SEAWAT code used to couple MODFLOW and MT3DMS models, and supplemental data used to simulate groundwater flow and tritium transport from the HANDLEY underground nuclear test, Pahute Mesa, southern Nevada

Three-dimensional numerical models were used to determine the permeable pathways
between the HANDLEY underground nuclear test and downgradient boreholes ER-20-12
and PM-3, Pahute Mesa, southern Nevada. The SEAWAT code was used to couple
MODFLOW and MT3DMS models, where the coupled model simulated groundwater-flow
and tritium migration from the HANDLEY test for 50 years, from the date of detonation
of the HANDLEY test (March 26, 1970) to March 26, 2020. Recharge, hydraulic-conductivity,
specific-storage, and effective-porosity distributions were estimated using parameter
estimation (PEST) by minimizing a weighted composite, sum-of-squares objective function.
Simulated water-level altitudes in 10 wells, vertical water-level differences between 5 well
pairs, aquifer-test transmissivity estimates in 9 wells, tritium concentrations in 7 wells,
and drawdowns in wells PM-3-1 and PM-3-2 from groundwater withdrawals in borehole
ER-20-12 were compared to measured equivalents during parameter estimation and
formally defined the goodness-of-fit or improvement of calibration. This USGS data release
contains data, analyses, and model files for the simulations and analyses described in
the associated model documentation report (https://doi.org/10.3133/sir20215032).

Supplementary data and analyses are in the ancillary directory, including transmissivity
estimates (Appendix A); water-level models to estimate drawdowns (Appendix B); tritium
data (Appendix C); the modified hydrostratigraphic framework in the numerical models
(Appendix D); effective-porosity estimates (Appendix E); the Pahute Mesa–Oasis Valley
groundwater model (Appendix F); model-calibration files (Appendix G); and pre-processing
routines to build model files (Appendix H).

The numerical model was calibrated using a multi-model approach and a two-step process.
First, the groundwater model was calibrated, which consists of (1) a steady-state model
simulating steady-state (predevelopment) groundwater flow; and (2) a transient model
simulating drawdowns from groundwater withdrawals during the drilling of borehole ER-20-12.
A transport model was developed that uses the best-fit estimated hydraulic-conductivity
field from the calibrated groundwater model and was used to estimate dispersion and effective
porosities.