Spatial variability of heavy metals in irrigated alfalfa fields in the upper Rio Grande River basin

Abstract

Reduced quality of wastewater discharged into the Rio Grande River is reported to be the cause of crop contamination and heavy metal transport onto fertile soils. This study was conducted to characterize Cd, Pb, Ni, Zn, Cr, and Co concentrations in two alfalfa ( Medicago saliva L.) fields irrigated with river water (in Texas) or wastewater mixed with river water (in Mexico) and to examine spatial variability of acid-extractable metal deposition in soil and in uptake by alfalfa. Multiple transects were established in both fields for intensive forage collection and soil sampling to depths of 1.2 m, with spacing every 7.6 or 15.2 m. Metal concentrations rarely exceeded 20 mg kg −1 with the exception of Zn. Relative metal concentrations were in the order Zn > Cr > Ni > Pb > Co > Cd in both fields, and were highly correlated with clay content within the plow zone. Linear and spherical variogram models best described surface metal deposition with spatial dependence > 100 m with regard to irrigation delivery. However, increasing metal concentrations below the plow zone at the Texas site, inconsistent geostatistical trends for soil Zn and Ni, and no association of Pb with soil texture suggested that irrigation delivery was not the exclusive transport source. Estimated metal loads from river water over a 50-year period suggested that irrigation may account for up to 31 % of surface metals. It is likely that atmospheric fallout from a local ore smelter and indigenous background levels significantly contributed to observed soil metal levels. Metal concentrations in unwashed alfalfa forage tissue were at least five times less than those in soil and showed no consistent association with soil concentrations. Metal concentrations in alfalfa forage posed no toxicity threat to animals or public health. Blending wastewaters in the Rio Grande River and canal system has diluted heavy metals to low concentrations for irrigation, but not to more stringent levels for fish and wildlife. Degraded waters could be diverted from the river and directly used for irrigation under careful water and soil management.