Background

One of the long-term goals of the National Water-Quality Assessment (NAWQA) Program is to identify changes in the quality of the Nation's groundwater resources over time and to provide a sound, scientific understanding of the primary natural and human factors contributing to these changes. Between 1991 and 2001, NAWQA completed groundwater-quality assessments in 51 study units across the United States. During 2002, NAWQA began resampling many of those wells to determine if groundwater quality has changed over time. NAWQA will continue to periodically resample those wells. As new data become available, NAWQA will continually analyze those data and publish those results. New results also will be published on this web site as they become available.

The NAWQA design for evaluating groundwater-quality trends incorporates two timescales (Rosen and Lapham, 2008). The first timescale consists of sampling all wells within a particular network (about 30 wells) at near-decadal time periods (7 to 13 years). Typically, all wells are resampled during the same season to help avoid the effects of any seasonal variations in groundwater quality. The second timescale consists of sampling a subset of at least five wells in each well network every two years (biennially) between the near-decadal sampling events to help define the temporal pattern of trends within the decadal sampling period.

It is important to point out the difference between the terms "change" and "trend." Groundwater-quality concentrations can change between two sampling events, but when concentrations change consistently over several sampling events, then there may be a trend in concentrations. The NAWQA program has collected sufficient data in some areas to identify changes in groundwater-quality concentrations, but it may be some time before the NAWQA program can identify actual trends in groundwater-quality concentrations and the reasons for those trends.

National-Scale Assessment

Factors Affecting Trends

Simulation and Forecasting

There are three general types of NAWQA studies to assess groundwater-quality trends:

1. The first is a national-scale assessment, where groundwater-quality trends are identified and investigated in the United States as a whole,
   
   
2. The second type investigates the factors affecting groundwater-quality trends, where groundwater age dating, geochemistry, and contaminant input are used to quantify the factors leading to observed groundwater-quality trends, and
   
   
3. The third type is simulation and forecasting, where geochemistry and groundwater flow and transport modeling are used to quantify the factors leading to observed groundwater-quality trends, and to project future groundwater-quality trends on the basis of various input scenarios.