Vadose zone – challenges in hydrochemistry

Abstract

Hydrochemical studies conducted in Bialystok, north-eastern Poland are used to illustrate the complexity of water chemistry in the vadose zone in clayey and silty sands. Field samples were collected from rain water, ceramic-cup lysimeters located at depths 0.6 m, 0.9 m, and 1.2 m and in a piezometer in the phreatic zone at a depth of 1.5 m. Analysis of chemical constituents in the water and mineral constituents in the sediments allowed saturation indices to be estimated for dominant minerals. Laboratory evaporation experiments showed significant affects of this physical processes on water chemistry in the vadose zone. Detailed mineralogic analysis allowed the cation exchange capacity of sediments to be estimated. More detailed study on Cu and Zn transport in the vadose zone indicates increasing diversification of Cu and Zn species with depth, and a degree of sorption saturation ranging from 9% to 90%.

A relatively dramatic change in the average TDS from 30.2 mg/L in rain water to 318 mg/L in the phreatic zone, and in the hydrochemical type from SO₄-Cl-Ca-NH₄ in rain water to HCO₃-SO₄-Ca-Mg confirms a hypothesis that in relatively uniform unconsolidated sediments, subsurface water chemistry is determined more by processes and reactions occurring in the vadose zone than in the phreatic zone. Significant complexity of these processes and reactions limit the applicability of available hydrochemical models based on chemical equilibrium at the present time, and emphasize the importance of new experimental techniques which would allow the in-situ interaction along water/mineral surfaces to be observed at preferably molecular level.

Results of this research characterize processes in the vadose zone in north-eastern Poland and can be compared to similar studies known from other regions and climatic zones.