National Water-Quality Assessment (NAWQA) Program
A total of 72 wells were sampled in 1993–1995 (Williamson and others, 1998) and again in 2002–2003 (Frans, 2008) in two monitoring well networks and one public supply well network. Twenty-nine pesticides were detected in the groundwater of the CCPT (Frans, 2008). The herbicide atrazine and its breakdown product, deethylatrazine (DEA), were detected far more frequently than any other pesticides. Because the majority of the pesticides were detected in an insufficient number of wells to detect a consistent change, the analysis was conducted only for atrazine and DEA.
Location of the Central Columbia Plateau area of Washington (CCPT) study area (from Frans, 2008).
Concentrations of DEA had a statistically significant increase in groundwater sampled from a row crop monitoring well network, a regional aquifer well network, and for the data set as a whole. Atrazine concentrations did not change significantly. No statistically significant changes were found for nitrate concentrations in any of the three well networks. Despite the lack of changes within these three well networks, a previous USGS study with a larger 474-well data set found a statistically significant decrease from 1998–2002 for wells with nitrate concentrations greater than 10 mg/L (Frans and Helsel, 2005).