Remediation of a Uranium-Contamination in Ground Water

Abstract

The former production site of NUKEM where nuclear fuel-elements were developed and handled from 1958 to 1988 was situated in the centre of an industrial park for various activities of the chemical and metallurgical industry. The size of the industrially used part is about 300.000m2. Regulatory routine controls showed elevated CHC (Chlorinated Hydro-Carbons) values of the ground water at the beginning of the 1990’s in an area which represented about 80.000 m2 down-gradient of locations where CHC compounds were stored and handled. Further investigations until 1998 proved that former activities on the NUKEM site, like the UF6 conversion process, were of certain relevance. The fact that several measured values were above the threshold values made the remediation of the ground water mandatory. This was addressed in the permission given by the Ministry for Nuclear Installations and Environment of Hesse according to §7 of the German atomic law in October 2000 [1]. Ground water samples taken in an area of about 5.000 m2 showed elevated values of total Uranium activity up to between 50 and 75 Bq/l in 2002. Furthermore in an area of another 20.000m2 the samples were above threshold value. In this paper results of the remediation are presented. The actual alpha-activities of the ground waters of the remediation wells show values of 3 to 9Bq/l which are dominated by 80 to 90% U-234 activity. The mass-share of total Uranium for this nuclide amounts to 0,05% on average. The authority responsible for conventional water utilisation defined target values for remediation: 20μg/l for dissolved Uranium and 10μg/l for CHC [2]. Both values have not yet been reached for an area of about 10.000 m2. The remediation process by extracting water from four remediation wells has proved its efficiency by reduction of the starting concentrations by a factor of 3 to 6. Further pumping will be necessary especially in that area of the site where the contaminations were found later during soil remediation activities. Only two wells have been in operation since July 2002 when the remediation technique was installed and an apparatus for direct gamma-spectroscopic measurement of the accumulated activities on the adsorbers was qualified. Two further remediation wells have been in operation since August 2006, when the installed remediation technique was about to be doubled from a throughput of 5 m3/h to 10 m3/h. About 20.000 m3 of ground water have been extracted since from these two wells and the decrease of their Uranium-concentrations behaves similar to that of the two other wells being extracted since the beginning of remediation. Both, total Uranium-concentrations and the weight-share of the nuclides U-234, U-235 and U-238 are measured by ICP-MS (Inductively Coupled Plasma – Mass Spectrometry) besides measurements of Uranium-Alpha-Activities in addition to the measurement of CHC components of which PCE (Perchlor-Ethene) is dominant in the contaminated area. CHC compounds are measured by GC (Gas Chromatography). Down-gradient naturally attenuated products are detected in various compositions. Overall 183.000m3 of ground water have been extracted. Using a pump & treat method 11 kg Uranium have been collected on an ion-exchange material based on cellulose, containing almost 100 MBq U-235 activity, and almost 15 kg of CHC, essentially PCE, were collected on GAC (Granules of Activated Carbon). Less than 3% of the extracted Uranium have passed the adsorber-system of the remediation plant and were adsorbed by the sewage sludge of the industrial site’s waste water treatment. The monthly monitoring of 19 monitoring wells shows that an efficient artificial barrier was built up by the water extraction. The Uranium contamination of two ground water plumes has drastically been reduced by the used technique dependent on the amounts of extracted water. The concentration of the CHC contamination has changed depending on the location of temporal pumping. Thereby maximum availability of this contaminant for the remediation process is ensured. If locations with unchanged water quality are detected electrochemical parameters of the water or hydro-geologic data of the aquifer have to be taken into further consideration to improve the process of remediation.