Environ. Sci. Technol. 1983, 17, 472-479 Behavior of Organic Compounds during Infiltration of River Water to Groundwater. Field Studies Red P. Schwarzenbach,” Walter Giger, Eduard Hoehn,+ and Jurg K. Schnelder Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-8600 Dubendorf, Switzerland fields of two rivers, a network of observation wells was The behavior of organic micropollutants during infil- installed that allowed the contaminants in the infiltrating tration of river water to groundwater has been studied at two field sites in Switzerland. In agreement with predic- water to be traced from the river to the groundwater. The tions from model calculations, persistent organic chemicals results of this 2-year field study contribute significantly exhibiting octanol/water partition coefficients smaller than to the limited field data on the behavior of trace organics about 5000 moved rapidly with the infiltrating river water in the groundwater environment (9-11). to the groundwater. The biological processes responsible for the “elimination” of various micropollutants (e.g., al- Theoretical Section kylated and chlorinated benzenes) occurred predominantly Prediction of Retardation Factors for Hydrophobic within the first few meters of infiltration. Alkylated Organic Compounds in the Ground. A rough estimate benzenes were “eliminated” at faster rates than 1,4-di- of the retention behavior of a given hydrophobic organic chlorobenzene. Anaerobic conditions in the aquifer near compound during infiltration may be obtained by treating the river hindered the biological transformation of 1,4- transport through the river bed and in the aquifer in a first dichlorobenzene. Among the compounds that were found approximation as a one-dimensional process with constant to be persistent under any conditions were chloroform, flow in a homogeneous porous medium. Assuming that l,l,l-trichloroethane, trichloroethylene, and tetrachloro- only the fine fraction of the aquifer material is relevant ethylene. With respect to such chemicals, bank filtration for sorption (5, 12) and assuming a linear sorption iso- is ineffective as a first step in the treatment of river water therm, an average retardation factor (Rf”= ratio of the for water supplies. residence time T, of the solute to the residence time T, of the water) can then be calculated for compound z for a Since in many European countries a significant fraction given segment of the aquifer (e.g., ref 8): of the groundwater is recharged through infiltration of river Rf“= T,/T, = 1 fK/p(l - t)/t water (1,2),the impact of river pollution on groundwater + (1) quality is of major concern. In addition, many waterworks where f = fraction of the aquifer material responsible for use natural or artificial bank filtration as a first step in the sorption (e.g., grain size range i$ < 125 pm; assumption: treatment of river water for water supplies (3, 4). homogeneous distribution), Kpz= equilibrium partition Therefore, the behavior of organic pollutants during in- coefficient of the compound z between water and the fine filtration is of great interest. fraction of the aquifer material at a given location in the The transport and fate of organic pollutants in a river ground (cm3/g), p = density of the aquifer material (g/ water-groundwater infiltration system is determined by cm3), and t = total porosity. As we have shown in a pre- several interacting processes, including advection, dis- vious study (5), for the compounds reported here, the persion, (ad)sorption/desorption, hydrolysis, redox reac- equilibrium partition coefficient, Kpz,may be estimated tions, and biological transformations. In laboratory ex- from the organic carbon content of the fine fraction of the periments, individual processes may be studied under aquifer material, f,, and from the octanol/water partition controlled conditions (5,6),and mathematical models may coefficient of the compound, KowZ: be developed to predict the effect of a particular process on the transport and fate of a compound in the environ- KpZ = 3.2foc(KowZ)0~72 (2) ment (7,8). However, comprehensive field investigations Similar relationships have been found for other types of are needed to evaluate the applicability of laboratory compounds and natural sorbents (12,13). Note that eq studies and model calculations to natural systems. 