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OLLI Course Field Trip to the Skytop/I-99 Project

Much of the material in these notes was taken from a report by David P. Gold and Arnold G. Doden on the Penn State Geosciences Website
http://www.geosc.psu.edu/news/features/gold/skytop.html

 
The Large Cut Face in section A-12: 1000 horizontal feet Bald Eagle containing 5% pyrite

Introduction.  The Skytop area sits atop Bald Eagle Ridge, which extends hundreds of miles from central Pennsylvania into West Virginia.  It marks the Allegheny Front, a zone of profound discontinuities in topography from the Valley and Ridge to the Allegheny Plateau, in surface rock structure, in ages of rocks, and in agricultural use and culture.  The rocks at the ridgetop sit stratigraphically 15,000 feet higher than the rocks at the crest of the Plateau, and in turn erosion has stripped off 15,000 feet of rocks once present above the level of the Ridge.

     Communication between valleys on either side of the Ridge has always been through wind gaps and water gaps.  The present course of US322 and I-99 follows earlier roads through the Skytop wind gap.  These wind gaps exist because rocks are less resistant to erosion than the Tuscarora sandstone on either side.  Some exist because of faults, and some because of the presence of veins of weaker material.  The Skytop gap exists because of a combination of faults cutting across the ridge, veins of pyritic material, and weak shales and limestones under some of the faults.  Some of these features were known to many geologists before construction, and some were not.

 Chemistry of pyrite alteration.  Pyrite at the Earth’s surface, in the presence of water and oxygen, spontaneously breaks down and produces sulfuric acid and an oxidized and hydrated form of iron called goethite:

     FeS₂ + 3.75 O₂ + 2.5 H₂O --> 4 H⁺ + 2 SO₄^2- + FeO(OH)     [1]

This is the same reaction that produces acid mine drainage, AMD.  Harmful heavy metals (Cd, As, Sb, Hg, Pb) are also released from minerals associated with the pyrite and in addition can be leached by the acid waters from other rocks.  Groundwater contamination can be monitored from sulfate content.

 The vein systems.  Prior to the recent road construction, virtually no veins were exposed at Skytop. However, their existence was suspected from the presence of arsenic and a phosphatic mineral in the old quarry workings along the ridge to the southwest, as well as gossan exposed along Old Route 322 in a road cut into the Bald Eagle sandstone near the site of the new Route 322 bridge.  Gossan is a punky, red-to-orange rock that results from reaction [1], the colors indicating the presence of goethite and its allied iron oxide mineral hematite, Fe₂O₃.  Decades of Penn State geology students visited that outcrop and were told that the gossan was underlain by a vein system containing pyrite. 
     The majority of veins are composed of pyrite and other sulfide minerals with Fe, Cu, As, and Zn.  Most are concentrated in the Bald Eagle Formation and to a lesser extent in Tuscarora and Juniata sandstones.  Few occur in shales.  Veins strike SSE, are typically 1 cm wide, and are spaced 50cm to 1m apart.
     Construction of I-99 cost about $10 million per mile.  Because of that cost, sections 4 miles long were put out for bid, and contracts were won by various companies.  Sections C-12 to the west and A-12 to the east join near the crest of Skytop.  Pyrite-bearing veins, well developed in the Bald Eagle sandstone, were exposed in the “Large Cut Face” (LCF) road-cut in the A-12 section over a distance of approximately 1000 feet. The LCF rock, overall, contains about 5% pyrite. Approximately 1 million cubic yards (2 million tons) of this rock was excavated. Pyrite veins are exposed for approximately 2/3 of the way up slope on the northern side of the road cut, before passing into gossan (oxidized pyritic rock).

 Formation of the vein systems.  The vein material was deposited from hot (150^oC to 350^oC), acidic, saline, reducing aqueous fluids migrating through the rocks at a depth of 5-8 km.  The joints through which they traveled were caused by a methane gas drive late in the Alleghenian orogeny, after formation of the Nittany anticlinorium.  Whether the sulfide-precipitating solutions were Alleghenian in age, or younger, has not been determined; one set has been dated at 35 my, considerably younger than Alleghenian, but it may be an exception. We will examine one locality in Juniata sandstones where the oxidized red sandstone has been reduced to green-gray in a zone of sulfide veins.

