Hydrologic, meteorological, and other environmental data were collected from water years 1998 to 2000 near two recently constructed storm-water retention basins near DuPont, Washington. A mixture of gypsum, grass seeds, and mulch was added to soils of one of the basins to slow infiltration by water. Six monitoring wells were installed in May 1997 near the two retention basins: one upgradient of the basins, four adjacent to the basin without gypsum, and one near the basin with the gypsum soil amendment. Water-quality samples were collected to monitor possible changes in ground-water chemistry. Ground-water levels in selected wells were measured both manually with a steel tape and with automatic monitoring systems. Water level of an intermittent pond in the gypsum-treated basin was monitored, as were selected meteorological and other environmental variables. The hydrologic, meteorological, and other environmental data are presented in this report.