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Groundwater pollution by manganese. Manganese speciation: Application to the selection and discussion of an in situ groundwater treatment
The Science of the Total Environment, 84 (1989) 169-183 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
169
G R O U N D W A T E R POLLUTION BY M A N G A N E S E . M A N G A N E S E SPECIATION: APPLICATION TO THE SELECTION AND D I S C U S S I O N OF AN IN SITU G R O U N D W A T E R T R E A T M E N T
P. JAUDON, C. MASSIANI, J. GALEA and J. REY Laboratoire Chimie et Environnement, Universitd de Provence, 3 place Victor Hugo, 13331 Marseille cedex 3 (France)
E. VACELET C.O.M. Station Marine d' Endoume, 13007 Marseille (France)
(Received March 15th, 1988; accepted January 24th, 1989)
ABSTRACT All over the world, drinking water pumped into an alluvial water table frequently becomes progressively polluted by dissolved manganese and iron, causing problems for regulatory authorities. It has been shown that, in addition to classical hydrogeological studies, manganese speciation and bacteriological experiments are essential to determine the origin of the manganese and the processes of water pollution in order to select the most appropriate treatment. In the aquifer studied here, speciation and mineralogical results showed that manganese originates from within the aquifer itself, mainly in the form of oxides (Z-disordered manganate type, 10/~ manganate, todorokite, birnessite) which settle on components of the pebble bed forming the aquifer. Manganese release is due to a decrease of dissolved oxygen in the groundwater. Under these conditions, Mn(IV) is reduced both chemically and bacterially into the Mn(II) soluble form. In such situations an in situ water treatment consisting of the oxygenation of the aquifer has proved to be both suitable and inexpensive. The risks of aquifer contamination are discussed in terms of manganese speciation.
INTRODUCTION
For a long time, in many countries, the presence of iron and manganese caused a problem for regulatory authorities in relation to industrial and main water supplies (Griffin, 1958; Riddick et al., 1958; Glensvick, 1976; Van Beek, 1985; Grombach, 1985; Jechlinger et al., 1985; Seyfried and Olthoff, 1985). For b o t h a e s t h e t i c ( b l a c k i s h w a t e r ) a n d e c o n o m i c r e a s o n s ( w a t e r flow d i m i n u t i o n i n t h e p i p e s d u e to i r o n a n d m a n g a n e s e o x i d e d e p o s i t s ) , t h e m a x i m u m a d m i s s i b l e c o n c e n t r a t i o n s of i r o n a n d m a n g a n e s e h a v e b e e n l i m i t e d to 0.3 a n d 0.05 p p m ( E E C , U S , C a n a d a ) , r e s p e c t i v e l y . In France, excessive concentrations have been observed in alluvial plains, p a r t i c u l a r l y i n t h e R h 6 n e V a l l e y ( M o s s e r , 1968; P o g g i , 1968; M a l e s s a r d , 1983; R e c o u l e s , 1984). I n o r d e r to i d e n t i f y t h e o r i g i n ( a u t o g e n o u s o r e x o g e n o u s ) of m a n g a n e s e a n d
0048-9697/89/$03.50
© 1989 Elsevier Science Publishers B.V.
170
Montfrin
GARD DERIVATION
I
PK258 ~ "O~ t
"PK259
"PK261 A~Vallabr~gues
CHANNEL I
• PK262
L'Aiguille
I
IHYDROELECTRI I C POWER PLANT
DAM
VALLOFRE6UES Caille I /%"~.,~,'.~L~%\~
RHONE BACKWATER I
/ I
CHANNEL
% ~, ~' ,',l~lli}" PK267
PUMPING WELLS
PK268
TARASCQN
PK269
°/"hl I/WI I,';Lq" eK27o
Fig. 1. Location of aquifer and pumping wells.
the processes whereby concentrations increase in water, it is necessary to obtain information about: the geology of the environment; aquifer flow and the physical chemical conditions (water and sediments); metal speciation in sediments; and the influence of micro-organisms.
171 The e x p e r i m e n t a l site is an alluvial w a t e r table of the R i v e r Rh6ne, s i t u a t e d d o w n s t r e a m from a h y d r o e l e c t r i c p l a n t and w h e r e the city of B e a u c a i r e pumps domestic w a t e r c o n t a i n i n g c o n t i n u o u s l y i n c r e a s i n g m a n g a n e s e c o n c e n t r a tions. THE STUDY SITE The s t u d y a r e a includes t h r e e s t r u c t u r a l geological units (Dumousseau, 1986): the C r e t a c e o u s massifs, the Mio-Pliocene hills located m a i n l y on the r i g h t b a n k of the River Rh6ne, and the alluvial plain. The aquifer is n o r t h of B e a u c a i r e , 2600 m long and 100-500 m wide, o r i e n t e d N W - S W , b o r d e r e d by the River R h 6 n e to the east, has an area of ~ 80 ha and an a v e r a g e altitude of 10 m (Fig. 1). F o l l o w i n g the c o n s t r u c t i o n of a h y d r o e l e c t r i c p l a n t in 1970 (Fig. 1), the part of the river c o n t i g u o u s with the aquifer b e c a m e a b a c k w a t e r . The reserve flow is 10m'~s 1; r e c o r d e d flows in the section of the River R h 6 n e t h a t feeds the aquifer are very w e a k and do n o t exceed 10m 3s 1 for most of the year. In all the drill holes we made into the aquifer, coarse a l l u v i u m with pebbles, gravel and sand were found, deposited d u r i n g the Q u a t e r n a r y and topped with s a n d y - c l a y loam. A detailed s t u d y of the sediments suggested t h a t the alluvial filling t o o k place d u r i n g different periods and came from several rivers: the Gard, previous shiftings of the River R h 6 n e and the present River Rh6ne. Thus the aquifer TABLE 1 Concentrations of carbon species (groundwater and River Rh6ne)
Rh6ne Well 1
Mineral dissolved carbon (ppm)
Total dissolved carbon (ppm)
Organic dissolved carbon (ppm)
Total carbon in suspended matter (ppm)
30.0 25.0
44.0 30.5
14.0 5.5
5.0 0.2~).5
TABLE 2 Average manganese concentrations (groundwater and River Rh6ne) (n = run number)
Rh6ne Well 1 Well 2 Well 3 P9 P 499 P 501
Mn (ppm)
n
0.075 i 0.16 0.29 + 0.44 1.09 + 0.9 1.29 + 1.46 0.50 + 0.44 0.17 _+ 0.13 0.57 +_ 0.82
11 48 48 48 7 7 7
