Nonpoint Source of Nutrients and Herbicides Associated with Sugarcane Production and Its Impact on Louisiana Coastal Water Quality.

A two-year watershed analysis of nonpoint source pollution associated with sugarcane production was conducted.  Runoff water samples following major rainfall events from two representative sugarcane fields (SC1 and SC2) were collected and analyzed for total suspended solids (TSS), N and P nutrients, and herbicides. The impact of sugarcane runoff on two receiving water bodies, St. James canal (SJC) and Bayou Chevreuil (BC), was studied. Runoff from a swamp forest (SFR) location was monitored to determine contribution of wetland on water quality. Results show that runoff flow/rainfall ratios at the SC1 were significantly higher (P < 0.0001, n = 14) than at the SC2, probably mainly due to higher soil sand content at the SC2. In runoff water samples, TSS showed a significant correlation with the concentrations of N and P. Sugarcane runoff showed a direct impact on the SJC and BC locations where seasonal variations of pollutant concentrations in the waters followed the patterns of runoff loadings. The ratios in total N/total P and in inorganic N/organic N in runoff waters indicated that fertilization in spring greatly contributed to the temporal increase of N loadings, especially in forms of inorganic N.  Isotope signature of ¹⁵N-nitrate in the water samples verified that the nitrate was derived from fertilizers and was consumed during transportation. Both N and P concentrations in the receiving water bodies were above the eutrophic level. Occurrence of low O₂ in the receiving water bodies was likely due to increase of O₂ demand and eutrophication supported by favorable nutrient conditions. During the study period, atrazine and metribuzin concentrations in the receiving water bodies rarely exceeded the drinking water standards. The SFR location showed a buffering effect of forested wetlands on water quality with the lowest measured pollutant concentrations. Best management practices (BMP) to reduce pollutant loadings are proposed.