The Heretaunga Plains, Hawke's Bay, New Zealand, is underlain by Quaternary fluvial, estuarine-lagoonal, and marine deposits infilling a subsiding syncline. Within the depositional sequence, river channel gravels form one of the most important aquifer systems in New Zealand. An interconnected unconfined-confined aquifer system contains groundwater recharged from the Ngaruroro River bed at the inland margin of the plain, 20 km from the coast. At the coast. gravel aquifers extend to a depth of 250 m. In 1993-95, 66 Mm(3) of high quality groundwater was abstracted for city and rural water supply, agriculture, industry, and horticulture. Use of groundwater, particularly for irrigation, has increased in the last 5 years. Concern as to the sustainability of the groundwater resource led to a research programme (1991-96). This paper presents the results and recommends specific monitoring and research work to refine the groundwater balance, and define and maintain the sustainable yield of the aquifer system. Three critical management factors are identified. These are (1) to ensure maintenance of consistent, unimpeded groundwater recharge from the Ngaruroro River; (2) to specifically monitor groundwater levels and quality at the margins of the aquifer system, where transmissivity is <5000 m(2)/d and summer groundwater levels indicate that abstraction exceeds recharge; (3) to review groundwater-quality programs to ensure that areas where contamination vulnerability is identified as being highest are covered by regular monitoring.
