Arbejdsrapport fra Miljøstyrelsen, 16/2004 – Videreudvikling af ådalstypologi

Development of Conceptual Tool for River Basin Typology

Summary and conclusion

The EU Water Framework Directive has outlined criteria for classification of the physical-chemical status of groundwater and surface water bodies, taking into account the quantitative status of a groundwater body having a potential impact on the ecological quality of surface waters interacting with that groundwater body. Good or bad status of individual groundwater bodies is expected to be defined in 2004. During the same period, a stream typology will be carried out in order to classify the ecological quality of different stream types in Denmark. A conceptual tool to identify groundwater bodies that interact with surface water bodies and wetlands has been developed, based on geological, hydrogeological and geomorphological concepts. The tool can be applied to evaluate wetland reduction of the nitrate flux from groundwater bodies and drains onto surface water.

The objectives of this study are:

To update the typology of Groundwater–Surface water Interaction in Danish river valleys, presented in Nilsson et al. (2003)
To compile relevant parameter sets from previous wetland studies in Denmark in order to evaluate wetland reduction of the nitrate flux from groundwater bodies and drains onto surface water.
To specify requirements for GIS tools enabling future typology of Groundwater–Surface water Interaction operational capability for water management authorities.

The updated typology covers the following three scales: 1) A landscape level on catchment scale (10s of km); 2) a river valley level on reach scale (100s of m); 3) a flow path level on local scale (10s of m). The landscape level is divided into five representative landscape deposits: moraine, outwash plain, marine deposits, large freshwater deposits, and dune deposits. The GSI typology comprises a total of sixteen River Valley Types, distributed on five types having moraine landscape characteristics; four types on outwash plain deposits; and six types on marine deposits. River Valley Types on freshwater deposits and dune deposits are covered by moraine and outwash River Valley Types.

The Flow Path Type characterises the four potential flow paths through the River Valley Aquifer. The potential of nitrate reduction in the individual flow paths has been evaluated based on the following criteria in pathways: Redox condition, content of organic matter (more or less than 3%) and the residence time of the seepage water. Based on compiled data from previous wetland studies in Denmark, the Diffuse Flow Path is estimated to reduce nitrate content with 10–97%. The overland Flow Path renders a maximum of 50% nitrate reduction whereas groundwater discharging directly through the stream bed (the Direct Flow Path) and groundwater bypassing river valley deposits through drains and ditches, show typically negligible nitrate reductions.

The goal of applying GSI typology is to enable classification of stream reaches on 100s m scale as : 1) Active reduction of nitrate, 2) potential reduction of nitrate, or
3) no reduction of nitrate. However, this goal has not yet been reached due to lack of documentation from a systematic field verification programme in representative River Valley Types. A final project in 2004 has been designed to catch up on said lack of documentation.