2 is valid only for sorbents exhibiting organic carbon To date, most of the field studies on natural river contents of greater than about 0.1% (foe > 0.001). For water-groundwater infiltration systems have been con- organic-poor sorbents, interactions of the chemical with ducted with respect to the use of bank filtrate for public the inorganic matrix of the sorbent may become important water supplies (e.g., ref 3). These studies have usually been (5). Combining eq 1 and 2 yields confined to monitoring selected water constituents in the river and in groundwater wells near the region of infil- Rf”= 1 + 3.2ff0c(K0wz)0~72p(l- e)/€ (3) tration. The temporal and spatial variations in concen- Retardation factors calculated from eq 3 are valid only at tration of organic compounds along the infiltration path sorption equilibria. At high groundwater-flow velocities, have not been thoroughly investigated. Consequently, the e.g., such as those encountered in the near field of a river results of such investigations provide only very limited during stormwater events (0.5 m/h; see ref 14), due to slow insights into the behavior of individual compounds during sorption kinetics, the compounds may be transported even infiltration. faster than would be assumed from equilibrium consid- In this paper, we report the results of two field studies erations (5,15). However, relationships such as eq 3 are aimed at investigating the transport and fate of organic very valuable for predicting the magnitude of the velocity micropollutants, including chlorinated hydrocarbons, al- at which a specific hydrophobic organic compound is kylated benzenes, and chlorinated phenols during natural transported in a given aquifer. infiltration of river water to groundwater. In the near Experimental Section +Presentaddress: Swiss Federal Institute for Reactor Research Description of the Field Sites. The main field site (EIR), CH-5303 Wurenlingen, Switzerland. of this investigation (field site I) is located in the lower 472 Environ. Sci. Technol., Vol. 17, No. 8, 1983 0013-936X/83/0917-0472$01.50/0 0 1983 American Chemical Society FIELD SITE I I a NW SE distance' 120m NW RIVER AARE PPOuNMEANWATER D- ,' LEVEL ! i H AQUIFER 5m LOWER CONFINING nn-n-------- BED Figure 1. Locations and layouts of the two field study sites: (a) lower Glatt Valley, Switzerland (field slte I); (b) lower Aare Valley, Switzerland (field site 11); (0 = sampling locations). Glatt Valley, Switzerland (see Figure 1). In this region, formation as the one in the lower Glatt Valley (19). At the River Glatt infiltrates over a distance of about 5 km the study site, the River Aare infiltrates through a satu- into a quaternary fluvioglacial valley fill aquifer composed rated zone. Figure lb shows the network of observation of layers of gravel and sand containing very little organic wells that were installed on the left bank of the river. At carbon (<0.1%). The River Glatt is a small, rather heavily this location the regional groundwater flows beneath the polluted perialpine river which has been studied exten- River Aare at an angle of between 45 and 90' to the flow sively (16, 17). The average discharge of the river is ap- direction of the river. proximately 8 m3/s, of which 15-20% is effluent from a Groundwater Observation Wells. All wells were lined number of mechanical-biological sewage treatment plants. with hard PVC tubes. In laboratory experiments, the PVC These treatment plants are the major source for organic material was found neither to contaminate the samples nor micropollutants in the river. At the study site, permanent to (ad)sorb the organic water constituents of interest. For infitration of the River Glatt through a saturated zone can technical details, see Hoehn et al. (18). be assumed. Sample Collection and Analytical Program. Be- Figure la gives a cross-sectional view of the study site tween May 1979 and Apr 1980 (field site I) and between on the right bank of the River Glatt. The groundwater Nov 1980 and Oct 1981 (field site 11), a program was flows beneath the river at an angle between 60 and 90° to conducted to determine temporal and spatial variations the flow of the river. The results presented in this paper in the water composition of the rivers and of the ground- have been obtained primarly from measurements in wells water in the observation wells shown in Figure 1. Samples Gl-G4 (see Figure la). These wells gave access to freshly were collected at approximately monthly intervals. In infiltrated water that stratified in the top layers of the addition to the trace organic compounds, a variety of other aquifer. Some data from observation well G15, which is chemical parameters were determined, mainly to charac- screened throughout the saturated thickness of the aquifer, terize the river water and the groundwater, as well as to will also be discussed. G15 is located in the center of the study the biogeochemical processes occurring during in- valley about 60 m downstream from G4. A detailed de- filtration. Results of these measurements are discussed scription of this field site is presented elsewhere (18). elsewhere (20). To check the general validity of conclusions drawn from The groundwater was sampled by using a small under- results obtained from the main field site, a second study water plunger pump as described by Kass (21). The small was conducted on a different type of river system: River discharge rate of this pump, typically between 0.5 and 1 Aare in the lower Aare Valley (field site II; see Figure lb). L/min, allowed sampling of the groundwater without River Aare is a moderately polluted alpine river with an causing a measurable drawdown of the groundwater level.