Acute and chronic aquatic toxicity structure-activity relationships for alcohol ethoxylates.

Acute and chronic toxicity tests using the cladoceran Daphnia magna were conducted on several alcohol ethoxylate surfactants. Exposure and homologue distributions were confirmed using specific analytical methods. These data were used to test currently available acute structure-activity relationships (SARs) and to develop a new chronic SAR to extrapolate test data to effluent or receiving water mixtures. Existing acute SARs adequately predicted the toxicity of these materials with an r(2) of 0.96 based on alkyl chain length and number of ethoxylates and 0.98 based on logK(ow). The additional chronic toxicity data allowed for development of a chronic SAR based on logK(ow) and produced an r(2) of 0.93. Slopes of new and published acute, chronic, and mesocosm SARs based on logK(ow) ranged from -0.61 to -0.87. These new data and refined SARs will assist in the toxicity prediction of mixtures in the environment.

Control of nitrogen export from watersheds by headwater streams.

A comparative (15)N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.