Domestic Water Boreholes and Deep Geothermal Wells
Drilling itself creates no discernible ground movement as the drill bit is formed of three cones that turn and grind away at the rock so slowly that you can’t feel the movement.
When drilling reaches the fault zone, drilling mud will be lost in the fractures and can cause some microseismicity. After drilling has been completed a ‘testing phase’ is carried out. It is common for small micro-seismic events to be felt during this period. At United Downs seismicity was felt at a maximum ground velocity of 0.8mm/s. This is ten times less than the Cornwall Council regulations for daytime blasting at quarries and could never cause any damage to nearby buildings or infrastructure.
For more information on induced seismicity and how GEL manages the risk, please see the separate Seismicity FAQ and the animation on YouTube.
If you think the water from your domestic borehole is contaminated, do not consume the water until you have received professional advice, either from your borehole supplier or Cornwall Council.
*The majority of Cornwall’s private borehole water supplies are used for human consumption. Private water supplies can be used for purely domestic purposes such as cooking and washing, or for large scale commercial operations such as farming or breweries. Ensuring clean, safe drinking water is the biggest concern and this is achieved in several ways including protecting the source and secondary treatment, such as filtration and disinfection. Private water supplies are regulated by national legislation:
*Cornwall Council: Private Water Supplies Webpage Cornwall Council provides a useful leaflet explaining the basics of the Private Water Supplies Regulations 2016.
Geothermal developments must follow regulations and guidance from the Environment Agency to ensure that groundwater is not impacted by drilling or operations. At first, a pilot hole is drilled approximately 30” wide and 11m deep and cased with a steel cylinder, to stabilise the top of the well. Air is used to extract the debris, so no fluid is injected or extracted. The next section of the well is then drilled to approximately 900m. In this section, clean water is used to lubricate the drill bit and no water is extracted from the ground at these depths; therefore, borehole water supply is never depleted. As drilling continues, the well is cased with more steel and cement to around 3,000m, effectively sealing of the entire well from ground water and preventing any possible interaction of well fluid with groundwater.
*Farming is an important industry in Cornwall, and this has a potential impact on the quality of private borehole water supplies. Use of pesticides can affect water quality, as can the presence of livestock.
*Cornwall Council: Private Water Supplies Webpage The geology of Cornwall also impacts on the quality of the groundwater. High levels of manganese, iron and arsenic have all been recorded in water samples taken from private borehole water supplies due to the natural presence of these within the rock.
There are many differences between deep geothermal wells and domestic water boreholes. Deep geothermal wells are cased in steel and cement from the surface for approximately ¾ of their depth and bring geothermal fluid contained in fractures at 4,000-5,000m depth to the surface. After the hot fluid has been used to produce renewable electricity, the water is reinjected underground to approximately 2,500m depth. In comparison, domestic water boreholes are rarely more than 200m deep and are cased with either steel or plastic with either an open hole section or perforated cased section in the part of the rock which contains the fresh water. The water from domestic boreholes is not deposited back underground, therefore the water extracted from a borehole is not replenished by the borehole user