Hydrodynamical and hydrochemical groundwater controls on abiotic environmental gradients in a nature reserve in Flanders (Belgium)

The biological value of natural and protected areas like nature reserves is often reflected in the diversity and combination of different vegetation associations. In settings where the vegetation is strongly groundwater dependent, the specific interrelation between groundwater flow characteristics (hydrodynamics) and groundwater composition (hydrochemistry) is usually the most important factor for controlling the ecological value and occurrence of site-specific vegetation. Consequently, small-scale variations in ecological gradients within the site rather than its overall size are the key features that control the biodiversity of groundwater-dependent ecosystems. This is demonstrated for a small nature reserve called "Gulke Putten" in the northwest of Belgium which is only a few km 2 large, but shows an interesting variety in groundwater types the occurrence and genesis of which are due to local and highly seasonal flow conditions. As a result, the site has a high ecological value which is rare in this part of the country. This paper shows how multi-year daily water level measurements were used to quantify seasonal and inter-yearly variation in water levels and flow situations and how these are related to the condition in groundwater chemistry. Different hydrochemical facies are found, ranging from evaporated rainwater and waters influenced in varying degree by calcite dissolution, pyrite oxidation, cation exchange and some marine influence originating from the marine sediments of Tertiary age in the phreatic aquifer. This paper relates the variations in flow conditions and groundwater seepage intensity to the groundwater chemistry and helps to understand the origin of the abiotic ecological gradients that are related to the specific ecohydrological value of the site