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.
Observations of hydraulic head response to lagoon leakage into an extremely heterogeneous aquifer allow qualitative identification of spatial patterns in heterogeneity. In a 150–hour leakage experiment over such an aquifer, the fluid mass balance was estimated using a stage‐fluctuation model while transient response was observed in downgradient wells and springs. Based on timing with respect to pond leakage, water‐level response at different locations within the aquifer may be identified as either within or outside the influence of preferred‐flow paths (PFPs). Flows at springs 150–200 m from the pond increased by 20–60% above baseline flows, showing initial increase within ca. 40 hours. Synoptic water‐level rise patterns observed during the test describe enlargement of a recharge mound from beneath the pond and suggest the spatial pattern of PFPs to > 150 m distant. An analytical model assuming homogeneity (Hantush, 1967), applied to water‐level response for wells > 30 m from the pond, overestimated K by > 1 order of magnitude in comparison to slug‐test estimates. This discrepancy is attributed to the inferred presence of linearly continuous heterogeneities causing preferential flow. Large‐scale hydraulic tests such as this sample the spatial structure for conductive pathways and may be of utility in parameterization of numerical flow models. In contrast, slug‐test results give local parameter estimates, but more limited information on PFP distribution.
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