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.
On July 5, 2010 the Rio Grande flooded peaking at 42 ft. above the river banks on July 8, 2010. Damage from the flood impacted approximately 250 miles of land between Amistad and Falcon Reservoirs. The flood plain, extending into both agricultural and municipal lands, was subject to contamination deposition from bi-national sources including overflowing sewage systems, industrial parks and shipping stations. These waters contained a mixture of both heavy metals and organic contaminants which can potentially affect both the water and soil quality within the flooded region. The quality of both water and soil is influenced by fluctuations/perturbations in environmental conditions. Immediate collection of samples was necessary to avoid changes in the physiochemical characteristics due to further natural and anthropogenic disturbances. As we gain in our understanding of the behavior of these compounds in the environment we can to begin to categorize those which are transported within the water column, those that will remain in soil, and those that are readily handled by the natural flora. This project has allowed us to better quantify the concentration and stability of hazardous materials from industrial/municipal sources in flood plain soils within the rhizosphere. We had two primary goals for this research: 1) to determine the concentration and partitioning of anthropogenic contaminants in the water column during both flood and non-flood conditions and 2) to determine the behavior of these contaminants once deposited on riparian soils post flooding. The behavior of these contaminants has been documented for biosolids used on agricultural soils; however, this project was one of the first to analyze the impact of contaminant deposition from primary water sources. Water samples were collected from 4 sites within the Laredo, TX city limits beginning in January 2010 and analyzed for general water quality and contaminant concentrations of triclosan, ciprofloxacin, atrazine, PBDE, antimony and arsenic. These samples along with those collected during the flood and the remainder of 2010 were used to determine the transport and cycling of contaminants in this region. We were able to determine that there are low levels of all contaminants, except PBDE, present in the watershed regardless of the flow rate of the river. Contaminant levels increased during the flood. Flood waters had a lower pH than typically found within the region resulting in greater partitioning of both triclosan and ciprofloxacin to sediments within the water column as noted. There was no change in the partitioning of either the heavy metals or atrazine during the flood. Soil samples were collected from two sites within the city limits adjacent to the river. Both sites were partially flooded, allowing for a comparison of flooded and non-flooded soils using transect sampling techniques. Sampling began as the flood waters receded from these lands, and continued over a 6 month period with collections at 6, 14 and 24 weeks. We were able to determine that contamination in the soils were due to sediment deposition on the surface as well as infiltration of the contaminated flood waters. Both triclosan and atrazine decayed away within 12 weeks, though ciprofloxacin remained throughout the study. There was no evidence that any of the organic compounds were mobile in the soils. Both antimony and arsenic were present in both flooded and non-flooded soils and cannot be attributed to the flood. Broader Impacts: The completion of this project has given us a better understanding of the behavior of contaminants mobilized by flooding events and deposited on soils. Our continued study of contaminate dynamics will have a greater impact on understanding agricultural lands as managed ecosystems. Most agricultural lands are within the floodplain of major rivers in the United States. Periodic flooding may potentially allow for contaminants to become available to commercial crops. In turn this would constitute an indirect exposure to the people relying on these food sources. More importantly we are now beginning to address behavior of organic contaminants in agricultural soils due to the use of recycled water from municipal/industrial sources used for irrigation in non-floodplain regions. Our research demonstrates that these compounds may be available to both soil bacteria and commercial crops.