172 shows some heterogeneity and can be described simply as a succession of stratified lenses of variable composition. At the top of the pebble bed the loam deposits are of variable depth. Their base is between - 2.60 and - 1.8 m (French Geodesical Level, FGL); geological section shows the close association that exists between the River RhSne and the Beaucaire aquifers. All the groundwater levels measured between 1965 and 1984 have shown a dual aquifer feed by karstic water supply from the calcareous strata that borders the west, particularly during the river's low-water periods, and by the River RhSne. The latter provides the main feed, which, though dominant, has clearly been decreasing since the dam was built. Results of physico-chemical analyses confirm the influence of the River Rh6ne in feeding the aquifer. Data obtained in previous work (Claire et al., 1986) showed that groundwater was hydrogenocarbonated-calcic; dissolved oxygen concentrations showed some geographical dispersion that might be related to the structure of the aquifer. The system showed an oxygen deficit, the origin of which is the dam built upstream. The average concentration was 2.6 + 1.9ppm for an average temperature of 13°C; the values were often < 1 ppm during the low-water season. We also noted an important decrease of organic matter content when the water ran from the river to the aquifer (Table 1). Manganese concentrations also reflected some dispersion in the whole aquifer (Table 2). No direct correlation was noted between the river flow and the metal concentrations in the groundwater, but the manganese concentrations were lower in the River Rh6ne than in the groundwater. Therefore, it seems unlikely that the river could be responsible for the high manganese concentrations in the groundwater, particularly as metals do not migrate through sediments and are adsorbed in the first decimetres of a river or lake bed (Ingolds and Wilroy, 1963; Delfino and Lee, 1968; Nembrini et al., 1982). This seems to be confirmed by results obtained using a P449 piezometer under the direct influence of the River Rh6ne, where the dissolved oxygen concentrations were twice that of other piezometers, which corresponds to lower manganese content. These results suggest an autogenous manganese origin, particularly as the important deficit in dissolved oxygen in groundwater favors the existence of soluble divalent manganese, mainly in its Mn(H20)62÷ , Mn(H20)5OH ÷ and Mn(H20)5(HCO3) ~ forms by creating a non-oxidizing environment. To verify this hypothesis and to explain the processes of constantly increasing manganese concentration in the groundwater, it was necessary to make further geological, chemical and microbiological analyses of the sediments. They were carried out by means of deep drilling into the aquifer. SAMPLINGAND ANALYTICALMETHODS Drilling (PS3), with a core extraction made in the pumping well region, provided samples of alluvium from the aquifer. The whole core was kept; granulometric analyses were made on sediments from different depths (from 10 to 40 m).
173
Geological analysis A complete granulometric analysis was made on all the washed and dried sediments. The elements, first selected with a stereo microscope, were observed with a scanning electron microscope (JEOL JSM35). The analysis by X-fluorescence and X-ray diffraction (Tracer Nothern energy dispersion spectrophotometer, TN-2000 series) made it possible to determine the structure and composition of iron and manganese oxides.
Chemical analysis The PS3 core sediments were dried at room temperature before being sieved. Only sediments < 5mm in diameter were kept for iron and manganese speciation measurements. Of all the techniques for specific chemical attack listed by Forstner and Whittman (1979) and by Fiszman et al. (1984), and whose reliability depends primarily on the element to be titrated and on the nature of the sediment, we chose the method recommended by Tessier et al. (1979) and by Balikungeri et al. (1985). In fact, interferences may occur between the different extracted phases (Tessier et al., 1979; Nembrini et al., 1982), however this method provides satisfactory results for many metals, including manganese and to a lesser extent iron. The determination of manganese concentration in each phase was performed by flameless atomic absorption spectrophotometry. Sequential extractions were made on 1 g of sediment. According to the depth and grain size, they made the selective removal of exchangeable, carbonate, oxide, organic and residual forms possible.
Biological analysis To assess the ability of groundwater to be enriched by manganese from the pebble bed, and to assess the part played by bacteria in the anaerobic process, in vitro anaerobic cultures were carried out. The manganese was incorporated into the culture by 5-mm diameter pebbles coated with microconcretions of manganate and were either sterilized (sterile blank) or unsterilized. The anaerobic environment was 5 g peptone, 0.5 g (NH4)3PO4, and 0.2 g cysteine for 250 ml of culture medium in a chamber where pyrogallol was present. The evolution of manganate on the pebbles (SEM), pH, reduction potential, bacteria numbers and manganese concentrations in the solution were monitored during the experiment. INFLUENCE OF MANGANESE SPECIATIONAND BACTERIALACTIVITY
Manganese speciation The percentage of each sediment fraction with depth is given in Table 3. The detailed manganese speciation results are given in Table 4.
174 TABLE 3 Percentage of each sediment fraction Fraction (mm)
Percentage according to sampling depth (m)
< 0.1 0.1-0.2 0.2-0.35 0.35-0.75 0.7~1.625 1.625-2.95 2.95-4 4-5 >5
10-15
15-22
23-25
25-28
28-30
30-32
32-34
34-36
36-40
0.08 0.26 1.06 2.56 4.44 8.15 1.99 2.64 78.82
0.098 0.22 0.7 1.68 5.95 7.85 0.94 1.22 81.35
0.05 0.14 0.40 1.09 4.27 6.67 1.08 1.76 84.54
0.022 0.17 0.64 0.99 3.14 3.47 1.33 1.96 88.28
0.13 0.59 1.70 1.73 2.29 3.57 2.16 2.89 84.94
0.17 0.60 1.70 2.92 3.83 8.06 4.78 5.77 72.17
0.13 0.35 1.31 1.91 2.57 5.60 3.60 6.94 77.59
0.09 0.30 4.50 8.07 3.6 5.67 2.87 4.49 70.4
0.015 0.04 0.07 0.09 0.23 0.75 3.60 6.94 88.27
According to Fig. 2, which illustrates the variations of total manganese concentrations in sediments, we can see that they reach their highest values in the deepest layers ( > 28 m depth), where well strainers are implanted. Analysis of the top alluvium and the substratum of the aquifer showed that these had - 1.3M
CLAY LOAM
SAND
.•1214°° O0 1500
-10M GRAVEL + 20 Z SAND
GRAVE--'~'L--T~ 50 % SANO
-
15.2
-
16.8 M
M
990
GRAVEL
(WITHOUT SAND: - 20.4
6RAVEL + 5 0 % SAND
- 24,5
1 800 M
70O SAND PEBBLES
I -30M
~kRLS
| -
i:
90M
LIME STONE
Fig. 2. Variation of total manganese concentration (ppm) in sediments according to depth (PS3).
