Around 18 million years ago, the Rangitīkei area was submerged beneath the sea and formed part of a large sedimentary basin where layers of marine silt, mud and clay were deposited between layers of shellbed and sand. As the sediments were uplifted, the strata were tilted and fractured by large faults and folds, trapping discrete layers of water-bearing sandstone and shell-rock between thick layers of siltstone and mudstone. Younger sediments were laid down as shallow marine or tidal deposits, and peat and organic layers are seen throughout the sequence -the remnants of ancient wetland and lagoons.
Groundwater is abundant in the upper part of the catchment throughout the Rangitīkei valley and surrounding hill country however, much of this water is moving slowly through impermeable siltstone and mudstone, known as papa, and is not accessible for use. Groundwater in this part of the region is generally drawn from gravels that have formed along the base of the valley. Groundwater in these gravels readily mixes with water from Rangitikei River due to the strong connection between the gravel deposits and the bed of the river.
Toward the coast, the mudstones and siltstones seen to the north are buried beneath a thick sequence of younger sediments deposited during the last few hundred thousand years. Here, water-bearing gravels and sands are layered with impermeable silts and clays to depths of hundreds of meters below surface. The silt and clay layers restrict the flow of groundwater within the larger leaky aquifer system. Productive water-bearing sediments are found at various depths, near surface to more than 200 m below surface, with further potential at greater depth.
Groundwater is in high demand in the coastal area of the Rangitīkei GMZ. Changing land use during the past decade has seen much of the existing exotic forestry and remnant dune farmland converted to grazed pasture and cropping for dairy farming and intensive sheep and beef. Low summer rainfall is common, and land owners are increasingly looking to groundwater to secure reliable water supply of water for summer irrigation.
Agriculture now accounts for 83% of consented water use in the Rangitikei GMZ. Public and commercial water supply accounts for 10% of the current allocation, with the remaining 10% utilised for industry and commercial operations. The total allocated annual volume of 37 million cubic meters now exceeds the water demands of the Whanganui GMZ and has the second highest consented use in the Region, following the Manawatū GMZ.
In recent years, the increasing demand for water supply for pasture irrigation has contributed to a gradual decline in seasonal groundwater levels in a number of Horizons monitoring bores. Near the coast, the Rangitīkei GMZ is home to a number of wetland and coastal lake ecosystems that rely on a consistent and stable supply of groundwater to sustain them. To ensure these lakes and wetlands and protected for future generations, Horizons works closely with local water users, iwi and the wider community to ensure water use is managed sustainably.
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