Evaluation of mercury cycling and hypolimnetic oxygenation in mercury-impacted seasonally stratified reservoirs in the Guadalupe River watershed, California

Abstract

Surface water reservoirs trap inorganic mercury delivered from their watersheds, create conditions that convert inorganic mercury to highly toxic methylmercury (MeHg), and host sportfish in which MeHg bioaccumulates. The Santa Clara Valley Water District (District) actively manages and monitors four mercury‐impaired reservoirs that help to serve communities in South San Francisco Bay, California. The Guadalupe River watershed, which contains three of those reservoirs, also includes the New Almaden mercury‐mining district, the largest historic mercury producer in North America. Monthly vertical profiles of field measurements and grab samples in years 2011–2013 portray annual cycling of density stratification, dissolved oxygen (DO), and MeHg. Monitoring results highlight the role that hypolimnetic hypoxia plays in MeHg distribution in the water column, as well as the consistent, tight coupling between MeHg in ecological compartments (water, zooplankton, and bass) across the four reservoirs. Following the 2011–2013 monitoring period, the District designed and installed hypolimnetic oxygenation systems (HOS) in the four reservoirs in an effort to repress MeHg buildup in bottom waters and attain regulatory targets for MeHg in water and fish tissue. Initial HOS operation in Calero Reservoir in 2014 enhanced bottom water DO and depressed hypolimnetic buildup of MeHg, but did not substantially decrease mercury levels in zooplankton or small fish.

Challenges and opportunities for managing aquatic mercury pollution in altered landscapes

“…In addition, eutrophication can alter organic carbon loads to surface waters, resulting in mixed effects that can increase or decrease MeHg levels in surface waters, as noted above. Managed alterations such as additions of nitrate or hypolimnetic oxygenation in the field have been attempted with some success to change redox conditions and decrease MeHg concentrations in water, but these manipulations do not always result in reductions of MeHg in biota (Matthews et al 2013 ; Austin et al 2016 ; Beutel et al 2016 ; McCord et al 2016 ).…”

“…Though not unique to just reservoirs, management strategies in waterbodies aimed at altering redox conditions through oxygen or nitrate addition can also be utilized to reduce MeHg production (Matthews et al 2013 ; McCord et al 2016 ). Other management actions aimed at decreasing MeHg production can include reducing the size and/or development of an anoxic hypolimnion using lake mixers or selective water withdrawals (Rask et al 2010 ; Perron et al 2014 ; Zouabi-Aloui et al 2015 ).…”

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

“…Spatial and temporal patterns of MeHg production within a reservoir will be influenced by the inflow of IHg, labile carbon, availability of terminal electron acceptors, and the timing and duration of thermal and redox stratification, among other variables. 11,12 In contrast, the fate of hypolimnetic MeHg released from a reservoir will be controlled by the timing and duration of thermal destratification. 8,13 Previous studies have reported seasonally elevated MeHg concentrations at reservoir outflow versus inflow locations, 1,8,14,15 suggesting that MeHg produced within a reservoir can be exported downstream.…”

Water Level Decline in a Reservoir: Implications for Water Quality Variation and Pollution Source Identification

“…In addition, the relatively high COD Mn in summer also indicated that the DO consumption was higher than in other seasons. DO in summer had a corresponding decrease trend in water in low water level operation [37].…”