WATER-QUALITY CHANGE DUE TO THE CERRO GRANDE FIRE, AND ITS POTENTIAL USE AS A RECHARGE TRACER

Portions of upper Pajarito Canyon watershed were severely burned during the Cerro Grande fire in May 2000. A perennial stream within this reach is supported by seeps that discharge from Tschicoma Formation dacites at an elevation of 9,000 ft. Flow extends eastward for approximately 1.5 miles and abruptly terminates at the surface expression of the Pajarito fault, where upper Bandelier Tuff units crop out at 8,100 ft elevation. Approximately 1.2 miles downgradient of the fault, several permanent springs emanate from these tuff units at about 7,640 ft elevation. Before the fire, the stream and spring waters generally showed the same hydrochemical signature. Additionally, the rate of stream loss across the fault was equivalent to the cumulative spring discharge. These observations suggest a hydrologic connection where the fault acts as a recharge conduit for ground water and the downgradient springs serve as discharge points. A tracer study was needed to test the conceptual model for connectivity between the surface water and the springs, and the Cerro Grande fire supplied ample chemical conditions for such a tracer. Surface-water samples were collected two weeks after the fire, and prior to major storm-runoff events. Hydrochemical changes to the surface water, such as elevated bicarbonate, were noticeable. The springs were sampled about one month after the fire; analytical results show that bicarbonate and calcium had already passed through the fault and spring system. Hence, the travel time from the point of recharge to discharge appears to be less than 30 days, assuming the referenced ions moved at the same velocity as ground water. Post-fire stream and spring sampling was conducted throughout the summer and into the fall, and data show variable hydrochemical-breakthrough times at the springs. Potentially, the post-fire surface water, if a source of recharge, can now be chemically traced to deeper saturated zones, which in turn should greatly improve the overall hydrogeologic conceptual model for the Los Alamos area.