THE SIGNIFICANCE OF VEGETATION KILLS AND EFFLORESCENT MINERAL BLOOMS FROM TRANSIENT GROUND WATER PLUMES DURING AND FROM HIGHWAY CONSTRUCTION

Perturbations of ground water paths associated with highway construction through toxic rocks can be both complex and transient. Newly created pathways with local perched water tables may be missed by monitoring wells. Some measure of the potential ARD of new regimes can be gauged from stressed vegetation on seeded cuts and embankments, as well as from the growth of efflorescent minerals such as gypsum (CaSO₄ •2H₂O), bassanite (2CaSO₄•H₂O), epsomite (MgSO₄•7 H₂O), rozenite (FeSO₄•4H₂O), jarosite (K Fe³⁺₃(SO₄)₂(OH)₆), hexahydrite (MgSO₄•6H₂O), fibroferrite (Fe³⁺ (SO₄)(OH)•5H₂O), melanterite (FeSO₄•7H₂O), epsomite (MgSO₄•7 H₂O), alunite (KAl₃(SO₄)₂(OH)₆), alunogen (Al₂(SO₄)₃•17 H₂O), pickeringite (MgAl₂(SO₄)₄•22 H₂O), halotrichite (Fe²⁺Al₂(SO₄)₄•22 H₂O), copiapite (Fe²⁺Fe³⁺₄(SO₄)₆(OH)₂•20 H₂O), and ettringite (Ca₆[Al(OH)₆]₂(SO₄)₃•26H₂O) in vadose zone intersections, seeps and ponds.

Such conditions have been documented near Skytop, Centre County, Pennsylvania, where ~1 x 10⁶ yards of pyrite-laced rocks were excavated and redistributed in 8 major and up to 100 minor sites along a 7 mile stretch of highway I99. Springs discharging acidic water (pH 2.0 – 3.4) developed at the base of large fills and vegetation kills occurred in acid seeps, vadose zone intersections on seeded areas over “shot rock” fills, and where pyritic rock had been crushed and used as a dressing aggregate in the berms. Efflorescent mineral blooms in soils, rubble, and bedrock are sufficient to cause a color change during evaporative dry periods. The high solubility of these efflorescent minerals leads to an initial drastic lowering of pH with a concomitant rise in sulfate ions in the runoff following rain.