USE OF H-3 HE-3 AGES TO EVALUATE AND IMPROVE GROUNDWATER-FLOW MODELS IN A COMPLEX BURIED-VALLEY AQUIFER/

Combined use of the tritium/helium 3 (H-3/He-3) dating technique and p article-tracking analysis can improve flow-model calibration. As shown at two sites in the Great Miami buried-valley aquifer in southwestern Ohio, the combined use of H-3/He-3 age dating and particle tracking l ed to a lower mean absolute error between measured heads and simulated heads than in the original calibrated models and/or between simulated travel times and H-3/He-3 ages. Apparent groundwater ages were obtain ed for water samples collected from 44 wells at two locations where pr eviously constructed finite difference models of groundwater flow were available (Mound Plant and Wright-Patterson Air Force Base (WPAFB)). The two-layer Mound Plant model covers 11 km-within the buried-valley aquifer. The WPAFB model has three layers and covers 262 km(2) within the buried-valley aquifer and adjacent bedrock uplands. Sampled wells were chosen along flow paths determined from potentiometric maps or pa rticle-tracking analyses. Water samples were collected at various dept hs within the aquifer. In the Mound Plant area, samples used for compa rison of H-3/He-3 ages with simulated travel times were from wells com pleted in the uppermost model layer. Simulated travel times agreed wel l with H-3/He-3 ages. The mean absolute error (MAE) was 3.5 years. Agr eement in ages at WPAFB decreased with increasing depth in the system. The MAEs were 1.63, 17.2, and 255 years for model layers 1, 2, and 3, respectively. Discrepancies between the simulated travel times and H- 3/He-3 ages were assumed to be due to improper conceptualization or in correct parameterization of the flow models. Selected conceptual and p arameter modifications to the models resulted in improved agreement be tween H-3/He-3 ages and simulated travel times and between measured an d simulated heads and flows.