Seasonal Dynamics and Interannual Variability in Mercury Concentrations and Loads through a Three-Reservoir Complex

Abstract

The Hells Canyon Complex (HCC) along the Snake River (Idaho–Oregon border, U.S.A.) encompasses three successive reservoirs that seasonally stratify, creating anoxic conditions in the hypolimnion that promote methylmercury (MeHg) production. This study quantified seasonal dynamics and interannual variability in mercury concentrations (inorganic divalent mercury (IHg) and MeHg) and loads at four reservoir inflow and outflow locations through the HCC (2014–2017). We observed (1) that the HCC is a net sink for both IHg and MeHg, (2) interannual variability in IHg and MeHg loads largely reflecting streamflow conditions, and (3) seasonal variability in particulate IHg loading at the inflow (greatest from February to April) and MeHg export from the outflow (greatest from September to December) of the HCC. Seasonal export of MeHg was evidenced by increases in monthly mean concentrations of unfiltered MeHg (approximately 2-fold) and the percentage of total mercury (THg) as MeHg (≥4-fold) coincident with reservoir destratification. Despite evidence of seasonal export of MeHg from the HCC, annual loads indicate a 42% decrease in unfiltered MeHg from HCC inflow to outflow. Results from this study improve the understanding of seasonal variability in mercury transport through and transformation within a reservoir complex.

Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA

“…The highest MeHg concentration (2.85 ng/L) was measured within the anoxic hypolimnion (depth of 71 m) and represented 56% of the THg concentration. Baldwin et al (2020) provide a more detailed assessment of seasonal and interannual variations in THg and MeHg concentrations and loads within all 3 of the Hells Canyon reservoirs.…”

“…The water column in the 2 deepest reservoirs (Brownlee and Hells Canyon) undergoes seasonal thermal stratification that leads to hypolimnetic anoxic conditions developing. The onset of anoxic conditions can vary between years depending on multiple factors such as water flow rates, temperature, and nutrient loadings (Baldwin et al 2020). During the spring and fall the mean epilimnion water temperatures were similar (19.1 ± 0.2 and 18.1 ± 0.2 °C, respectively).…”

“…The reservoirs were built over 50 yr ago and are well past the point where MeHg concentrations would be expected to be elevated as a result of recent impoundment (Hall et al 2005; Bodaly et al 2007). Brownlee is the largest of the 3 reservoirs (93 km long and up to 91 m deep); it is located upriver from the other 2 and therefore receives the largest inputs of nutrients, DOM, and Hg from the upstream watershed, which includes large agricultural areas (Baldwin et al 2020). Fish consumption advisories for elevated MeHg concentrations have been in place over the last 2 decades in the Hells Canyon complex (Clark et al 2016).…”

“…Water samples were collected from 2 locations within Brownlee Reservoir (at river mile [RM] 300 and RM286), one location in Oxbow Reservoir (RM273), and one location in Hells Canyon Reservoir (RM248; Figure 1). Most sample locations correspond to areas where previous and ongoing studies by the US Geological Survey (USGS) and Idaho Power have been conducted (Baldwin et al 2020). The RM300 location was selected because this is a key location in the reservoir where discharge velocity decreases and nutrients and DOM tend to drop out of suspension.…”

In-Reservoir Physical Processes Modulate Aqueous and Biological Methylmercury Export from a Seasonally Anoxic Reservoir

“… 23 Additionally, the destratification period (August to December) coincides with the elevated MeHg concentrations in water samples at the reservoir outflows, suggesting that some portion of the metalimnetic and hypolimnetic MeHg is exported downstream. 17 …”