Acid-Rock Drainage at Skytop, Centre County, Pennsylvania, 2004

Recent construction for Interstate Highway 99 (I?99) exposed pyrite and
associated Zn-Pb sulfide minerals beneath a >10-m thick gossan to oxidative
weathering along a 40-60-m deep roadcut through a 270-m long section of the
Ordovician Bald Eagle Formation at Skytop, near State College, Centre County,
Pennsylvania. Nearby Zn-Pb deposits hosted in associated sandstone and
limestone in Blair and Centre Counties were prospected in the past; however,
these deposits generally were not viable as commercial mines. The pyritic
sandstone from the roadcut was crushed and used locally as road base and fill
for adjoining segments of I?99. Within months, acidic (pH<3), metal-laden seeps
and runoff from the exposed cut and crushed sandstone raised concerns about
surface- and ground-water contamination and prompted a halt in road
construction and the beginning of costly remediation. Mineralized sandstones
from the cut contain as much as 34 wt. % Fe, 28 wt. % S, 3.5 wt. % Zn, 1% wt.
Pb, 88 ppm As, and 32 ppm Cd. A composite of <2 mm material sampled from the
cut face contains 8.1 wt. % total sulfide S, 0.6 wt. % sulfate S, and is net
acidic by acid-base accounting (net neutralization potential ?234 kg CaCO3/t).
Primary sulfide minerals include pyrite, marcasite, sphalerite (2 to 12 wt. %
Fe) and traces of chalcopyrite and galena. Pyrite occurs in mm- to cm-scale
veinlets and disseminated grains in sandstone, as needles, and in a locally
massive pyrite-cemented breccia along a fault. Inclusions (<10 ?m) of CdS and
Ni-Co-As minerals in pyrite and minor amounts of Cd in sphalerite (0.1 wt. % or
less) explain the primary source of trace metals in the rock and in associated
secondary minerals and seepage. Wet/dry cycles associated with intermittent
rainfall promoted oxidative weathering and dissolution of primary sulfides and
their oxidation products. Resulting sulfate solutions evaporated during dry
periods to form intermittent ?blooms? of soluble, yellow and white efflorescent
sulfate salts (copiapite, melanterite, and halotrichite) on exposed rock and
other surfaces. Salts coating the cut face incorporated Fe, Al, S, and minor
Zn. They readily dissolved in deionized water in the laboratory to form
solutions with pH <2.5, consistent with field observations. In addition to
elevated dissolved Fe and sulfate concentrations (>1,000 mg/L), seep waters at
the base of the cut contain >100 mg/L dissolved Zn and >1 mg/L As, Co, Cu, and
Ni. Lead is relatively immobile (<10 ?g/L in seep waters). The salts sequester
metals and acidity between rainfall events. Episodic salt dissolution then
contributes pulses of contamination including acid to surface runoff and ground
water. The Skytop experience highlights the need to understand dynamic
interactions of mineralogy and hydrology in order to avoid potentially negative
environmental impacts associated with excavation in sulfidic rocks.

[Summary provided by the USGS.]