High-frequency sensor data capture short-term variability in Fe and Mn concentrations due to hypolimnetic oxygenation and seasonal dynamics in a drinking water reservoir

• Published in: Water research (Oxford) 2023-07, Vol.240, p.120084-120084, Article 120084
• Main Authors: Hammond, Nicholas W., Birgand, François, Carey, Cayelan C., Bookout, Bethany, Breef-Pilz, Adrienne, Schreiber, Madeline E.
• Format: Article
• Language: English
• Subjects: Hypolimnetic oxygenation; Iron; Manganese; Sensors; Spatiotemporal resolution; Spectrophotometer
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2023.120084

•We used UV–vis spectral data and PLSR to quantify Fe and Mn in a reservoir.•High-resolution Fe and Mn predictions were made during fall turnover and summer oxygenation.•Fe and Mn show hourly to daily variability and are sensitive to redox and stratification.•Our sensor approach holds promise for drinking water monitoring of metals.•We suggest opportunities for future improvements to the system. The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and partial least squares regression (PLSR) modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two transitional periods: reservoir turnover (Fall 2020) and HOx initiation (Summer 2021). Our sensor system successfully predicted mean Fe and Mn concentrations and trends, ground-truthed by grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 h after full water column mixing. During the initiation of HOx in Summer 2021, Fe and Mn displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are sensitive to changes in redox conditions induced by stratification and oxygenation, although their responses to these changes differ. We also show that high spatio-temporal resolution predictions of Fe and Mn can improve drinking water monitoring programs and reservoir management pract