Summary

End-member mixing analysis (EMMA) was used to determine the relative mixing amounts of water samples collected from wells at the Terrace Farm site and was robust in predicting the concentrations of the conservative solutes. The relative proportions of each end member were not inconsistent with the hydrogeologic setting. Nitrate and several other major ions (potassium and sodium), however, did not appear to be conservative.

Piper diagrams and constituent-constituent plots were used to determine which samples were affected by the wastewater at the North Farm application site.

The delta nitrogen-15 (δ¹⁵N) of nitrate generally ranged between +2 and +9 parts per thousand and the delta oxygen-18 (δ¹⁸O) of nitrate generally ranged between -2 and -7 parts per thousand. No relation was determined between δ¹⁵N and δ¹⁸O values and the proportion of a sample attributed to food-processing wastewater at the Terrace Farm application site. Additionally, no differences in nitrate-isotope values were detected between samples affected by the wastewater at the North Farm application site and samples that were not affected by the wastewater. These observations indicated that the nitrate derived from the food-processing wastewater likely was not distinct from other nitrate, because it overlaps with nitrate from ammonium fertilizer, animal and septic waste, and soil nitrogen. Values of δ¹⁵N and δ¹⁸O of nitrate provided no additional information about sources of nitrate in the Umatilla River basin than did a hydrologic and geochemical understanding of the ground-water system derived from interpreting water-level and major-ion chemistry data.