Environ. Sci. Technol. 1994, 28, 2087-2093
Chemical Processes in Infiltrated Riverbed Sediments Hans R. von Gunten,'qtf* Georgios Karametaxae,tfB and Rolf KeN* Laboratorium fur Radiochemie, Universitat Bern, CH-3000 Bern 9, Switzerland, and Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Redox and other chemical processes were investigated in situ in the sediments of the River Glatt, Switzerland, and were qualitatively reproduced in laboratory column experiments. At the study site, river water infiltrates continuously through the sediments of the riverbed into the groundwater aquifer, In the summer, a temperatureinduced seasonal cycle was observed with concentration minima of oxygen and nitrate and maxima of dissolved organic carbon (DOC), phosphorus, manganese, and iron. The seasonal changes in the concentrations of these elements and compounds were due to an enhanced oxidation of organic matter in the sediments during the summer. This reaction released DOC and phosphorus and lowered the redox potential within the sediments of the riverbed. The lower redox potential enabled a bacteriamediated dissolution of manganese and iron (hydrloxides within the first few centimeters of the infiltrated sediments of the riverbed. The release of manganese led also to a significant increase of its dissolved concentrations in the groundwater close to the river bank. On the basis of the observed reactions, a redox potential of 5-100 mV was estimated in the interstitial water of the sediments during summer conditions. Significant quantities of manganese and iron were also released in the spring during a period of enhanced calcite dissolution due to COZproduction by higher biological activity.
nitrate, dissolved organic carbon (DOC), phosphorus, manganese, and other heavy elements (e.g.,cadmium) were observed between the contaminated River Glatt and the hydraulically connected groundwater (7, 11-13). The concentration differences in elements and compounds between river water and groundwater were attributed to microbial mediated processes in the sediments of the riverbed. These processes proceeded with highest intensities during the warm summer months (13). These authors postulated that in summer the oxidation of organic matter in the riverbed by oxygen (aerobic degradation) and nitrate (denitrification) created reducing conditions, which led to a dissolution (mediated by bacteria, e.g., ref 14) of manganese (hydr)oxides and related other compounds. In winter, bacterial and biological activities declined, and the conditions in the sediments became oxidizing again. A direct proof for the occurrence of these processes in the sediments of the riverbed was still missing, and furthermore, it was not clear from the observations in the groundwater whether the decrease of the redox potential in the sediments was large enough to dissolve even ferrihydrites andlor other iron compounds. In the following, we describe in situ observations of cyclic redox processes in the infiltrated sediments of the River Glatt, Switzerland.
* Address correspondence to this author a t the Paul Scherrer Institut. e-mail address:
[email protected] t Universitat Bern. t Paul Scherrer Institut. f Present address: EAWAG, CH-8600 Dubendorf, Switzerland.
Experimental Section Field Site. The study site is located in the lower River Glatt Valley north of Zurich, Switzerland. The River Glatt and the study site have been described in detail in a number of publications (e.g., refs 5-7 and 15-17). The small River Glatt (mean discharge rate -8 m3 s-l) flows through a heavily populated and industrialized area and is, despite modern sewage treatment plants, still polluted and loaded with nutrients. Seasonal changes in the riverbed due to biological activity were evident: during summer, the riverbed was covered with a carpet of algae; in winter, its appearance was much cleaner. At the field site, the river recharges the aquifer a t a rate of 0.4-0.8 m3 d-1 and m2 of infiltration area (15). Sample Collection. Riverbed Sediments and Interstitial Waters. The sediment samples were collected once every month between April 1989 and May 1990 close to the river bank from a location with a very slow, almost stagnant water flow. Here, the sediments were always covered by 10-30 cm of river water and were only slightly affected by faster flow conditions during storm events. The fast response in the interstitial water of the sediments to variations in the concentrations of sodium and chloride ions in the river water showed that the sediments of this location were connected to the infiltrating water. However, we have no proof that the infiltration occurs homogeneously throughout the riverbed. About 10 cm of the uppermost sediments was recovered with a plastic corer (3 cm i.d.1. They were sealed in the corer tube and were transported vertically to the laboratory. The river water above the sediments was discarded, and the solid material and its interstitial water were separated by centrifuging
0 1994 American Chemical Society
Envlron. Sci. Technol., Voi. 28, No. 12, 1994 2087
Introduction Groundwater resources and their management are of prime interest for modern society. Groundwater aquifers are often recharged by river water which infiltrates through the sediments of riverbeds into unconsolidated geological formations (11, e.g., into glaciofluvial deposits of alpine, prealpine, and other regions. Infiltration of river water is also used during bench filtration in technical installations for the purification and production of drinking water (2). In contrast to the extensive investigations of processes in lake sediments (e.g., refs 3 and 4), not much is known about biogeochemical processes in the sediments of riverbeds despite the fact that these processes may significantly influence the quality of water when it infiltrates into hydraulically connected aquifers. Chemical changes in the water quality between rivers and groundwater reservoirs have been observed by several authors (5-13). However, no direct in situ measurements were made in the sediments of the riverbed. One of the well investigated infiltration sites is located close to Glattfelden, in northern Switzerland, where groundwater is extensively used for domestic, industrial, and agricultural purposes. Here, marked seasonal changes in alkalinity and in the concentrations of calcium, oxygen,
0013-936X/94/0928-2087$04.50/0
Table 1. Mean Annual Composition (mg/g of Dry Sample) of Suspended (>0.45 pm) Particles from the River Glatt (Mean Particle Load 14 mg/L), of Riverbed Sediments, of River Sand, of Algae (mainly Potamogeton natans), and of Deposits from Well G2 (5 m Distance from River Bank)# material
Mn
C (erg)
Fe
suspended river particles >0.45 pm (annual mean) 0.88 28.9 sediments (fine fraction, 40 pm) 0.73 28.4 sediments (sand fraction, 200 pm) 0.28 6.7 algae 0.52 5.3 7.1 deposits in well G2 1.20 0 Sediments, sand, and algae collected in September 1989; accumulation of and Sigg (22).c nd = not determined.
(15 min at 12000 rpm). Centrifugation started about 2 h after sampling. The interstitial water was filtrated (0.2 pm); water and sediments were used for chemical analysis. The sampling of the interstitial water involved uncertainties. It was very difficult and often impossible to recover undisturbed samples from exactly the same place, and some oxidation may have occurred during sampling and transport to the laboratory. In order to minimize these uncertainties and to improve reproducibility, we observed a strict sampling and timing protocol. River W a t e r and Groundwater. River water and groundwater (from sampling wells in the experimental field) were collected with submersible plastic pumps and plastic tubings. The water was filtrated inline (
