Dr. Stefan Huber
Drinking Water - Example 12
Humic Substances diagram (HS-diagram)

The HS-diagram plots the SAC/OC ratio of the HS-fraction against its nominal average molecular weight (Mn-value). SAC is the Spectral Absorption Coefficient obtained with the UVD. SAC/OC is the specific UV absorption of the HS peak, and a measure for HS aromaticity, considering that the response in UVD reflects aromatic and unsaturated structures. For the range of water samples measured, the SAC/OC ratio was plotted against Mn-values to produce a diagram for which we introduced in 1996 the term Humic Substances diagram or HS-diagram (Huber and Frimmel 1996). As shown, a close correlation exists between HS-aromaticity and HS-molecularity.

HS standards of the IHSS, which were used for calibration, originate from a dystrophic brown water lake, Suwannee River. These HS are of pedogenic
origin. In the HS-diagram the positions for IHSS-FA and IHSS-HA are in the upper centre and upper right of the diagram (full dots). As both species were isolated from the same water it is clear that the position of the non-isolated, original IHSS-HS must be between the positions for IHSS-FA and IHSS-HA. Therefore, HS in the upper centre and upper right of the HS-diagram are of pedogenic origin. This is confirmed by the positions of other HS isolates of pedogenic origin (e. g. BM-10 FA, NOR-1 FA) and HS from brown water rivers whose catchment areas are bogs (e. g. river Westend, Lady Bower reservoir).

Below the position for IHSS-FA, HS of other isolates are found, but also HS from surface waters (e. g. Steinbach, Kleine Kinzig). This area reflects small rivers and creeks whose HS originate from areas with strong slopes. It is assumed that the high hydraulic energy allows relatively high molecular weight FA from soils to be washed into the water body. Often the aromaticity of these FA is lower in comparison to FA from brown water lakes. This is presumably due to sunlight induced bleaching of HS in the photic zone. Although in brown water lakes HS in the upper layer may also be subject to photobleaching, deeper layers are not accessible and the impact will be relatively low.

At the left side of the HS-diagram FA derived from lakes are found (e.g. Lake Sosa, Lake Galilee, Caspian Sea). It is known that in lakes FA are also produced in-situ by microbial activity and are lower in molecular weight than soil-derived FA (Her et al. 2002b). HS from marine environments are also found here. Thus, this area reflects aquagenic, autochthonous FA in contrast to pedogenic, allochthonous FA.

In between the area of aquagenic and pedogenic FA, HS of large rivers are found (e.g. River Rhine, River Seine). To understand their position in the HS-
diagram we have to consider that a large proportion of NOM in many rivers originate from treated and untreated sewage, or Effluent Organic Matter (EfOM). HS in EfOM are a mix of town water HS which are typically of pedogenic FA type. These FA are superimposed by aquagenic FA produced either in sewage plants or in the river itself by microbial activity. Superposition of both species defines this area in the HS-diagram: a mixing zone containing both aquagenic and pedogenic FA. Thus, it can be concluded that the HS-diagram gives information about the origin of HS. The specificity of HS is remarkable and ”hallmarks” natural water bodies. The HS-diagram also can be used to assess the reactivity of HS in treatment processes.

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