Geologic, tectonic, and geochemical signatures leading to arsenic in groundwater in the Gettysburg Basin

• Main Author: Blake, Johanna M. T
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2014
• Subjects: Geochemistry; Geology; Geomorphology
• ISBN: 9781321222319; 1321222319

Exposure of humans to arsenic through drinking water may cause health impairment ranging from stomach pain and circulatory problems to skin, lung, bladder, and kidney cancer. Arsenic can enter groundwater from anthropogenic sources, but in many areas of the world, arsenic in groundwater is derived from naturally occurring geologic sources. Elevated arsenic (As) concentrations in groundwater and rocks have been found in crystalline and sedimentary aquifers from New England to Pennsylvania, USA. The Triassic Newark rift basin in New Jersey and Pennsylvania has elevated arsenic concentrations in groundwater and rocks, but little is known about the stratigraphically similar Gettysburg Basin. Researching where arsenic is abundant, why it is abundant and developing simplified ways to monitor arsenic location are important to improving human health, understanding geologic history, and refining contamination assessment techniques. This research uses geochemical signatures, provenance, and iron oxide coated stream pebbles to answer where, why, and how we find arsenic. The Gettysburg basin has 18%-39% of rock samples with arsenic concentrations greater than the crustal average of 2 mg/kg, while the Newark Basin has 73% to 95% of rock samples above the crustal average. The strongest controls on arsenic in rocks of the Gettysburg Basin are the relationship between arsenic and iron and silicon concentrations while the strongest controls in the Newark Basin are the relationship between arsenic and iron and organic carbon concentrations. The groundwater arsenic concentrations follow similarly with 8-39% of water samples from the Gettysburg Basin above 10 μg/L and 24-54% of water samples from the Newark Basin above 10 g/L. The strongest controls on arsenic in water of the Gettysburg Basin are pH, alkalinity and silicon, while the strongest controls in the Newark Basin are pH and alkalinity. The dichotomy between the arsenic concentrations in the rocks and water of the Gettysburg and Newark Basin is likely from provenance of sediments. The Newark Basin sediments are from terranes equivalent to the Central-Maine and Avalon terranes and Precambrian Grenville basement rocks, which have higher amounts of arsenic. NASC normalized rare earth element spider diagrams show patterns of the Gettysburg Basin data most similar to Iapetus Continental Slope Rise. Plots of La/Th versus Hf, Zr/Sc versus Th/Sc, Eu/Eu* versus La N/SmN, and Eu/Eu* versus GdN/YbN all show the most overlap with Iapetus Continental Slope Rise, Iapetus Passive Margin, and Accretionary Complex terranes with less from Iapetus Rift Volcanic terranes. Iron-manganese oxide coatings on streambed pebbles in Pennypack Creek, southeastern PA have a significant relationship (p<0.15) with zinc, copper, nickel, and arsenic. Two segments from Pennypack Creek used pebbles and relative discharge from the main stem and two tributaries to predict the arsenic source in the main stem. Results suggest that iron-manganese coated stream pebbles are useful indicators of zinc, copper, nickel and arsenic location within a watershed, but the source of arsenic differs from that of the other metals of interest.