Population growth, climate uncertainties, and unsustainable groundwater pumping challenge aquifer sustainability worldwide. Efficient and data-driven groundwater supply management is a necessity to maintain essential water-dependent functions. Currently, managers lack the cost-effective, scalable, and reliable groundwater monitoring systems needed to collect vital groundwater data. Existing automated groundwater monitoring systems tend to be cost-prohibitive, and manual methods lack the spatial or temporal resolution to sufficiently meet critical water modeling, management, and policy objectives. In this study, we developed a fully automated, open source, low cost wireless sensor network (LCSN) for real-time groundwater data acquisition, processing, and visualization in the South American Subbasin Groundwater Observatory (GWO), located in California, USA. We demonstrate the steps taken to create the GWO, including field, hardware, software, and data pipeline components so that it may be easily reproduced in new areas. We find that the GWO is comparable in cost to manual measurements at a weekly measurement frequency, and costs between three and four times less than comparable commercially available telemetry and dashboard systems, largely due to the use of free open source software to acquire, clean, store, and visualize data. The open source-powered GWO thus lowers the financial and technical barrier of entry for real-time groundwater monitoring, creating the potential for more informed water management worldwide, particularly in regions whose managers are restricted by the high capital costs of commercial monitoring systems.

Anoxic conditions within reservoirs related to thermal stratification and oxygen depletion lead to methylmercury (MeHg) production, a key process governing the uptake of mercury in aquatic food webs. Once formed within a reservoir, the timing and magnitude of the biological uptake of MeHg and the relative importance of MeHg export in water versus biological compartments remain poorly understood. We examined the relations between the reservoir stratification state, anoxia, and the concentrations and export loads of MeHg in aqueous and biological compartments at the outflow locations of two reservoirs of the Hells Canyon Complex (Snake River, Idaho-Oregon). Results show that (1) MeHg concentrations in filter-passing water, zooplankton, suspended particles, and detritus increased in response to reservoir destratification; (2) zooplankton MeHg strongly correlated with MeHg in filter-passing water during destratification; (3) reservoir anoxia appeared to be a key control on MeHg export; and (4) biological MeHg, primarily in zooplankton, accounted for only 5% of total MeHg export from the reservoirs (the remainder being aqueous compartments). These results improve our understanding of the role of biological incorporation of MeHg and the subsequent downstream release from seasonally stratified reservoirs and demonstrate that in-reservoir physical processes strongly influence MeHg incorporation at the base of the aquatic food web.