DEVELOPMENT OF A HYDROSTRATIGRAPHIC FRAMEWORK MODEL (HFM) TO SUPPORT ENVIRONMENTAL CHARACTERIZATION OF MORTANDAD CANYON, LOS ALAMOS NATIONAL LABORATORY (LANL), NEW MEXICO

A three-dimensional (3D) hydrostratigraphic framework model (HFM) of the Mortandad Canyon Project Site was developed to support ongoing environmental characterization and restoration activities. The Mortandad Canyon watershed encompasses 8.4 km2 (3.3 mi2) that drains eastwards across the Pajarito Plateau and crosses both LANL and San Ildefonso Pueblo land to its confluence with the Rio Grande. Research activities at LANL resulted in effluent discharges to Mortandad Canyon containing inorganic and organic chemicals and radionuclides.

The Mortandad Canyon HFM was constructed using multiple types of field data to constrain the intricate stratigraphy that underlies the watershed. Bandelier Tuff from the Jemez volcanic field overlies interbedded Pliocene basalt and alluvial fan deposits and Miocene sedimentary deposits resulting in heterogeneous and discontinuous hydrostratigraphic units. These units control the migration of pore waters and dissolved contaminants through a thick (approx. 245-m) vadose zone. Multiple data integration and gridding techniques were utilized to render the 3D aquifer framework consisting of a shallow alluvial system, intermediate vadose zone containing localized perched ground water, and a deep regional aquifer. The HFM illustrates that the alluvial aquifer is a losing system due to preferential infiltration resulting in higher pore water contents extending downward towards two perched-intermediate ground water zones associated with dense basalt units. The HFM facilitated the calculation of contaminant mass inventories utilizing grids that represent the distribution of volumetric water content and associated contaminants (specifically nitrate and perchlorate). The mass inventories were within 15% of historic release estimates, indicating that the model accounts for the bulk of contaminant mass residing in the subsurface.

The HFM is presently being used to convey the site conceptual models of pore water and contaminant distribution to LANL regulators and the general public, facilitating their participation in critical decisions about site characterization and remediation. Future HFM applications may include supporting risk-based modeling and remedial design evaluations.