Recharge systems for protecting and enhancing groundwate

TOPIC 1
River/lake bank filtration and pond infiltration issues / Recharge systems 19
diffusive heat transport within the model. The calibration of the heat transport parameters enabled the modelling of
further hydrochemical processes linked to temperature.
The absorption capacity versus the temperature of the aquifer has a significant impact on the drinking water. The
mixing with the landside groundwater which showed a constant temperature of 13 °C leads to a raw water temperature
less than 17 °C. Despite the exceptional long period with high river water temperatures in the summer of
2003 the obtained drinking water remained cool and fresh.
30
25
measured values Rhine
model calculation conservative transport
measured values bank filtrate
model calculation temperature retardation
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Temperature [˚C]
15
10
5
0
Jan. 03 Mrz. 03 Mai. 03 Jun. 03 Aug. 03 Okt. 03 Dez. 03 Feb. 04
Figure 2. Comparison of temperature in the river water, the bank filtrate,
conservative and reactive model calculation
The yearly changing river water temperature has a direct influence on the oxygen concentration of the river water.
While during the winter in the cold river water the oxygen concentration ranged between 11 and 13 mg/l, the concentration
decreased to 7 mg/l in the summer due to lower solubility of oxygen in the warm river water (Figure 3).
Oxygen consumption is induced by aerobic biodegradation of organic substances during bank filtration. The disagreement
of the measured oxygen values of the bank filtrate with the model results while considering a fixed
consumption of 5 mg/l oxygen, indicates a more complex reaction scheme between May and December. The model
fitting process revealed that the microbial oxygen consumption is a function of the temperature and the oxygen
concentration of the infiltrating river water. During spring time the biological activity increased with the rising
temperature leading to maximum oxygen consumption of 11 mg/l. Despite of still increasing temperature during
the summer the microbial activity decreased again. Obviously the lower oxygen concentration became the limiting
factor. Nevertheless, the biological activity was so high that anaerobic conditions appeared within the aquifer over a
period of nearly three months.
The changed redox conditions are of particular interest for the drinking water treatment. During the anaerobic period
additional micropollutants were already degraded within the aquifer (Schmidt et al. 2004). Only a part of the
low concentrated nitrate (7–10 mg/l) in the infiltrated river water was reduced. Because of the incomplete denitrification
no dissolved iron or manganese appeared in the raw water.
10 – 16 June 2005, Berlin ■ 5th International Symposium ■ AQUIFER RECHARGE ■ ISMAR 2005