A major challenge in understanding the environmental implications of nanotechnology lies in studying nanoparticle uptake in organisms at environmentally realistic exposure concentrations. Typically, high exposure concentrations are needed to trigger measurable effects and to detect accumulation above background. But application of tracer techniques can overcome these limitations. Here we synthesised, for the first time, citrate-coated Ag nanoparticles using Ag that was 99.7 % ¹⁰⁹Ag. In addition to conducting reactivity and dissolution studies, we assessed the bioavailability and toxicity of these isotopically modified Ag nanoparticles (¹⁰⁹Ag NPs) to a freshwater snail under conditions typical of nature. We showed that accumulation of ¹⁰⁹Ag from ¹⁰⁹Ag NPs is detectable in the tissues of Lymnaea stagnalis after 24-h exposure to aqueous concentrations as low as 6 ng L^–1 as well as after 3 h of dietary exposure to concentrations as low as 0.07 μg g^–1. Silver uptake from unlabelled Ag NPs would not have been detected under similar exposure conditions. Uptake rates of ¹⁰⁹Ag from ¹⁰⁹Ag NPs mixed with food or dispersed in water were largely linear over a wide range of concentrations. Particle dissolution was most important at low waterborne concentrations. We estimated that 70 % of the bioaccumulated ¹⁰⁹Ag concentration in L. stagnalis at exposures <0.1 µg L^–1 originated from the newly solubilised Ag. Above this concentration, we predicted that 80 % of the bioaccumulated ¹⁰⁹Ag concentration originated from the ¹⁰⁹Ag NPs. It was not clear if agglomeration had a major influence on uptake rates.