THE IMPACT OF THE LAS CONCHAS FIRE ON DISSOLVED OXYGEN CONCENTRATIONS IN THE ALBUQUERQUE REACH OF THE RIO GRANDE IN NEW MEXICO

The recent Las Conchas (LC) fire, the largest fire in recorded history in New Mexico (NM), burned 156,593 acres in the Santa Fe National Forest, as well as Los Alamos and Rio Arriba Counties; the Santa Clara, Jemz, Cochiti, and Santo Domingo Pueblos; Bandelier National Monument; and Valles Caldera National Preserve during June and July of 2011. Water quality (WQ) findings indicate the fire negatively impacted the Rio Grande (RG) at four WQ monitoring stations in the Albuquerque area, and may do so for years to come, as precipitation events move ash, soil, charcoal, and fire debris into streams and rivers. The spatial and temporal variability of the RG necessitates continuous collection of temperature, specific conductivity, dissolved oxygen (DO), pH and turbidity data in order to provide an accurate assessment of the response of RG WQ to inter-annual climate changes, episodic disturbance events and resulting changes in river flow and river chemistry. While WQ data for several years has indicated the presence of DO sags related to urban runoff; initial analyses of data gathered before and after the LC fire indicate even more severe DO sags associated with upstream tributary inputs resulting from July and August 2011 monsoon rains in the burned areas. These storm events transported pulses of fire products through the Albuquerque reach of the RG in a relatively short period of time. Pulses were identified by examining RG discharge rates after monsoon rains in the LC area and evaluating changes in DO concentration when the fire-impacted discharge pulse arrived at the monitoring stations. Comparison of measured DO concentrations at the northernmost research monitoring station with discharge rates (Q) for the San Felipe, Alameda and Central RG USGS gauges after thunderstorms in northern NM indicate that river discharge pulses travel downstream and are correlated with significant DO sags. The DO sags were also distinct at all four monitoring stations and became smaller as they traveled downstream. Sags in DO, especially below the NM Water Quality standard of 5.0 mg/L including hypoxic waters with DO concentrations of 2.0 mg/L or less, resulted after the LC fire. These fire-impacted discharge pulses have potentially significant implications for river biota.