The GEMS (Geohydrologic Experimental and Monitoring Site) alluvial aquifer is composed of 11-12 m of sand and gravel confined by about the same thickness of mud and silt. Multi-level samplers at GEMS contain ports (0.5 cm diameter) every 0.6 m through the sand and gravel. Chemistry of water from these ports shows that small-scale differences correspond to heterogeneities in hydraulic conductivity within the sand and gravel aquifer, as deduced from grain-size analysis, laboratory permeameter measurements, and an induced-gradient tracer test. The variations in the dissolved species, however, are not all coincident and differences in the patterns reveal vertical trends in chemical reactions. In particular, the distribution of ammonium and nitrate is the reverse of the expected pattern, with the reduced form of nitrogen (ammonium) found in the upper part of the sand and gravel aquifer and the oxidized form (nitrate) found in the lower part.
Comparison of water chemistry from the multi-level samplers with wells completed with 0.6 m screened intervals and a fully-penetrating well shows that, in this highly permeable and heterogeneous aquifer, the fine-scale variation in chemistry is slightly to completely obscured in samples from the screened wells. Fluids produced from the wells are mixtures of waters from different zones with the result that, for example, the ammonium and nitrate stratification is not evident.
Kansas Geological Survey
Updated April 17, 1997
URL="http://www.kgs.ku.edu/Conferences/GSA96/macpherson.html"
Send comments and/or suggestions to
webadmin@kgs.ku.edu