Land-Use Studies (LUS)
Investigations of the concentrations and distribution of water-quality constituents in recently recharged ground water (generally less than 10 years old) associated with the most important regionally significant Environmental Settings of land use and hydrogeologic conditions in each Study Unit. For each study, usually a combination of 20 to 30 shallow existing and observation wells are sampled. Two to four studies typically are completed in each Study Unit during the first cycle of NAWQA.

Reconnaissance Study
A reconnaissance sampling effort to determine if recently recharged water containing agricultural chemicals was present at the water table under irrigated fields in the central High Plains study area.

Regional Transect
The study of vertical gradients in ground-water chemistry and age of water in the aquifer to develop an understanding of ground-water residence times and recharge rates and to provide a better understanding of contaminant sources/sinks in the aquifer.

Retrospective Analysis
The review and analysis of existing water-quality data to provide a historical perspective on the water quality in the Study Unit, to assess strengths and weaknesses of available information, and to evaluate initial implications for water-quality management and study design.

Major Aquifer Study (MAS)
Broad assessment of the water-quality conditions of the major aquifer systems of each Study Unit. The Sub-Unit Survey relies primarily on sampling existing wells and, wherever possible, on existing data collected by other agencies and programs.

* Modified from Gilliom and others (1995).