175 TABLE 4 C o n c e n t r a t i o n of different m a n g a n e s e species (#g g - l ) in t e r m s of d e p t h a n d s e d i m e n t g r a i n size (mm) G r a i n size
Fraction Exchangeable
Carbonate
Oxide
Organic
Residual
Total
B e t w e e n 10 a n d 1 5 m d e p t h < 0.1 29 0.1-0.2 17 0.2-0.35 13 0.35-0.75 8 0.75-1.625 6 1.625-2,95 3 2.954 3 4 5 13
6 2 1.5 0.5 0.7 1 0.5 3
136 113 95 97 88 78 63 170
7 4.5 3 3 3.5 2.5 2 10
50 43 24 33 41 38 25.5 93
228 179.5 136.5 141.4 139.2 122.5 94 289
B e t w e e n 15 a n d 2 2 m d e p t h < 0.1 12 0.1-0.2 18 0.2-0.35 10 0.354).75 5 0.75-1.625 4 1.62~2.95 5.8 2.95-4 4.4 4-5 5
16 3 1 1 1 0.9 0.7 1
147 106 101 89 96 72 43 73
12 6 4 3 3 1.7 1.4 2
65 32 25 19 32 25 25 26
25 165 141 117 136 74.5 74.5 107
B e t w e e n 22 a n d 25 m d e p t h < 0.1 27 0.1-0.2 117 0.2-0.35 10 0.35-0.75 25 0.75-1.625 13 1.625-2.95 21 2.954 22.5 4 5 103
31 117 67 27 50 21.5 121 79
245.5 124 103 60.5 88 70.5 78 200
10 12 7 11 4 10 2.5 12
23 20.5 27 10 12 9 1 18.5
336 390.5 214 133.5 167 132 225 412.5
B e t w e e n 25 a n d 2 8 m d e p t h < 0.1 63 0.1-0.2 28 0.2-0.35 10 0.354).75 9 0.75-1.625 15 1.625-2.95 29 2.954 14
73 51 83 32.5 42.5 61 50
132 85 104 86 190 265.5 170
9 10 5 1.2 4.3 4 4
12 13 12 11.7 16.7 15 5
289 187 214 140.4 268.5 374 243
B e t w e e n 28 a n d 30 m d e p t h < 0.1 89 0.1-0.2 60 0.2-0.35 48 0.35-0.75 36 0.75--1.625 22.5 1.625-2.95 17.5
13 5.5 2 2 2 1.5
280 189 100 124 127 265
31 10 9 11 17 18
95 57 23 68 62 66
508 321 182 241 230 368
(continued)
176
(continued)
TABLE 4
Concentration of different manganese species (pg g- 1) in terms of depth and sediment grain size (mm) Grain size
Fraction Exchangeable
2.95-4 4-5
41 47
Carbonate 3.5 2.5
Between 30 and 32 m depth < 0.1 117 0.1~0.2 32.5 0.2~).35 45.5 0.35~.75 28 0.75-1.625 42 1.625-2.95 22.5 2.95-4 4 4-5 9.5
22 14 2 1.5 1.5 1 0.5 1
Oxide
Organic
Residual
Total
126 189
10.5 6
82 53
263 297.5
380.5 396 116 126 139.5 163.5 53 72
18.5 39 6.5 4 4 3 3 5.5
112 63 136 40 28 24 7 34.5
650 544.5 306 199.5 215 214 67.5 122.5
high manganese contents of 1200-1500 ppm from 7 to 10 m depth (top alluvium), and of 2200-2800 ppm from 45 to 51 m depth (substratum). From 7 to 10 m depth, manganese was found essentially as the carbonate (60~5%) and to a lesser extent as the oxide (Fig. 3). However, the low permeability of the top alluvium did not allow access to the deepest regions, even during strong water leaching. The results were more interesting for manganese speciation in aquifer sediments, extending from 10 to 32 m depth, according to grain size and depth (Table 4). The concentration depended on grain size, with the finest particles containing the most manganese (Forstner and Whittman, 1979), although it was only 10% of the total (balanced concentration according to the representative weight of the fraction).
80 70
..........................................
e' EXCHRN
I
-7 -22
C~RBON OXIDE
ORGRHI RESIDU
EXTRRCTED FRe~CTI ON I ~ -10 TO -22 N M -28 TO -32 M TO - 2 8 I'1
TO - 1 8
Fig. 3. Percentages of different chemical species of manganese according to depth.
177
÷
÷+
+ + + ÷ + +
0
+1
6 x Z~
2
0
~D
-+-i +1
+i
+l
+1
I
I
I
I
]
178 Independent of depth and grain size the oxide form was dominant, reaching 50-60% of the total manganese concentration (Fig. 3). The organic form was less important, and the concentrations of exchangeable and carbonate forms were similar to the residual form, i.e.: from 10 to 22 m depth, oxide > residual > exchangeable from 22 to 28 m depth, oxide > carbonate > exchangeable from 28 to 32m depth, oxide > residual > exchangeable. The mineralogical study (SEM and X-ray diffraction) confirmed the speciation results. The manganese was found on the pebbles as the oxide with an average size of 10 #m. It was found on all siliceous or calcareous alluvium types, mainly on those having rough surfaces. Oxides were usually represented on siliceous components by massive microconcretions belonging to the Z-disordered manganate type (Birnessite), often accompanied by lamellae (sometimes in isolated concretions) of manganate type 10A or todorokite. On the calcareous components, manganates attributable to the 7A or Birnessite manganate type prevail (but the other two types coexist). Minor elements are also present in these manganates: A1, Fe, K, Ca and Ba.
Role of bacterial population The results are presented in Table 5. During the experiment a reducing environment was maintained and the pH remained constant. The increase with time of dissolved manganese concentrations was slight in the sterile culture, but significantly higher in the presence of bacteria. Manganese release does not seem to be a function of the number of bacteria, but probably their respiratory activity. In the presence of bacteria the gravels were progressively cleared of manganese deposits. The physical-chemical conditions of manganese reduction were achieved in the two cultures, and the part played by bacteria in manganese release kinetics was obvious.
