In situ impacts of a flooding event on contaminant deposition and fate in a riparian ecosystem

Abstract

Floodwaters contain a mixture of organic contaminants which affect both the water and soil quality within flooded regions. This study is the first to describe the chemical behavior of organic contaminants mobilized during the 2010 flooding event of the Rio Grande. The study compares two sites: one directly impacted by outfall from municipal sources and one primarily impacted by rural runoff. This study characterized the in situ cycling of organic contaminants (triclosan, ciprofloxacin, and atrazine), their depositional mechanisms, and the post-flood dynamics within riparian soils over 6 months. Water quality (pH, salinity, alkalinity, turbidity, N, and P) of the Rio Grande at four sites within Webb County, TX, was monitored for 3 months pre-flooding and 6 months post-flooding in order to determine contaminant levels along the Webb County border. Soil physicochemical properties (pH, CEC, and EC) were assessed for 6 months at two sites post-flooding: within the municipal area of both Laredo, TX, USA, and Nuevo Laredo, Mexico (LCC site), and upstream of the site near the edge of Laredo, TX (MR site). Low levels of contaminants were detected in the watershed year-round with levels increasing during the flood. Floodwaters had a lower pH than nonflooding conditions, resulting in greater partitioning of both triclosan and ciprofloxacin to particles within the water column with no change detected in atrazine behavior. Soil samples (0–2-cm depth) showed an increase in clay-sized particles (up to 6–14 %) and contaminant concentrations similar to that of water column particles. Organic contaminants were uniform in concentration throughout the soil profile (2–30 cm), demonstrating the impact of infiltration. Both triclosan and atrazine decayed away at rates of 23 ± 2 and 16 ± 1 days, respectively, with no decay of ciprofloxacin detected. Vertical mobility of contaminants within soils was not detected. This study was able to distinguish the influx of contamination contributed by particle deposition from that due to infiltration of the contaminated floodwaters. Further, our data indicates that outfall/effluent contaminants, such as ciprofloxacin, may persist in riparian soils post-flooding. Agricultural management should consider potential exposure and accumulation of floodwater contaminants in crops.