{"id":1257,"date":"2026-05-12T21:47:13","date_gmt":"2026-05-12T21:47:13","guid":{"rendered":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/reactions-and-retardation\/"},"modified":"2026-05-13T03:19:22","modified_gmt":"2026-05-13T03:19:22","slug":"reactions-and-retardation","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/basic-hydrogeology-an-introduction-to-the-fundamentals-of-groundwater-science\/chapter\/reactions-and-retardation\/","title":{"raw":"12.3 Reactions and Retardation","rendered":"12.3 Reactions and Retardation"},"content":{"raw":"
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\r\n\r\nSome constituents in groundwater are relatively stable and do not undergo significant reactions in the subsurface. These are called conservative<\/em> constituents. An example is the chloride ion\u2014chloride doesn\u2019t stick to anything, it doesn\u2019t undergo biological reactions involving micro\u2011organisms in the subsurface, and it is generally one of the last ions to precipitate out of groundwater into a mineral phase. Not all solutes and contaminants are like that; in fact, most solutes and contaminants are reactive<\/em> in the subsurface. This complicates our understanding of mass transport because, in addition to dilution of the contaminant from dispersion, the mass of the solute or contaminant in the system changes with time.\r\n\r\n<\/div>\r\n
\r\n\r\nReactions in the groundwater usually involve degradation or decay of the contaminant. For radioactive contaminants, decay of the contaminant will occur based on the half-life of the radioactive constituent. For non-radioactive contaminants, decay and degradation is typically related to some sort of biologically controlled process, or biodegradation<\/em>, that involves microorganisms in the groundwater breaking down contaminants as part of their metabolism. Biologically controlled degradation is much more difficult to understand and quantify, as there are often multiple types of organisms that are \u201cconsuming\u201d the contaminant and they are all connected within complex subterranean ecosystems. In addition, biological degradation involves changing the contaminant to a different molecule or type of molecule, which reduces the mass of the contaminant in the system. This can be problematic when the daughter products<\/em>, or the molecules that result from degradation, are also considered contaminants; for example, the degradation of chlorinated solvents can result in the production of additional chlorinated compounds that are just as toxic or dangerous as the original compound.\r\n\r\n<\/div>\r\n
\r\n\r\nMore information on microbial ecosystems in groundwater and on biodegradation can be found in Groundwater Microbiology<\/em><\/span><\/a>\u2197 by Ferris and others (2021), Biotic Transformations of Organic Contaminants<\/em><\/span><\/a>\u2197 by Rittmann (2023), and Processes Controlling the Natural Attenuation of Fuel Hydrocarbons and MTBE in the UK Chalk Aquifer<\/em><\/span><\/a>\u2197 by Thornton and others (2006); each of these books is available as a free download from The Groundwater Project. More information on chlorinated solvents in groundwater can be found in the classic text Dense Chlorinated Solvents and other DNAPLs in Groundwater<\/em><\/span><\/a>\u2197 by Pankow and Cherry (1996), which is also available as a free download from The Groundwater Project.\r\n\r\n<\/div>\r\n
\r\n\r\nSome of the reactions do not involve degradation of the contaminant; rather they slow down the overall movement of the contaminant through the groundwater. This process is called retardation<\/em>. Most retardation reactions involve sorption<\/em>, which is a general term that describes how one entity sticks to another. Sorption is usually reversible, so there is no loss in mass, just a delay in travel time (hence the term retardation). Sorption works in two ways.\r\n\r\n<\/div>\r\n