Acid Sulfate Soils Management

What are acid sulfate soils?

Acid sulfate soils form when sulfide-bearing materials are excavated from below the Earth’s surface and are exposed to the atmosphere. The sulfides (primarily pyrite) oxidize to produce sulfuric acid, iron oxides/hydroxides, and sulfate precipitates. The resulting soil is typically highly acidic (pH less than 3.0), and is often associates with acidic, metal-laden surface runoff.

If you  think you have acid sulfate soils and would like more information, please contact us with any questions, and we can provide guidance on reclamation of your site. You can call us at (540) 231-7175 or email Dr. Lee Daniels at wdaniels@vt.edu.

Grey reduced sulfidic materials are commonly encountered during active construction in the Fredericksburg/Stafford area of Virginia. These materials will usually acidify over time to pH less than 3.5 unless large amounts of lime are added and incorporated.

What problems are associated with acid sulfate soils?

In construction sites such as road corridors, industrial parks, airports, and housing developments, the exposure of sulfidic materials can present a number of technical, environmental, and social problems:

  • Acids degrade metal and concrete building materials.
  • Weathering of fill material and precipitation of sulfates compromise structural stability.
  • Highly acidic soils cannot support roadside vegetation resulting in increased erosion and acid runoff.
  • Highly acidic and metal-laden runoff impairs surface water quality and aquatic life.
  • Visible pollution and adverse conditions for aquatic life limit recreational use of impacted surface waters.
Acid sulfate materials also produce acidic soil and runoff conditions that can damage concrete, metal, block walls and cause serious local water quality problems.
Typical surficial expression of impacts of exposure of acid sulfate soil materials near Fredericksburg, Virginia.  Highly acidic soil and water  conditions (< pH 4.0) have killed or stressed planted vegetation while iron oxides and sulfate salts in leachates and runoff are etching and staining concrete infrastructure.

Which geologic formations in Virginia are associated with acid sulfate soils?

Acid sulfate soils occur at several sites in different geologic and geomorphic settings across the state, including:

  • Coastal Plain: Tertiary marine sediments and the Tabb formation
  • Piedmont: Phyllite and slate of the Quantico Formation
  • Blue Ridge: Alum phyllite
  • Valley and Ridge: Devonian black shale including the Marcellus, Millboro, and Chattanooga shale and Needmore Formation
  • Appalachian Plateau: Coal seams and shale including the Wise, Kanawha, Norton, New River, Lee and Pocahontas Formations

By following this link, you will download a Google Earthtm map file (.kmz) for the Commonwealth of Virginia that portrays the relative risk of encountering acid sulfate soil conditions. Open Google Earth, projects, and then select import KMZ file. (If you don’t have the Google Earth application or software on your device or PC, go to googleearth.com or app store to install. For more help, type in How to Import KMZ file into Google Earth in your browser’s search bar, then select videos. You will find many tutorials that will be helpful.) Once opened, the map will allow you to zoom into your location of interest and once you click on it, information on the potential risk of encountering these materials in near-surface excavations will appear in a pop-up note.
This map is based on the Digital Representation of the 1993 Geologic Map of Virginia (VDMR, 2003), and is best interpreted with an understanding of the local geology and landforms for a given area.

Please keep in mind the following:
1) This map is based on small-scale (state-wide) geologic mapping and therefore provides only a rough guideline for site-specific planning.
2) This map is based on surficial geology, so:
a) Significant geologic changes may occur with depth – i.e. an area mapped as no- or low-risk may have problematic sulfide-bearing formations occurring within excavation depth beneath the surficial geologic formation, and;
b) a “high-risk” geologic unit which has undergone significant geologic weathering may no longer be acid forming through the depth of weathering.

Presentations: Recognition and Remediation of Acid Sulfate Soil Conditions in the Fredericksburg Area. 24 Jan 2020

Acid Sulfate Soils: Recognition, Remediation and Impact Prevention, MAPSS/VAPSS Technical Session, Fredericksburg, VA. Nov. 3-4, 2011

Acid Sulfate Information