Carbon dioxide emissions from active subaerial volcanoes represent 20–50% of the annual global volcanic CO₂ flux (Barry et al., 2014). Passive degassing of carbon from the flanks of volcanoes, and the associated accumulation of dissolved inorganic carbon (DIC) within nearby groundwater, also represents a potentially important, yet poorly constrained flux of carbon to the surface (Werner et al., 2019). Here we investigate sources and sinks of DIC in groundwaters in the Lassen Peak region of California. Specifically, we report and interpret the relative abundance and isotopic composition of helium (³He, ⁴He) and carbon (¹²C, ¹³C, ¹⁴C) in 37 groundwater samples, from 24 distinct wells, collected between 20 and 60 km from Lassen Peak. Measured groundwater samples have air-corrected ³He/⁴He values between 0.19 and 7.44 R_A (where R_A = air ³He/⁴He = 1.39 × 10⁻⁶), all in excess of the radiogenic production value (~0.05 R_A), indicating pervasive mantle-derived helium additions to the groundwater system in the Lassen Peak region. Stable carbon isotope ratios of DIC (δ¹³C) vary between −12.6 and − 27.7‰ (vs. VPDB). Measured groundwater DIC/³He values fall in the range of 2.2 × 10¹⁰ to 1.1 × 10¹². Using helium and carbon isotope data, we explore several conceptual models to estimate surface carbon contributions and to differentiate between DIC derived from soil CO₂ versus DIC derived from external (slab and mantle) carbon sources. Specifically, if we use ¹⁴C to identify soil-derived DIC (assuming decadal-to-centennial groundwater ages and a soil CO₂ ¹⁴C activity equal to that of the atmosphere), we calculate that a hypothetical external carbon source would have an apparent δ¹³C signature between −10.3 and − 59.3‰ (vs. Vienna Pee Dee Belemnite (VPDB)) and an apparent C/³He between 7.0 × 10⁹ and 1.0 × 10¹². These apparent δ¹³C and C/³He values are substantially isotopically lighter than and greater than canonical MORB values, respectively. We suggest that >95% of any external (non-soil-derived) DIC in groundwater must thus be non-mantle in origin (i.e., slab derived or assimilated organic carbon). We further investigate possible sources of external DIC to groundwater using two idealized conceptual approaches: a pure (unfractionated) source mixing model (after Sano and Marty, 1995) and a scenario that invokes fractionation due to calcite precipitation. Because the former model requires carbon contributions from an organic source component with unrealistically low δ¹³C (~ − 60‰), we suggest that the second scenario is more plausible. Importantly, however, we caution that all conceptual models are dependent on assumptions about initial ¹⁴C activity. Thus, we cannot rule out the possibility that the true fraction of non-surface-derived DIC in these samples is lower or negligible, despite the pervasive mantle-derived He isotope signatures throughout the region. Following the ¹⁴C approach to deconvolving sources of DIC, we determine that the maximum passive carbon flux could be up to ~2.2 × 10⁶ kg/yr, which is lower than previous magmatic carbon flux estimates from the Lassen region (Rose and Davisson, 1996). We find that the passive dissolved carbon flux could represent a maximum of ~4–18% of the total Lassen geothermal CO₂ degassing flux (estimated to be ~3.5 × 10⁷ kg/yr Rose and Davisson, 1996; Gerlach et al., 2008), which is still more than an order of magnitude smaller than soil gas CO₂ flux estimates (7.3–11 × 10⁷ kg/yr) for nearby volcanoes (Sorey et al., 1998; Gerlach et al., 1999; Evans et al., 2002; Werner et al., 2014). We conclude that passive dissolved carbon fluxes should be combined with geothermal fluxes and soil gas fluxes to obtain a complete picture of volcanic carbon emissions globally. Our approach highlights the utility of measuring helium isotopes in concert with the full suite of noble gas abundances, tritium, δ¹³C and ¹⁴C, which when interpreted together can be used to better elucidate the various sources of DIC in groundwater.