tag:blogger.com,1999:blog-4228590560998272503.post7041372018641772635..comments2024-03-10T08:20:29.607+00:00Comments on GEOBLOGY: Happy as a Hog in ChalkAnonymoushttp://www.blogger.com/profile/05297797356322244437noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-4228590560998272503.post-52777688219035691702014-11-18T14:46:53.697+00:002014-11-18T14:46:53.697+00:00This post prompted a really interesting question o...This post prompted a really interesting question on Facebook... <br />"We are extracting a massive amount of water from the chalk for drinking, irrigation, etc. It comes from our taps saturated with hard water minerals - calcium and magnesium, which precipitate out in the kettle, etc. I would like to know how many tonnes per year of chalk is removed from the strata in this way, and whether it could or has led to the formation of sinkholes!" from Sarah J<br /><br />Here's the comprehensive answer compiled by a whole raft of BGS experts...<br /><br />The Chalk is a soluble, porous rock that is dissolved through contact with water that is not already saturated with calcium carbonate (the main component of chalk). Rainwater that has passed through the soil is often weakly acidic and most dissolution occurs near the soil-chalk boundary, i.e. in the upper parts of the chalk strata. So it is as water enters the chalk that most dissolution occurs. In general, human abstraction of Chalk groundwater does not increase the amount of recharge that occurs, it just reduces natural discharge volumes, so is not likely to increase the amount of chalk dissolution.<br /><br />It is possible to make a rough estimate of the amount of chalk dissolved in the water we abstract. The typical hardness of Chalk groundwater is between 200-300 mg per litre. The UK Groundwater Forum estimates that 1250 million cubic metres of Chalk groundwater is abstracted per year. Using these figures, a rough approximation of the amount of Chalk contained in this water is: 1250,000,000,000 litres x 300 mg = 375,000 tonnes per year. The typical intact dry density of Chalk is 1.53 tonnes per cubic metre which equates to 245,000 cubic metres of Chalk! However, this is spread out across the whole Chalk outcrop, resulting in a gradual lowering of the chalk surface, around a few mm per thousand years. This is unlikely to cause sinkhole formation. However, where water flow is concentrated, and particularly if it is relatively acidic, for example where water flows off adjacent impermeable strata such as sand and clay, or flows through a thick clay soil such as the ‘clay-with-flints’, then this can cause enhanced dissolution and the gradual formation of cavities. These can collapse to form sinkholes if triggered by heavy rain or flooding, for example through leaking water mains (e.g. at Fontwell in 1985). Anonymoushttps://www.blogger.com/profile/05297797356322244437noreply@blogger.com