A COMPARISON OF NUTRIENT INPUTS WITH NUTRIENT LOADS IN RIVERS FOR SEVEN COASTAL PLAN BASINS

By Clyde E. Asbury and Edward T. Oaksford


ABSTRACT

Basin inputs were analyzed and compared with nutrient loads in rivers draining the Altamaha, Suwannee, St. Johns, Satilla, Ogeechee, Withlacoochee, and Ochlockonee River basins. This study was done in conjunction with the National Water-Quality Assessment program in the Georgia-Florida Coastal Plain study unit for the period 1986-90. Nutrient constituents considered included nitrate, ammonia, organic nitrogen, and total phosphorus. Sources of nutrients considered in this analysis included atmospheric deposition, fertilizer, animal waste, wastewater discharge, and septic discharge.

The mean nitrogen input per unit area ranged from 2,402 kg per km2 per year in the Withlacoochee basin to 5,467 kg per km2 per year in the Altamaha basin. The Satilla and Ochlockonee basins also had large amounts of nitrogen input per unit area totalling 5,425 kg per km2 per year and 4,919 kg per km2 per year respectively.

Agricultural sources of nitrogen input (fertilizer and animal waste) predominated in all basins. Atmospheric deposition comprised less that one-quarter of the mean total nitrogen input to all basins and was consistently the third largest input in all but the Ogeechee basin where it was the second largest.

The mean total phosphorus input per unit area ranged from 331 kg per km2 per year in the Withlacoochee basin to 1,381 kg per km2 per year in the Satilla basin. The Altamaha and Ochlockonee rivers also had high phosphorus inputs per unit area totalling 1,377 kg per km2 per year and 1,137 kg per km2 per year respectively.

Per unit area, the Suwannee River discharged the highest mean total nitrogen and phosphorous loads of any of the seven basins. The Suwannee River also discharged higher nitrate loads per unit area than the other six rivers. Phosphorus loads in streamflow were highest in the Suwannee and Ochlockonee Rivers.

The percentage of mean total nitrogen loads in rivers to mean total nitrogen basin inputs for the seven studied rivers varied from 4.2% to 14.9%, with the percentage from the St. Johns (14.9%) and Suwannee (12.1%) rivers significantly higher than the other basins. The same percentages for mean total phosphorous varied from 1.0% to 7.0%, again with the Suwannee (7.0%) and St. Johns (6.2%) rivers exporting the highest proportion of phosphorous inputs. Although nutrient loads carried in rivers constitutes only one of the various pathways nutrients may take in leaving a river basin system, it is clear that the bulk of nutrients do not leave the coastal river basins examined in this study by this route. The actual amount of nutrient leaving a river basin depends greatly on the ways in which nutrients are physically handled, geographically distributed, and chemically assimilated within a river basin system.


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