Underground nuclear testing via deep
vertical shafts was conducted at the Nevada Test Site (NTS) from 1951 until
1992. The Pahute Mesa area of the NTS was used for 27 years, and was an area
where many of the larger yield tests were conducted. As a consequence of testing,
radionuclides were left behind in unsaturated and saturated rocks. The Underground
Testing Area (UGTA) Project is currently conducting characterization investigations
to ensure the protection of the public and the environment, and one of these
activities is the construction and calibration of a flow model, which is described
here. The Pahute Mesa area has a great depth to groundwater, and exhaustive
characterization is not possible. The issue is further compounded by the complexity
associated with multiple calderas and basin-andrange faulting in the area. In
addition, recharge is highly uncertain in this arid region. Thus, two highlevel
conceptual uncertainties, in the geologic framework and in the water balance,
are present. The UGTA Project recognizes this high-level uncertainty, and has
addressed it by considering a matrix of 7 geologic models and 3 estimates of
recharge from widely differing methods (chloride mass balance, empirical Maxey-Eakin,
and distributed rainfall-runoff). Fifteen different combinations of geologic
and recharge models were calibrated.
Several of the alternative models produced flow fields that, considering calibration
metrics alone, were nearly indistinguishable from the expected model. However,
some of the models were substantially different. Extensive use of automated
parameter estimation software (PEST) to run the FEHM forward models was required,
along with the use of distributed computing clusters.