No abstract

During 1976‐1977 nitrate‐nitrogen (NO3‐N) concentrations exceeded 10 mg/1 in 183 of the 256 ground‐water samples collected from parts of Buffalo, Hall, and Merrick counties in Nebraska. Comparison of the isotopic values with those of potential nitrate sources suggested that the primary source of contamination in most wells was fertilizer and that a small percentage of the wells contained significant concentrations of NO3‐N derived from animal wastes. A combination of two or more sources of contamination was assumed for those samples for which the source could not be identified. Significant negative correlations between δ15N and NO3‐N (‐0.35 in Buffalo and Hall Counties) and between δ5N and depth to water (–0.55 in Merrick County) indicate that denitrification is the major factor in isotopic fractionation of nitrogen from the nitrate sour

The areal distribution of 293 samples from the ground water of Merrick County, Nebraska has definite patterns of high (>20 ppm), intermediate (10–20 ppm), and low (<10 ppm) nitrate‐nitrogen concentrations. Where contamination is present the nitrate‐nitrogen concentrations are relatively homogeneous indicating large diffuse nonpoint sources. SYMAP (gray‐scale mapping) indicates exceptionally good correlation between the irrigated coarse‐textured soils and the higher nitrate‐nitrogen levels. The obvious implication is that the nitrate‐nitrogen levels are directly dependent on the leaching of nitrogenous material dispersed in or on the coarser‐textured soils. The widespread use of commercial nitrogen fertilizer on irrigated corn acreages is suspected of being the major contributor of nitrate to the ground water.

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The concentration, type and thermal maturity of organic matter from the Imo Shale, Nkporo Shale and the upper parts of the Nkalagu Formation which were penetrated by the Akukwa‐2 well are presented. The organic carbon content ranged from 0.4–3.5%. The kerogens from the various formations were of the humic and mixed varieties. Evidence from independent thermal maturity parameters (namely, average vitrinite reflectance (R0%), Tmax and transformation ratio from Rock‐Eval, n‐alkanes distribution pattern, and hopane “fingerprint”) combined to locate the top of the ‘oil window’ within the Nkporo Shale at a depth of approximately 2,000 m from the surface. This corresponds to a threshold temperature of circa 65°C. Sedimentary organic matter which is favourable for both gas and oil generation is therefore probably abundant in the Anambra basin. Hydrocarbon reservoirs are possibly located at relatively shallow depths where accumulations of heavy oils may also be expected.