PAGEOPH, Vol. 120 (1982) 0033-4553/82/020330-18501.50 + 0.20/0 9 1982 Birkh~iuser Verlag, Basel Estimation of the Percentage of Annual Groundwater Recharge with Bomb Tritium Using a Cumulative Mass Balance Method By BONIFACE C. E. EGBOKA1 JOHN A. CHERRY 2 and ROBERT N. FARVOLDEN 2 Abstract - The bomb tritium (3H) distribution patterns in the aquifer beneath an abandoned landfill at the Canadian Forces Base (CFB) Borden, Ontario, and in a sandy aquifer at Whiteshell Nuclear Research Establishments (WNRE) Pinawa, Manitoba, all in Canada, were delineated in great detail. A sampling and monitoring network of multilevel samplers and bundle piezometers were used. The directions of groundwater flow were established, and the boundary between the tritiated and non-tritiated zones of the two aquifers were closely demarcated. Using a cumulative mass balance method, the 3H input mass into the aquifers was compared with the 3H mass in groundwater storage to estimate the percentages of annual groundwater recharge from 1953 to 1978. Two recharge calculations for the effective recharge zone and the total recharge area of the aquifers as established from the flownet analysis, and the distributions of dissolved geochemical constituents showed that the effective recharge zone calculations gave higher values of 30.6 cm/yr for CFB Borden and 20.1 cm/yr for WNRE while the total recharge areas gave lower values of 19.1 and 10.1 cm/yr for the Borden and WNRE aquifers respectively. The two recharge values provide possible minimum and maximum recharge estimates for the two study areas. Keywords: Groundwater recharge; Bomb tritium in groundwater. Introduction The distribution patterns of bomb 3H in two unconfined sandy aquifers at an abandoned landfill site at Canadian Forces Base (CFB) Borden, Ontario, and a recharge area at Whiteshell Nuclear Research Establishments (WNRE) Pinawa, Manitoba, all in Canada were investigated. Seven hundred sampling points were installed at various locations and depths at the Borden aquifer while 650 points were similarly mounted at WNRE. The sampling and monitoring network consisted of multilevel samplers (PICKENS et al., 1978) and bundle piezometers (EGBOKA, 1980). The sampling and monitoring network was used to establish well-defined boundaries between the tritiated and non-tritiated zones of the aquifers. The suction-technique of sampling and the small diameter of the intake zones of both samplers, 0.0028 and 0.0095 m for the multilevel samplers and bundle piezometer respectively, ensured that little or no mixing occurred between waters at different depths in the aquifers during sampling. Groundwater samples were also collected at narrowly-spaced depths. The tritiated zones of the aquifers were then used to estimate the percentage of the annual groundwater recharge. 1) Department of Geology, University of Nigeria, Nsukka, Nigeria. 2) Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3GI.