Environ. Sci. Technol. 1994, 28, 868-876
Seasonal and Spatial Trends in Manganese Solubility in an Alluvial Aquifer Alaln C. M. Bourg' and Clotilde Bertin Geochemistry Department, National Geological Survey, BRGM, BP 6009, F-45080 Orleans Cedex 2, France The hydrogeochemistry of river water and groundwater in a hydraulic configuration where river water infiltrates into an alluvial aquifer was monitored monthly over a 14-month period. Trends could be seen for temperature, pH, and dissolved 0 2 and Mn. Wells located within 15m of the river show a significant seasonal variation in dissolved Mn. A threshold temperature of 10 OC seems to be necessary in order to trigger and maintain Mn solubility (microbiologically mediated reactions). Farther from the river, where the temperature is relatively constant, there is little seasonal variation in dissolved Mn. Some of the boreholes contain little Mn (0.5 pmol L-l) while in others the Mn concentration is very high (up to 25 pmol L-l). Hypotheses proposed for the high dissolved Mn content in these wells are (a) a local control of the redox conditions by organic matter in aquifer sediments and (b) a different mineralogical source of Mn.
Introduction Alluvial aquifers hydraulically connected to neighboring rivers supply a significant amount of the drinking water in many countries (1-4). Well fields located near rivers are both highly productive and can provide a groundwater of higher quality than that of the river. In the case of highly polluted rivers or of small communities which cannot afford sophisticated water treatment plants, river banks can serve as a mechanical, biological, and chemical filter ( 1 , 5 1 3 ) . However, a decrease in water quality has been observed in the first meters or tens of meters of the infiltration flow path (10, 14). The particular characteristics of alluvial aquifers (i.e., spatial heterogeneity of alluvions) could provoke the appearance of undesirable substances, especially the redox-sensitive elements, manganese and iron (15-17). Distinct annual cycles were observed for several parameters in the Glattfelden rivedgroundwater system in Switzerland (10). Variations were explained by seasonal changes in the temperature and their incidence on the biological activity in the river and in its banks and on the solubility of minerals. Manganese and other trace elements were solubilized in the summer in the first few meters of infiltration. During the winter, under more oxidizing conditions, dissolved Mn in the infiltrating river water precipitated in the bank sediments and in the aquifer. These amorphous Mn deposits might dissolve the following summer. In a well field, using chloride as a natural tracer, we studied the mixing of infiltrating river water with groundwater (from upstream of the pumping wells) and revealed the presence of a slightly reduced zone near the river bank (18). Depletion of 02,DOC (dissolved organic carbon), Nos, Na, and K and enrichment in Mn, Ca, Mg, bicarbonate, and silica were observed. The solubilization of Mn in some of the wells led to concentrations above the drinking water limit (0.91 pmol L-l). These phenomena
* To whom correspondence should be addressed. 868
Environ. Sci. Technol., Vol. 28, No. 5, 1994
Flgure 1. Area investigated.Dotted lines: piezometriclevels (in meters above sea level) in November 1992. Outlines of zones are schematic only (based on data from the boreholes labeled in the map).
could all be explained by the bacterial degradation of organic matter in the river bank sediments and by the weathering of minerals along the infiltration flow path. In the present paper, we report monthly observations of the water chemistry in the same well field, in the infiltrating river, and in the surrounding alluvial aquifer for a period of slightly over 1 year. The main emphasis of this study is the solubility of Mn, but we also investigated other trace metals (Cd, Zn) and their potential mobility during the summer months under the local conditions (strong hydraulic gradient created by pumping in the well field, eutrophication of the river).
Study Area and Sampling Sites The well field studied is located next to the Lot River in southwestern France (Figure 1). This river has been extensively studied because it has been polluted by heavy metals for many decades, and its bottom sediments are still significantly contaminated (19-24). The well field is made up of six wells (each pumping ca. 40 m3 h-l) and is used mainly for the drinking water supply and food processingactivities of the nearby town of Capdenac-Gare. The alluvial aquifer system is made up of gravel and clay lenses of riverine origin overlying marly-limestone and dolomite from the Lias (relatively impermeable Lower Hettangian formations). The aquifer is unconfined. The unsaturated zone and the aquifer formationsare each about 0013-936X/94/0928-0868$04.50/0
0 1994 American Chemical Society
5 m thick (25). Several piezometers, drilled during the reconnaissance survey for the well field siting or for the needs of our investigation, and private domestic wells were also used to monitor the hydrodynamic behavior of the groundwater. Monthly surveys (water characterization and piezometry) were carried out in all of the pumping wells, five piezometers, and the river near the bank (close to Pzl) (Figure 1). Two of the piezometers were drilled for this study 6 months after the beginning of our investigation in order to more accurately observe the biogeochemical processes occurring in the aquifer: Pzl, closer to the river bank and, Pz2, at a location in the well field where there is little mixing with river water. It rapidly became clear that piezometer F6 was influenced by a nearby cannery and slaughterhouse. Sampling in this borehole was therefore discontinued after 3 months.
Materials and Methods Physicochemical monitoring began in February 1992.
