Characterizing the dynamics of groundwater flow in alosing stream
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For quantitative reasons, drinking water wells are often located close to rivers, resulting ina large fraction of freshly infiltrated river water in the extracted water. Rivers bear the riskof being contaminated, e.g. by runoff from agriculture, and may thus contaminate the waterextracted from the well. Soil acts as a decontamination medium, where transport processes as advection, diffusion and mechanical dispersion lead to a dilution of the contamination,and microbial activity leads to degradation of organic pollutants. Therefore, travel times and fraction of freshly infiltrated water are key parameters in the management of alluvial aquifers. Time series of natural tracers as temperature and electric conductivity (EC) from a research field located in Niederneunforn, North-East Switzerland, have been analyzed to determine travel times of freshly infiltrated groundwater. Cross-correlation and non-parametric deconvolution were performed to quantify mean residence time and mixing ratio. Nonparametric deconvolution yielded, in addition, the travel time distribution curve which reflects the transport processes along the flow path. Radon-222 was used as an independent measurement to determine groundwater residence times. Hydraulic conductivity and hydraulic gradient are parameters that affect the travel times, and were determined by performing slug tests and water level measurements in selected observation wells. The chemistry in the groundwater is dependent on the residence time due to mixing with older infiltrated water and mineralization of the water; The degree of mineralization was interpreted with linear regression of the cross-correlation results. Furthermore, the concentration of the major ions from 18 water samples were compared with the residence times along the flow path. Results from the time series analysis showed increased travel time with increasing distance to the river. However, not all the wells agreed to this. Slug tests indicated a heterogeneityin the hydraulic conductivity in these wells. The time series results were validated by the calculated residence time from the radon concentrations in most of the wells, although some wells deviated from this. All the observation wells contained more than 50% fresh infiltrated river water. Calcium and bicarbonate dominated the water chemistry, and followed the pattern of increasing travel times along the flow path.