sulfidic sites reported by vdot · virginia: the tabb formation. sample locations hampton and...
TRANSCRIPT
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o Prior to past 15 years: acid sulfate soil studies in Virginia typically were related to coal mining.
o During 1980’s and 1990’s the Virginia Department of Transportation (VDOT) became increasingly aware that discrete occurrences of acid roadcuts around the state were manifestations of the same problem – exposure of sulfidic materials.
o From 1997 – 2001 researchers at VT (CSES) worked on identifying, characterizing, and delineating sulfide bearing materials across the state (VDOT funding).
o Stafford Regional Airport (2001 – 2006) and other miscellaneous projects – mainly in the Coastal Plain.
Acid-S soils in Va: historical perspective……
Sulfidic sites reported by VDOT
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Coastal Plain: the Tabb Formation.
Roadcut along 64E in Suffolk.
Soil pH ~ 3.3
Coastal Plain: the Tabb Formation.
Pond at Hampton Roads Center
Runoff from stockpiled material at Hampton Roads Center.
pH = 3.09
pH = 3.05
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Compiling a state-wide sulfide hazard map for Virginia: the Tabb Formation.
Samplelocations
Hampton and Suffolk
PPA < 10 Mg CaCO3/1000 Mg material
Total-S < 0.2%
pH 3 - 4
water pH = 3.1; Fe = 12 mg/L; Mn = 2 mg/L
Coastal Plain: the Tabb Formation.
The Tabb Formation (Sedgefield member) may be considered likely to produce moderately problematic roadside management conditions which could require special reclamation efforts.
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Coastal Plain: Tertiary marine sediments.
Chesterfield County: Proctor’s Creek was redirected to this excavated channel during road construction.
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Coastal Plain: Tertiary marine sediments.
Further upstream, Proctor’s Creek
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Coastal Plain: Tertiary marine sediments.
Interchange of I-295/Rt-360 in Mechanicsville.
Soil pH ~ 3
0 9.14
18.28
27.43
1.21
3.04
4.87
6.70
8.53
36.57
41.14
45.7 50.29
54.86
59.43
64.00
68.58 73.15 82.29 91.44 100.58Distance (meters) along base of roadcut
Height (m)
1.5‐2.0
1.0‐1.5
0.5‐1.0
%S
82.29 91.44
100.58
40‐5030‐4020‐30
10‐20
1.21
3.04
4.87
6.70
8.53
36.57 41.14 45.72
50.29 54.86 59.43 64.00 68.58
73.150 9.14 18.28 27.43
Distance (meters) along base of roadcut
PPA: Mg CaCO3/1000 Mg material
Height
(m)
Quaternary sands and gravels overlie reduced Tertiary marine sediments of the Chesapeake Group. The upper portion of the roadcut was undisturbed by road construction and is vegetated.
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Coastal Plain: Tertiary marine sediments.
Within 5 years, erosion has removed over 30 cm of sediment...
…and the guardrail is severely corroded.
Coastal Plain: Tertiary marine sediments.
I-95 in Spotsylvania County. The stability of this sign is compromised by concrete degradation and erosion. Acid conditions (soil pH ~ 2.5) limit vegetation.
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Caroline County:500,000 tires were excavated from this ravine, but in doing so sulfidic materials were exposed.
Compiling a state-wide sulfide hazard map for Virginia: Tertiary marine sediments.
Samplelocations
Greensville, Prince George, Chesterfield, Henrico,Hanover, and Spotsylvania Counties; City of Richmond.
PPA 10 – 20 (oxidized) 30 – 50 (reduced)Mg CaCO3/1000 Mg material
Total-S < 1.0% (oxidized) 1.0 – 2.5% (reduced)
pH 2.5 – 3.5 (oxidized) 5.5 – 8.0 (reduced)
water pH = 3.2; Fe = 17.5 mg/L; Mn = 2.1 mg/L
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Coastal Plain: Tertiary marine sediments.
Tertiary marine sediments may be considered highly likely to produce problematic roadside management conditions which require intense reclamation efforts.
Fredericksburg
Piedmont: Quantico Formation.
Roadcut along Mine Road in Stafford County.
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Piedmont: Quantico Formation.Iron-staining along curbs and sidewalks through the Hampton Oaks subdivision (~1996). Homeowners in this neighborhood apply the equivalent of 2 Mg agricultural lime per hectare (1~ton/ac) per month to maintain soil pH above 5.5.
Concrete etching occurs when acid drainage “dissolves” out the cement, deteriorating the concrete.
Piedmont: Quantico Formation.
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Compiling a state-wide sulfide hazard map for Virginia: the Quantico Formation.
SampleLocations
Stafford and Prince William Counties
PPA 6 – 100Mg CaCO3/1000 Mg material
Total-S 0.2 – 4.0%
pH 2 - 4
water pH = 2.5; Fe = 249 mg/L; Mn = 18 mg/L
Piedmont: Quantico Formation.
