Nitrate in the upper groundwater of 'De Marke' and other farms

Groundwater-nitrate concentrations are compared between the experimental farm 'De Marke' - which was designed to minimize nutrient surpluses - and farms being representative for the sandy region of the Netherlands. Samples were collected during the period 1991-1999 at 'De Marke' and during 1992-1995 at 94 representative farms. Between 1991 and autumn 1992 groundwater-nitrate at 'De Marke' decreased from 193 mg l(-1) to 115 mg l(-1). No decrease was found at the representative farms. The decrease at 'De Marke' was attributed to new farm management. After autumn 1992, groundwater nitrate at 'De Marke' fluctuated between 30 and 115 mg l(-1). A comparable pattern in time was found at other farms. This variation is attributed mostly to variation in groundwater table and precipitation. After autumn 1992, farm management did not result in a further reduction in groundwater nitrate. Only 9 of the 94 representative farms had an average nitrate concentration lower than that at 'De Marke' during 1992-1998. If differences in groundwater table and precipitation are taking into account, it is estimated that only three of the representative farms would have had a lower average nitrate concentration than 'De Marke'.