Early warning systems for deep groundwater exploitation based on environmental tracer measurements

This report demonstrates the possibility of the application of environmental-isotope measurements to construct 'early-warning system (EWS)'. EWS should help to prevent changes of water quality in deep groundwater bodies subject to long term exploitation. A systematic numerical study of flow and transport patterns was performed in system having a depth of 400 m and length of 15 km, and stands for a 'regional aquifer' with stable water divides and constant recharge of 150 mm/y. Results of modelling were shown in more detail for one representative class of multi-layered aquifer. The broader representation underlying the class of models for a shallow aquifer highly exposed to (diffuse-source) contaminations (mainly from agriculture) separated from a deep, 'protected' aquifer by a poorly-conductive layer, with hydraulic conductivity contrasts in the range (from top to bottom) 100:1:10 or 1000:1:10 and with layer height ratios 1:2:5. The anisotropy factor kz/kx=4 was assumed. The discharge boundary was approximated by a perfect hydraulic contact over the whole depth underneath the effluent. Generally, it was first more important to obtain qualitative picture of the system's behaviour during, and following, long-lasting water abstractions from the deep layer. In most cases the aquifer was exploited at constant rate for three decades then allow it to relax, and simulate its recovery for further three decades. The abstraction volumes of 10%, 20%, 30% of total recharge were considered. The comparison of turnover balances for various abstraction ratios has shown that, with increasing pumping rate, ever more water is drawn from the upper (contaminated) to the lowest (allegedly protected) layer. Disrupted turnover balances in the presence of pumping were not without effect upon the downward migration of contaminants from the shallow aquifer and it is of particular relevance in the case of persistent components of diffuse-source agrochemicals. It was shown how such a component would, over decades, migrate towards greater depths, and how its migration is accelerated by pumping water out of the deep layer. The computer simulations have shown that 39Ar concentrations at depths of 150-300 m are the adequate indicators for an early warning system. Simultaneously it is shown that tritium measurements can be used for the EWS purposes when the monitoring net is installed in the depths of 100-150 m. (author)