The study concluded that although considerable unexplained variability remained, marijuana strontium isotope ratios did retain a primary geologic signal based on bedrock age. This suggests that strontium isotopes in marijuana have significant promise for application to geographic sourcing. The sensitivity and accuracy of these procedures can be improved through finer-scale sampling of plants and soils, so as to establish local signatures prior to an application to unknowns. In addition, common origins could be inferred from similar strontium isotope ratios, even absent additional local information. This would occur if the range of possible origins could be constrained, such that a unique isotope ratio could be identified for a given location. This study analyzed the 87Sr/86Sr of marijuana samples grown in 79 different counties across the United States, using thermal ionization mass spectrometry. These results were compared with modeled bedrock 87Sr/86Sr values based on rubidium decay rates and a generalized geologic map provided by the United States Geological Survey (USGS). Strontium isotope ratios vary spatially, with this variation largely dependent on soil mineralogy, specifically the time since crystallization and Rb content; however, atmospheric deposition, complex soil-forming processes, and fertilizer inputs all have the potential to obscure the expected geographic signal. 3 figures, 2 tables, 50 references, and appended references for published 87Sr/86Sr values for soils or rocks from within sample county-of-origin for Oregon and Tennessee for those sample counties with published data