Daniels and Orndorff, 2003 ICARD Acid Rock Drainage

W L DANIELS and Z W ORNDORFF
draining a much larger watershed to pH 0.2 per cent) in all future road corridors
passing through known risk zones.
Once potentially acidic materials are exposed in cuts and
disposed of in oxidised fill environments (unsaturated), the
thermodynamics of pyrite oxidation will inevitably lead to acid
generation. In extreme examples, such as those found at the
Stafford airport, widespread soil and surface water acidification
and associated environmental damage will ensue. Currently, the
only known proven technique for permanently remediating these
situations is to bulk-blend lime or other alkaline materials with
the cut surface, or with the bulk of the disposal fill based upon
appropriate acid-base accounting procedures. Where feasible,
placement of the sulfidic materials below the:
1. water table; or
2 beneath an impermeable engineered cap will also
drastically limit or prevent acid generation.
Obviously, avoidance of sulfidic materials in the road planning
process is clearly the preferred mitigation alternative. However, it
is obvious that in many instances:
1. road corridors or construction projects cannot be
economically relocated sufficiently to miss sulfide-bearing
strata; and
2. the increasing depth of cut in modern road designs in
rolling topography will lead to increased probability of
intercepting sulfidic materials.
While the barren and erosive slopes resulting from
acidification of cut roadbanks are the most obvious indicator of
this problem, the long term emission of acidic drainage from fills
is clearly the most serious environmental compliance problem
that VDOT and other land developers will face with sulfidic
materials over time. It is clear that acid seepage from fills is
causing local damage to Virginia’s streams at various locations.
While the extent of this damage is very localised and not
extensive to date, the costs of capturing and treating these
discharges could represent significant long-term costs if and
when they are identified as point source discharges. Therefore,
the true cost of identifying, handling and disposing of potentially
acid-forming materials must be rigorously assessed and designed
for in the overall construction process.
ACKNOWLEDGEMENTS
We are indebted to the Virginia Transportation Research Council
(Mike Fitch and Mike Perfater) and to the District of Columbia
Water and Sewer Authority (Chris Peot) for their support of
various components of the work reported here. We also
appreciate the field support of Wright Trucking Inc. (Lloyd and
Milton Wright), Synagro (Steve McMahon), Adam Crist with
Stafford County, and the collective efforts of Campbell and Paris
(Tim Harms, Tony DiLuca, Cindi Martin and Ed Wallis) at
Stafford airport. Thanks also to Pat Donovan and Katie Haering
of Virginia Tech for assistance on the Stafford airport project.
Finally, we greatly appreciate the long-term persistence of Dr
Delvin Fanning of the University of Maryland in pointing out the
nature of acid-sulfate soils to our colleagues around the world.
REFERENCES
Adams, C B, Klamke C A and Hollabaugh, C L, 1999. Geochemical
monitoring of Kiser Creek, near Buchanan, Harallson County,
Georgia: the effects of pyrite-rich rocks on the pH, iron, and sulfate
content of surface waters, Georgia Journal of Science,
57(2):113-122.
Anderson, R, Bell B and Reynolds, J, 1991. Induced polarization used for
highway planning, Investigations in Geophysics, 4:397-410.
Barnhisel, R I and Harrison, J, 1976. Estimating lime requirement by a
modified hydrogen peroxide potential acidity method, Kentucky
Agric Exper Sta, Soil Testing Laboratory, Lexington, KY, USA.
486 Cairns, QLD, 12 - 18 July 2003 6th ICARD