Global amphibian declines have often been attributed to disease, but ignorance of the relative importance and mode of action of potential drivers of infection has made it difficult to develop effective remediation. In a field study, here we show that the widely used herbicide, atrazine, was the best predictor (out of more than 240 plausible candidates) of the abundance of larval trematodes (parasitic flatworms) in the declining northern leopard frog Rana pipiens. The effects of atrazine were consistent across trematode taxa. The combination of atrazine and phosphate--principal agrochemicals in global corn and sorghum production--accounted for 74% of the variation in the abundance of these often debilitating larval trematodes (atrazine alone accounted for 51%). Analysis of field data supported a causal mechanism whereby both agrochemicals increase exposure and susceptibility to larval trematodes by augmenting snail intermediate hosts and suppressing amphibian immunity. A mesocosm experiment demonstrated that, relative to control tanks, atrazine tanks had immunosuppressed tadpoles, had significantly more attached algae and snails, and had tadpoles with elevated trematode loads, further supporting a causal relationship between atrazine and elevated trematode infections in amphibians. These results raise concerns about the role of atrazine and phosphate in amphibian declines, and illustrate the value of quantifying the relative importance of several possible drivers of disease risk while determining the mechanisms by which they facilitate disease emergence.

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Sorption to dissolved macromolecules and colloidal particles may enhance the transport of environmental contaminants in subsurface environments. Some properties of sorbent and solute that may influence the adsorption behavior, especially in saturated soils with low organic C concentrations, were examined in a series of stirred reaction cell experiments, using acetophenone and ionizable 4‐chloroaniline and pentachlorophenol as solutes and substituted bio‐ and geopolymers and suspended clays as sorbents. The distribution coefficient was most variable, by a factor of 5500 from one sorbent to another, for the anionic pentachloropbenol and least variable, by a factor of 250, for acetophenone. These variations could be ascribed to differences in binding properties of the solute and sorbent functional groups. Sorption to clay minerals was generally low and approximated sorption to an aquifer soil suspension; the exception was a local clay with 12% organic C, which increased sorption of the phenolate ion more than 30 times compared with the low‐organic‐C clays. The most efficient sorbent was cells of bacteria isolated from aquifer material. The linear distribution coefficient for sorption on bacteria varied from 0.5 × 103 mL g−1 for acetophenone to 5 × 104 mL g−1 for neutral pentachlorophenol. The implications of sorption varying with solute and sorbent functional groups on movement of solutes through an aquifer were examined by a transport model, which divides the mobile system into an aqueous and a colloidal phase. The relative mobility of pentachlorophenol associated with colloidal sorbents can vary by as much as 90% as a result of variations of the distribution coefficient to the polymers and clays examined.