Effect of Long Term Land Disposal by Spray Irrigation of Food Processing Wastes on Some Chemical Properties of the Soil and Subsurface Water

Food processing waste waters at two irrigated land disposal sites and subsurface waters (perched and ground waters) were monitored at daily to monthly intervals over one annual cycle of production. Soil profiles were sampled to depths up to 6.6 m in the early fall. Yearly inputs were calculated at 487 kg/ha total N (Kjeldahl plus NO³-N) and 101 kg/ha soluble PO₄-P (orthophosphate) from cannery wastes at site 1. Estimates for milk wastes at site 2 were 562 kg/ha total N and 522 kg/ha PO₄-P. The range for NO₃-N in subsurface waters was 7 to 16 ppm at site 1 (perched water at 1.5 m) and 2 to 41 ppm at site 2 (ground water at 0.9 m). Maximum concentrations, found in summer, were essentially the same as the average for total N in the input wastes (16 ppm at site 1 and 38 ppm at site 2). Nitrate was stable in the percolation stream below the root zone. Annual additions to subsurface waters were estimated at 76% of input N at site 1 and 65% at site 2. The range of PO₄-P in subsurface waters was 0.5 to 1.5 ppm at site 1 and 0.04 to 1.8 ppm at site 2; average waste water concentrations were 3 and 35 ppm. The highest concentrations in subsurface water were found in spring. Annual subsurface discharge was estimated at 27% of input P at site 1 and 2% at site 2. The extensive removals of PO₄ and the similar concentrations encountered in subsurface waters are of theoretical and practical interest since PO₄-P had already accumulated in soil profiles at both sites in quantities which exceed the Langmuir maxima for nonirrigated control soils. During seasons of major irrigation input, NO₃ appeared in subsurface waters in concentrations exceeding public health standards; PO₄ concentrations exceeded environmental guidelines at all times except where irrigation was discontinued during the winter at site 2. Soil systems appeared poised to discharge at the observed rates because of the large quantities of organic N and fixed P which had accumulated in the profiles over 20 years operation at site 1, and 10 years at site 2. The rate of residual accumulation in soil could have been reduced by harvest, to extend system life materially. The harvest potential of three grass clippings per season removed for silage, was estimated experimentally at 31% of input N at both sites and 80% of input PO₄ at site 1; 27% at site 2.