Tufa

Tufa calcite deposits below the vast lime waste tips at the former Harpur Hill limeworks of Buxton Lime Firms Ltd. Great care has to be taken around these deposits due to the high alkilnity of the water which exceeds pH 12 in the lagoon immediately below the tips.

The following explanation is taken from this fascinating report.

The Harpur Hill Site: its geology, evolutionary history and a catalogue of materials present

Report number: CR/13/104 British Geological Survey Commissioned Report, May 2015.

Authors:

Antoni Edward Milodowski, British Geological Survey.

Douglas I Stewart, University of Leeds.

www.researchgate.net/publication/289335354_The_Harpur_Hill_Site_its_geology_evolutionary_history_and_a_catalogue_of_materials_present

The site at Harpur Hill was developed in the 1830’s for the large scale production of lime for the agricultural, building and chemical industries, which continued until the early 1950’s when production on this site ended. Lime production was an inefficient process leading to the creation of large amounts of waste in the form of under- and over-burnt limestone, lime fines, and ash from the low quality local coal used in the process. This waste has been deposited in tips at a number of locations at Harpur Hill, one of the larger tips being created by ‘tumble tipping’ over the side of the Brook Bottom valley, close to one of the banks of lime kilns. The valley carries an ephemeral stream at this location, which has been culverted under the tip, though the culvert is now largely blocked by calcium carbonate deposited within it.

Because the waste is partly comprised of calcined limestone, it reacts with water which hydrates the burnt lime and forms a calcium hydroxide-dominated leachate with a pH of greater than 12. The leachate migrates through the tip and emerges from its base into the Brook Bottom valley. Here, contact with atmospheric carbon dioxide and mixing with local bicarbonate water leads to precipitation of calcium carbonate. This has led to the development of an extensive tract of fine grained and soft tufa deposits of calcium carbonate over a large area of the valley bottom downstream of the spoil tip. Where largely undiluted by fresh water, the fluid in the resulting lagoon in the vicinity of the tip base maintains a high pH (>12). Mixing with fresh water inflows and uptake of atmospheric carbon dioxide occurs as the surface water flows downstream, resulting in continued calcium carbonate deposition and pH reduction. Most of the precipitation of calcium carbonate occurs in the first 400 m or so and the alkalinity, except in dry weather conditions, drops to around pH 8 over this distance. However small amounts of calcium carbonate deposition occurs for up to 2 km downstream.

The leaching of the lime waste by percolating rainwater and shallow groundwater, and the development of the ‘lagoon’ of calcium carbonate precipitate, has probably been occurring since soon after waste tipping on the side of the valley commenced. Precipitation is continuing today, over 60 years since lime production ceased at Harpur Hill, and around 100 years after the bulk of the waste on this part of the site had been tipped and this tip ceased to be utilised for waste disposal (more recently waste has been tipped elsewhere in the quarry complex to the east, including into worked out areas of the quarry as production methods and kiln locations changed).