Hydrogeology and water quality of the unconfined gravel aquifer beneath the Maraekakaho feedlot, Hawkes Bay

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Rosen, M.R.; White, P.A.; Reeves, R.R. 1995 Hydrogeology and water quality of the unconfined gravel aquifer beneath the Maraekakaho feedlot, Hawkes Bay . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/01 66 p.

Abstract: The geology, hydrogeology, and hydrochemistry of the Maraekakaho area, Hawkes Bay, have been integrated to construct flow directions and determine hydrochemical trends in the groundwater directly below a sheep feedlot. The geology of the Maraekakaho area consists of greywacke boulders, gravels, and sands formed from Holocene and Pleistocene river terrace deposits derived from the Ngaruroro River. The terrace deposits are highly porous and permeable but are intermixed with clay and bentonite stringers that may reduce permeability in some areas. Water levels in multi-level piezometers and stream elevations indicate that the groundwater flow direction is almost due east. A limestone block to the south of the feedlot area is a major source of alkalinity (HCO3-) to the Maraekakaho River and the groundwater. The Maraekakaho River is everywhere elevated along its course relative to the Ngaruroro River, so that groundwater flows from the Maraekakaho to the Ngaruroro River. River flow measurements indicate that the Maraekakaho River loses water to the ground before it reaches the confluence with the Ngaruroro River. This water mixes with groundwater that is low in HCO3- under the feedlot area. The source of the low HCO3- groundwater may be rainwater, groundwater derived from non-limestone area, Ngaruroro River water, or a combination of all of these sources. The degree of mixing depends on the amount of water in the Ngaruroro River and rainfall in the area. The chemistry of all the major ions in the groundwater beneath the Maraekakaho area indicates that it is the product of variable mixing between the Maraekakaho and the Ngaruroro Rivers, and direct rainfall recharge. The long-term spatial variation in natural groundwater quality of bores in the Maraekakaho area is dominated by the mixing of the above mentioned groundwater sources because rainfall infiltration is too fast and the amount of rainfall is too low in the area to account for the relatively constant inter-bore variations in water quality. Nitrate-nitrogen (NO3- -N) concentrations are highest down-gradient of the feedlot site (range 15 - 40 cubic g/m) and the distribution of the high concentrations are consistent with calculated groundwater flow directions originating from the feedlot. However, NO3- -N concentrations measured before the feedlot was in operation (range 10 - 27 cubic g/m) indicated that in some bores the nitrate level was up to 3 times above the current background level (0.7 - 3.0 cubic g/m) for the area up gradient of the feedlot. This suggests that other factors must be taken into account before evaluating the source(s) of nitrate to the groundwater. Elevated nitrate and lower HCO3- levels in bores and streams coincide with rainfall events with almost no lag time, indicating that rainwater infiltration is the main short-term process for washing accu mulated soil nitrate into the aquifer. Alkalinity contours o ver the feedlot area show that there is little evidence to call on soil ammonification reactions to contribute HCO3- to the groundwater. (auth)