HIGH EXPLOSIVES (HE) TRANSPORT IN FRACTURED BANDELIER TUFF - HYDROGEOLOGIC CONCEPTUAL MODEL AND CONTAMINANT TRACKING

Outfalls from HE production sites at Los Alamos National Laboratory (LANL) discharged RDX, TNT, HMX and barium contaminated waters onto the mesatops of the Quaternary upper Bandelier Tuff (UBT) from 1944 to 1996. HE concentrations in surface soils ranged up to 137 000 ppm HMX, 118 000 ppm RDX, and 102 000 ppm TNT. HE in waters range to 160 ug/L HMX, 818 ug/L RDX and 7.41 ug/L TNT. HE in water is present in springs, surface waters, alluvial waters and deep perched (> 700 ft. depth) and possibly regional (> 1200 ft. depth) groundwaters. HE has migrated further than would be predicted using porous flow models with observed values of volumetric moisture content and unsaturated hydraulic conductivities. Drilling results, spring flow data, stable isotope data, pore-water anion profiles, and contaminant data suggest a hydrogeologic conceptual model in which fast pathways such as fractures and surge beds facilitate contaminant transport within the UBT vadose zone. HE ratios, particularly RDX/HMX, may provide insights into the sources and vadose-zone transport of contaminants in the subsurface. Older, cast explosives contain higher proportions of TNT and RDX than do modern plastic bonded explosives. Both RDX and HMX behave conservatively during transport; they have similar retardation factors. Their ratio will be minimally effected by differential retardation during flow. SWSC spring has statistically higher RDX/HMX than Burning Ground and Martin springs. However, the RDX/HMX of the latter two springs is quite similar to the solubility ratio of RDX/HMX, suggesting buffering of this ratio by solid HE. This observation also suggests differing mixtures of contaminant sources for the former spring than the latter two springs. Surprisingly, spring flow, stable isotopes, and major element chemistries show similarities between SWSC and Burning Ground springs. Deep groundwater RDX/HMX is fairly constant in the regional aquifer whereas the overlying perched zone shows heterogeneous values. HE sources and flow paths to deep groundwaters also appear heterogeneous.