Introduction

Stable isotopes of nitrate have been used for more than 30 years to determine nitrate sources in water supplies (Kohl and others, 1971; Kreitler, 1975; Kreitler and Jones, 1975; Kreitler and others, 1978; Gormly and Spalding, 1979; Kreitler, 1979; Wolterink and others, 1979; Mariotti and others, 1980; Spalding and others, 1982; Heaton and others, 1983; Kreitler and Browning, 1983; Flipse and others, 1984; Heaton, 1984; Mariotti and others, 1984; Exner and Spalding, 1985; Flipse and Bonner, 1985; Heaton, 1986; Hubner, 1986; Amberger and Schmidt, 1987; Mariotti and others, 1988; Aravena and others, 1993; Komor and Anderson, 1993; Clark and Fritz, 1997; Aravena and Robertson, 1998). Although the delta oxygen-18 (δ¹⁸O) and delta nitrogen-15 (δ¹⁵N) of nitrate derived from various nitrate sources, including nitrate and ammonium fertilizers, soil nitrogen, precipitation, and manure and septic waste, are known (Kendall, 1998), values from nitrate derived from food-processing wastewater applications have not been published.

Food-processing activities related to french-fry production generate large volumes of nutrient-rich wastewater on a daily basis (Grondin and Camacho, 1995). Food-processing wastewater often is applied as a treatment/disposal method to crop land in the study area in north-central Oregon near Umatilla. Since the early 1980s, more than 1 million pounds of nitrogen have been produced annually by food-processing activities in the Umatilla River basin. These activities are a potential source of nitrate to the ground-water system of the basin. The objective of this study was to measure the δ¹⁵N and δ¹⁸O of nitrate in ground water beneath food-processing wastewater application sites in the Umatilla River basin to determine if the nitrate resulting from the food-processing wastewater-application could be distinguished from other potential sources of nitrate.

Site Descriptions

Two food-processing wastewater-application sites in the Umatilla River basin study area in north-central Oregon were studied (fig. 1): the Terrace Farm site and the North Farm site. At both sites, a shallow, unconfined aquifer occurs in the alluvial sediment that overlies the Columbia River Basalt Group; the sediment comprises primarily silt, sand, and gravel that resulted from the floods of glacial Lake Missoula (Wozniak, 1995). The aquifer mainly consists of basaltic fragments of plagioclase, pyroxene, and volcanic glass. Secondary minerals, such as calcite, smectite, gypsum, and dolomite, occur as well. The ground-water chemistry is generally classified as a mixed-cation/bicarbonate type (Grondin and Nelson, 1995).

The Terrace Farm site is on an upland terrace between Emigrant Buttes to the east and the Butter Creek flood plain to the west (Richerson, 2004). The Hunt Irrigation Ditch runs along the western and northern edges of the site. Food-processing wastewater had been applied to the site since 1981 on what previously was dryland and then irrigated farmland. Irrigated agriculture is the dominant land use upgradient of the Terrace Farm site. Immediately downgradient of the Terrace Farm site is a cattle-feedlot that has been in operation since before 1985. Columbia River water is the source of the process water for the Terrace Farm site.

The North Farm site is on a northeast/southwest trending topographic ridge that includes rangeland areas surrounding a landfill in the northwest section, the Umatilla Ordnance Depot to the west, and the North Farm wastewater-application areas to the east. Food-processing wastewater has been applied to the North Farm site since 1972 on land that previously was dryland (Richerson, 2004). The Umatilla Ordnance Depot landfill began accepting solid waste (mostly packing material) in 1968 and ceased receiving waste in 1993. Some munitions-contaminated soils that were solidified with cement were placed in the landfill in 1994. A slight downward vertical gradient generally has been observed beneath the wastewater application area (Phil Richerson, Oregon Department of Environmental Quality, written commun., September 2005), which is consistent with recharge occurring due to the wastewater application. The vertical gradient on the western edge of the wastewater-application area, however, occasionally reverses itself indicating that there are brief periods of upward movement of water at that location.

Food-processing wastewater, characterized by high levels of total Kjeldahl nitrogen (greater than 100 mg/L) and low amounts of nitrate (about 1 mg/L), was being applied to the two sites at the time of the study in 2004. The wastewater is sprayed on the crops by using center pivot sprinkler systems during the crop growing season, which typically runs from March through October. Various crops are grown, including corn, wheat, alfalfa, barley, forage grasses, potatoes, and peas.