Moutere Valley groundwater : nature and recharge from isotopes and chemistry

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Stewart, M.K.; Thomas, J.T. 2002 Moutere Valley groundwater : nature and recharge from isotopes and chemistry . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2002/22 26 p.

Abstract: Recharge to the Moutere Gravel aquifer system in the Moutere Valley was investigated by means of isotopic and chemical measurements. Bores up to 500 m deep tap three Moutere Gravel aquifers underlying the area. Shallow bores (50-100 m), tapping the shallowest of the three aquifers, have delta18O in the range -6.8 ± 0.4‰ as expected for present-day (Holocene) rainfall. Their carbon-14 concentrations are generally 90 ± 10 pmC indicating modern ages, i.e. water residence times of up to hundreds of years. Deeper bores have more negative delta18O values and lower 14C concentrations resulting from input of much older water from depth in the western and eastern zones. The old deep water from the deepest of the aquifers is believed to have been recharged in the Pleistocene during the last glaciation. This ‘glacial’ water has delta18O of -7.6‰, and 14C concentration of 0 pmC. Mixing of glacial and modern waters gives rise to the variations observed in the oxygen-18, carbon-14 and chemical concentrations in the bore waters. The chemical characteristics of the glacial water have been determined as end members on mixing plots between chemical components and delta18O. Sea level was much lower when the glacial water was recharged, and the sea would have been far from its present position. A large body of glacial age water may be resident in the Moutere Gravel under the sea. Recharge is provided by modern water penetrating the groundwater system at shallow levels. Measurements on this water give evidence on the patterns and rates of recharge. The distribution of delta18O and chloride suggest that water has been recharged through both the tm1 and tm2 units of Moutere Gravel in the past few hundred years, but evidently at low rates because of the ages. Young recharge is observed only on the hills west of the valley floor, but observations are lacking in the most probable recharge zone (the tm1 outcrop area in the Rosedale Hills). Under the natural situation, it is believed recharge water flowed at shallow or intermediate levels, leaving an undisturbed body if ice age water at deep levels. This pattern resulted from the present sea level and the expected lack of offshore outlets for water in the deep aquifer. However, groundwater exploitation from deep bores will now be tending to draw shallow water deeper into the system. (auth)