Date of Thesis

Summer 2026

Description

Abandoned mine drainage (AMD) from the Excelsior Mine Strip (SR12) in the Shamokin Creek watershed, Northumberland County, Pennsylvania, emits mildly acidic to circumneutral, Fe-rich water towards Shamokin Creek. This work aimed at evaluating the geochemical processes that govern trace metal mobility, colloidal transport, and sequestration into solid phases along the Excelsior overflow channel and at its confluence to Shamokin Creek.

Water samples were collected at eight locations spanning the Pond Outlet, overflow channel, mixing zone, and upstream Shamokin Creek reference sites. Both 0.45 µm filtered and 3 kDa ultrafiltered fractions were analyzed by ICP-MS and ion chromatography to evaluate dissolved and operationally defined colloidal metal fractions. PHREEQC modeling was used to assess mineral saturation, with emphasis on barite. Additionally, a 24 cm sediment core was obtained from near the Pond Outlet and was characterized by XRF, EPA 3051A-extractable digestion followed by ICP-MS and XRD for the identification of solid-phase metal speciation and Fe mineralogy.

Field measurements showed that pH remained mildly acidic to circumneutral across the transect (5.08-5.98), while ORP increased from approximately 40-60 mV in the Excelsior overflow to a maximum of 309 mV near the mixing zone. This shift indicates a rapid redox transition as mine-influenced water approaches and mixes with Shamokin Creek. Dissolved Fe was substantially higher in the Excelsior overflow than in the upstream Shamokin Creek reference sites. In contrast, dissolved Al was higher in Shamokin Creek upstream of the confluence than in the Excelsior overflow.

Comparison of the 0.45 µm and 3 kDa fractions showed that Al had the clearest colloidal component, with 0.45 µm concentrations consistently higher than ultrafiltered concentrations. Fe showed localized evidence of a colloidal or nanoparticulate component near the mixing zone, although this signal was not consistent at the Excelsior overflow and the Shamokin creek sampled locations. Most other trace metals, including Co, Ni, As, and Cu showed limited separation between the two fractions and were largely present in the truly dissolved fraction under the sampled conditions. PHREEQC calculations indicated that waters were near equilibrium but slightly undersaturated with respect to barite (SI = -0.13 to -0.08), suggesting that barite-related processes are chemically plausible but not confirmed by the aqueous data alone.

The Pond Outlet sediment core was dominated by Fe-rich material, with goethite identified by XRD as the main crystalline Fe-bearing phase. Goethite peak character varied with depth, with broader and less defined peaks in the upper portions of the core and sharper peaks in the mid- bottom depth zone. This pattern suggests depth-dependent variation in Fe mineral crystallinity and ordering. As and Cu were enriched in upper Fe-rich intervals, Zn showed a more complex  distribution, and Co remained low and stable throughout the core. These patterns indicate that trace metal retention at the Pond Outlet is controlled not only by Fe abundance, but also by the mineralogical character and surface reactivity of Fe-bearing phases.

Together, these findings show that trace metal mobility in the Excelsior AMD system is element-specific and controlled by interacting aqueous and solid-phase processes, including redox-driven Fe precipitation, selective colloidal association, dilution and mixing with Shamokin Creek, and retention in Fe-rich sediments. This study highlights the importance of integrating aqueous chemistry, size-fractionation, geochemical modeling, and solid-phase mineralogy when evaluating trace metal fate in AMD-impacted watersheds.

Keywords

Abandoned mine drainage, trace metals, Schwertmannite, goethite, PHREEQC, Shamokin Creek

Access Type

Masters Thesis (Bucknell Access Only)

Degree Type

Master of Science

Major

Chemistry

First Advisor

Molly M. McGuire

Second Advisor

Ellen K. Herman

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