Natural organic matter fate and transport in Quabbin Reservoir
The concentration and character of natural organic matter (NOM) in samples collected from Quabbin Reservoir an oligotrophic drinking water source were studied. In general, higher in total and dissolved organic carbon (TOC and DOC), UV-254 absorbance, trilialomethane formation potential (THMFP) and specific UV absorbance (SUVA) were observed in the tributary samples than in the reservoir samples. Analysis of the apparent molecular weight distribution (AMWD) indicated that the reservoir samples were enriched in low molecular weight compounds (<1000 daltons) and high molecular weight compounds (>30,000 daltons). Consistent with the oligotrophic state of the system, algal densities and nutrient concentrations are low. Regression analysis of water quality parameters yielded meaningful positive relationships between DOC and UV, THMFP and TOC and THMFP and UV for the complete data set. Relationships were significantly weaker using only the tributary or only the reservoir data. Material balance analyses showed that the most significant allochthonous input of NOM is tributary water from the largest subwatershed in the system, but that direct runoff and direct precipitation are also significant allochthonous inputs. Material balance analyses indicated that over a two year period, the net change in NOM in the reservoir water was 26%, 48%, and 71% less for TOC, THMFP and UV than what would have occurred based on the total inputs and outputs. Thus, waterbody processes remove and transform NOM. Fluxes of organic carbon due to waterbody processes including the net algal effects of algal growth, excretion and death, as well as sedimentation, biodegradation and photodegradation, were estimated from field and modeling studies. The mass balance analysis including these processes indicated that autochthonous inputs of NOM by net productivity were as much as 3 times the total allochthonous inputs. Furthermore, the analysis showed that the sources and sinks of TOC roughly balance, a finding that is consistent with historical records that suggest the organic carbon concentration in the system is relatively constant. The importance of autochthonous inputs in this oligotrophic reservoir suggests that watershed management related to the control of NOM should be focused on the control of algal populations through control of nutrient inputs.