Evaluatie van de grondwaterkwaliteit in de provincie Zuid-Holland

Groundwater has been sampled yearly since 1989 in the province of Zuid-Holland in the Netherlands at approximately 85 locations at depths of 10 en 25 m below soil surface. The current study had the objective of: (1) describing and explaining groundwater quality in Zuid-Holland, with Dutch target values as a reference and (2) calculating trends in the groundwater quality over the 1989-1996 period. Information on hydrology, land use and soil type was used to classify the data into so-called homogenic sub-areas (hsa). The study yielded mean concentrations and 95% confidence intervals per hsa, along with estimates of areal percentages target value exceedance per hsa. These estimates are presented in maps. Trends were calculated for each individual location and per hsa. Concentrations of chloride, potassium, phosphate and ammonia exceeded target values due to marine influence and soil type. Concentrations of nitrate and heavy metals in groundwater were found to be low. No trends were detected for any of the hsa. The groundwater quality in Zuid-Holland was concluded to be mainly governed by natural processes. Anthropogenic influence is (still) relatively small