Spatial and temporal variations of water quality in drainage ditches within vegetable farms and citrus groves

Water quality is influenced by agricultural development and varies with surrounding land management. Seasonal and spatial patterns in the concentrations of nitrogen (NH4-N and NO3-N), phosphorus (P), and heavy metals were investigated at six drainage ditches distributed in commercial vegetable farms and citrus groves in St. Lucie County, Florida. The concentrations ranged from nondetectable to 9.13 mg l−1 for NH4-N, to 283 mg l−1 for NO3-N, and to 4.86 mg l−1 for total P. The concentrations of Cu and Zn ranged from nondetectable to 63.7 and 121.7 μg l−1, respectively. Spatial and temporal variations of water quality were noted among the different drainage ditches. Since fertilizer rates were higher on the vegetable farms than in the citrus groves, the concentrations of N, P and K were generally higher in the ditches on the vegetable farms than in the citrus groves. Seasonally, the higher concentrations of N, P, K, Cu, and Zn occurred in the wet season than in the dry season, which might result from higher nutrient input from the adjacent field. Mean N, P, Zn, and Cu concentrations in the ditch water were significantly correlated with soil test N, P, Zn, and Cu levels and annual amounts of N and P applied in the adjacent fields. However, the concentrations of Cd, Co, Cr, Ni, Pb, Mo, and As were generally low, with small seasonal variations, probably due to limited water-soluble Cd, Co, Cr, Ni, Pb, Mo, and As in the soil of adjacent fields. The results indicate that nutrients and heavy metals accumulated in the soils and fertilization practices have significant influences on water quality in adjacent drainage ditches, and water quality of drainage ditch can vary with season.