River influence on groundwater – head changes vs. chemical changes

High correlations between river stages and groundwater levels are often seen as an indicator of surface water influence on groundwater. However, such a simple correlation does not necessarily provide information about the nature of said influence, i.e. whether the groundwater hydraulic head only follows the changes in river level or if there is also significant inflow of surface water into the aquifer.

In two large sampling campaigns (early summer and late autumn) covering 45 groundwater and 11 river sites stretching from the alpine region to the foreland basins of the Mur river, Austria using surface water-borne wastewater indicators, stable isotopes of water and selected microbial indicators [1], we show that the influence of surface water intrusion into the shallow aquifer often can be traced hundreds of meters away from the river. Still, at some wells in close vicinity to the river (< 50m) with high correlation of water levels (R > 0.9), isotope data and wastewater indicators hint at no direct surface water influence.

However, one could argue that even at these locations it is plausible that a flood event in the river might reverse flow temporarily, signs of which will not be found by irregular sampling at an inappropriate temporal scale, as the river-borne substances will be quickly flushed out of the shallow aquifer due to the generally effluent conditions. Operating a high-resolution UV-Vis sensor, monitoring nitrate and other key components, we show that direct river influence in the given case still is unlikely.

[1] https://doi.org/10.5194/egusphere-egu21-13111