Dissolved Organic Matter Loads in Surface Runoff Water Under Different Land Uses.

Water quality throughout south Florida has been a major concern for many years. Transport of dissolved organic matter (DOM) from land to water has significant negative impacts on water quality of St. Lucie Estuary (SLE) and Indian River Lagoon (IRL), including increased inputs of nutrients (N and P) and heavy metals (Cu and Zn), increased inputs of pesticides, accelerated depletion of dissolved oxygen in water, and accumulation of muck sediments in the SLE and IRL. The objectives of this study were to quantify the amounts of DOM losses in surface runoff from major agricultural production systems and to compare the differences of dissolved organic carbon (DOC) under different types of land uses. Twelve sampling sites were selected for this study, which represent important sources of dissolved organic matter in the St. Lucie watershed, including two citrus groves, two vegetable farms, two ranches, two golf courses, one landscape nursery, two residential communities (home lawn) and one site in the upstream of Ten Mile Creek. Water sampling was conducted at the twelve locations on a monthly basis from Aug., 2008 to Apr., 2009. Both water physical and chemical parameters were investigated on site or in laboratory. Monitored data suggested a neutral to weak alkaline, low salinity water condition in the 12 sampling locations (pH 6-8, salinity 0.1-3.5 PPS), but such water condition may not  pose any apparent influence on DOC concentrations in the drainage water,  since DOC had poor correlations with pH and salinity in water. However, DOC positively correlated to nutrients total P (P <0.01, N=120) and total N (P <0.05, N=120), metals Zn (P <0.05, N=120) and Fe (P <0.01, N=120) in drainage water. Available data did not support a close relationship between DOC and metal Cu in drainage water as expected.

DOC in drainage water varied temporally and spatially. Storm season apparently resulted in low DOC in drainage water due to dilution from high flushing water, while normal water period high in DOC. Spatially, drainage water from ranch area usually had highest DOC concentration during this period, however, no significant differences in concentrations of water DOC were observed among other locations.  In surface runoff water, loads of DOC demonstrated positive correlations with loads of solid (r = 0.996, N=56, P <0.01), total P (r= 0.998, P <0.01, N=56), and total N (r= 0. 997, P <0.01, N=56), respectively. This suggested a synchronous migration of DOC with P and N particulates in surface running off. Extractable total organic carbon (TOC) concentrations in soils varied greatly with different land uses. But they accounted for less than 4% of soil TOC. Residential area and ranch, which are connected to human or animal activities, were observed to have the most impact on the availability of TOC in soils.

Key words: Dissolved organic carbon (DOC); Land use; Drainage water; Surface runoff; Soil total organic carbon (TOC).