It included the on-site measurement of pH, conductivity, E h (redox potential between a Pt electrode and an Ag/ AgCl reference electrode), temperature, dissolved oxygen concentration, and alkalinity. Water samples were collected to determine the concentration of cations, anions, and dissolved organic carbon at the various sampling points. Piezometric variations in the aquifer and water levels in the Lot River were recorded. The measurements in and sampling of the wells were done at the production flow rate. The water was collected with a tube linked to a tap located near the well head. For the piezometers, we used a submersible pump whose flow rate was ca. 1 m3 h-1. Since the pumps in the wells are located just above the impermeable Lower Hettangian formation, we placed the piezometer pump at about the same depth. In the piezometers, sampling and measurements were carried out only after a pumping time corresponding to a volume of at least three times the capacity of the borehole. In order to minimize exchanges between the atmosphere and the water coming out of the wells or the piezometers, the probes and electrodes used for measurement of the various physicochemical parameters were placed at the outlet of the sampling tube, at the bottom of a tall plastic recipient. River water was collected from the bank at a depth of 15-20 cm under water. Alkalinity was measured on-site on filtered samples by the Gran titration method with 0.1 M HC1. Water samples for laboratory analysis were filtered on-site (0.45 pm, single-use Sartorius ester cellulose filters). Samples for cation and dissolved organic carbon (DOC) analyseswere acidified with suprapur HN03 and HzS04, respectively. Samples were stored at 4 "C until they were analyzed. Chloride, sulfate, and nitrate were measured by ion chromatography. DOC was measured by infrared spectrometry. Mn, Cd, Zn, Ca, and Mg concentrations were analyzed by atomic absorption spectroscopy (flame and/ or graphite furnace). The average analytical precision for the range of concentrations was 5% for anions, 0.5% for alkalinity, 10% for DOC, and from a few to 100% (at the detection limit) for trace metals. Aquifer solids (from Pzl, drilled for this survey) were sieved (nylon sieve) to eliminate grains larger than 2 mm. Their geochemicalcomposition was determined by multielemental ICP.
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MIXING INDEX /% ofaquifer water) Figure 2. Conductivityas a function of mixing of infiltratingriver water and groundwater (in April 1992).
Results and Discussion Aquifer Hydrogeology and Hydrobiogeochemical Processes. Before pumping was begun in the well field, the alluvial aquifer drained toward the river with an average hydraulic gradient of 1-1.5%0(25). The direction of the flow of groundwater has now reversed (Figure l), and the river infiltrates the aquifer (with, in places, hydraulic gradients far exceeding 30%). The water level of the Lot River is now always higher than that of the pumping wells, and all of the piezometric variations with time in the aquifer are synchronous with the hydraulic head of the river. The high water levels recorded in June and November 1992 are related to heavy rainfall. Corresponding high piezometric levels are observed even in the wells located farthest from the river, indicating that river water infiltration into the aquifer is rapid and/or that rain percolates efficiently and rapidly through the vadose zone to recharge the aquifer. These two hypotheses are coherent with our previous investigations of this well field: mean pore velocity of 3.3 m d-I between the river bank and the FE5 well and reoxygenation in the aquifer after the first 20-30 m of infiltration (18, 26). The extent of recharge of the well field by infiltrating river water is quantified using chloride concentrations as an index of mixing or river water and groundwater (18, 27). For all of the operating wells except FE9, the water pumped comes predominantly from the river (the ranges of mixing, in percent of river water, are 91-100% for FE2, 93-100% for FE3,95-100% for FE4,93-100% for FE5, and 78-98% for FE6). For the other boreholes, groundwater origin depends on their location in the aquifer (ranges of mixing: 99-100% for Pzl, 59-100% for Pz2, 72-94% for F4). The large fraction of river water in the wells close to the river (FE2, FE3, FE4, FE5, FE6) is confirmed by their sulfate pattern, which is similar to that of the Lot with in some cases a slightly higher concentration due to the (slight) mixing with aquifer water (FE9). Some of the parameters measured in the groundwater are not only a result of the mixing of river water and alluvial water but also result from reactions with aquifer minerals (18). Infiltrating river water, low in TDS (total dissolved solids), becomes increasingly mineralized (Figure 2) as it flows through the aquifer sediments and mixes with aquifer water (18). The water conductivity increases from the river value (e200 p S cm-l) to the aquifer value (>400 p S cm-9. Close to the Lot River (wells FE3, FE4, and FE5), the conductivity is closer to river values (e240 p S cm-1). As Lot River water infiltrates the aquifer, the pH decreases and tends to stabilize at around 6.7-7.0 (Figure 3). Values for dissolved manganese measured in some of the boreholes (Figure 4) are almost 3 orders of magnitude higher than Envlron. Sci. Technol., Vol. 28, No. 5, 1994
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Figure 8. Seasonal variations in dissolved oxygen: (A)FE2, (0)FE3, (0)FE6, (M) FEQ,(X) Lot River.
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in the Lot River or in the aquifer. Observed DOC concentrations (Figure 5) range between the values of the aquifer and those of the Lot River, but there is a large spatial variability in the well field. Redox potential values in the river are very similar to those in the aquifer upstream from the pumping wells, while in the well field the E h (measured with a Pt electrode) can be in places much lower, especially for wells FE2 and FE6. Dissolved oxygen concentrations are always highest in the Lot River (Figure 6). In the well field, the wells closest to the river (FE3, FE4, FE5) have a variable dissolved 0 2 content, sometimes higher than that of the aquifer upstream from the well Environ. Scl. Technol., Vol. 28, No. 5, 1994
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Flgure 7. Seasonal variations In water temperature: (A)FE3, (0) FE5, (0)FE6, (M) FE9, (0)F4, (X) Lot River.
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Figure 4. Seasonal variations in dissolved manganese. (Note the different scale for FE2 and FE6.)
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field. In some of the wells (FE2,FE6),the dissolved oxygen is always low (
