Date of Award

Winter 1-1-2021

Document Type

Thesis: Open Access

Degree Name

MS Marine Science

Department

Environmental and Ocean Sciences

Committee Chair

Bethany O'Shea, Ph.D.

Committee Member

Suzanne Walther, Ph.D.

Committee Member

Eric Cathcart

Abstract

Anomalously elevated arsenic concentrations occur at an abandoned artisanal mine site in an open space recreational park in urban San Diego County, California. Although mining at the site ceased 100 years ago little to no remediation efforts took place leaving shafts, adits, waste piles and derelict infrastructure accessible by inquisitive hikers and adventurers frequenting Black Mountain Open Space Park. This study evaluates the geochemical mechanisms of As transport downgradient from the mine and examines the contribution of both climate and urbanization on the restriction of As mobility. Sediment As concentrations reach 2,320 mg/kg (483 times background crustal concentrations; n=73) along drainage pathways and sequential extraction experiments indicate As overwhelmingly partitions into the least mobile phase (mean 83% As in residual fraction; n=30). These geochemical results indicate there is little redistribution of As from primary minerals into secondary or dissolved phases despite the sediments being exposed to a century of weathering. Arsenic transport in dissolved phase does occur however, with surface water runoff during peak rain events reporting As up to 272 μg/L. Both sediment and water As decrease to background concentrations approximately 1,000 m from the mines, attributable to the semi-arid climate limiting weathering and transport, and the encroaching urbanization limiting further dissolved As transport off-site. Similar patterns of As migration downgradient of abandoned mines have been measured at other mining sites in arid and semi-arid regions; however, off-site As transport at these other sites extended over far greater distances. The Black 17 Mountain site shows it is possible that a combination of geochemical, geomorphological, climatic, and urbanization controls can work together to retard the surface transport of As over large distances from artisanal and un-remediated mine sites.

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