Freiburger Schriften zur Hydrologie

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Band/volume 16: KÖNIGER P. (2003):

Tracerhydrologische Ansätze zur Bestimmung der Grundwasserneubildung

The aim of this study was to estimate groundwater recharge at the Forest Meteorological test site Hartheim during the investigation period from November 1997 to October 2000. The Meteorological Institute of the Freiburg University has conducted continuous micro-meteorological measurements of water balance and radiation components at this test site for more than thirty years. Small amounts of rainfall, high evapotranspiration and lower-ing of the groundwater level lead to classifying the area as almost semi arid. The soil is characterized by a low water storage capacity and a high permeability. A distance of seven meters to the groundwater table suggests that capillary rise and groundwater uptake are neglectable.
Tracer methods are known to be the most precise methods to estimate groundwater re-charge. In this work, environmental tracer methods were applied in the saturated and un-saturated zone, and artificial tracers were used in the unsaturated zone (sprinkling experi-ments and dry powder deposition). A main focus was the application of the light stable water isotopes deuterium and oxygen-18. In this study they were used as environmental and artificial tracers. The derived recharge results were compared with water balance cal-culations using the Bowen Ratio Energy Balance method for calculation of evapotranspi-ration.
Micrometeorological and hydrometeorological investigations of evapotranspiration, pre-cipitation and soil water storage point out relatively wet conditions during the three-year period of investigation. These were mainly caused by high rainfall amounts during the year 1999. Water balance calculations resulted in 5 mm recharge during the hydrological year 1998, 200 mm for 1999 and 100 mm for 2000. The water balance components showed a high temporal variability. Isotope contents in precipitation measured at the test site showed a clear seasonal variability and were thus used as input for studies of the satu-rated and unsaturated zone.
Investigations carried out in the saturated zone showed that environmental tracers could not be used for an estimation of direct recharge in this case. The influence of the nearby Rhine river by causing bank filtration during flood events dominated the isotopic signature in the flood plain aquifer. However, isotopic variations in the groundwater were observed over the whole period and were interpreted for bank filtration amounts.
Experiments with artificial tracers allowed an interpretation of penetration depths and flow velocities in the unsaturated zone. Infiltrated water at the study site reached a depth of 140 cm where soil water loss due to transpiration can be neglected. The root depth is smaller than 40 cm. The observed results for applied deuterium are similar to that of fluorescein and showed that deuterium is a useful tracer for studies in the unsaturated zone.
The isotope content of soil water in deep profiles usually reflects the influence of infiltrated precipitation and evaporation and can be interpreted for groundwater recharge using the signature method. Four profiles at the Hartheim test site were excavated to a depth of 180 cm. Between two profiles that were taken in August and November 1999 the water moved to the entire depth of the profiles.
Continuous observations of isotope content in the upper 40 cm of the soil layer allowed an estimation of the dampening depth of the isotope signature and suggests a dampening to 1% within 200 cm. Flow velocities calculated using a sine curve fit resulted in values of 1 to 2 cm per day. In contrast to the artificial tracer experiments these flow velocities reflect slower matrix flow.
Finally, the analysis of oxygen-18 deuterium relation and deuterium excess values revealed the influence of evaporation on the different water components of the investigated area. The soil water of the uppermost layer is influenced most strongly. Variations of the deuterium excess observed in the soil profiles could not be definitively interpreted as seasonal variations because these variations were not observable in precipitation. The parameters of the local meteoric water line at the test site are close to those of two other sta-tions in the upper Rhine valley.
During the investigation period a quantitative estimation of direct groundwater recharge was possible using water balance calculations for the Hartheim pine forest. Tracer methods proved that direct recharge occurred but could not be evaluated quantitatively. How-ever as opposed to some earlier hypotheses, it can be concluded that direct groundwater recharge is not neglectable at the Meteorological test site. In fact, in relation to the variable water balance components it even is considerable.

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