National Water-Quality Assessment (NAWQA) Program
Studies investigating the factors affecting groundwater-quality trends incorporate groundwater age dating, geochemistry, and contaminant input to quantify the factors leading to observed groundwater-quality trends. For example:
1) Groundwater age dating has become an essential tool used to understand trends in groundwater. The study by Hinkle and others (2010) provides a summary of the methods used, and the estimated recharge dates, of groundwater from wells sampled by the NAWQA program.
2) Groundwater age dating, in conjunction with groundwater-quality data, can be used to better understand the history of a contaminant in groundwater, and the development of trends over time (Puckett and others, 2011).
3) In order to understand trends in groundwater quality, it is essential to also understand the factors affecting degradation rates of contaminants. The study by Tesoriero and Puckett (2011) examines the rate of O2 reduction in groundwater and denitrification rates in various settings across the country, as well as the environmental factors that control these rates.
4) In selected areas, multiple sources of information are used to better understand the factors affecting temporal and spatial distributions of contaminants (Landon and others, 2011).
5) Redox conditions are known to affect degradation rates of many contaminants, and dissolved organic carbon is known to affect redox conditions. Thus an understanding of the bioavailability of DOC is important in being able to understand degradation of contaminants and to predict future concentration patterns, both spatially and temporally (Chapelle and others, 2011).
NAWQA has been investigating the factors affecting trends in groundwater quality in eight study areas. Those eight study areas are (from west to east): the San Joaquin Valley in California (Burow and others, 2007; Burow and others, 2008; Tesoriero and others, 2007), the Central Columbia Plateau area of Washington (Frans, 2008), the South Platte River basin in Colorado and Nebraska (Paschke and others, 2008), the Red River of the North basin (Puckett and Cowdery, 2002), the Western Lake Michigan drainages in Wisconsin (Saad, 2008; Tesoriero and others, 2007), the Apalachicola-Chattahoochee-Flint River basin in Alabama, Florida, and Georgia (Dalton and Frick, 2008; Tesoriero and others, 2007), the Albemarle-Pamlico drainages (Tesoriero and others, 2007) and the Great Valley carbonate rock areas of the Potomac River Basin and surficial aquifers on the Delmarva Peninsula (Debrewer and others, 2008).
The results from all eight study areas indicate that determining groundwater-recharge dates is one of the most important components of a groundwater-quality trends investigation. Knowledge of groundwater-recharge dates allowed correlations between changes in groundwater quality and changes in historical chemical use and climate data that otherwise would not be possible.