ASSESSING SHORT-TERM VARIABILITY OF IRON AND MANGANESE CYCLING IN A DRINKING-WATER RESERVOIR USING A HIGH-FREQUENCY WATER QUALITY SENSOR

The biogeochemical cycles of iron (Fe) and manganese (Mn) can strongly influence water quality in freshwater lakes and reservoirs. Dissolved oxygen (DO) is a primary driver of Fe and Mn cycling as there is a well-established correlation between seasonal anoxia in lakes and reservoirs and elevated metals concentrations. However, less is known about the short-term (i.e. hourly to daily) variability of Fe and Mn cycling.

We applied a novel high-frequency monitoring system to assess the dynamics of metals chemistry in a seasonally-stratified drinking water reservoir located in Vinton, VA, USA. The reservoir contains a hypolimnetic oxygenation (HOx) system, which is designed to increase the hypolimnetic DO concentration. Routine operation of the HOx system involves a daily “blowoff” of air to clear the system of debris. We conducted experiments during two distinct periods in Fall 2020: a.) “baseline” conditions with the HOx system active and b.) reservoir turnover. The monitoring system includes an in-situ spectrophotometer connected to a multiplexor pump that can retrieve samples from multiple depths in the water column. The spectrophotometer measures UV-visible absorbance spectra for 216 wavelengths at a 10-minute resolution, and then partial least-squares regression (PLSR) models, calibrated on manual water chemistry sampling data, are used to predict total and soluble Fe and Mn concentrations from absorbances.

Experimental results from the “baseline” period show significant changes in total and soluble Fe occurring over a 24-hour period. Total Fe increased in the hypolimnion simultaneously with the HOx “blowoff,” which indicates that this procedure may cause resuspension of particulate Fe. Soluble Fe fluctuated significantly in the anoxic metalimnion, suggestive of biogeochemical hot moments (i.e. short periods of high Fe reactivity). During reservoir turnover, mixing processes led to Fe and Mn being well-distributed throughout the water column and a decrease in Fe and Mn in the hypolimnion. The high-frequency monitoring system made predictions that matched the general trends in the manual sampling data. High-frequency monitoring of Fe and Mn chemistry in drinking water reservoirs will enable managers to identify hot moments of metals cycling and predict when high metals concentrations may occur.