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Abstract. Combined use of the tritium/helium 3 (3H/3He) dating technique and particletracking 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 3H/3He age dating and particle tracking led 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 3H/3He ages. Apparent groundwater ages were obtained for water samples

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Simulating groundwater flow in a water-table (unconfined) aquifer can be difficult because the saturated thickness available for flow depends on model-calculated hydraulic heads. It is often possible to realize substantial time savings and still obtain accurate head and flow solutions by specifying an approximate saturated thickness a priori, thus linearizing this aspect of the model. This specified-thickness approximation often relies on the use of the "confined" option in numerical models, which has led to confusion and criticism of the method. This article reviews the theoretical basis for the specified-thickness approximation, derives an error analysis for relatively ideal problems, and illustrates the utility of the approximation with a complex test problem. In the transient version of our complex test problem, the specified-thickness approximation produced maximum errors in computed drawdown of about 4% of initial aquifer saturated thickness even when maximum drawdowns were nearly 20% of initial saturated thickness. In the final steady-state version, the approximation produced maximum errors in computed drawdown of about 20% of initial aquifer saturated thickness (mean errors of about 5%) when maximum drawdowns were about 35% of initial saturated thickness. In early phases of model development, such as during initial model calibration efforts, the specified-thickness approximation can be a very effective tool to facilitate convergence. The reduced execution time and increased stability obtained through the approximation can be especially useful when many model runs are required, such as during inverse model calibration, sensitivity and uncertainty analyses, multimodel analysis, and development of optimal resource management scenarios.

iv "Water Budget Myth" (Bredehoeft, 1997). Even further misinterpretations sometimes are made that if groundwater withdrawals are less than the recharge rate, then water levels and groundwater storage will not decline. To understand the fallacy of the misperception described above, one needs to consider how groundwater systems respond to withdrawals. Under natural (non-pumping) conditions, a groundwater system is in long-term equilibrium. That is, averaged over some period (and without considering possible long-term climate change), the amount of water recharging the system is approximately equal to the amount of water leaving or discharging from the system. Groundwater withdrawals by pumping change the flow system. The water that is withdrawn must be supplied by some combination of (1) increased recharge, (2) decreased discharge, and (3) removal of water that was stored in the system. Regardless of the amount of water withdrawn, the groundwater system will undergo some drawdown in water levels in pumping wells to induce the flow of water to these wells. Thus, some water is removed from storage. For most groundwater systems, the change in storage in response to pumping is a transient phenomenon that occurs as the system adjusts to the withdrawals. The relative contributions of changes in storage, changes in recharge, and changes in discharge evolve with time. If the system can come to a new equilibrium, the changes in storage will diminish to zero (at a new reduced level of groundwater storage), and inflows will again balance outflows. Thus, the long-term source of water to discharging wells becomes a change in the amount of water entering or leaving the system. The time that is required to bring a groundwater system into equilibrium is a function of the characteristics of the aquifer system, such as permeability, thickness, distance to hydrologic boundaries, and the placement of pumping wells. In summary, natural recharge is a critical element for understanding the water balance, an important parameter in many computer-model simulations of aquifer systems, and is fundamental to understanding contaminant transport from the land surface. All of these aspects may factor into an analysis of the sustainability of a groundwater system. An estimate of natural recharge, by itself, however, should not be used to determine the amount of ground water that can be withdrawn on a sustained basis. The quantity of ground water available for use depends more upon how the changes in inflow and outflow that result from withdrawals affect the surrounding environment and the acceptable tradeoff between groundwater use and these changes. Achieving this tradeoff in the long term is a central theme in the evolving concept of sustainability (Alley and others, 1999; Alley and Leake, 2004).