Document Type

Thesis

Date of Degree Completion

Winter 2025

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Carey Gazis

Second Committee Member

Dr. Christopher Mattinson

Third Committee Member

Dr. Hannah Shamloo

Abstract

Arsenic (As), a common contaminant in groundwater, is toxic to human health and has a maximum contaminant level of 10 ppb set by both the World Health Organization and the U.S. Environmental Protection Agency (ATSDR, 2007). In Toutle, WA, USA (30 km west of Mount Saint Helens), a private well, labelled TOU1, was previously identified with ~670 ppb of As and was the impetus of this study. This study targeted private wells in the vicinity of TOU1 to identify the extent of the arsenic contamination, identify the aquifer(s) of concern, and identify the mechanism(s) of mobility.

The wells were screened in Mount St. Helens lahar deposits (Qsl), the Wilkes Formation (Tw), the Toutle Formation (Tto), and a basaltic andesite flow interfingering Tto (Ttob). 17 wells drilled in Tto and Ttob, all within 3.4 km of TOU1, had moderate to high As concentrations (11.0-610 ppb), 5 wells in the Qsl had low to moderate arsenic (2.1-15.4 ppb), and 7 wells in the Tw had low arsenic (< 1 ppb) The majority of the groundwaters were magnesium-bicarbonate type waters and have major ion compositions that indicate recharge from precipitation as the dominant influence. In addition, there are two sodium-bicarbonate type groundwaters (including TOU1) and one intermediate composition whose major ion composition is dominated by basic ion exchange. Stable oxygen and hydrogen isotope compositions and the major ion chemistry suggest the wells in this region tap small aquifers within the units with significant barriers to flow between adjacent wells. In general, the wells were circumneutral to slightly alkaline, low SO42-, and high Fe; low arsenic wells were either in Tw or were under oxic conditions while moderate to high arsenic wells were under reducing conditions and in Tto/Ttob or Qsl, which were determined to be the aquifers of concern. Based on pH and redox conditions, the mechanism of mobility was reductive dissolution of Fe-oxides. The role of sulfate reducing bacteria and possible enrichment mechanisms in lignite and associated high alumina clay in the Tto should be explored as possible explanations of the distribution of SO42-, iron, and arsenic in the study area.

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