Interrelationships of Water Quality, Nutrients, and Major Ions in a Stream Draining a Mixed-Use Catchment, NW Spain

Major cations [calcium (Ca), magnesium (Mg), sodium (Na), potassium (K)], anions [nitrite (NO2), nitrate (NO3), choride (Cl), sulfate (SO4)], dissolved copper (Cu) and zinc (Zn), total nitrogen (TN), dissolved phosphorous (DP), total phosphorus (TP), suspended solids (SS), chemical oxygen demand (COD), dissolved organic carbon (DOC), and O-2 were analyzed in a small stream draining an agroforestry catchment located in northwest Spain. The mean concentrations found were relatively low compared to global "average" values. The results are consistent with the low population density and the nonintensive agriculture developed in the catchment. The nutrient balance offers relatively low loss rates for nitrogen (N) and phosphorus (P) (14.6% and 1.8%, respectively), although losses could have been underestimated by the use of regular sampling. Moreover, soil analysis reveals a possible P accumulation in cultivated fields of the catchment. Correlation analysis shows that components with a greater presence in fertilizers have generally positive correlations between them and with flow values. On the other hand, major ions show a negative correlation with discharge levels because their main sources have a geological origin.