ABSTRACT

Long-term monitoring data from over 10 000 shallow drinking water wells in the county of Osnabrück (2121 km²) were used for generating regionalized time-averaged maps of top aquifer contamination. Using this approach local impacts can be distinguished from widespread contamination with agrochemicals (nitrate, pesticides). Spatial information of landuse patterns, livestock figures, soil, and meteorological data are stored in a Geographical Information System (ARC/INFO) to facilitate the risk mapping of top aquifer contamination.

INTRODUCTION

Groundwater is the main source of drinking water in Western Lower Saxony (Germany) which is an area of intensive agricultural land use. The water quality of shallow drinking water wells in the county of Osnabrück has routinely been measured by the local public health department for nitrate, ammonia, and other chemical ingredients and physical properties since 1983. These wells supply approximately 10 % of the inhabitants. Over 10 000 wells were monitored, over 3000, i.e. 30 % of which were closed due to contamination above the legal standards.

DATA AND METHODS

The locations of all wells were digitized and median nitrate values (Figure 1) were built from 32 833 nitrate measurements. The medians were interpolated with Inverse Distance Weighting to create a regionalized view yielding a time-invariant background contamination (Figure 2). By computing differences local impacts can be separated (Figure 3).

Meteorological data were used to compute the daily distribution of recharge and evapotranspiration. Vectorized soil data of a scale of 1:50.000 from the Lower Saxony Soil Information System NIBIS (Figure 4) enabled the determination of horizontal specific field capacity. From field capacity and water recharge the potential nitrate leaching for arable land in the winter period of September 1994 to April 1995 was assessed by using regression curves (Figure 5).

RESULTS AND DISCUSSION

Figure 2 shows that the top-level aquifer, especially in the central northern part of the Osnabrück county, exceeds the legal standard of 50 mg/l, and sometimes even 90 mg/l. Most shallow wells in the southern county have lower nitrate levels; however, several locations approach or exceed the standard. The lowest nitrate contents were detected in hilly areas covered with forests and in deep wells. Figure 3 allows the determination of well locations exceeding the regional level. Furthermore, local impacts, e.g. from dung heaps or leaking sewers can be selected.

The potential nitrate leaching on arable land (Figure 5) shows in the center and eastern part of the district a small vertical nitrate transport because of clay soils in the hilly landscapes. Higher values were computed for the southwest. In the flat area of the northern district with sandy and locally distributed loamy and silty soils, nitrate transport is much higher but also more variable. The winter period 1994/95 was a season with high precipitation (516 mm) and low evapotranspiration (113 mm).
 

Figure 1	Figure 2	Figure 3	Figure 4	Figure 5

CONCLUSION

High production ratios of nitrogen by livestock and a high risk of nitrate leaching are reasons for the considerable exceeding of legal standards in the central northern part of the district. Geographically referenced data can be used for the classification of areas with a high contamination risk in the top aquifer. The comparison of detected contaminations and the areas of high contamination risks show accordances in several areas but also some discrepancies. A consideration of long-term averaged data seems to be necessary for the verification of the approach. Hence the prediction of contamination risks to regions without monitoring data is facilitated.

 ACKNOWLEGEMENT

The cooperation with the Public Health Department of the District of Osnabrück is gratefully acknowledged. Financial support was provided by the German Environmental Foundation.