Discussion The results of manganese speciation, and of the mineralogical study, show that the oxides settled onto the pebble bed after its formation, essentially as oxides (Z-disordered manganate type, 10 A manganate, todorokite, birnessite); the manganese is of autogenous origin. Because of changes in the physicalchemical conditions of the system, the oxide progressively dissolves in the groundwater. The rate of manganese dissolution is considerably enhanced by bacterial processes. When the environment is low in oxygen, the energy for this bacterial reduction is obtained through the oxidation of the organic matter existing in the system. From this the reduction of oxides linked to sediments into manganous compounds follows, which are then released into the groundwater. These results on the origin of the manganese and on its dissolution processes in groundwater make it possible for us to propose an in situ treatment,
179
-..--- ~ 11 METER~ I
WATE_._.RROXIGENATION ~ - - I ------'-"~"
VYRED~-X P~
EXPLOITATION PUMPS
nFC~.S.SINGTANK h
1
~
P2
-P3
Fig. 4. Application of the Vyredox process.
involving the most suitable direct oxygenation of the groundwater appropriate for the water table conditions APPLICATION OF THE VYREDOX PROCESS
Principle This technique consists of eliminating the dissolved iron and manganese before pumping, by creating a highly oxidized zone around the well to be treated by periodic injections of aerated water (Glensvig, 1976; Descroix, 1978; Foliot, 1978 Descroix and Laurent, 1979; Van Beek, 1985; Grombach, 1985; Jechlinger et al., 1985; Seyfried and Olthoff, 1985). With this proposal there is a risk of filling the aquifer pores. This will be discussed in terms of the results on manganese origin and dissolution processes previously obtained. The different stages in groundwater treatment with this process are the following: - - pumping is stopped in the well to be treated - - water from another well is aerated by mixing it with atmospheric air the aerated water is degassed in a tank to eliminate non-dissolved oxygen and other undesirable gases - - t h e oxygen-saturated water is then returned by gravity to the aquifer through the well to be treated and through satellite wells. This operation is continuous and lasts for ~ 20 h - - t h e process is halted for 4 h in order to allow the establishment of a high-potential redox zone in the aquifer after a recharging sequence, water with low concentrations of iron and manganese can be pumped and sent directly into the water mains. It can be used until the dissolved iron and manganese concentrations return to their normal levels (until five to ten times more water has been pumped than has been recharged). The treatment operation is then repeated. In practice, for each site, a preliminary assay is necessary to verify whether -
-
-
-
180 TABLE 6 Main results for P2 after a recharging sequence (Cycle No. 6) (injection of 2120 m3 aerated water and a 18h pause). Pumping flow 135m3h 1 Volume (m3)
Fe
Mn
pH
02
171 467 2530 2972 3538 3983 6007 6533 7103 7549 9658 10105 10676 11123 13235 13682 14253 14699 16810 17257 17828 18275 18381
0.09 0.02 0.02 0:025 0.017 0.04 0.015 0.02 0.02 0.01 0.01 0.03 0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.02 0.02 0.025
0.03 0.03 0.03 0.03 0.04 0.035 0.045 0.045 0.045 0.055 0.065 0.07 0.075 0.08 0.085 0.09 0.12 0.12 0.10 0.134 0.16 0.11 STOP
7.6 7.6 7.6 7.6 7.3 7.3 7.3 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.8 7.8 7.8 7.8 7.8
8.5 7.4 2.5 2.2 1.9 2.0 1.8 1.8 1.7 1.6 1.5 1.4 1.4 1.3 1.3 1.3 1.3 1.3 1.4 1.3 1.3 1.3
t h e p r o c e s s c a n be u s e d a n d to d e t e r m i n e t h e m o d e a n d size of a p e r m a n e n t installation.
Assay A t t h e B e a u c a i r e s i t e a n e x p e r i m e n t w a s c a r r i e d o u t u s i n g P1 a n d P2 w e l l s (Fig. 4). T h e w a t e r w a s p u m p e d f r o m P1, a e r a t e d , d e g a s s e d a n d i n j e c t e d i n t o P2. A f t e r a t e m p o r a r y i n t e r r u p t i o n , t h e o p e r a t i o n w a s r e v e r s e d a n d P2 w a s p u m p e d a n d w a t e r w a s i n j e c t e d i n t o P1. D u r i n g t h e p r e l i m i n a r y tests, t h i s s c h e m e m a d e it p o s s i b l e to t r e a t t h e t w o w e l l s s i m u l t a n e o u s l y w i t h o u t b o r i n g a n y s a t e l l i t e wells. T h e t r e a t m e n t d e v i c e w a s s i t u a t e d a t a n e q u a l d i s t a n c e f r o m t h e t w o w e l l s (Fig. 4), t h e o x y g e n a t i o n s t a t i o n w a s f u l l size, w i t h a flow s i m i l a r to t h a t of t h e w o r k s ( 1 5 0 m 3 h 1). T h e a e r a t e d w a t e r v o l u m e to be i n j e c t e d w a s 2500 m 3 for e a c h 24-h cycle, w h i c h w a s d i v i d e d as follows: - - p u m p i n g f r o m P1 a n d i n j e c t i o n i n t o P2 for 1 8 h - - i n t e r r u p t i o n of p u m p i n g a n d p a u s e for P2 (6 h) D u r i n g t h e n e x t c y c l e t h e o p e r a t i o n w a s r e v e r s e d , a n d so on. E v o l u t i o n of dissolved oxygen, i r o n a n d m a n g a n e s e c o n c e n t r a t i o n s was
181 followed in order to specify the pumped volume beyond which the manganese concentration would be above the limit for drinking water. From one cycle to another we noticed the regular decrease of iron and manganese concentrations owing to injections of recharged water that was less and less loaded with these elements and to the creation of a zone enriched in dissolved oxygen around the treated wells. Normal manganese concentrations for drinking water were reached after four injection cycles. During a given cycle, pH and dissolved oxygen concentration decreased in the pumped water. The return of the system to its initial state accompanied a progressive increase in the iron and manganese concentrations. Long lasting pumping (cycle 6) (Table 6) showed that iron and manganese concentrations first increased slowly, while the pH and dissolved oxygen decreased. The latter stabilized at the low value of 1.3 mg 1 1 characteristic of the aquifer. The concentration of manganese then increased more quickly. The ratio of pumped water volume (concentrations ~< manganese limit for drinking water) to injected aerated water volume was 3.5.
