Scientific Investigations Report 2005-5255
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5255
Fecal bacteria are present in the intestinal tract and feces of warm-blooded animals, some of which can cause diseases in humans, such as cholera and dysentery when spread as waterborne pathogen originating in sewage from infected humans (Craun, 1986). Because fecal coliforms can be readily detected in water samples and are present in untreated human sewage, their presence in samples of surface water is considered an indicator of the potential presence of other pathogenic bacteria species that can occur in human sewage and are not easily detected. Escherichia coli (E. coli) is a major component of the fecal coliform bacteria group and typically are harmless to humans. In recent years however, several atypical strains of E. coli have been identified that are linked to a variety of diseases in humans, most notable being the strain E. coli (O157:H7) that cause hemorrhagic colitis (Nataro and Kaper, 1998). Cattle have been shown to be a reservoir for E. coli O157:H7 (Hancock and others, 1994; Hussein and Sakuma, 2005) and while most recorded outbreaks of E. coli O157:H7 infections have resulted from consumption of contaminated food products or contact with farm animals, manure handling and contact with contaminated surface waters are considered potential routes of E. coli O157:H7 infection in humans (Petridis and others, 2002).
Most area residents living outside of the towns of Lynden and Everson use on-site septic systems to treat domestic sewage. Livestock farms in the lower Nooksack River basin generate large quantities of manure and use various disposal methods including manure application to pasturelands and croplands either by direct application, field spraying, or direct injection below the soil surface. Direct injection reduces the potential for surface runoff to transport bacteria from agricultural fields to surface water. However, this method also enhances the likelihood that viable bacteria are transported to shallow aquifers by eliminating exposure of fecal bacteria to the sterilizing effects of oxygen and sunlight and reducing the distance of the pathway from land surface to the shallow aquifer.
The potential for fecal bacteria to occur in ground water is governed by the quantity of bacteria (manure application) deposited on agricultural fields, the infiltration capacity and rate of transport through the shallow subsurface soils, and the fate and transport of the bacteria once they have entered the aquifer. Die-off of bacterial cells and filtration within sediment pore spaces reduce bacteria concentrations as water flows through porous media (Bitton and Harvey, 1992), reducing the number of organisms that are transported to, and remain viable in ground water.
Subsurface tile drains in many fields of the lower Nooksack River basin increase the discharge rate of shallow ground water to surface water and lower the water-table elevation early in the year thereby allowing earlier spring planting and extending the growing season. In similar agricultural settings, subsurface tile drains have been shown to be a significant pathway for fecal coliform transport to adjacent surface-water streams (Patni and others, 1984; Joy and others, 1998).
The accumulation of nitrate in ground water primarily is dependent on the input of nitrogen and the presence or absence of oxygen. Nitrogen is applied to agricultural production fields primarily as ammonium in fertilizer or as a component of barnyard manure. Within aerobic soils, ammonium is converted to nitrate by nitrifying bacteria. Nitrate dissolves easily, and remains stable in water under aerobic conditions. Nitrate is transported with infiltrating soil water and if not assimilated by plants within the soil-root zone, nitrate is leached to the underlying ground-water system. Nitrate does not react readily with aquifer materials and remains in ground water throughout the ground-water flow path unless utilized by denitrifying microbes. Attenuation of elevated nitrate concentrations in ground water can occur under chemically reducing conditions by microbially mediated processes if sufficient carbon is available (Korom, 1992). Facultative denitrifying microbes that are capable of this process generally are present throughout most surficial aquifers (Chapelle, 1993).
Much of the Sumas aquifer typically is aerobic and contains limited organic carbon (Cox and Kahle, 1999) and thus may not be widely conducive to nitrate degradation by microbial processes. Geochemical conditions that support nitrate degradation are clearly present in some parts of the aquifer (Tesoriero and others, 2000) as well as in some soils and near the water table (Carey, 2002). Tesoriero and others (2000) observed nitrate degradation in carbon rich sediments adjacent to a stream in the lower Nooksack River basin and postulated that in other parts of the aquifer, iron minerals also might be used for denitrification. In locations where anaerobic conditions are present along with a suitable source of electrons, these naturally occurring facultative denitrifying bacteria can reduce nitrate to nitrogen gas.
There have been extensive efforts over many years to develop and implement farm-management practices that reduce migration of contaminants from farms to the surrounding environment. Local and State efforts to reduce the agricultural loading of nitrate to ground water and fecal bacteria to surface water are supported by regulatory requirements that are intended to bring water bodies into compliance with the Clean Water Act standards. Whatcom County adopted a Manure Control Ordinance in 1998 that restricts the field application of manure for forage production to April through September. The Washington State Department of Ecology has developed and is currently implementing a Total Maximum Daily Load Detailed Implementation Plan for reduction of fecal bacteria in surface-water drainages. A consortium of local governmental agencies is currently developing watershed-scale models within a decision-support system to increase water-resources management capabilities of local decisionmakers. A smaller-scale nitrate ground-water transport model is currently being developed by Environment Canada in coordination with Simon Fraser University for the trans-boundary Abbotsford-Sumas aquifer. This is relevant to the lower Nooksack River basin efforts because the headwaters of some of the northernmost tributaries to the Nooksack River (Bertrand and Fishtrap Creeks) lie in Canada. In addition, the direction of ground-water flow trends southward toward the Nooksack River transporting nitrogen in ground water across the border, and contributing to the high nitrate concentrations measured at some locations in the United States. Due to a lack of field data, the specific mechanisms describing the fate and transport of nitrate and nitrate degradation in the models being developed are based on assumptions.
Washington State mandated the development of Nutrient Management Plans for all dairy farming operations that handle more than 700 mature dairy cattle under their delegated Clean Water Act authority. These plans require an on-farm nitrogen budget that accounts for the production, distribution, and plant and animal uptake of nitrogen, so that overall increases in nitrogen concentrations in soil and ground water are avoided. The effectiveness of these and future management strategies in the reduction of nitrate in ground water would benefit from a more detailed understanding of the processes affecting fate and transport of nitrate in the study area.
For more information about USGS activities in Washington, visit the USGS Washington Water Science Center home page .
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