Document Type

Thesis

Degree Name

Master of Science (MSc)

Department

Biology

Program Name/Specialization

Integrative Biology

Faculty/School

Faculty of Science

First Advisor

Dr. James McGeer

Advisor Role

Supervisor

Second Advisor

Dr. Scott Smith

Advisor Role

Co-supervisor

Abstract

Gadolinium (Gd) is a rare earth element (REE), primarily used in contrast agents for magnetic resonance imaging (MRI), with additional applications in metallurgy, phosphors, neutron shielding, and other niche applications. Both Gd and Gd-based contrast agents (GBCAs, the form used for MRIs) have been detected downstream of wastewater effluent discharges and there is currently little research into the risk this poses to aquatic ecosystems. This study investigated the potential environmental risks of Gd and GBCAs using the Grand River watershed (GRW) in southwestern Ontario as a model ecosystem due to it containing a variety of land uses (urban, industrial, agricultural) and multiple MRI facilities using GBCAs. Total and dissolved REE concentrations were measured throughout the GRW with a focus on potential sources of contamination such as wastewater treatment plants. Sampling results showed low but measurable levels of multiple REEs in the watershed, with Gd anomalies downstream of wastewater discharge points with concentrations up to 9 times the watershed average (176 ng/L at outflow & 19 ng/L in GRW). The potential for adverse effects of inorganic Gd and GBCAs were assessed using standard toxicity test methods with Daphnia magna as well as tissue bioaccumulation studies with fathead minnows. We determined an acute EC50 of 231 µg Gddissolved/L of inorganic Gd for D. magna in a soft water media, but were unable to calculate an EC50 for GBCAs due to insufficient mortalities at the tested concentrations, concluding than it would be >2.5 mg Gddissolved/L of GBCA. Chronic 21d reproduction tests with D. magna, in the same soft water media showed no significant effect on survival or reproduction in concentrations up to 200 μg Gdnominal/L inorganic Gd, however this test was limited by low dissolved concentrations (<20% of nominal). Bioaccumulation tests showed that dissolved organic carbon (DOC) reduced the accumulation of Gd into gill tissues (32% decrease at highest tested concentration), while Ca and Na did not. When combining our environmental Gd and D. magna results into a risk assessment, we found risk quotients below 1, indicating minimal risk, even before considering mitigating conditions such as protective DOC being present or less toxic GBCAs composing some of the Gd present.

Convocation Year

2024

Convocation Season

Spring

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