Discussion According to the theoretical model proposed by different authors, the Vyredox system results in precipitation at the limit of the enriched oxygen zone; the secondary effect would be to fill the pores of the aquifer. Conversely, according to this model, the injection of aerated water helps to maintain a population of oxidizing bacteria, which contributes to the precipitation of the metals and, by its activity, maintains a high pH and redox potential. Even for this hypothesis, a theoretical estimate (Descroix, 1978; Foliot, 1978) shows that for a 5 mg 1 1 iron concentration, the life of the aquifer would be greater than 100 years, despite the risk of its pores becoming progressively filled. However, our studies on the origin and the experimental evidence of manganese dissolution by bacteria make us believe that this risk is non-existent or minimal, and therefore we propose another model. In natural conditions, the release of iron and especially of manganese stems from bacterial activity owing to oxygen deficiency. In this case, the facultative anaerobic bacteria of the aquifer use the metallic oxides as final acceptors of electrons, leading to the dissolution of iron and manganese. The oxygen supplied by the Vyredox process stops the release of Mn, both by a redox increase and through a modification of bacterial respiration: the bacteria readily use the dissolved oxygen molecules. It is likely that the bacterial and chemical processes are superposed. The metallic oxides would remain on their substrate (pebbles) in the aquifer, which eliminates any risk of pores becoming filled, at least in the oxidizing zone. We also note that the pores in aquifers in Finland, Sweden, and in the Federal Republic of Germany, where Vyredox stations have been working for more than 10 years, show no sign of becoming filled.
182 CONCLUSION
An understanding of the processes involved in groundwater contamination by manganese reveals the usefulness of determining its speciation on aquifer sediments. This information is necessary both for preventive and curative treatment. If the aquifer is drilled and a chemical analysis reveals the presence of manganese on the sediments, it would be advisable to adopt the necessary recommendations for aquifer protection. To prevent manganese release into the groundwater and then into the main water supply, it is necessary to avoid enrichment of the aquifer by organic matter, which should induce a reducing environment. When it is too late for prevention, as in the case of autogenous manganese, an inexpensive in situ treatment can be carried out. The success we have had must be added to the numerous other successes involving permanent Vyredox stations around the world (in 1984 more than 50 in eight European countries and in the U.S.A.). Therefore, although some critics of this method (Mouchet and Rault, 1979) doubt the reliability of the process, it seems that its suitability for solving the problem of contamination of groundwater by autogenous manganese and iron has been proven. The use of this technique requires an initial study of the origin of these metals, as we have done in the case described here. When the manganese and (or) iron origin is autogenous, the artificial oxygenation of the aquifer allows the metallic oxides to remain on the components of the strata forming the aquifer, which eliminates any risk of the pores becoming filled at least at the periphery of the wells.
REFERENCES Balikungeri, A., D. Robin and W. Haerdi, 1985. Manganese in natural waters. 1. Speciation of manganese in running waters and sediments. Int. J. Environ. Anal. Chem., 19: 227-231. Beek, C.G.E.M van, 1985. Experiences with underground water treatment in the Netherlands. Water Supply, 3 (1): 1-11. Claire, Y., B. Dumousseau, P. Jaudon, J.C. Landry, C. Massiani, J. Rey and E. Vacelet 1986. Manganese dans l'eau d'alimentation de la ville de Beaucaire: origines et solutions. Int. J. Environ. Anal. Chem., 24: 37-54. Delfino, J.J. and F.G. Lee, 1968. Chemistry of manganese in lake Mendota (Wisconsin). Environ. Sci. Technol., 2 (12): 1094-1100. Descroix, P., 1978. La m~thode Vyredox pour l'enl~vement du fer et du manganese des eaux souterraines. Tech. Sci. Munic. l'eau, 8-9: 437-442. Descroix, P. and J. Laurent., 1979. R~ponse ~ rarticle de MM P. MOUCHET et B. RAULT. Tech. Sci. Munic. l'eau, 2: 101-104. Dumousseau, B., 1986. Le manganese dans une nappe alluviale: ~tude du site de Beaucaire (Gard, France). Thesis, Univ. Aix-Marseille, France, 201 pp. Fiszman, M., W.C. Pfeiffer and L.D. Lacerda, 1984. Comparison of methods used for extraction and geochemical distribution of heavy metals in bottom sediments from Sepetiba Bay. Environ. Technol. Lett., 5 (12): 567-571. Foliot, B., 1978. Un proc~d~ "~cologique" de traitement du fer et du manganese contenus dans les eaux souterraines. Genie Rur., 4:4
183 Forstner, U. and G.T.V. Wittman, 1979. Metal Pollution in the Environment. Springer Verlag, 486 pp. Glensvig, L., 1976. The first Vyredox water supply plant in Denmark, Vandteknik, 42 (2): 3-8. Griffin, A.E., 1958. Problems caused by manganese in water supplies. J. Am. Water Works Assoc., 50 (10): 1386-1382. Grombach, P., 1985. Groundwater treatment in situ in the aquifer. Water Supply, 3(1): 13-18. Ingols, S. and D. Wilroy, 1963. Mechanism of manganese solution in lake water. J. Am. Water Works Assoc., 3: 282-290. Jechlinger, G., W. Kasper, F. Scholler and F, Seidelberger, 1985. The removal of iron and manganese in groundwaters through aeration underground. Water Supply, 3 (1): 19-25. Malessard, G., 1983. Sur la presence de fer et de manganese dans l'aquif~re alluvial de la plaine d'Avignon. Thesis, Facult~ des Sciences d'Avignon, Avignon, France, 122 pp. Mosser, C., 1968. Le fer et le manganese dans la nappe phr~atique du Rhin au nord de Strasbourg. Thesis, Facult~ des Sciences de Strasbourg, Strasbourg, France. Mouchet, P. and B. Rault, 1979. Apropos du proc~d~ Vyredox. Tech. Sci. Munic. l'eau 2: 99-101. Nembrini, G.O., F. Rapin, J.L. Garcia and U. Forstner, 1982. Speciation of Fe and Mn in a sediment core of the baie de Villefranche (Mediterranean Sea, France). Environ. Technol. Lett., 3: 545-552. Poggi, J.P., 1968. Contribution h l'etude hydrog~ologique alluviale du RhSne entre Beaucaire et Arles. Thesis, Universit~ des Sciences et Techniques du Languedoc, Montpellier, France, 198 pp. Recoules, A., 1984. Evolutions des teneurs en fer et en manganese dans la nappe de la Sa6ne aux environs de Gray (Haute SaSne). Thesis, Facult~ des Sciences et Techniques de Franche-Comt~, Besancon, France, 153 pp. Riddick, T.M., N.L. Lindsay and A. Tomassi, 1958. Iron and manganese in water supplies. J. Am. Water Works Assoc., 50 (5): 688~96. Seyfried, C.F. and R. Olthoff, 1985. Underground removal of iron and manganese. Water Supply, 3 (1): 117 142. Tessier, A., P.G.C. Campbell and M. Bisson, 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem., 51 (7): 844-851.
