In 1989, 48 new groundwater monitoring sites were installed in the province of Zuid-Holland. Together with sites from the National Groundwater Monitoring Network (with which they are compatible) they constitute the provincial groundwater quality monitoring network. The results were processed in different ways. Cumulative frequential distributions show that concentration levels in Zuid-Holland exceed concentration levels in the rest of the country in various types of area. In particular this holds true for ortho.P, HCO3, total.P and NH4 are higher as in corresponding areatypes outside the province. Concentration levels of these parameters as well as those of K, Na and C1, frequently turn out to exceed national standards. In this province, however, the quality of groundwater is strongly effected by natural processes like the deformation of organic matter in the underground (peat) and marine influences. Thus, it will be difficult to discriminate between anthropogenetic parts and natural parts of high concentration levels.<br>