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<p>Carbon-14 (<span class="inline-formula">¹⁴</span>C) is routinely used to determine mean residence times (MRTs) of groundwater. <span class="inline-formula">¹⁴</span>C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input <span class="inline-formula">¹⁴</span>C activity. However, multiple studies have shown that unsaturated zone <span class="inline-formula">¹⁴</span>C activities are lower than atmospheric values. Despite the availability of unsaturated zone <span class="inline-formula">¹⁴</span>C data, no attempt has been made to generalise initial <span class="inline-formula">¹⁴</span>C activities with depth to the water table. We utilise measurements of unsaturated zone <span class="inline-formula">¹⁴</span>C activities from 13 studies to produce a <span class="inline-formula">¹⁴</span>C–depth relationship to estimate initial <span class="inline-formula">¹⁴</span>C activities. The technique only requires the depth to the water table at the time of sampling or an estimate of depth to water in the recharge zone to determine the input <span class="inline-formula">¹⁴</span>C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 <span class="inline-formula">¹⁴</span>C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the <span class="inline-formula">¹⁴</span>C–depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of unsaturated zone <span class="inline-formula">¹⁴</span>C to MRTs derived from <span class="inline-formula">¹⁴</span>C data.</p>