Dissolved organic matter quality variations in drinking water reservoirs and their catchment waters – Scientific knowledge and research gaps

Dissolved organic matter (DOM) is ubiquitous in aquatic systems. Discharge of DOM to reservoirs via shallow ground and surface waters from the catchment poses major problems for drinking water production. Knowledge had been generated about mobilization and discharge of DOM in catchments based on the bulk parameter dissolved organic carbon (DOC) (1-3) or on bulk optical parameters describing its quality (4). The decomposition of DOC in catchments and reservoir waters was reported using DOC, bulk optical and carbon isotope analysis (5, 6).

For drinking water treatment, removal of humic substances by coagulation / flocculation and the formation of disinfection byproducts are the most pressing challenges. The treatment success depends strongly on the chemical quality of DOM, which probably consists of thousands or even millions of different molecules. The identification of the isomeric structure of each molecule is still far from any instrumental analytical realization. From the analytical point of view the highest resolution of DOM can be achieved by Fourier Transform-Ion Cyclotron Resonance Mass Spectroscopy (FTICR-MS). This analytical tool generates elemental compositions of thousands of DOM components which are water extractable (solid phase extractable DOM, SPE-DOM) and which are ionizable (electrospray ionization, ESI).

Using FTICR-MS, knowledge has been generated about the formation potential of disinfection byproducts and its composition (7) and about the flocculation behavior as function of the raw water DOM quality (8).

Only few knowledge exists about DOM quality variations in the reservoirs and their catchments based on sum formulas from FTICR-MS analysis (8 - 11). Also little is known about transformations of drinking water treatment relevant sub fractions within the complex DOM in catchments and reservoir waters.

As a first result of FTICR-MS measurements we observed that few components (sum formulas) showed high abundance differences as function of depth during reservoir stratification. Some poly-phenol-like components (relevant for flocculation) declined in the epilimnion of a drinking water reservoir potentially due to photo degradation. Some of the (more aliphatic) photo products, which were enriched in the epilimnion, are suspected to be disinfection byproduct precursors. This knowledge can be used to investigate the adaptation of the raw water subtraction depth in the reservoir.

 

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2) Werner BJ et al., Biogeosci. (2019), 16, 4497-4516

3) Musolff A et al., J. Hydrol. (2018), 566, 205-215

4) Da Silva MP et al., Biogeosci. (2020), 17, 5355-5364

5) Kamjunke N et al., Sci. Tot. Environ. (2016), 548-549, 51-59

6) Morling K et al., Sci. Tot. Environ. (2017), 577, 329-339

7) Phungsai P et al., Environ. Sci. Technol. (2018), 52, 3392-3401

8) Raeke J et al., Wat. Res. (2017), 113, 149-159

9) Da Silva MP et al., J. Geophys. Res. Biogeosci. (2021), 126, e2021JG006425

10) Herzsprung P et al., Environ. Sci. Technol. (2020), 54, 13556-13565

11) Wilske C et al., Water MDPI (2021), 13, 1703