Freshwaters systems surface waters

Exposure concentrations. Concentrations of NP and NPEO occurring in the aquatic environment in the EU have been summarised in Chapters 6.2.1 and 6.3.1. It can be concluded that in both freshwater and marine surface waters, concentrations of NP still exceed the PNEC of 0.33 pig L-1 derived in the EU document cited above [10]. Whereas in several freshwater systems in NW Europe (notably The Netherlands... 

Dissolved humic substances (DHS) are the main constituents of the dissolved organic carbon (DOC) pool in surface waters (freshwaters and marine waters), groundwaters, and soil porewaters and commonly impart a yellowish-brown color to the water system. Despite the different origins responsible for the main structural characteristics of DHS, they all constitute refractory products of chemical and biological degradation and condensation reactions from plant or animal residues and play a crucial role in many biogeochemical processes. 

Evaporation and Evaporative Fractionation of Water. Evaporation from standing water bodies is the principal fractionation mechanism in most hydrological systems. Evaporative isotopic enrichment is a function of numerous factors (e.g., temperature, salinity, and relative humidity) that cause considerable variation in the lsO/ ieO and D/H ratios of natural surface waters. Craig and Gordon (22) evaluated isotopic effects on precipitation and evaporation in the ocean-atmosphere system. Much of what was developed in that work is directly applicable to the freshwater systems discussed here. 

Surface water half-lives range from t/2 4-11 d in freshwater systems, t/2 = 0.1-10 d in cloud water, t,/2 > 1000 d in oceans for PCBs with as many as 8 chlorines for OH- oxidation (Sedlak Andren 1991) t/2 = 13-38 min for dichlorobiphenyl in St. Lawrence River water containing 3 ng/mL of total PCB irradiated by sunlight in the presence of 100 pg/mL TiOz (Huang et al. 1996)... 

This research topic is quite recent and could lead to a better understanding of how natural freshwater systems work and hazardous compounds are formed in photochemical reactions. The remaining part of the chapter will be devoted to the reactions involved in the photochemical generation of some radical species in surface waters, and to their reactivity toward organic compounds of both natural and anthropic origin. 

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