Calibration and verification of a regional groundwater flow model by comparing simulated and measured environmental isotope concentrations. An application to the Alnarp aquifer system in southwestern Sweden

Environmental isotope studies and computerized groundwater flow modelling and radioisotope transport modelling have been applied to the large system of reservoirs in the sedimentary deposits of southwestern Scania, Sweden. The stable isotopes 2H, 18O and 13C and the radioactive 3H and 14C have been measured and the results obtained can improve the estimations of the periods of recharge and the average circulation times of the groundwater reservoirs studied. A groundwater flow model based on finite difference techniques and a continuum approach has been modified by data from traditional hydrogeological studies. A calibration against piezometric records has been made for 1970, assuming steady-state conditions, and also for the transient evolution from 1840 to 1988. The computer code NEWSAM has also been used to simulate steady-state and transient isotope transport in the area studied, taking into account advective transport with radioactive decay. The interacting groundwater reservoirs studied have been represented by a three-dimensional grid in the numerical model. A major merit of this combination of isotope hydrogeology and regional flow and transport modelling is that the isotope transport simulations help to demonstrate where zones particularly vulnerable to pollution are situated. However, there are several difficulties concerning the detailed, quantitative interpretations of the results, and these difficulties must be the focus of future work. Even if the aquifer system studied here can be classified as unusually homogeneous and well-defined, the number of isotope measurements is small compared to the extension of the system. Furthermore, the calculated concentrations are similar over large areas. Therefore, it is at present not possible to calibrate the isotope transport model by measured isotope concentrations in order to verify or reject the results of the underlying groundwater flow model. (author). 38 refs, 13 figs, 1 tab