Carbon Dioxide Measurements for Monitored Natural Attenuation

Carbon Dioxide (CO2) is the carbon product of biodegradation for every respiration process other than methanogenesis. As such, when seeking to evaluate monitored natural attenuation (MNA) progression, tracking the concentration of CO2 in ground water is very important. Unfortunately, it can also be very elusive. CO2 participates in an equilibrium with water to form carbonic acid. In turn, carbonic acid splits into carbonate ions, bicarbonate ions and carbonic acid. The fraction of each is a function of pH. To further complicate matters, some of the carbonate and bicarbonate forms insoluble salts, and these salts further shift the partitioning of the acid. In turn, this shifts the amount of dissolved CO2. There are many different names for each of these, and tracking them is confusing.

Standard Methods for the examination of Water and Waste water, 17 th ed, 1989, 4500- CO2, suggests that a nomographic or calculation method, which uses field data and estimates the amount of each CO2 derived species, be used. They set strong requirements for gathering the appropriate field data: pH, alkalinity (method 2320 of the same text) and total dissolved solids (TDS) measurments must be done in the field. Often it is not possible to perform all of these analyses in the field. Also, the calculation, or the nomograph, must rely on certain assumptions that may be inappropriate. Finally, all of this work produces more information than is needed.

For MNA what is really wanted is the mg/l of C present as CO2, carbonate, bicarbonate, carbonic acid or suspended, insoluble salt in the water. The best way to get this is to take two measurements, normalize them and combine them. The first measurement is the CO2 in the water as a dissolved gas. The concentration of the free CO2 is denoted C(CO2) and measured via Microseeps AM25 or equivalent. The second measurement is that of the CO2 present as a salt or ionic species. The concentration here is denoted as C(alk) and it can be measured either as bicarbonate alkalinity or as total inorganic carbon. The bicarbonate alkalinity is an indirect measurement, but is typically a good indicator for MNA. The total inorganic carbon is not always available from a laboratory.

Typically C(CO2) is in mg/l. To convert it to mg C/l multiply it by 0.27. Typically bicarbonate alkalinity is expressed as mg CaCO3/l. To convert it to mg C/l multiply it by 0.12. Sum the two converted numbers and compare across the site. Be sure to include background wells. An elevation in inorganic mg C/l should be evident and telling.