New Mexico Geological Society Annual Spring Meeting — Abstracts

Mortandad Canyon receives effluent from Los Alamos National laboratory's liquid radioactive-waste treatment facility at Technical Area 50 (TA-50). As the stream does not normally leave the canyon, the fate of the effluent is a concern. Although the hydrology of Mortandad Canyon has been fairly well studied, including some modeling it is still not completely understood. Thus a modeling project was undertaken. The first phase, reported here, consisted of a steady-state simulation of the shallow aquifer in one dimension for pre-TA-50 conditions. The model focused on the alluvium between wells MCO-1 and MCO-8, which was discretized as 1 layer, 1 column (scaled for valley cross-sectional dimensions) and 30 rows. MODFLOW, utilizing the recharge, evapotranspiration (ET) and streamflow routing packages, was the code used. With ET maximized and no leakage to the underlying tuff allowed, the model showed that the stream flowed the entire length of the canyon. As this does not normally occur in nature and ET was maximized to a reasonable value for the vegetation present, the volume of water the model showed to be leaving the system via streamflow (0.185 cfs) must be dispersed either by leakage or underflowing ground water. Since no underflow has been detected in previous studies, the excess water must be lost through downward leakage. This project showed the need for additional data on various parameters: runoff or interflow from slopes to the canyon floor; distribution, ET rate and extinction depth for vegetation types; and hydraulic conductivity of the alluvium. The next phase of the project, involving a fully three-dimensional simulation of the system, should eliminate the constraints placed on this model by using a single layer and column.