TRENDS IN GROUNDWATER QUALITY

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.