Exposures of the Quantico Formation may be considered highly likely to produce severely problematic roadside management conditions which require intense reclamation efforts.
Fredericksburg
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Blue Ridge: Ashe Formation.Rt-750, Floyd Co.
Drainage from this roadcut flows to the
culvert on the far right.
Acid produced by pyrrhotite.
~ 2 mm
the Ashe Formation
I-77, Carroll Co.
pH of road drainage is near neutral - local bedrock provides enough alkalinity to neutralize acid drainage !
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Compiling a state-wide sulfide hazard map for Virginia: the Ashe Formation.
SampleLocations
Floyd and Carroll Counties
PPA 1 – 20Mg CaCO3/1000 Mg material
Total-S 0.0 – 2.0%
pH 3.5 – 4.5
water pH = 3.2; Fe = 39.7 mg/L; Mn = 1.4 mg/L
Blue Ridge: Ashe Formation.
Exposures of phyllitic material from the Ashe Formation may be considered somewhat likely to produce moderate to severely problematic roadside management conditions.
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Valley and Ridge: Devonian black shales.
Exposure of the Millboro shale and NeedmoreFormation along Rt-250 in Highland County.
Landslide along bentonite bed – did sulfide weathering contribute to slope failure ?
Valley and Ridge: Devonian black shales.
Culvert beneath I-64 in Clifton Forge
pH upstream end of culvert: 6.6pH downstream end of culvert: 5.3pH drain into culvert: 2.7
Fe- and Al-precipitates heavily coat the bed of this stream which is biologically dead for at least 0.5 km downstream of the roadcut.
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Inside the culvert!
Compiling a state-wide sulfide hazard map for Virginia: Devonian black shales.
Sulfate salts precipitated in rock fractures.
Copiapite (yellow) precipitated along the roadcut.
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EXPANSIVE PYRITIC SHALES
Sulfides may convert to sulfate minerals that occupy several times more volume than the original sulfide volume.
The forces exerted by this expansive crystal growth may result in heaving which affects overlying structures.
Heaving also may result in slope instability and landslides along roadcuts.
Cracks in basement floor due to sulfide/sulfate-induced shale heave.
A Canadian consumers group (ACQC) publishes this pamphlet to promote public awareness of expansive pyritic shales. According to this group about 10,000 houses in eastern Canada are thought to be affected by pyrite oxidation.
Expansive pyritic shales are abig problem around Montreal, Canada!
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Pavement surface distortionDisplaced curb and gutter
above culvert
Damaged culvert pipe Shale borrow site
Sulfide-induced shale heaving in Bristol, VAPictures from report to VDOT by Thomas E. Freeman P.E., available online
Compiling a state-wide sulfide hazard map for Virginia: Devonian black shales.
Samplelocations
Frederick, Highland, Alleghany, and Wise Counties
PPA 0 – 60*Mg CaCO3/1000 Mg material
Total-S 0 – 3.3%
water pH = 2.7; Fe = 59.0 mg/L; Mn = 38.5 mg/L
* numerous samples contained carbonates which neutralize acidity.
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Valley and Ridge: Devonian black shales.
Exposure of the Marcellus, Millboro, and Chattanooga shales, and the Needmoreformation, may be considered somewhat likely to produce severely problematic roadside management conditions.
Appalachian PlateauThe majority of this material is relatively low in pyritic-S, and some strata contain carbonate cementing agents which help neutralize acidity.
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Our approach to producing a sulfide hazard risk map
o Sulfides are a function of geologic setting.
o If sulfides are identified in several samples from a specific geologic formation then the entire formation is considered “at risk”.
o On a statewide scale, delineation of acid-producing materials is best identified as the boundaries of sulfide-bearing formations using the State Geologic Map of Virginia.
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Compiling a state-wide sulfide hazard map for Virginia: the final map.
All formations, based on draft version of
digital geologic map of Virginia.
CharacterizedNot characterized
Sulfides documented. Acid potential unknown.
Sulfides not documented.
1: PPA < 10; S < 0.5%.
2: PPA < 10; S > 0.5%.
3: PPA 10 - 60.
4: PPA - more than 10% of samples > 60.
Sulfide hazard risk map for Virginia
Sulfides undocumented
1
2
3
4
Sulfides documented
NSulfides undocumented
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This map is based on surficial geology. Sulfide-bearing materials not mapped at the surface may lay within excavation depths. (i.e. “no risk” may become “high risk” with depth).
A "high-risk" geologic unit which has undergone significant geologic weathering may no longer be acid forming (through the depth of weathering).
The nature and extent of problems associated with exposure of sulfidic materials are site specific, and therefore the map is best interpreted with an understanding of the geology and landscape for a given area.
Section from Stafford quad geologic map