EBTAG Annual Workshop and Field Trip
May 16-17, 2012

¹Los Alamos National Laboratory, MS M997, TA-00, Bldg 1237, Los Alamos, NM, 87545, abwhite@lanl.gov

²Adelante Consulting, MS M997, TA-00, Bldg 1237, Los Alamos, NM, 87545

³Los Alamos National Laboratory, MS D452, TA-03, Building 215, Los Alamos, NM, 87545

⁴Los Alamos National Laboratory, MS M992, TA-00, Bldg 1237, Los Alamos, NM, 87545

The effect of the 2011 Las Conchas fire was particularly evident at gage station E109.9, which measures discharge from a total drainage area of 37,800 acres, with 11% impermeable surface area before the fire and an additional 13% of the watershed experiencing high- or moderate-severity burn during the fire. A small portion of this watershed is downstream of Los Alamos National Laboratory (LANL), and is a tributary of the Rio Grande. The total and permeable surface areas in this watershed were more highly correlated to suspended sediment concentrations (SSCs) than the impermeable surface area, suggesting the amount of sediment in the runoff is strongly related to the permeable surface area contributing to the station. This is counter to the well-documented observations that converting permeable surfaces to impermeable surfaces can increase the peak and shorten the duration of a hydrograph, thereby creating a conduit for sediment to reach the stream and increasing SSC. This effect is most likely because of the influence of the Las Conchas fire, which created a similar impermeable “conduit” for sediment transmission and greatly increased the supply of sediment, in part ash-laden, from the upper watersheds of Los Alamos and Guaje Canyons. Runoff from burn areas typically has high concentrations of suspended sediment (partly related to the entrainment of ash), and the runoff volume typically increases. Indeed, SSC was elevated at E109.9 as a result of floods in Guaje Canyon from the Las Conchas burn area, and E109.9 recorded discharge greater than 5 cfs only 4 times during 2010 (pre-fire) and 15 times during 2011 (post-fire). When the Guaje Canyon gage station (E099) was operational, increases in stage height were measured during all 10 events when E109.9 also measured discharge, indicating runoff from Guaje Canyon was the dominant contribution to discharge at E109.9. As for the water chemistry, elevated concentrations of manganese, selenium, total cyanide, total polychlorinated biphenyls (PCBs), dioxins, gross alpha, americium-241, plutonium-239/240, and strontium-90 measured at E109.9 are similar to those measured in upper Los Alamos Canyon, above LANL-impacted sites, and are consistent with the transport of these constituents from the Las Conchas burn area down Guaje Canyon. Analyses of ash, sediment, and stormwater in Guaje and Rendija Canyons after the 2001 Cerro Grande fire indicated that these elevated constituents also had a source in the Cerro Grande burn area.