Field Studies on the Behaviour of Organic Micropollutants During Infiltration of River Water to Ground Water

The fate of organic micropollutants during ground water infiltration is of great interest since many water works use bank filtration as a first step in the treatment of river water for public water supplies. Field and laboratory studies are necessary to enhance our knowledge on the behaviour of organic chemicals during infiltration of river water to ground water. In an earlier study nonpolar volatile compounds (e.g. tetrachloroethylene, 1,4-dichlorobenzene and 1,3-dimethylbenzene) were investigated in natural river-ground water infiltration systems in Switzerland (Schwarzenbach et al., 1983). In this poster we report on recent work using one of these field sites and studying pentachlorophenol (PCP), nonylphenol (NP), nonylphenol monoethoxylate (NP1EO), nonylphenol diethoxylate (NP2EO) and nitrilotriacetate (NTA), Preliminary results will also be reported for ethylenediaminetetraacetate (EDTA) and polycyclic aromatic hydrocarbons. The field site for our investigation was in the lower Glatt Valley, Switzerland where the Glatt River infiltrates into a quarternary fluvioglacial valley fill aquifer. The Glatt River is a small, rather heavily polluted perialpine river which receives effluents from ten mechanical-biological treatment plants of municipal waste water (Ahel et al., 1984). At the field site the average discharge of the river is approximately 8 m3/sec and permanent infiltration of the river through a saturated zone can be assumed. Observation wells allowed the sampling of freshly infiltrated water at various distances (2,5 - 14 m) from the river. During one year seventeen sample series were collected at approximately monthly intervals including samples from the river and from four ground water observation wells. Pentachlorophenol was determined by a method based on the procedure by Renberg and Lindstróm (1981), Detailed descriptions of the analytical methods for NP, NP1EO, NP2EO and NTA are given elsewhere (Ahel and Giger, 1985; Schaffner and Giger, 1984). The observed averages and ranges of concentrations are given in Table 1. It was concluded that NTA is eliminated rapidly during ground water infiltration. Starting from a range of 8 to 83 mg/m3 and an average of 27 mg/m3 in the river, after 7 m of infiltration only 0.5 mg/m3 are left corresponding to an elimination of 98%. Low temperatures in winter (4 – 6 °C) and reduced oxygen contents in summer had no effect on the efficient elimination of NTA, This result is highly important in addressing the question as to what extent NTA might reach bank filtrated waters from polluted rivers. The phenolic pollutants were eliminated according to the sequence: NP1EO ≈ NP2EO > NP > PCP. This is based on the decrease of the average concentrations over the first seven meters of infiltration. In particular, PCP turned out to be rather persistent in the ground water but not to such a degree as tetrachloroethylene and other chlorinated solvents which had been studied earlier.