A Quantitative Assessment of Trace Metals in Subsurface Soils and Groundwater in Agricultural Fields of El Paso, Texas

Intensive agricultural practices such as irrigation, application of pesticides and the use of fertilizers, as well as contamination from urban settings and industrial activities contribute to the input of trace metals onto soils and groundwater. Consequentially, biological uptake and accumulation concentrate these toxic metals in fruits, vegetables, and grains that will be used for human consumption. In southern New Mexico and western Texas, cropland is developed along the Rio Grande valley. These farmers are concerned about the quality of their irrigation water and soils in order to produce adequate crops and maintain healthy soils to maximize revenue. To evaluate loading and mobility of trace metals through agricultural practices, irrigation water (from the Rio Grande River and a local well), groundwater, soil water, agricultural soils, and soil amendments were sampled and analyzed for As, Pb, P, Zn, V, B, and Fe concentrations at a pecan orchard in Tornillo, Texas. In addition, soils were collected at a natural site near Fabens, Texas for comparison. The emphasis was placed on sources and sinks of these trace metals and different fluxes that added to or removed them from soils. A mass balance approach spanning 90 years estimated the loading of trace metals onto soils during the lifetime of the pecan orchard situated in the study area, through irrigation, and soil amendments. Soluble As, Pb, P, Zn, V, B, and Fe were predominantly loaded onto soils through irrigation water with some addition from soil amendments. Irrigation water sourced from the river water and from the local well were observed to have different trace metal concentrations; indeed, concentrations of B (504 +/- 7 μg/L; n=2) Fe (14 +/- 0.5 μg/L; n=2), and Pb (7 +/- 1 μg/L; n=2) were higher in groundwater while those of P (923 +/- 709 μg/L; n=8), V (10 +/- 2 μg/L; n=8), Zn (8 +/-6 μg/L; n=8), and As (6 +/- 1 μg/L; n=8) were higher in Rio Grande water. Soil amendments added a smaller amount of trace metals to soils than irrigation, but continuous application of fertilizers and others would load trace metals to soils over long-term. This is especially true for Zn and P that were added as nutrients. Soils were characterized for both water leachable fraction (salt phases) and acetic acid leachable fraction (pedogenic carbonate and Fe-oxyhydroxide) to understand trace metals that have been sequestered in soils at different mobility. The acid leachable fractions had much higher trace metal concentrations than water leachable fractions. For example, almost all P and Fe that were loaded through irrigation and soil amendments were estimated to be present in the acid leachable fractions, at almost 390 g/m2. The trace metal concentrations in groundwaters were variable, but generally much lower for P and much higher for Zn than those in irrigation waters or soil waters. The depth variation of these trace metals in the natural waters (soil waters and groundwaters) are in the acid leachable fractions was controlled by their geochemical properties and gradients of soil conditions. For example, V and Fe concentrations were controlled by redox conditions, and dictated by fluctuations in groundwater table. Clayey soils, where salts and pedogenic carbonates accumulate, have limited the mobility of trace metals to pecan trees and to drainage canals that lead to nearby orchards and back to the Rio Grande River. Assessing the presence of trace metals in agricultural fields provides a better understanding of the quality of irrigation water that can affect crop quality, crop production, soil health, and the need for soil management by farmers