169
G R O U N D W A T E R POLLUTION BY M A N G A N E S E . M A N G A N E S E SPECIATION: APPLICATION TO THE SELECTION AND D I S C U S S I O N OF AN IN SITU G R O U N D W A T E R T R E A T M E N T
P. JAUDON, C. MASSIANI, J. GALEA and J. REY Laboratoire Chimie et Environnement, Universitd de Provence, 3 place Victor Hugo, 13331 Marseille cedex 3 (France)
E. VACELET C.O.M. Station Marine d' Endoume, 13007 Marseille (France)
(Received March 15th, 1988; accepted January 24th, 1989)
ABSTRACT All over the world, drinking water pumped into an alluvial water table frequently becomes progressively polluted by dissolved manganese and iron, causing problems for regulatory authorities. It has been shown that, in addition to classical hydrogeological studies, manganese speciation and bacteriological experiments are essential to determine the origin of the manganese and the processes of water pollution in order to select the most appropriate treatment. In the aquifer studied here, speciation and mineralogical results showed that manganese originates from within the aquifer itself, mainly in the form of oxides (Z-disordered manganate type, 10/~ manganate, todorokite, birnessite) which settle on components of the pebble bed forming the aquifer. Manganese release is due to a decrease of dissolved oxygen in the groundwater. Under these conditions, Mn(IV) is reduced both chemically and bacterially into the Mn(II) soluble form. In such situations an in situ water treatment consisting of the oxygenation of the aquifer has proved to be both suitable and inexpensive. The risks of aquifer contamination are discussed in terms of manganese speciation.
INTRODUCTION
For a long time, in many countries, the presence of iron and manganese caused a problem for regulatory authorities in relation to industrial and main water supplies (Griffin, 1958; Riddick et al., 1958; Glensvick, 1976; Van Beek, 1985; Grombach, 1985; Jechlinger et al., 1985; Seyfried and Olthoff, 1985). For b o t h a e s t h e t i c ( b l a c k i s h w a t e r ) a n d e c o n o m i c r e a s o n s ( w a t e r flow d i m i n u t i o n i n t h e p i p e s d u e to i r o n a n d m a n g a n e s e o x i d e d e p o s i t s ) , t h e m a x i m u m a d m i s s i b l e c o n c e n t r a t i o n s of i r o n a n d m a n g a n e s e h a v e b e e n l i m i t e d to 0.3 a n d 0.05 p p m ( E E C , U S , C a n a d a ) , r e s p e c t i v e l y . In France, excessive concentrations have been observed in alluvial plains, p a r t i c u l a r l y i n t h e R h 6 n e V a l l e y ( M o s s e r , 1968; P o g g i , 1968; M a l e s s a r d , 1983; R e c o u l e s , 1984). I n o r d e r to i d e n t i f y t h e o r i g i n ( a u t o g e n o u s o r e x o g e n o u s ) of m a n g a n e s e a n d
0048-9697/89/$03.50
© 1989 Elsevier Science Publishers B.V.
170
Montfrin
GARD DERIVATION
I
PK258 ~ "O~ t
"PK259
"PK261 A~Vallabr~gues
CHANNEL I
• PK262
L'Aiguille
I
IHYDROELECTRI I C POWER PLANT
DAM
VALLOFRE6UES Caille I /%"~.,~,'.~L~%\~
RHONE BACKWATER I
/ I
CHANNEL
% ~, ~' ,',l~lli}" PK267
PUMPING WELLS
PK268
TARASCQN
PK269
°/"hl I/WI I,';Lq" eK27o
Fig. 1. Location of aquifer and pumping wells.
the processes whereby concentrations increase in water, it is necessary to obtain information about: the geology of the environment; aquifer flow and the physical chemical conditions (water and sediments); metal speciation in sediments; and the influence of micro-organisms.
171 The e x p e r i m e n t a l site is an alluvial w a t e r table of the R i v e r Rh6ne, s i t u a t e d d o w n s t r e a m from a h y d r o e l e c t r i c p l a n t and w h e r e the city of B e a u c a i r e pumps domestic w a t e r c o n t a i n i n g c o n t i n u o u s l y i n c r e a s i n g m a n g a n e s e c o n c e n t r a tions. THE STUDY SITE The s t u d y a r e a includes t h r e e s t r u c t u r a l geological units (Dumousseau, 1986): the C r e t a c e o u s massifs, the Mio-Pliocene hills located m a i n l y on the r i g h t b a n k of the River Rh6ne, and the alluvial plain. The aquifer is n o r t h of B e a u c a i r e , 2600 m long and 100-500 m wide, o r i e n t e d N W - S W , b o r d e r e d by the River R h 6 n e to the east, has an area of ~ 80 ha and an a v e r a g e altitude of 10 m (Fig. 1). F o l l o w i n g the c o n s t r u c t i o n of a h y d r o e l e c t r i c p l a n t in 1970 (Fig. 1), the part of the river c o n t i g u o u s with the aquifer b e c a m e a b a c k w a t e r . The reserve flow is 10m'~s 1; r e c o r d e d flows in the section of the River R h 6 n e t h a t feeds the aquifer are very w e a k and do n o t exceed 10m 3s 1 for most of the year. In all the drill holes we made into the aquifer, coarse a l l u v i u m with pebbles, gravel and sand were found, deposited d u r i n g the Q u a t e r n a r y and topped with s a n d y - c l a y loam. A detailed s t u d y of the sediments suggested t h a t the alluvial filling t o o k place d u r i n g different periods and came from several rivers: the Gard, previous shiftings of the River R h 6 n e and the present River Rh6ne. Thus the aquifer TABLE 1 Concentrations of carbon species (groundwater and River Rh6ne)
Rh6ne Well 1
Mineral dissolved carbon (ppm)
Total dissolved carbon (ppm)
Organic dissolved carbon (ppm)
Total carbon in suspended matter (ppm)
30.0 25.0
44.0 30.5
14.0 5.5
5.0 0.2~).5
TABLE 2 Average manganese concentrations (groundwater and River Rh6ne) (n = run number)
Rh6ne Well 1 Well 2 Well 3 P9 P 499 P 501
Mn (ppm)
n
0.075 i 0.16 0.29 + 0.44 1.09 + 0.9 1.29 + 1.46 0.50 + 0.44 0.17 _+ 0.13 0.57 +_ 0.82
11 48 48 48 7 7 7
