Abstract
The development of best management practices (BMP) and allocation of pollution among land users in a watershed through the total maximum daily load process (TMDL) requires an understanding of the effect of cultural practices on both crop yields and nutrient leaching below the root zone. Nitrate concentrations in ten wells screened at the 7-m depth located up-gradient, inside and around a 2-ha vegetable field were monitored every three weeks between January 2001 and December 2005. Crop sequences were a rye (Secale cereale L.) cover crop in winter, watermelon (Citrullus lanatus (Thumb.) Mastum. Nakai; 2001-2003) or tomato (Lycopersicum esculentum Mill.; 2004-2005) in spring, and pumpkin (Cucurbita spp.) in fall. Treatments were different each year, but ranged from 100% to 300% of N and irrigation UF/IFAS recommended rates. Wells could not be assigned to a specific fertilization treatment, but instead reflected the overall impact of all combined treatments on shallow ground water quality. The objectives of this study were to (1) describe the seasonal trends in nitrate-N (NO3-N) concentration, (2) identify annual peaks in NO3-N concentration, and (3) determine if rainfall affects NO3N concentration in the wells. Overall, NO3-N concentrations ranged between 0 and 45 mg L-1 during the 5-year period. Nitrate concentration was <5 mg L-1 in two wells up-gradient from the field. Maximum yearly concentrations were greater in the East and South wells (down gradient from the field) and were 42, 34, 29, 23, and 29 mg L-1 NO3-N for 2001 to 2005, respectively. Nitrate concentration in the inside wells tended to be higher than in the perimeter wells. No clear seasonal trends were found except in 2003 when concentration in all wells remained <10 mg L-1 NO3-N during the spring watermelon crop. The high rainfall events associated with hurricanes Frances and Jeanne in 2004 (>12 cm of rain each) did not reduce NO3N concentration by dilution as was expected. Thus, although NO3-N concentration in shallow wells has shown to respond to land use and rainfall, the positions of the well in relation to ground water flow may affect that response.
