Upland active acid sulfate soils from construction of new Stafford County, Virginia, USA, Airport

Abstract

This paper reports on a situation where severe active acid sulfate soils were brought into existence by the construction of a new (opened in 2002) airport in Stafford County, VA, approximately 60 km south-west of Washington, DC. About 290 ha of new land surface was brought into existence that consisted of both scalped land surfaces on steep slopes, and spoil (fill), some of which was graded to provide level land surfaces for paved runways. Over 150 ha of ultra acidic (pH <3.5 at soil surface) post-construction acid sulfate soils remained barren for over 2 years before the acid sulfate soil situation was properly recognised. Construction took place in an originally dissected landscape with about 30 m of local relief. The construction was designed to balance the cut and fill areas so that soil materials would not need to be taken from the area or brought to it from other locations. This resulted in some deep cuts (scalped surfaces) in the higher parts of the landscapes, which retained slopes of about 25%. Great difficulty was encountered in establishing vegetation on these surfaces. The exposed sulfidic materials were dense, commonly on steep slopes, and developed low pHs, some <pH 2, after exposure. After a dry period in the autumn of 2001, sulfuric horizons crusted over with bitter hydrated sulfate salt minerals had formed in the surface of sulfidic materials originally exposed in 1999. By X-ray diffraction, halotrychite, Fe2+Al2(SO4)4.22H2O, was identified as a main white salt mineral and copiapite group minerals, e.g. Al2/3Fe3+4(SO4)6(OH)2.20H2O for aluminocopiapite, were identified as a yellow salt minerals. Information about, and photographs of, the site, soils, and drainage waters are presented, including examples of deleterious environmental impacts. Intensive reclamation/revegetation measures were initiated in 2002. These involved the application of high rates of lime stabilised biosolids (sewage sludge) incorporated to a depth of about 0.15 m to neutralise acidity and add organic matter and nutrients to the soils. These measures permitted the establishment of acid- and salt-tolerant grasses on the acid sulfate soils and caused dramatic increases in pH and drops in Fe and Al levels in stream waters leaving the site. However, they also caused initial large increases in ammonia/ammonium-N in the waters and subsequent increases in NO3-N in the waters. Experience with this and other similar sites demonstrates the need for engineers involved with earth-moving construction activities to be educated in the principles of acid sulfate soils so that the number of such disturbances that result in the creation of active acid sulfate soils can be lessened or, preferably, eliminated. Plans for recognition and reclamation of acid sulfate soil situations should be built into the construction plans and designs when it is necessary to disturb sulfidic materials.