* Sensitivity of chemical weathering and dissolved carbon dynamics to hydrological conditions in a typical karst river

Scientific Reports volume 7, Article number: 42944 (2017)

https://www.nature.com/articles/srep42944

Abstract To better understand the mechanisms that hydrological conditions control chemical weathering and carbon dynamics in the large rivers, we investigated hydrochemistry and carbon isotopic compositions of dissolved inorganic carbon (DIC) based on high-frequency sampling in the Wujiang River draining the carbonate area in southwestern China. Concentrations of major dissolved solute do not strictly follow the dilution process with increasing discharge, and biogeochemical processes lead to variability in the concentration-discharge relationships. Temporal variations of dissolved solutes are closely related to weathering characteristics and hydrological conditions in the rainy seasons. The concentrations of dissolved carbon and the carbon isotopic compositions vary with discharge changes, suggesting that hydrological conditions and biogeochemical processes control dissolved carbon dynamics. Biological CO2 discharge and intense carbonate weathering by soil CO2 should be responsible for the carbon variability under various hydrological conditions during the high-flow season. The concentration of DICbio (DIC from biological sources) derived from a mixing model increases with increasing discharge, indicating that DICbio influx is the main driver of the chemostatic behaviors of riverine DIC in this typical karst river. The study highlights the sensitivity of chemical weathering and carbon dynamics to hydrological conditions in the riverine system.

Issam Moussa Thank you

In many if not most cases Ca+Mg are the only earth alkalines in shallow groundwater and bicarbonate is the only species of inorganic carbon. If this is true, you can estimate:

mHCO3(-)[mmol(eq)/l]=mCa(2+)[mmol(eq)/l]+mMg(2+)[mmol(eq)/l]

with m = equivalent molar concentration

or

cHCO3(-)[mg/l]={(cCa(2+)[mg/l]*2/40,078[mmol/l])+(cMg(2+)[mg/l]*2/24,3050[mmol/l]}*61,02[mmol/l]

using the molar weights

Ca = 40,078 [mmol/l]

Mg = 24,3050 [mmol/l]

HCO3 = 61,02 [mmol/l]

But always you must be sure that the initial assumption is true!! Groundwater interacting with saline groundwater (deep gw or marine intrusion) is Na dominated and Ca depleted.

Anyway: Analysis of DIC is not difficult but should be performed in the field, because the carbonate equilibrium is temperature dependant.

Peter Faber Thank you for your reply. Do you have any publications so that I can cite for the relation you have posted.

I was asking this question for Nepalese rivers and I have concentrations for Ca and Mg for water samples.

Rajendra, certainly, you should try and understand some chemical basics of natural aqueous chemistry, especially the carbonate system which in most cases dominates groundwater chemistry! I propose to study one of the newer textbooks on aqueous chemistry, i.e., Patrick L. Brezonik and William A. Arnold, Water Chemistry: An Introduction to the Chemistry of Natural and Engineered Aquatic Systems, 2011 (Oxford University Press). Ch. 8.5 (p. 283 ff) deals with the carbonate system. Anyway, the relation I sent to you with my answer to your question is quite basic hydrochemistry. Just be sure to understand the hydrochemical system you’re dealing with through a sufficient amount of “complete” chemical analyses; after that you can work on simplify the analytical efforts. Always glad to be of assistance! Peter

Peter Faber Many Thanks. Greatly Appreciated!

Peter Faber lets try for a set of data, I have Ca (µmol/L), Mg (µmol/L) and Na (µmol/L) with values of 603, 311 and 241, respectively. How do you calculate in this case.

Thank you in advance.

Ok, what are your results?

Peter Faber

Ca (µmol/L), Mg (µmol/L) and Na (µmol/L) with values of 603, 311 and 241 respectively.

It may be easier to determine DIC, Dissolved Inorganic Carbon directly: Alkalinity is practically the same as the sum of HCO3- and CO32- in non-alkaline water. At a pH slightly above 7, one can probably assume that most of it is HCO3-. Alkalinity is measured by titration with dilute acid such as HCl or H2SO4 to pH 4.5. Possibly for cover in methyl orange indicator. The method requires few resources, and is described in handbooks for the analysis of fresh water. Calculating DIC from Ca+Mg can be difficult. Normally they will be "Ca(HCO3)2" and "Mg(HCO3)2". Sodium on the other hand often comes from salt NaCl.

Arne Andersen Thank you so much.