Efflorescent minerals in Bald Eagle fill

 Efflorescent minerals. A number of efflorescent [blooming] minerals have been identified, most of which are hydrated sulfates, gypsum being the only name you would recognize. These grow in moist seeps as saturated ground water evaporates at the surface to form cauliflower head like blooms. These salts are extremely soluble in water and do not survive the next rain. Their dissolution results in an immediate increase in sulfate ions in the runoff.

Chevron folds in the C-12 section.  The top cross-section for Skytop is an interpretation that would have been made before I-99 excavations.  The bottom cross-section is based on new exposures.  The complex series of thrust faults and chevron folds uncovered in the C-12 domain was unanticipated.  Unfortunately, bedding on the limbs of these folds was subparallel to the slopes of the roadcuts, so that small landslides developed.  To compensate, Penn DOT decreased the slope angles and raised the northbound road bed 17 feet with, as it turned out, pyritic sandstone.

Cross-sections through Bald Eagle Ridge, drawn by D.P. Gold.

Avoiding the problem in the first place.  The geotechnical consultants on the A-12 section took drillcore samples every 100 ft, a standard practice.  In retrospect, that spacing was inadequate to characterize the steeply-dipping strata and the vein nature of the pyrite.  If the geology had been correctly analyzed, the roadbed could have been moved west of the veins, and at a slightly higher elevation, and nearly all the subsequent problems would have been avoided.

The Bauxsol fiasco.  Early on a $1M contract was issued to an Australian company to test remediation of bank material in the C-12 section with Bauxsol, an alkaline byproduct of aluminum refining.  Bauxsol slurries proved to be difficult to handle.  Bauxsol did not penetrate far into the fill, and therefore was ineffective.

Covering the pyritic areas.  Cuts through pyritic rock, notably the LCF, have been or will be covered with geotextile materials.  These will prevent access of water to the material and thus prevent reaction (1) from occurring.  There will be some access of water to the material by infiltration from rain above the covered areas, however, so runoff will have to be collected and treated a long time into the future.  Areas of pyritic fill not under the roadbed in the C-12 section have also been covered.  The roadbed in the LCF area is now entirely underlain by geotextiles until, southeastward, the bedrock is non-pyritic Reedsville shale. 

The ERPA.  Pyritic sandstone from the LCF was used as roadbed material farther south on the A-12 section and as roadbed and bank material in the chevron-fold C-12 section.  Several stockpiles of excavated pyritic material, totaling 1-2 million tons, sat onsite, primarily from the LCF, but have now (May, 2008) been moved to ERPA, an "engineered rock placement area."  The ERPA is a secure containment structure in Worth Township sitting on shale bedrock, lined, where pyritic rock is mixed with lime kiln dust, an alkaline agent.  Penn State geologists had proposed a less-expensive and geologically feasible solution to put the containment structure on Juniata Formation shales on Skytop itself, but state regulations require that a contaminant to a high-quality stream (Buffalo Run) be entirely removed.

     A controversy existed, for a while, concerning how much lime kiln dust, if any, to add to ERPA rock.  The PennDOT permit approved by the state Department of Environmental Protection called for 1,100 pounds of lime per cubic yard of pyrite-laced rocks.  But Prof. Hu Barnes of Penn State says that the spoil piles already contained enough lime, and that addition of any more would produce a "dry tomb" with far too much alkalinity should it ever fail.  The mixing of the lime kiln dust was also inefficient.  Reaction of acidic solutions with the dust produce solid coatings of gypsum, effectively isolating the dust areas and preventing any further reaction.

     A stop-gap system has been installed to collect acidic runoff and acidic groundwater that are pumped to containment ponds, where the water is neutralized and filtered.  Leakage from ERPA is presently (May, 2008) collected in a tank and hauled out of the area.

     The total cost of remediation will be $80M - $130M.