulated more arsenic than those grown in soil
  from the middle of the bed
• All tissue arsenic levels were below the limit
  for arsenic content in fresh fruit, crops, and
  vegetables set by the United States Public Health
  Service (2600 ppb FW) as well as below the 1000
  ppb limit set in Canada

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Minimize Arsenic Uptake

• Grow plants at least 15 inches from treated wood
• Place edging at 12 inches to prevent roots from
  growing in higher arsenic levels
• In new beds or when replacing old soil, line the
  wood with plastic to prevent arsenic from
  entering soil

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Bean leaves
2100
1800
Soil A
1500
Soil B
As concentration f.w. (ppb)
1200
900
600
300
0
0-1 inch
45-50 inches
Distance from treated wood