Scott Hensel

Date of Award

Spring 5-2023

Degree Type


Degree Name

Master of Science (MS)



Committee Chair

Alysia Cox

First Advisor

Chris Gammons

Second Advisor

Glenn Shaw


Uncontrolled chemical releases of molybdenum into the subsurface at the Kerr-Mcgee Chemical Plant near Soda Springs, ID over the course of several decades led to the site being added to the Superfund National Priority List in 1989. Various subsequent site studies have resulted in a high-resolution dataset that enables the geochemical modeling of molybdenum in the subsurface. Thermodynamic calculations are used to model how aqueous molybdenum is affected by mineral precipitation, oxygen, carbon, and pH in context of the Kerr-Mcgee Superfund Site. Geochemical modeling of molybdenum in groundwater using publicly available Site data from 2020 has been achieved in two ways: water-rock interaction modeling with basalt (EQ3/6), and plume flow path modeling in an aquifer (Geochemist’s Workbench). Molybdenum speciated primarily as two species: dissolved molybdate and the mineral powellite (CaMoO4). The precipitation of powellite removed molybdate from solution, and when dissolved, re-released molybdate as a secondary source. The control of powellite stability relied on the concentrations and speciation of dissolved calcium, which, in turn, depended on the precipitation/dissolution of carbonate minerals. Changes in the modeled groundwater pH to <4 resulted in no powellite precipitation due to molybdenum speciating as molybdic acid. The low pH conditions also affected calcium speciation, which greatly increased the predicted persistence time of powellite. Reactive transport indicated powellite precipitation during mass loading, followed by slow dissolution when flushed with background groundwater that may remobilize concentrations high enough to exceed the molybdenum regional screening level. These modeling results help define how molybdenum may be affected by a wide variety of subsurface conditions. A previously unknown contaminant-bearing secondary precipitate in the form of an unidentified cubic mineral has been observed in rock samples collected from beneath the primary source area. Maximum weight percents of Site contaminants found in the cubes are 21.25 weight percent of molybdenum and 45.42 weight percent of vanadium. This cubic mineral is strong evidence that secondary precipitates are present and may be acting as a secondary source for Site constituents. The role that water-rock interactions involving this mineral currently have an unknown effect on the fate and transport of molybdenum and vanadium in groundwater.


A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geoscience: Geochemistry