Geochemical response of acid groundwater to neutralization by alkaline recharge

Abstract

Solute transport and chemical neutralization (pH 3 to 7) within a shallow heterogeneous aquifer producing acid mine drainage (AMD) are examined at an abandoned surface coal mine in West Virginia. The aquifer is undergoing partial neutralization by mixing with alkalinity from a leaking sludge disposal pond, extending in preferential zones controlled by aquifer heterogeneity. Hydraulic heads interpolated from wells indicate leakage from a central alkaline (pH 7.1, 0.72 meq/L alkalinity) sludge pond is a principal source of recharge. Chemically-conservative sodium, added to AMD during treatment and leaked into the aquifer with the sludge, develops a dispersion plume over a restricted portion of the aquifer that correlates with pH, hydraulic head, and dissolved metals distributions. Concentrations of aluminum, iron, sulfate and acidity display higher concentrations downgradient from the pond as sludge alkalinity is consumed along flow paths. Before reaching springs, most dissolved iron is oxidized and hydrolyzed, likely precipitating in the aquifer as a ferric hydroxide or hydroxysulfate phase. The spatial pattern of iron and aluminum concentrations suggests accelerated oxidation caused by gas transport along the outer slopes of the spoil. Dissolved aluminum concentrations increase with total acidity, suggesting that dissolution of silicate minerals results from acidity released by iron hydrolysis. Neutralization reactions and higher pH are favored in more highly permeable portions of the spoil, where ferrihydrite and aluminum hydroxysulfate minerals (such as basaluminite) are supersaturated, In acid-producing zones at pH < 4.5, jurbanite is near equilibrium and an aluminum-sulfate phase with similar properties may limit aluminum concentrations, but become undersaturated in zones of advancing neutralization. At this particular site, ferrous iron produced by pyrite oxidation is almost completely oxidized over short transport distances, allowing hydrolysis of iron and aluminum should sufficient alkalinity be added to these acid waters.

Isotopic composition of dissolved inorganic carbon in underground mine discharges of the northern appalachian basin

“…All 20 sites were analyzed for inorganic geochemistry (Fe, Al, Mn, Na, K, Mg, Ca, Ba, Sr, NO 3 , S, F, Cl, and Br; Table 13). Since the geochemistry of mine discharges remains relatively consistent throughout the year (Donovan et al, 1997) All 20 sites were analyzed for δ 13 C DIC with a precision of 0.1 ‰ (Table 15).…”

“…Equations used to construct the Bjerrum plot are also listed in Table A1. underground mine discharges stays relatively the same throughout the year (Donovan et al, 1997).…”

Quantification of acid loads and delineation of hydrochemical facies at a phreatic mine spoil aquifer

“…Sludge is pumped from the settling ponds to disposal ponds ( Fig. 1-3) in the central portion of the spoil piles (Donovan et al, 1997).…”

“…Numerous hydrological and geochemical studies have focused on the X Bench as well as at similar mine-spoil aquifers in the region (Sturm, 1986;Frysinger, 1995;Maher, 1996;Avery, 1997;Donovan et al, 1997;Ketchum, 1998;Ketchum et al, 2000;Neuman, 2013). In situ chemical treatment of sodium hydroxide was attempted in numerous investigations (Aljoe and Hawkins, 1993;Canty and Everett, 2006;Werner et al, 2008).…”

“…Hawkins (2004) concluded that the driving force for groundwater flow in a mine-spoil aquifer is the geometry of the pit floor. Donovan et al (1997) characterized the geochemistry of the X Bench and noted the effect of alkaline recharge from the sludge disposal pond. They concluded that the flux of alkalinity from sludge disposed in the pond was insufficient to neutralize the acidity generated in the aquifer.…”