The Dragonfly Mercury Project spans nearly every state in the US, and represents samples collected across over 100 diverse national parks. Since 2009, parks have sampled dragonfly nymphs for total mercury (Hg) analyses, often coupled with water and sediment quality metrics to determine: (1) baseline Hg concentrations in national parks prior to large-scale implementation of national and international mercury emission controls; and, (2) how vulnerability food web Hg accumulation varies among habitats and with landcover characteristics. Dragonfly nymphs are useful biosentinels for Hg spatial patterns because they inhabit many freshwater habitats, are relatively sedentary, and as predators, contain almost all of their Hg as MeHg. As of 2017, a total of 127 freshwater lakes, ponds, wetlands, streams, and river sites have been sampled across 34 national parks. Preliminary results indicate that the mean (± SE) THg concentration in dragonfly nymphs was 141.1 ± 2.5 ppb dry weight (dw). We observed 76­fold variation between the sites with the greatest (>1000 ppb, dw) and least (~20 ppb, dw) THg concentrations across parks, and up to 44­fold variation among sites within a single park. These preliminary findings highlight the importance of spatial variability at the scale of individual water bodies. By linking dragonfly THg concentrations with data on water and sediment Hg, water chemistry, and watershed characteristics, the DMP will provide valuable insights into the drivers of, and potential vulnerability to, MeHg bioaccumulation in the NPS’s aquatic resources. Although benchmarks that define how these dragonfly Hg concentrations translate to risk for the animals that consume them are still in development, our early analyses suggest that dragonfly larvae with Hg concentrations less than about 315 ppb, dw are likely to be in the lowest risk category. This proposed benchmark reflects potential toxicological risk to fish and wildlife based upon a combination of published effects thresholds and trophic enrichment factor estimates; further interpretation requires site-specific information on foodweb structure and community composition that is rarely available. The DMP is implemented within each park via engagement of citizen scientists, sparking public interest in biodiversity and participatory science, as well as communicating key messages about air quality and mercury risk. Since 2009, over 3,705 citizens have participated in the project, providing the scientist team with 8,769 dragonfly samples and making the research possible across this broad spatial scale.