Range of horizontal transport and residence time of nitrate in a mature karst vadose zone

Abstract

Nitrate concentrations in drips in Amaterska, Spolecnak, and Holstejnska caves situated below a 25 to 120 m thick vadose zone in the Moravian Karst, Central Europe were studied during several periods from 1992. Each cave runs below a land-use boundary between fertilized lands and forest, which enabled study of the range of horizontal components of nitrate transport in the vadose zone. Parts of the fertilized land were turned into grassland in 1998 and 2003, and the cave drips were sampled both prior and after the changes in land use. The mean residence time of nitrate is <10 years in the 25-30 m thick vadose zone, but >16 years in the 105-120 m thick vadose zone. The maximum range of horizontal nitrate transport (Hmax) is 18 m in the 105-120 m thick vadose zone. Hmaxwas normalized by the vadose zone's thickness (T). In the Moravian Karst the Hmax/T ratio is <0.2. A low Hmax/T between 0.1 and 0.6 was observed in the Czech and Slovenian karst areas, unaffected by glaciations, and with an epikarst having evolved at least from the Pliocene. On the contrary, a high Hmax/T (1.6-24) was reported from those areas affected by glaciations or mining activities, where the epikarst zone might be partly removed or sealed, and where shallow soils do not store much water after heavy rains. More data on the horizontal transport of tracers within the vadose zone are needed in order to test the potential relationship between epikarst development and Hmax/T.