The term 'trend' takes on a variety of meanings for groundwater qualit
y in both a temporal and spatial context. Most commonly, trends are th
ought of as changes over time at either a regional or localized spatia
l scale. Generally water quality managers are most interested in chang
es associated with some form of human activity. Carefully defining wha
t is meant by 'trend' is a critical step in trend analysis and may be
accomplished by formulating a statistical model which includes a trend
component. Although there are a great many regional groundwater studi
es which provide a 'snapshot' description of water quality conditions
over an area at one point in time, there are relatively few which cons
ider changes over time and fewer still which include a statistical ana
lysis of long-term trend. This review covers both regional and localiz
ed studies of groundwater quality around the world, including a few sn
apshots, but focusing primarily on those studies which include an eval
uation of temporal changes in groundwater quality. The studies include
national assessments, agricultural case studies (the largest group, m
ostly regional in scope), urban case studies, and point source and haz
ardous waste case studies.