As the Pacific plate subducts beneath the Mariana forearc it releases water that hydrates the overlying mantle wedge, converting it to serpentinite that protrudes to form mud volcanoes at the seafloor. Excess H2O ascends through these mud volcanoes and exits as cold springs at their summits. The composition of this deep‐slab derived water has been determined by drilling on two of these seamounts. It has a pH of 12.5 and, relative to seawater, is enriched in sulfate, alkalinity, Na/Cl, K, Rb, B, light hydrocarbons, ammonia, 18O, and deuterium, and depleted in chloride, Mg, Ca, Sr, Li, Si, phosphate, and 87Sr. Within the upper 20 m below seafloor at South Chamorro Seamount a microbial community operating at pH 12.5, made up overwhelmingly of Archaea, is oxidizing methane from the ascending fluid to carbonate ion and organic carbon, while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia.

Nitrate concentrations in excess of national drinking‐water standards (10 mg/1 as N) are present in certain sand‐plain aquifers in central Minnesota. To investigate nitrate sources in the aquifers, nitrogen‐isotope values of nitrate (δ15NNO3No3) were measured in shallow ground water from 51 wells in five land‐use settings. The land‐use settings and corresponding average nitrate concentrations (as N) and δ15NNO3 values are: livestock feedlots, 12.7 mg/1, 21.3%0; cultivated‐irrigated fields, 13 mg/1, 7.4%0; residential areas with septic systems, 8.3 mg/1, 6.0%0; cultivated‐nonirrigated fields, 15.5 mg/1, 3.4%0; and natural, undeveloped areas, 3.8 mg/1, 3.1%0. Values of δ15NNO3 less than 2%0 suggest that nitrogen from commercial inorganic fertilizers exists in ground water beneath all settings except the feedlots. Values of δ15NNO3 greater than 10%0 suggest that nitrogen from animal waste is present in ground water beneath certain feedlots, cultivated‐irrigated fields that are fertilized with manure, and residential areas with septic systems. Values of δ15NNO3 between 22 and 43%0 in ground water beneath the feedlots probably result from denitrification. Values of δ15NNO3 increase with depth in many locations in the sand‐plain aquifers. These increases may be caused by progressive denitrification with depth or by changes with depth in the proportions of nitrate from different sources. Similarly, variations of δ15NNO3 values from 1986 to 1987 in certain locations may be due to temporal variations in the amounts of denitrification or to changes in the proportions of nitrate from different sources. Ambiguities in the interpretation of changes in δ15NNO3 values could be eliminated by increasing the spatial and temporal frequency of sampling.

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1973].

Boron‐isotope (δ11B) 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 δ11B 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 δ11B 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 δ11B 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‐km2 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 δ11B values of 5.3 to 15.1‰; 27 stream samples had average B concentrations of 0.05 mg/L and δ11B values of 1.0 to 19.0‰; and eight groundwater samples had average B concentrations of 0.09 mg/L and δ11B values of −0.3 to 23.0‰. Values of δ11B 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.