SPATIAL VARIABILITY OF HEAVY-METALS IN IRRIGATED ALFALFA FIELDS IN THE UPPER RIO-GRANDE RIVER BASIN

Reduced quality of wastewater discharged into the Rio Grande River is reported to be the cause of crop contamination and heavy metal transpo rt onto fertile soils. This study was conducted to characterize Cd, Pb , Ni, Zn, Cr, and Co concentrations in two alfalfa (Medicago sativa L. ) fields irrigated with river water tin Texas) or wastewater mixed wit h river water tin Mexico) and to examine spatial variability of acid-e xtractable metal deposition in soil and in uptake by alfalfa. Multiple transects were established in both fields for intensive forage collec tion and soil sampling to depths of 1.2 m, with spacing every 7.6 or 1 5.2 m. Metal concentrations rarely exceeded 20 mg kg(-1) with the exce ption of Zn. Relative metal concentrations were in the order Zn > Cr > Ni > Pb > Co > Cd in both fields, and were highly correlated with cla y 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 concen trations below the plow zone at the Texas site, inconsistent geostatis tical trends for soil Zn and Ni, and no association of Pb with soil te xture suggested that irrigation delivery was not the exclusive transpo rt source. Estimated metal loads from river water over a 50-year perio d suggested that irrigation may account for up to 31% of surface metal s. It is likely that atmospheric fallout from a local ore smelter and indigenous background levels significantly contributed to observed soi l metal levels. Metal concentrations in unwashed alfalfa forage tissue were at least five times less than those in soil and showed no consis tent association with soil concentrations. Metal concentrations in alf alfa forage posed no toxicity threat to animals or public health. Blen ding wastewaters in the Rio Grande River and canal system has diluted heavy metals to low concentrations for irrigation, but not to more str ingent levels for fish and wildlife. Degraded waters could be diverted from the river and directly used for irrigation under careful water a nd soil management. (C) 1998 Elsevier Science B.V.