Riparian Dendrochemistry: Detecting Anthropogenic Gadolinium in Trees along an Effluent-Dominated Desert River

This research documents spatial and temporal patterns of effluent uptake by riparian trees through development of a new and innovative application for dendrochronology, specifically dendrochemistry. The rare-earth element (REE) gadolinium (Gd) is a known micro-pollutant in its anthropogenic form and enters streams from wastewater treatment plants. Anthropogenic Gd was first used in select medical procedures in 1988 and has since been used as a contrast agent for medical imaging. It is naturally flushed from the body following procedures and is subsequently discharged via treatment plants into waterways. Riparian trees that utilize effluent-dominated surface water take up Gd, which then remains in annual growth rings. The year 1988 serves as presence/absence date stamp for Gd in tree rings, thereby making Gd an ideal marker for this dendrochronological study. Results from this study along the Upper Santa Cruz River in southeastern Arizona show levels of Gd in effluent-dominated surface flows to be elevated above the threshold that distinguishes an anthropogenic anomaly from natural GdSN abundance in freshwater, thereby confirming that anthropogenic Gd is present. Gd was found in the growth rings of cottonwood trees (Populus fremontii var. arizonica (Sarg.) Jeps.) that are growing in the floodway adjacent to the effluent-dominated portion of the stream. The presence of Gd in cottonwood annual rings confirms that the trees are utilizing effluent over the course of the growing season. Furthermore, temporal patterns of Gd concentrations in trees directly adjacent to the stream may be reflective of high-frequency changes in surface water quality. Information on the impacts of effluent quality on the chemical composition of tree rings can be a useful monitoring tool to evaluate the spatial and temporal patterns of effluent use in riparian trees and to identify high-frequency changes in surface water quality.