Nitrate concentrations and migration in groundwater beneath the Maraekakaho sheep feedlot, Hawkes Bay

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SR_1996-002-pdf
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Rosen, M.R.; McNeill, W.J. 1996 Nitrate concentrations and migration in groundwater beneath the Maraekakaho sheep feedlot, Hawkes Bay . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/02 81 p.

Abstract: Water quality measurements from the unconfined gravel aquifer below the Maraekakaho sheep feedlot, Hawke's Bay, show elevated concentrations of nitrate-nitrogen. The pattern of elevated concentrations strongly indicates that the feedlot is the major source of excess nitrogen to the groundwater. The spatial distribution of nitrate in the aquifer is consistent with the calculated groundwater flow direction. Groundwater flow and nitrate plume directions are slightly north of east and flow into the Ngaruroro River northwest of the town of Maraekakaho. Consequently, groundwater wells in the town of Maraekakaho are not greatly affected by the high nitrate plume from the feedlot. Time series analyses of nitrate, chloride, and alkalinity concentrations in the groundwater after two feedlot-use events (one in August 1994 and one in October 1994) and one rainfall event (August 1995) indicate that groundwater concentrations of these chemical species did not immediately increase after these events. Possible reasons for no immediate increase include: 1) the groundwater table is 10 m below the surface and there was insufficient time for contaminants to reach the groundwater, and 2) the rainfall event may not have been large enough to provide sufficient direct recharge to the aquifer. However, between events nitrate and chloride concentrations increased and alkalinity decreased. These long-term responses suggest that nitrate and chloride are stored in the unsaturated zone and infiltrate to the groundwater after large recharge events. The decrease in alkalinity below the feedlot suggests that ammonification reactions may be important directly below the feedlot area. Excess nitrate and chloride from the feedlot have changed the chloride/nitrate-nitrogen and alkalinity/chloride ratios in the local aquifer. The molar chloride/nitrate- nitrogen ratio of the Maraekakaho Stream, Ngaruroro River, and groundwater outside of the influence of the feedlot is greater than about 4. Groundwater within the influence of the feedlot has a chloride/nitrate-nitrogen ratio of less than 1, indicating the addition of excess nitrogen from the feedlot. The molar alkalinity/chloride ratio of the Maraekakaho Stream, Ngaruroro River, and groundwater outside of the influence of the feedlot is greater than about 2.5. Groundwater within the influence of the feedlot has a alkalinity/chloride ratio of less than 1, indicating the increase of acid waters and enrichment of chloride due to infiltrating water from the feedlot. (auth)