Ecohydrological parameters indicating different types of shallow groundwater

Ecohydrology, the interdisciplinary field of research directed to the application of hydrological knowledge to landscape ecology, is introduced to evaluate a test data set of 412 analyses of groundwater samples from 60 locations in a Dutch coversand area, numerically classified into 17 water types. Three types of simple groundwater quality classification are identified: single parameter, combined parameter and multiparameter.

Single quality parameters like pH, electrical conductivity (EC) and ion concentration values, are rarely used in ecohydrology. The representation of the complex conditional relationships between groundwater and the living part of ecosystems with these parameters is unsatisfactory, as also proved with the test data set. It is difficult to differentiate groundwater types reliably on the basis of one single parameter.

Combined quality parameters which are widely used are a pH-EC plot, a plot of pHCO₃ vs. (pH-pCa), after Kemmers, and an IR-EC plot after van Wirdum, where the ionic ratio (IR) is a Ca/(Ca + Cl) ratio. The latter two parameters indicate the residence time of the groundwater sampled as well as the hydrochemical conditions related to vegetation distribution and development. The IR is very popular for discriminating between atmocline (precipitation-like) and lithocline (long-residence groundwater-like) water types. In the test data set, objections to its use are discussed with regard to the conditions assumed in its application. Especially in the intermediate IR range, which should indicate potentially marked ecologic gradients, its theoretical basis is narrow. However, general relationships can be deduced from the IR-EC plot. In the test data set as well as in the general Dutch setting, the transformation of atmocline into lithocline water types determines to a large extent ecosystem diversity, although a third, metacline, pollution-induced water type increasingly interferes.

Finally, multiparameter classification of groundwater types is discussed. Statistical classification gives good results but is site specific. It is recomended that efforts be combined in the search for an internationally applicable composite classification adapted to ecohydrological research.