RAPID - Evaluation of Soil Contamination due to Rio Grande Flooding July 2010

Texas A&M International University, Laredo TX

Investigators

Abstract

The primary goal of this research is to assess contaminant stability within soils impacted by recent flooding of the Rio Grande near Laredo, Texas. The Rio Grande flood peaked on 8 July 2010 with a flow exceeding 4,000 m3/s (410,000 cfs) at 12.8 m (42 ft.) above the average flow stage. [Note that major flood stage is considered to be 9 m (30 ft) above the average flow stage.] Agricultural, industrial, mining, shipping and municipal lands were flooded and served as sources which mobilized a suite of contaminants including heavy metals and organics. As flood waters receded, these contaminants were deposited into soils adjacent to the river. This project will assess the retention and movement of heavy metals (full panel), and representative organic compounds (PBDE, Triclosan, Ciprofloxacin, and Atrazine) within the Rio Grande and soils impacted by the flood. Water samples have been collected from six locations beginning in May 2010, with more frequent sampling during the flood to better quantify the contaminant deposition rate, and will continue throughout this project. In situ water quality parameters, flow rate, nutrients and the chemicals of interest (aqueous and particle bound) will be assessed for each sampling to determine the level of contamination during both high-flow (flood) conditions and average-flow conditions. Two sites were selected for sampling - undeveloped lands and those used for a nature walk adjacent to the river. Three transects were established at each site with half of each transect covering impacted (1 mm - 2 cm of sediment deposition) and non-impacted soils. Initial samples were collected within 2 days of the flood waters receding, and will be collected again at 6 weeks, 12 weeks and 24 weeks. Samples are collected and segmented by horizon (surface/sediment deposition, root penetration depth and deep soils). Each will be evaluated for overall soil conditions and the contaminants of interests. This research will allow us to better assess the physiochemical behavior of these compounds within the rhizosphere due to further natural and anthropogenic disturbances. Field sampling of the watershed floodplain and adjacent lands will help us to map contaminant distributions in both water and soil in the region. This project will contribute to a whole system assessment of actual contaminant levels in the lower Rio Grande agricultural region and the potential effects of these contaminants on commercially important crops. This flooding event will allow us to better model contaminant behavior within the rhizosphere with limited manipulations of the natural system. Working directly with farmers, this will also help us to better understand the potential impacts on soil health and crop productivity of using recycled water for irrigation in the Rio Grande Valley. Eventually this research will extend to the examination of the microbial community to support sustainability of nutrient dynamics and soil quality in the presence of anthropogenic contaminants.