The Stable Isotope (H,O) and Chemical Composition of Individual Precipiation Events

J. Hoefs Geochemisches Inst., Universität Göttingen, Goldschmidtstr.1, D-37077 Göttingen, Germany

K. Simon Geochemisches Inst., Universität Göttingen, Goldschmidtstr.1, D-37077 Göttingen, Germany

T. Schirmer Geochemisches Inst., Universität Göttingen, Goldschmidtstr.1, D-37077 Göttingen, Germany

The chemical and isotope compositions of rain waters are usually determined on monthly averages. Such an approach does not allow to constrain the origin of water masses of individual precipitation events. Frequently changing wind directions
are typical for Western and Central Europe, which lead to precipitations of different sources. Thus it appears possible that a combination of different analytical techniques may help to elucidate the origin of different water masses and to constrain
anthropogenic contaminants.

We have determined the stable isotope and chemical composition of individual precipitation events in the period from May 93 to March 94 at 2 locations (Göttingen and Clausthal-Zellerfeld) being not far away from each other (50 km), but different in altitude. Productive precipitations have been split into different portions and analyzed separately.

Isotopes (H, O) and chemical elements (Na, K, Ca, Mg, Sr, Ba, Zn, Pb) are extremely variable, even within individual rain showers. dD- and d18O-values range from -16 to -150 ” and -2 to -20 ”, respectively, with large deviations from the Meteoric Water Line. The deuterium excess factor varies from +40 to -15. Northernly derived samples are generally lighter in isotope composition than southernly derived samples. Within a single shower variations can be extremely large and often show a

The contents of the respective elements may vary by a factor of 100 or more. Northernly derived rain samples have relatively high Na- and K- concentrations and are enriched in heavy trace metals (Sr, Ba, Zn; Pb) relative to southernly derived samples, which are more Ca-rich. Elemental concentrations show a negative correlation with rain intensity.

The analytical data are discussed in combination with the path of air masses (-24h trajectories). To deduce the origin of the air masses, samples have been classified into 4 sectors: NW, NE, SE and SW. The vast majority of the samples belong to the NW and SW sectors.

The observed variations depend on various parameters such as amount, humidity, wash- and rain-out processes, weather conditions, i.e. warm or cold front and last not least on the origin and path of the air mass. It is extremely difficult "to look through" these various parameters, but the "best" samples to match the composition of the original air mass seem to be the final stage of a precipitation event. In a few suitable cases a striking correlation between deuterium and lead content has been observed, in the way that low D-contents correspond with low lead contents. Additional analyses are however necessary to support this conclusion.