A validation of the 3H/3He method for determining groundwater recharge

Tritium and He isotopes have been measured at a site where groundwater flow is nearly vertical for a travel time of 100 years and where recharge rates are spatially variable. Because the mid‐1960s 3 H peak (arising from aboveground testing of thermonuclear devices) is well‐defined, the vertical groundwater velocity is known with unusual accuracy at this site. Utilizing 3 H and its stable daughter 3 He to determine groundwater ages, we compute a recharge rate of 0.16 m/yr, which agrees to within about 5% of the value based on the depth of the 3 H peak (measured both in 1986 and 1991) and two‐dimensional modeling in an area of high recharge. Zero 3 H/ 3 He age occurs at a depth that is approximately equal to the average depth of the annual low water table, even though the capillary fringe extends to land surface during most of the year at the study site. In an area of low recharge (0.05 m/yr) where the 3 H peak (and hence the vertical velocity) is also well‐defined, the 3 H/ 3 He results could not be used to compute recharge because samples were not collected sufficiently far above the 3 H peak; however, modeling indicates that the 3 H/ 3 He age gradient near the water table is an accurate measure of vertical velocities in the low‐recharge area. Because 3 H and 3 He have different diffusion coefficients, and because the amount of mechanical mixing is different in the area of high recharge than in the low‐recharge area, we have separated the dispersive effects of mechanical mixing from molecular diffusion. We estimate a longitudinal dispersivity of 0.07 m and effective diffusion coefficients for 3 H ( 3 HHO) and 3 He of 2.4×10 −5 and 1.3×10 −4 m 2 /day, respectively. Although the 3 H/ 3 He age gradient is an excellent indicator of vertical groundwater velocities above the mid‐1960s 3 H peak, dispersive mixing and diffusive loss of 3 He perturb the age gradient near and below the 3 H peak.