Abstract
Water samples from 16 deep wells (10-40 ft) and from 14 shallow wells (1-2 ft deep), from drainage ditches, creeks and ponds adjoining citrus groves, were collected every 30 days from November 1994 to June 1999 and analyzed for nitrates. There were great differences in NO[sub3] concentrations among the five locations: St. Cloud in the eastern flatwoods, Lake Wales and Frostproof on the Ridge, Dade City and Manatee County on the upper and lower west coast. The highest nitrate levels were found in Manatee County, in a bedded grove with an 8-ft hardpan; the lowest levels, rarely exceeding 10 mg/L NO[sub3]-N were found near Dade City. Attempts to influence groundwater nitrate levels by changes in fertilization were successful only in Frostproof and to some extent in Manatee County and St. Cloud; they had no effect in Lake Wales. Unintentional overapplication of nitrogen resulted in elevated groundwater nitrate levels lasting about 12 months. Liquid fertilizer spills in drainage ditches dissipated in less than 60 days. In spite of sometimes high groundwater nitrate levels within the groves, the water seeping into drainage ditches contained little nitrate when the general water table of the area was high Of 12 deep (150 ft or more), irrigation wells, only one contained 2 mg/L NO[sub3]- N. The great variability of groundwater nitrate and the lack of NO[sub3]-N in the drainage water should be taken into account in regulating nitrogen application.
