Chemical and isotopic evidence for irrigation return flow in Avra Valley, Arizona.

Abstract

Nineteen ground-water samples were collected from municipal drinking water wells and private irrigation wells in Avra Valley, Arizona, for the purpose of identifying ground water that has been impacted by irrigation return flow. Nitrate concentrations, stable isotopic ratios of carbon, oxygen, and hydrogen, chloride/bromide ratios, electrical conductivity, and carbon-14 activity were evaluated with respect to land-use patterns in order to determine the effectiveness of using these data to identify irrigation return water. An areal plot of nitrate data indicated that peak concentrations are located in the north-central portion of the valley, where agricultural activity is the most intense. Chloride/bromide ratios in ground water of this area are within the range of natural ground water, indicating that the source of the nitrate is not effluent recharge along the Santa Cruz River. Analyses of the stable isotopes of hydrogen and oxygen were used to identify waters that have been influenced by varying degrees of evaporation. In a plot of 6D versus 60-18, data points representing ground water in irrigated areas fell to the right of the Craig Meteoric Line. Geographically, the distribution of 60-18 values showed trends similar to the distribution of nitrate concentrations. Deuterium-excess parameters calculated from 60-18 and 6D values were lower in irrigated areas than in areas directly downgradient from pristine desert. Carbon-14 analyses indicated that recent recharge has occurred in irrigated areas in northern Avra Valley where elevated nitrate concentrations and heavy 60-18 values are found. The 6C-13 values for Avra Valley ground water appear to be random and are not influenced by irrigation return water.