Foreword
Dissolved organic carbon (DOC) in groundwater is usually expressed as milligrams of organic carbon per liter of filtered water (mg/L). DOC in both ground- and surface water systems is made up of thousands of individual compounds that are difficult to identify. However, they rarely need to be identified for most groundwater applications and more inexpensive DOC measurements typically suffice. For that reason, DOC measurements are thus a routine component of many general-purpose groundwater analyses. DOC is present in all surface water and groundwater, but in groundwater the concentrations are small relative to their concentration in surface water. In uncontaminated (pristine) groundwater, DOC is generally much less than 1 percent by weight of total dissolved solids (TDS) present. However, DOC in groundwater is important despite the typically low concentration. Active bacteria of natural origin are present nearly everywhere in groundwater and they foster biochemical reactions that commonly influence or control groundwater quality (i.e., water usefulness). For example, the presence and nature of the DOC may be a controlling factor in the biochemical processes that govern the pH and oxidation-reduction state of the water. pH and oxidation-reduction state, in turn, governs concentrations of important water-quality constituents such as iron, manganese, chromium, arsenic and nitrate. It is useful to read the Groundwater Project book, Groundwater Microbiology (2021) as background for reading this book.
One reason for measuring the amount of DOC in groundwater is to determine whether the water has a normal concentration, with normal being between 0.3 and 1 mg/L. Exceptionally abnormal levels are above 5 mg/L. Abnormality usually indicates something important, for example the DOC may come from anthropogenic sources or from direct infiltration of water from a river or lake into an aquifer. Alternatively, it may indicate that agricultural or industrial activity has diminished DOC attenuation in water percolating through the vadose zone. Any of these situations would indicate the need to consider possible causes of contamination that would be worthy of further investigation.
The author of this book, Dr. Francis Chapelle is an emeritus Research Hydrologist at the United States Geological Survey where his research has focused on how microbial processes affect pristine and contaminated groundwater. He authored the textbook Groundwater Microbiology and Geochemistry that was published as a second edition in 2003 and he has received numerous awards for his life’s work. The Groundwater Project feels fortunate that Dr. Chapelle freely provided this synthesis of humankind’s knowledge of DOC for all who want to know more about groundwater.
John Cherry, The Groundwater Project Leader
Guelph, Ontario, Canada, August 2022