FATE OF ORCHARD DERIVED ARSENICAL PESTICIDES

While flume studies suggest that flow interactions between a stream and its bed can potentially impact the transport and fate of dissolved and suspended contaminants, these impacts are less well documented in the field. Of particular interest in New England is the transport and fate of arsenic in alluvial hyporheic sediments derived from orchards where arsenic-based pesticides were widely used in the early 1900's. We are investigating the distribution of arsenic contamination in hyporheic sediments not only longitudinally downstream from their presumed source, but also vertically in the stream bed. We have identified two field sites in southern New Hampshire that drain disturbed orchard lands, as well as one control site where no agricultural influence has occurred. At the two sites below orchards we have taken, at downstream intervals of several hundred meters, vertical profiles in 5cm increments down to 25cm. In a companion study we are also investigating the temporal variation in the vertical distribution of an atmospherically-derived short-lived radioisotope beryllium-7. We have found that the expected longitudinal decrease in sediment contaminant concentration with distance from the source due to dilution and preferential transport can be interrupted by changes in stream character. We have also found relatively little variation in vertical distribution of beryllium-7, possibly suggesting significant penetration and more or less uniform mixing of contaminant load to depths of several centimeters and more in the stream bed. These results demonstrate that hyporheic sediments can filter and store potentially large amounts of contaminants for long periods of time and thus stream bed interactions have a significant impact on the transport and fate of suspended contaminants.