Encyclopedia Of Engineering Geology

A Acid Mine Drainage Paul L Younger School of Engineering, University of Glasgow, Glasgow, Scotland Synonyms Acid rock drainage; Acidic mine drainage; Acidic rock drainage; Mine water pollution; Polluted mine water Definition Water encountered in and/or draining from active or abandoned mines which has a low pH and/or highly elevated concentrations of potentially ecotoxic metals Mining disrupts the natural hydrogeological conditions in the subsurface often increasing the through-flow of aerated waters, resulting in oxidative dissolution of sulfide minerals. The ferrous sulfide (FeS2) minerals (pyrite and its less common polymorph marcasite) release acidity when they dissolve. (This is not true of the nonferrous sulfide minerals.) This acidity can attack other minerals, releasing further metals to solution. Clay minerals commonly dissolve to release Al3+, with Mn2+, Zn2+, and (less commonly) Ni2+, Cu2+, Cd2+, Pb2+, and the metalloid As also being mobilized where mineralogical sources for these are present. Above the water line, dissolution is often incomplete, and the products of sulfide oxidation accumulate as efflorescent hydroxysulfate minerals. Later dissolution of these will release acidity. The resultant water is “acid mine drainage” (albeit “acidic” is # Springer International Publishing AG 2017 P.T. Bobrowsky, B. Marker (eds.), Encyclopedia of Engineering Geology, DOI 10.1007/978-3-319-12127-7_5-1 more correct). In addition to low pH and elevated concentrations of iron and (possibly) other metals, acid mine drainage is invariably rich in sulfate (Younger et al. 2002). The total acidity in mine drainage has two components: “proton acidity” due to the presence of high concentrations of hydrogen ions (H+) that manifest in a low pH (below 6 would typically be regarded as “acidic” in this context) and “metal acidity” due to the presence of the metals listed above that tend to react with any available alkalinity to form hydroxide minerals, releasing further protons in the process. In many mine waters, the total acidity is exceeded by the total alkalinity, which in the relevant pH range is predominantly accounted for by dissolved bicarbonate (HCO3 ). Such mine waters are termed “net-alkaline.” Where the total acidity exceeds the total alkalinity, the mine water is termed “net acidic.” This distinction is important: many net-acidic mine waters actually have a near-neutral pH (>6) where they first flow out at surface, but after prolonged oxidation and hydrolysis of their metal acidity, pH drops to strongly acidic levels (< 4.5).="" misidentification="" of="" net-acidic="" waters="" as="" net-alkaline="" on="" the="" basis="" of="" ph="" alone="" can="" be="" a="" costly="" mistake.="" the="" principal="" concern="" with="" acid="" mine="" drainage="" is="" ecological,="" as="" it="" often="" devastates="" aquatic="" life="" in="" receiving="" watercourses.="" in="" engineering="" terms,="" the="" high="" acidity="" poses="" heightened="" risks="" of="" corrosion="" of="" steel="" and="" other="" materials,="" thus="" demanding="" careful="" galvanic="" protection.="" the="" high="" sulfate="" concentrations="" pose="" a="" risk="" of="" rapid="" weathering="" of="" concretes="" based="" on="" ordinary="" portland="" cement.="" sulfate-resistant="" cements="" must="" be="" specified="" for="" structures="" likely="" to="" contact="" acid="" mine="" drainage.="" acidic="" attack="" can="" weaken="" many="" rocks="" and="" engineering="" soils.="" passive="" and="" active="" treatment="" methods="" are="" routinely="" used="" to="" treat="" acid="" mine="" drainage="" (fig.=""> 2 Acid Mine Drainage Acid Mine Drainage, Fig. 1 A typical acid mine drainage outflow – Bardon Mill Colliery, Northumberland, UK Cross-References Reference ▶ Acidity ▶ Contamination ▶ Drainage ▶ Hydr