Dynamics of selenium in Cibola Lake, Arizona

Abstract

Selenium bioaccumulates along the lower Colorado River and may impact fish and waterfowl. Selenium may be reduced in lakes or reservoirs by flushing (increasing the water exchange rate). Therefore, I monitored selenium levels in water, sediment, and bluegill (Lepomis macrochirus) in response to flushing on the Cibola NWR in the lower Colorado River Valley, California and Arizona. Selenium in the lakewater was below the detection level of 5.6 ppb wet weight (WW) but data from the US Geological Survey (1990-1995) showed that selenium levels in the water ranged from 1 to 3 ppb WW in the mainstem Colorado River 135 km upstream from Cibola Lake. There was no predictable trend in selenium in the sediment after flushing. It was the same after the first flushing, lower after the second flushing, and higher after the third flushing. Selenium levels in the biota also did not vary in a predictable way after flushing. It went up after the first flushing, down after the second flushing, and up after the third flushing. One might speculate that higher flushing rates, or continuous flushing might result in lower selenium levels in biota and sediment. However, selenium levels in the sediment (1.10 ppm DW) and bluegill (4.93 ppm DW) in Cibola Lake were not significantly different than levels in the sediment (0.86 ppm DW) and bluegill (4.83 ppm DW) in a comparable backwater lake (Mittry Lake, Arizona) that was continuously flushed. Therefore, flushing does not appear to be a viable strategy for managing selenium levels in backwater lakes along the lower Colorado River.