NUCLEAR-DETONATION ALTERATION OF HYDROGEOLOGY: LARGE-SCALE, POST-BLAST EFFECT ON FLOW AND TRANSPORT

Underground nuclear detonations have the potential to alter regional hydrogeology by rupturing confining units. The altered hydrogeology can permanently change the rate and direction of groundwater flow, which can affect radionuclide migration. At Pahute Mesa, on the Nevada National Security Site, large-scale effects of underground nuclear testing on the groundwater system, such as altered hydrogeology and permanent changes to groundwater levels, have gone unnoticed until recently (2018). This study documents a large-scale, underground, nuclear-test effect, termed a “breaching event”. A breaching event occurs when a nuclear detonation fractures or ruptures a confining unit that hydraulically separates two aquifers with different heads. A period of transient flow occurs after the confining unit is ruptured, during which time hydraulic head equilibrates between the affected aquifers. Groundwater velocities temporarily increase as water flows through the new pathways. Hydraulic and nuclear-test data are presented that support the occurrence of a breaching event at Pahute Mesa. A numerical model simulating a breaching event illustrates the nature of the transient flow field and the changes in groundwater velocities following a breach. An understanding of breaching events can lead to better forecasts of radionuclide migration away from underground nuclear test sites.