Scientific Investigations Report 2005-5255
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5255
Widespread and persistent elevated concentrations of fecal bacteria and nitrate have been documented in lowland stream waters of the Nooksack River basin, Whatcom County, Washington. The application of large quantities of livestock manures to fields and pastures overlying a shallow and highly permeable aquifer has resulted in elevated fecal-bacteria concentrations in surface waters and high nitrate concentrations in ground water. Because of the high permeability of the shallow aquifer and the persistence of fecal bacteria in surface waters during the late summer base-flow period, discharging ground water may be a source of bacteria in stream water analyzed from this area. In this study, spatial and temporal variations in ground-water discharge were characterized, and water-quality analyses and laboratory experiments were conducted to better understand the processes that control the transport and fate of these contaminants at the interface between ground water and surface water.
Ground-water discharge is nearly continuous at many locations in the Nooksack River lowland except during periods of high river stage or late summer low flow. Ground-water discharge generally is greater during the wet winter months and decreases during the dry summer months. In some cases, the direction of water flow reverses for brief intervals so that surface water recharges ground water. Detailed site studies in various hydrologic settings in the Nooksack River lowland show small local-scale spatial variations and large temporal variations in ground-water discharge. Locations of ground-water discharges appear to be related to the distribution of permeable geologic materials adjacent to the streambed.
Samples of discharging ground water collected from the shallow sediments beneath streams and rivers rarely contained measurable concentrations of the fecal bacteria Escherichia coli or nitrate, indicating that ground-water transport was not a major source of bacteria and nitrate to surface waters of the reaches that were studied. In samples of ground-water discharge from most study sites, concentrations of dissolved oxygen typically were less than 1 milligram per liter and concentrations of dissolved ferrous iron typically ranged from 0.1 to 17 milligrams per liter. These geochemical oxidation-reduction (redox) conditions in ground water within the monitored reaches are conducive to denitrification. Concentrations of dissolved argon and nitrogen gas measured in discharging ground water indicate that 5 to 15 milligrams per liter of nitrate were denitrified prior to its discharge to surface water.
A laboratory microcosm experiment was used to evaluate the potential for high bacteria concentrations measured in streamflow during late summer periods that may have originated from field sediments washed into streams during preceding wintertime overbank flow events. The microbial attenuation experiment, conducted with manure amended native stream sediment, showed that less than 1 percent of fecal coliform bacteria survived beyond 65 days and that at 100 days the sediment coliform concentration had attenuated to near pre-amendment levels. Although small numbers of bacteria remained viable in sediments at the end of the experiment, it is unlikely that enough bacteria survive the many months between winter and summer to make up the substantial fraction of large fecal coliform concentrations measured during late summer.
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