Nutrient and organic carbon trends and patterns in the upper Rio Grande, 1975–1999

Nutrient patterns and trends were analyzed using USGS water quality data collected from 1975 to 1999 along the uppermost 600 km of the Rio Grande in Colorado and New Mexico. Data on discharge, pH, organic carbon (total), N–NH4 ++organic N (total), NH4 + (dissolved), N–NO2 +N–NO3 (dissolved), phosphorus (total), and P-orthophosphate (dissolved) came from six USGS stations—Lobatos, Taos Junction, Otowi, San Felipe, Isleta and Bernardo—ranging from the Colorado–New Mexico border to about 80 km below Albuquerque, NM. Kendall’s S and Seasonal Kendall’s SV were used to measure trend, and ANOVA and Tukey’s multiple comparison test were used to analyze spatial differences between stations. Temporal trend analyses show widespread decreases in N and P concentrations at most stations, likely due to improvements in sewage treatment and dilution from increasing discharge. N–NO2 +N–NO3 (dissolved) and total nitrate load increases at Isleta and Bernardo, likely due to improved nitrification in sewage treatment and to increasing human population. Spatial analyses show large increases for most parameters at Isleta. All parameters show decreases again at Bernardo, about 50 km downstream from Isleta, except for N–NO2 +N–NO3 (dissolved), which continues to increase. Urbanization in the Albuquerque area significantly impacts downstream river nutrient levels.