Ba/Sr, Ca/Sr and Sr-87/Sr-86 ratios in soil water and groundwater: implications for relative contributions to stream water discharge

Barium/Sr and Ca/Sr ratios have been used to model the relative importance of different sources of stream water. Major and trace element concentration s together with Sr-87/Sr-86 ratios were measured in precipitation, soil wat er, groundwater and stream water in a small (9.4 km(2)) catchment in northe rn Sweden. The study catchment is drained by a first order stream and mainl y covered with podzolized Quaternary till of granitic composition. It is un derlain by a 1.8 Ga granite. A model with mixing equations used in an itera tive mode was developed in order to separate the stream water into 3 subsur face components: soil water, shallow groundwater, and deep groundwater. Con tributions from precipitation are thus not included in the model. This sour ce may be significant for the stream water generation, but it does not inte rfere with the calculations of the relative contributions from the subsurfa ce components. The results show that the deep groundwater constitutes betwe en 5 and 20% of the subsurface water discharge into the stream water, The h ighest values of the deep groundwater fraction occur during base flow. Soil water dominates during snowmelt seasons, whereas during base flow it is th e least important fraction. Soil water accounts for 10-100% of the subsurfa ce water discharge into the stream water. Shallow groundwater accounts for up to 80% of the subsurface water discharge with the lowest values at peak discharge during snowmelt seasons and the highest values during base flow. The validity of the model was tested by comparing the measured Sr-87/Sr-86 ratios in the stream water with the Sr-87/Sr-86 ratios predicted by the mod el. There was a systematic difference between the measured and modelled Sr- 87/Sr-86 ratios which suggests that the fraction of soil water is overestim ated by the model, especially during spring flood. As a consequence of this overestimation of soil water the amount of shallow groundwater is probably underestimated during this period. However. it is concluded that the diffe rences between measured and predicted values are relatively small, and that element ratios are potentially effective tracers for different subsurface water flowpaths in catchments. (C) 1999 Elsevier Science Ltd. All rights re served.