A VALIDATION OF THE H-3 HE-3 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 rech arge rates are spatially variable. Because the mid-1960s H-3 peak (ari sing from aboveground testing of thermonuclear devices) is well-define d, the vertical groundwater velocity is known with unusual accuracy at this site. Utilizing H-3 and its stable daughter He-3 to determine gr oundwater 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 H-3 peak (me asured both in 1986 and 1991) and two-dimensional modeling in an area of high recharge. Zero H-3/He-3 age occurs at a depth that is approxim ately equal to the average depth of the annual low water table, even t hough 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 t he H-3 peak (and hence the vertical velocity) is also well-defined, th e H-3/He-3 results could not be used to compute recharge because sampl es were not collected sufficiently far above the 3H peak; however, mod eling indicates that the H-3/He-3 age gradient near the water table is an accurate measure of vertical velocities in the low-recharge area. Because H-3 and He-3 have different diffusion coefficients, and becaus e the amount of mechanical mixing is different in the area of high rec harge than in the low-recharge area, we have separated the dispersive effects of mechanical mixing from molecular diffusion. We estimate a l ongitudinal dispersivity of 0.07 m and effective diffusion coefficient s for H-3 ((HHO)-H-3)) and He-3 of 2.4 x 10(-5) and 1.3 x 10(-4) m/day , respectively. Although the 3H/3He age gradient is an excellent indic ator of vertical groundwater velocities above the mid-1960s 3H peak, d ispersive mixing and diffusive loss of He-3 perturb the age gradient n ear and below the H-3 peak.