<p>Mountain system recharge (MSR) represents an important source of water for many basin aquifers that border mountains. However, characterizing the 3D spatial distribution of water inputs remains a significant challenge. This study employed water stable isotopes to investigate sources of recharge for the Denver Basin Aquifer System (DBAS), a series of vertically stacked sandstone aquifers adjacent to the Rocky Mountain Front Range. The sampling network spanned the mountain–basin interface and included samples of precipitation, mountain-block groundwater, basin aquifer groundwater, and basin stream water. While precipitation exhibited large seasonal variability (δ¹⁸O standard deviation of 7.3 ‰ at one site with a 12-month record), the isotopic composition of mountain-block groundwater was remarkably stable (δ¹⁸O standard deviation of 0.03 ‰ at the same site). Basin aquifer samples were analyzed to identify the input of mountain-block groundwater for basin aquifer units at different depths. Results show a clear connection between the mountain block and the Dawson aquifer, the uppermost sedimentary rock aquifer unit in the DBAS, as evidenced by low δ²H and δ¹⁸O values that overlap with mountain-block groundwater values. Deeper aquifer units, notably the Denver and Arapahoe aquifers, were characterized by higher δ²H and δ¹⁸O values, suggesting a recharge source other than modern MSR within the study area. This study highlights the importance of directly sampling mountain-block groundwater for MSR studies that utilize environmental tracers. Results for the DBAS show clear variation in recharge inputs with depth, providing an improved understanding of the hydrogeologic connectivity between the mountain block and basin aquifer system.</p>