Parallel observations of groundwater and stream water chemistry through the critical zone: new insight into the concentration-discharge relationships

Abstract:

Catchment hydrogeochemists have long been puzzled by the ‘chemostatic’ behavior of rivers: solute concentrations remain nearly invariant while discharge fluctuates several orders of magnitude. Past studies have tended to focus on stream chemistry measurements. Little attention has been paid to source waters (i.e. groundwater). In this study, we simultaneously monitored water chemistry of stream and groundwater at 1-3 days intervals for four years in the Elder Creek catchment, the Eel Critical Zone Observatory, California. At our study site, all runoff occurs as groundwater flow. During the winter high-flow regime, cation (Ca, Mg, Na, K, Ba and Sr) concentrations in stream and groundwater were remarkably similar throughout the study period and were 5-100 times higher than that of throughfall. In contrast, during the low-flow regime, the cation concentrations in groundwater were higher than that in stream, and concentration differences varied by element. We propose that during the high-flow regime, rainwater rapidly increases solute concentrations by cation exchange reactions enhanced by pCO₂ in the vadose zone, recharging groundwater and draining into the stream. During the dry season, groundwater reaches thermodynamic equilibrium with the argillite, particularly with carbonates, at high pCO₂. As this groundwater enters the stream, CO₂ degasses and consequently carbonates precipitate, decreasing Ca, Mg, Sr and Ba concentrations at the hillslope-stream interface. These cation concentrations are further decreased as carbonates likely mediated by biology-induced processes. These observations suggest that the apparent chemostatic behavior of Elder Creek does not arise from the failure of high flows to progressively dilute, as concentration in the creek and in the source groundwater are quite similar. Rather at progressively lower flow, stream concentration does not increase to match the low flow concentration in the groundwater. Instead as groundwater emerges, degassing and precipitation causes concentrations to drop and depress concentration increase with decreasing flow in the creek. This process can only be assessed by direct measurement of source groundwater: even emergent spring water will have already degassed and loss solutes to precipitation.