Boron Contents and Isotopic Compositions of Hog Manure, Selected Fertilizers, and Water in Minnesota

Abstract

Abstract Boron‐isotope (δ 11 B) values may be useful as surrogate tracers of contaminants and indicators of water mixing in agricultural settings. This paper characterizes the B contents and isotopic compositions of hog manure and selected fertilizers, and presents δ 11 B data for ground and surface water from two agricultural areas. Boron concentrations in dry hog manure averaged 61 mg/kg and in commercial fertilizers ranged from below detection limits in some brands of ammonium nitrate and urea to 382 mg/kg in magnesium sulfate. Values of δ 11 B of untreated hog manure ranged from 7.2 to 11.2‰ and of N fertilizers were −2.0 to 0.7‰. In 22 groundwater samples from a sand‐plain aquifer in east‐central Minnesota, B concentrations averaged 0.04 mg/L and δ 11 B values ranged from 2.3 to 41.5‰. Groundwater beneath a hog feedlot and a cultivated field where hog manure was applied had B‐isotope compositions consistent with the water containing hog‐manure leachate. In a 775‐km 2 watershed with silty‐loam soils in southcentral Minnesota: 18 samples of subsurface drainage from corn ( Zea mays L.) and soybean ( Glycine max L. Merr.) fields had average B concentrations of 0.06 mg/L and δ 11 B values of 5.3 to 15.1‰; 27 stream samples had average B concentrations of 0.05 mg/L and δ 11 B values of 1.0 to 19.0‰; and eight groundwater samples had average B concentrations of 0.09 mg/L and δ 11 B values of −0.3 to 23.0‰. Values of δ 11 B and B concentrations, when plotted against one another, define a curved mixing trend that suggests subsurface drainage and stream water contain mixtures of B from shallow and deep groundwater.