An extensive study focusing on irrigation drainwater was carried out in the vicinity of Laguna Atascosa National Wildlife Refuge in the summer of 1990. During three sampling periods of 5 days each, up to 20 samples per day were collected for Microtox testing, along with basic water quality measurements from each regular site.
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Sampling periods coincided with intensive aerial applications of pesticides and herbicides on cotton and other crop fields in this predominately agricultural region. Rainfall, and consequently runoff, was lower than anticipated, and perhaps accordingly, no dramatic indications of toxicity were observed for any of the 257 samples screened. The most commonly observed effect was a modestly to substantially greater light output in samples relative to controls. This increased light output may be an indication of a nutrient effect, or it may be a result of stimulatory effects (hormesis) caused by exposure to low levels of potentially toxic agents. An additional study is recommended to more accurately assess the water and sediment quality of these agricultural drains.