Spectrophotometric characterisation of surface lakewater samples: implications for the quantification of nitrate and the properties of dissolved organic matter.

Filtered lakewater samples, mainly collected in the province of Torino (Piedmont, NW Italy) were characterised from a spectrophotometric point of view. Spectral data were then used for the direct determination of nitrate by three-wavelength photometry, which should account for the spectral interference by dissolved organic matter (DOM), and the results compared with nitrate quantification by ion chromatography. The spectrophotometric method proved very suitable for nitrate measurement, with unity slope (micro +/- sigma = 0.99 +/- 0.03) of the correlation plot (spectral vs. ion chromatography data) up to 0.1 mM nitrate, and with r2 = 0.97 for 26 data points. Lakewater spectra were also used for the characterisation of DOM by means of the specific absorption at 285 and 254 nm (absorbance vs. NPOC, the latter to quantify the DOM amount), and the E2/E3 and E3/E4 indexes. The latter two make only use of radiation absorption data (250 vs. 365 and 300 vs. 400 nm). It could be concluded that lakewater DOM is mainly composed of autochthonous material (biologically produced aliphatic compounds and only a minor fraction of aromatic groups), with generally low molecular weight and degree of aromaticity. Some exceptions could be found in high-mountain lakes, but it should also be considered that NPOC measurement cannot be avoided if DOM origin is to be studied. From the absorption spectrum alone it is possible to get indication on the aromaticity degree of radiation-absorbing DOM, but most of the autochthonous DOM would escape spectrophotometric characterisation.

Sources and sinks of hydroxyl radicals upon irradiation of natural water samples.

Hydroxyl radical formation rates, steady-state concentration, and overall scavenging rate constant were measured by irradiation of surface lake water samples from Piedmont (NW Italy) and nitrate-rich groundwater samples from Moldova (NE Romania). Dissolved organic matter (DOM) was the main source and sink of *OH upon lake water irradiation, with [*OH] being independent of DOM amount. Water oxidation by photoexcited DOM is a likely *OH source in the presence of very low levels of nitrate and dissolved iron. Under different circumstances it is not possible to exclude other processes, e.g., DOM-enhanced photo-Fenton reactions. Under the hypotheses of no interaction and absence of mutual screening of radiation, nitrate would prevail over DOM as *OH source for a NO3-/DOM ratio higher than 3.3 x 10(-5) (mol NO3-) (mg C)(-1), DOM prevailing for lower values. Substantial DOM photolability was observed upon irradiation of nitrate-rich groundwater, mainly due to the elevated *OH generation rate. For the first time to our knowledge, evidence was also obtained of the photoformation of potentially toxic and/or mutagenic nitroaromatic compounds upon irradiation of natural lake water and groundwater samples, proportionally to the nitrate levels.