Review of subsidence at Wairakei field, New Zealand

Abstract

Maximum ground subsidence of 14 m occurred at Wairakei geothermal field between 1950 and 1997 due to development of the field for power generation. The centre of the subsidence bowl is now subsiding at 220 mm/year, after having reached a maximum of 480 mm/year during the mid-1970s. Most of the subsidence is concentrated in a circular area of less than 1 km 2, centred 500 m from the edge of the production borefield. Smaller rates of subsidence (10–100 mm/year) have occurred over a 30 km 2 area of the field affected by fluid withdrawal for the power plant. Horizontal ground movements ranging to over 200 mm/year, and extensional strain rates of 3×10 −4/year, were measured around the flanks of the subsidence bowl during the mid-1970s. Fissures developed in the areas of greatest extension rate and rigid structures such as pipelines, roads, concrete drains and transmission lines have required maintenance where they traverse the subsidence bowl. A pond has formed in the centre of the bowl, and has influenced groundwater levels in the adjacent borefield. The cause of the subsidence is considered to be compaction of a high-porosity, low-permeability lacustrine mudstone at 100–200 m depth. These sediments continue to drain at a slow rate despite pressure stabilisation in the underlying aquifers and the main reservoir during the early 1980s. The small areal extent of the subsidence bowl compared to that of the mudstone is attributed to under-compaction of these lake sediments as they accumulated over the main hot spring outflow zone during the last 100,000 years.