172 shows some heterogeneity and can be described simply as a succession of stratified lenses of variable composition. At the top of the pebble bed the loam deposits are of variable depth. Their base is between - 2.60 and - 1.8 m (French Geodesical Level, FGL); geological section shows the close association that exists between the River RhSne and the Beaucaire aquifers. All the groundwater levels measured between 1965 and 1984 have shown a dual aquifer feed by karstic water supply from the calcareous strata that borders the west, particularly during the river's low-water periods, and by the River RhSne. The latter provides the main feed, which, though dominant, has clearly been decreasing since the dam was built. Results of physico-chemical analyses confirm the influence of the River Rh6ne in feeding the aquifer. Data obtained in previous work (Claire et al., 1986) showed that groundwater was hydrogenocarbonated-calcic; dissolved oxygen concentrations showed some geographical dispersion that might be related to the structure of the aquifer. The system showed an oxygen deficit, the origin of which is the dam built upstream. The average concentration was 2.6 + 1.9ppm for an average temperature of 13°C; the values were often < 1 ppm during the low-water season. We also noted an important decrease of organic matter content when the water ran from the river to the aquifer (Table 1). Manganese concentrations also reflected some dispersion in the whole aquifer (Table 2). No direct correlation was noted between the river flow and the metal concentrations in the groundwater, but the manganese concentrations were lower in the River Rh6ne than in the groundwater. Therefore, it seems unlikely that the river could be responsible for the high manganese concentrations in the groundwater, particularly as metals do not migrate through sediments and are adsorbed in the first decimetres of a river or lake bed (Ingolds and Wilroy, 1963; Delfino and Lee, 1968; Nembrini et al., 1982). This seems to be confirmed by results obtained using a P449 piezometer under the direct influence of the River Rh6ne, where the dissolved oxygen concentrations were twice that of other piezometers, which corresponds to lower manganese content. These results suggest an autogenous manganese origin, particularly as the important deficit in dissolved oxygen in groundwater favors the existence of soluble divalent manganese, mainly in its Mn(H20)62÷ , Mn(H20)5OH ÷ and Mn(H20)5(HCO3) ~ forms by creating a non-oxidizing environment. To verify this hypothesis and to explain the processes of constantly increasing manganese concentration in the groundwater, it was necessary to make further geological, chemical and microbiological analyses of the sediments. They were carried out by means of deep drilling into the aquifer. SAMPLINGAND ANALYTICALMETHODS Drilling (PS3), with a core extraction made in the pumping well region, provided samples of alluvium from the aquifer. The whole core was kept; granulometric analyses were made on sediments from different depths (from 10 to 40 m).
173
Geological analysis A complete granulometric analysis was made on all the washed and dried sediments. The elements, first selected with a stereo microscope, were observed with a scanning electron microscope (JEOL JSM35). The analysis by X-fluorescence and X-ray diffraction (Tracer Nothern energy dispersion spectrophotometer, TN-2000 series) made it possible to determine the structure and composition of iron and manganese oxides.
Chemical analysis The PS3 core sediments were dried at room temperature before being sieved. Only sediments < 5mm in diameter were kept for iron and manganese speciation measurements. Of all the techniques for specific chemical attack listed by Forstner and Whittman (1979) and by Fiszman et al. (1984), and whose reliability depends primarily on the element to be titrated and on the nature of the sediment, we chose the method recommended by Tessier et al. (1979) and by Balikungeri et al. (1985). In fact, interferences may occur between the different extracted phases (Tessier et al., 1979; Nembrini et al., 1982), however this method provides satisfactory results for many metals, including manganese and to a lesser extent iron. The determination of manganese concentration in each phase was performed by flameless atomic absorption spectrophotometry. Sequential extractions were made on 1 g of sediment. According to the depth and grain size, they made the selective removal of exchangeable, carbonate, oxide, organic and residual forms possible.
Biological analysis To assess the ability of groundwater to be enriched by manganese from the pebble bed, and to assess the part played by bacteria in the anaerobic process, in vitro anaerobic cultures were carried out. The manganese was incorporated into the culture by 5-mm diameter pebbles coated with microconcretions of manganate and were either sterilized (sterile blank) or unsterilized. The anaerobic environment was 5 g peptone, 0.5 g (NH4)3PO4, and 0.2 g cysteine for 250 ml of culture medium in a chamber where pyrogallol was present. The evolution of manganate on the pebbles (SEM), pH, reduction potential, bacteria numbers and manganese concentrations in the solution were monitored during the experiment. INFLUENCE OF MANGANESE SPECIATIONAND BACTERIALACTIVITY
Manganese speciation The percentage of each sediment fraction with depth is given in Table 3. The detailed manganese speciation results are given in Table 4.
174 TABLE 3 Percentage of each sediment fraction Fraction (mm)
Percentage according to sampling depth (m)
< 0.1 0.1-0.2 0.2-0.35 0.35-0.75 0.7~1.625 1.625-2.95 2.95-4 4-5 >5
10-15
15-22
23-25
25-28
28-30
30-32
32-34
34-36
36-40
0.08 0.26 1.06 2.56 4.44 8.15 1.99 2.64 78.82
0.098 0.22 0.7 1.68 5.95 7.85 0.94 1.22 81.35
0.05 0.14 0.40 1.09 4.27 6.67 1.08 1.76 84.54
0.022 0.17 0.64 0.99 3.14 3.47 1.33 1.96 88.28
0.13 0.59 1.70 1.73 2.29 3.57 2.16 2.89 84.94
0.17 0.60 1.70 2.92 3.83 8.06 4.78 5.77 72.17
0.13 0.35 1.31 1.91 2.57 5.60 3.60 6.94 77.59
0.09 0.30 4.50 8.07 3.6 5.67 2.87 4.49 70.4
0.015 0.04 0.07 0.09 0.23 0.75 3.60 6.94 88.27
According to Fig. 2, which illustrates the variations of total manganese concentrations in sediments, we can see that they reach their highest values in the deepest layers ( > 28 m depth), where well strainers are implanted. Analysis of the top alluvium and the substratum of the aquifer showed that these had - 1.3M
CLAY LOAM
SAND
.•1214°° O0 1500
-10M GRAVEL + 20 Z SAND
GRAVE--'~'L--T~ 50 % SANO
-
15.2
-
16.8 M
M
990
GRAVEL
(WITHOUT SAND: - 20.4
6RAVEL + 5 0 % SAND
- 24,5
1 800 M
70O SAND PEBBLES
I -30M
~kRLS
| -
i:
90M
LIME STONE
Fig. 2. Variation of total manganese concentration (ppm) in sediments according to depth (PS3).
