Modeling and prediction of the natural decontamination of the mining-impacted Geul River floodplain

Abstract

Heavy metal contamination due to historical metal mining is an important problem in many rivers around the world. The contamination is very persistent and it is accumulated and spread in the floodplain sediments. This paper presents a study of the mining-impacted Geul River in The Netherlands. The Geul is a meandering river which has been contaminated since Roman times due to metal mining. Especially during the 19th and 20th century zinc and lead mining in Belgium caused contaminated sediments to be deposited on the floodplains of the Geul River.

This study aims to investigate and predict the temporal dimensions of the natural decontamination of zinc and lead at the Geul River floodplain between Cottessen and Meerssen.

The area of study is the floodplain of the Geul River between Cottessen situated at the Dutch- Belgian border and Meerssen located close to its confluence with the Meuse River, approximately the total channel length of the area of study is 20 km. Nine transects along the catchment were chosen out to cover the 20 km. The transects were separately every 4 km in the downstream direction. Sediment cores were taken along each transect every 10 cm in a vertical profile of an average of 1.5 meters depth. The soil samples were later on analyzed to obtain the concentration of zinc and lead with the use of a hand-held X-Ray fluorescence spectrometer. Results from the soil sample analysis were used in a regression analysis to provide information of the actual situation of contamination.

CAESAR model (Cellular Automaton Evolutionary Slope and River Model) was used to simulate two scenarios in which erosion, deposition and remobilization of sediments occur. Two different case scenarios were used to predict the decontamination of 70% of the total excess of contamination of the floodplain. First case scenario used results from regression analysis and sediment output from CAESAR model simulation. Second case scenario used a map of total excess of contamination and an output map of elevation differences from CAESAR model simulation. By the use of an exponential decay formula prediction of the natural decontamination of 70% of the total excess contamination was calculated and compared with the persistence of the metal contamination of a study at River Swale, Northern England.

Results of fieldwork and regression analysis show that upstream floodplain areas contain higher contamination in lead and zinc than downstream areas. Nevertheless, although upstream sediments are more contaminated downstream sediments also show a noticeable content in lead and zinc contamination indicating that the contamination has been already highly widespread along the sediments of the Geul. According with the first case scenario in order to decontaminate 70% of the actual heavy metal contamination of the Geul floodplain will take 1241 years for lead and 1095 years for zinc. The results from the second case scenario are 2140 years for lead and 1745 years for zinc. Therefore, both results show that for the Geul reach to be higher than 70% decontaminated it will take more than thousands years, indicating the environmental problem which causes metal mining at the banks of the river.