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Pollutant Contributions from Irrigation Surface Return Flows¹

ABSTRACT

The purpose of this investigation was to examine the causes and sources of irrigation surface return flow pollution and the interrelationships among pollution constituents. Irrigation applications and surface return flows were metered, net applied water was computed, and the volume of infiltrated water was determined at four sites in the Carson Valley of Nevada during the 1974–75 irrigation seasons. Concentrations of TDS, BOD, NO₃-N, TN, PO₄-P, and TP were combined with flow volumes to compute the change in surface loading per irrigation and seasonally. Of those parameters studied, the major pollutants contributed by irrigation surface return flows were BOD and PO₄-P, ranging from 13.6–52 kg/ha and 1.5–3.0 kg/ha, respectively. Minor contributions of TP (0.4 kg/ha) were recorded only at Site 3 during 1975. Multiple regression equations were determined in 1975 for concentrations of BOD as functions of the variables NO₃-N, TN, PO₄-P, and TP, and for DO_d/DO_s as a function of temperature and BOD. F-tests for all multiple regressions were significant. BOD = f(TN, TP) appeared the best predictor. Regression coefficients were significant at the 99% level (t-test) and R² values ranged from 0.51 to 0.74. The coefficients of regression for DO_d/DO_s = f(temperature, BOD) were also significant at the 99% level; however, the R² was only 0.30. It may be necessary to include a rate factor to obtain greater predictability of such a relationship. Differences in water quality and load computations between the two years was attributed, in part, to changes in water quantities.

Key Words: agricultural pollutant loads • pollutant correlation and regression • percent dissolved oxygen deficit • water quality

¹ This paper is a contribution of the Nevada Agric. Exp. Stn., and the Nevada Coop. Ext. Serv., Max C. Fleischmann College of Agric., Univ. of Nevada, Reno. Exp. Stn. J. Series no. 358. Financial support from the Nevada State Bur. of Environ. Health, Health Div., Dep. of Human Resour. is gratefully acknowledged.

² Assistant Professor of Soil and Water Science, Associate Professor of Irrigation Engineering, Extension Irrigation Specialist, and Staff Water Chemist, respectively.

Received for publication January 26, 1977.