ENVIRONMENTAL GEOCHEMICAL CHARACTERISATION OF THE NETHERLANDS: BUFFERING CAPACITIES AND BACKGROUND COMPOSITIONS OF GROUNDWATER AQUIFERS

The fate of contaminants in groundwater aquifers is determined by the buffering capacity of aquifers together with the background composition of groundwater. A nationwide characterisation of the environmental geochemistry of the shallow subsurface (down to 30 m below surface) of the Netherlands is performed to yield background information on these aspects. It covers 3 aspects:

1. chemical composition of groundwater;
2. the buffering capacity of sediment;
3. elemental association between trace and major elements.

The Netherlands is subdivided into 27 geotop regions each having a unique geological build-up. The regions are statistically characterised where the geogenic controls on the composition are indicated. Marked differences in salinity, redox status and pH-range are found among the 27 geotop regions. Natural background concentrations of the nutrients NH₄ and PO₄ also vary widely and relate to mineralisation of sedimentary organic matter. Reaction capacity is considered as a series of characteristics that control acid/base condition, redox condition and sorption processes. Five primary compound variables are characterised: 1. pyrite, 2. non-pyrite, reactive iron compounds, 3. clay fraction, 4. organic matter and 5. Ca-carbonate. Secondary reaction capacity variables are calculated from these: 1. potential reduction capacity, 2. cation-exchange capacity, 3. carbonate buffering upon pyrite oxidation. Statistical investigation of 8000 sediment analyses for series of geotop regions points out that the lithological class is the major control on reaction capacity and that significant differences among the medians are found between geological formations as well as geotop regions for in particular non-sandy classes. Classification according to regional geohydrological layers within the geological formations also shows promising results as does classification based on sedimentary facies. Total element analyses are used to investigate the association between trace elements and major elements. Factor analysis is performed where samples are classified according to geological formation. Important differences in elemental associations are, for example, found between marine and non-marine sediments. The geo-availability of metals thus varies among geological units in the Netherlands.