A 60-year database of nitrate concentrations from more than 200 wells in and around Phoenix, Arizona, was studied using geographic information system tools. This information, augmented with land-use maps, ground water levels, and well construction details, was used to examine ground water nitrate concentrations and changes in concentration and their relationship to land use, land-use changes, and changes in hydrologic conditions (as indicated by ground water level changes). Spatially integrated data for the 1960s to 1990s time period suggest slow and subtle temporal changes in the nitrate mass and overall average nitrate concentration in the aquifer, with increases associated with increased nitrogen fertilizer application rates. More recently, declines in nitrate concentration have been observed, which are thought to reflect the combination of declining ground water levels and transitions from agricultural to urban land use in many areas. Temporal trends in ground water nitrate concentration are presented for wells under desert, agricultural, and urban land use and desert-to-agriculture, desert-to-urban, and agriculture-to-urban land-use change scenarios.