GROUNDWATER RECORD OF HALOCARBON TRANSPORT BY THE DANUBE RIVER

Groundwater dating studies have supported the concept that aquifers wi th low coefficients of dispersion may contain coherent records of past conditions in recharge areas. Groundwater records can provide unique information about natural or anthropogenic changes in the atmosphere a nd hydrosphere where long-term monitoring data are not available. Here we describe a 40-year record of halocarbon contamination in the Danub e River that was retrieved from a shallow aquifer in northwest Hungary . The time scale is based on H-3 and He isotope dating of groundwaters that were recharged by the Danube River and moved horizontally away f rom the river in a surficial gravel aquifer with minor dispersion at a maximum rate of at least 500 m/yr. Analyses of dated groundwaters alo ng a flow path indicate that the river loads of selected compounds (in cluding CFC-12, CFC-113, and trichloroethane) were negligible before a bout 1950, rose rapidly to peak values in the 1960s and 1970s, and the n decreased by varying degrees to the present. Peak concentrations are tentatively attributed to point sources in upstream urban-industrial centers; while recent decreases presumably resulted from declining man ufacturing rates and(or) improvements in control of urban-industrial r unoff and sewage effluent entering the river in upstream areas.