As part of the National Water Quality Program, USGS scientists are investigating why, in some areas and at some depths, groundwater quality changes at short timescales: years to months to days to even hours, rather than decades. These fluctuations often occur in areas where groundwater and surface water interact. The Enhanced Trends Network study is evaluating these short-term fluctuations, identifying what causes them, and determining whether the water-quality changes are just part of a seasonal trend or are part of an overall long-term trend. For those chemical constituents with human-health benchmarks (thresholds for drinking-water quality), changes in constituent concentrations are being evaluated in the context of those benchmarks. This web page provides an overview of USGS's research about fluctuations in groundwater quality. It includes related research, publications, and a collection of data and tools (Introduction).