175 TABLE 4 C o n c e n t r a t i o n of different m a n g a n e s e species (#g g - l ) in t e r m s of d e p t h a n d s e d i m e n t g r a i n size (mm) G r a i n size
Fraction Exchangeable
Carbonate
Oxide
Organic
Residual
Total
B e t w e e n 10 a n d 1 5 m d e p t h < 0.1 29 0.1-0.2 17 0.2-0.35 13 0.35-0.75 8 0.75-1.625 6 1.625-2,95 3 2.954 3 4 5 13
6 2 1.5 0.5 0.7 1 0.5 3
136 113 95 97 88 78 63 170
7 4.5 3 3 3.5 2.5 2 10
50 43 24 33 41 38 25.5 93
228 179.5 136.5 141.4 139.2 122.5 94 289
B e t w e e n 15 a n d 2 2 m d e p t h < 0.1 12 0.1-0.2 18 0.2-0.35 10 0.354).75 5 0.75-1.625 4 1.62~2.95 5.8 2.95-4 4.4 4-5 5
16 3 1 1 1 0.9 0.7 1
147 106 101 89 96 72 43 73
12 6 4 3 3 1.7 1.4 2
65 32 25 19 32 25 25 26
25 165 141 117 136 74.5 74.5 107
B e t w e e n 22 a n d 25 m d e p t h < 0.1 27 0.1-0.2 117 0.2-0.35 10 0.35-0.75 25 0.75-1.625 13 1.625-2.95 21 2.954 22.5 4 5 103
31 117 67 27 50 21.5 121 79
245.5 124 103 60.5 88 70.5 78 200
10 12 7 11 4 10 2.5 12
23 20.5 27 10 12 9 1 18.5
336 390.5 214 133.5 167 132 225 412.5
B e t w e e n 25 a n d 2 8 m d e p t h < 0.1 63 0.1-0.2 28 0.2-0.35 10 0.354).75 9 0.75-1.625 15 1.625-2.95 29 2.954 14
73 51 83 32.5 42.5 61 50
132 85 104 86 190 265.5 170
9 10 5 1.2 4.3 4 4
12 13 12 11.7 16.7 15 5
289 187 214 140.4 268.5 374 243
B e t w e e n 28 a n d 30 m d e p t h < 0.1 89 0.1-0.2 60 0.2-0.35 48 0.35-0.75 36 0.75--1.625 22.5 1.625-2.95 17.5
13 5.5 2 2 2 1.5
280 189 100 124 127 265
31 10 9 11 17 18
95 57 23 68 62 66
508 321 182 241 230 368
(continued)
176
(continued)
TABLE 4
Concentration of different manganese species (pg g- 1) in terms of depth and sediment grain size (mm) Grain size
Fraction Exchangeable
2.95-4 4-5
41 47
Carbonate 3.5 2.5
Between 30 and 32 m depth < 0.1 117 0.1~0.2 32.5 0.2~).35 45.5 0.35~.75 28 0.75-1.625 42 1.625-2.95 22.5 2.95-4 4 4-5 9.5
22 14 2 1.5 1.5 1 0.5 1
Oxide
Organic
Residual
Total
126 189
10.5 6
82 53
263 297.5
380.5 396 116 126 139.5 163.5 53 72
18.5 39 6.5 4 4 3 3 5.5
112 63 136 40 28 24 7 34.5
650 544.5 306 199.5 215 214 67.5 122.5
high manganese contents of 1200-1500 ppm from 7 to 10 m depth (top alluvium), and of 2200-2800 ppm from 45 to 51 m depth (substratum). From 7 to 10 m depth, manganese was found essentially as the carbonate (60~5%) and to a lesser extent as the oxide (Fig. 3). However, the low permeability of the top alluvium did not allow access to the deepest regions, even during strong water leaching. The results were more interesting for manganese speciation in aquifer sediments, extending from 10 to 32 m depth, according to grain size and depth (Table 4). The concentration depended on grain size, with the finest particles containing the most manganese (Forstner and Whittman, 1979), although it was only 10% of the total (balanced concentration according to the representative weight of the fraction).
80 70
..........................................
e' EXCHRN
I
-7 -22
C~RBON OXIDE
ORGRHI RESIDU
EXTRRCTED FRe~CTI ON I ~ -10 TO -22 N M -28 TO -32 M TO - 2 8 I'1
TO - 1 8
Fig. 3. Percentages of different chemical species of manganese according to depth.
177
÷
÷+
+ + + ÷ + +
0
+1
6 x Z~
2
0
~D
-+-i +1
+i
+l
+1
I
I
I
I
]
178 Independent of depth and grain size the oxide form was dominant, reaching 50-60% of the total manganese concentration (Fig. 3). The organic form was less important, and the concentrations of exchangeable and carbonate forms were similar to the residual form, i.e.: from 10 to 22 m depth, oxide > residual > exchangeable from 22 to 28 m depth, oxide > carbonate > exchangeable from 28 to 32m depth, oxide > residual > exchangeable. The mineralogical study (SEM and X-ray diffraction) confirmed the speciation results. The manganese was found on the pebbles as the oxide with an average size of 10 #m. It was found on all siliceous or calcareous alluvium types, mainly on those having rough surfaces. Oxides were usually represented on siliceous components by massive microconcretions belonging to the Z-disordered manganate type (Birnessite), often accompanied by lamellae (sometimes in isolated concretions) of manganate type 10A or todorokite. On the calcareous components, manganates attributable to the 7A or Birnessite manganate type prevail (but the other two types coexist). Minor elements are also present in these manganates: A1, Fe, K, Ca and Ba.
Role of bacterial population The results are presented in Table 5. During the experiment a reducing environment was maintained and the pH remained constant. The increase with time of dissolved manganese concentrations was slight in the sterile culture, but significantly higher in the presence of bacteria. Manganese release does not seem to be a function of the number of bacteria, but probably their respiratory activity. In the presence of bacteria the gravels were progressively cleared of manganese deposits. The physical-chemical conditions of manganese reduction were achieved in the two cultures, and the part played by bacteria in manganese release kinetics was obvious.
Discussion The results of manganese speciation, and of the mineralogical study, show that the oxides settled onto the pebble bed after its formation, essentially as oxides (Z-disordered manganate type, 10 A manganate, todorokite, birnessite); the manganese is of autogenous origin. Because of changes in the physicalchemical conditions of the system, the oxide progressively dissolves in the groundwater. The rate of manganese dissolution is considerably enhanced by bacterial processes. When the environment is low in oxygen, the energy for this bacterial reduction is obtained through the oxidation of the organic matter existing in the system. From this the reduction of oxides linked to sediments into manganous compounds follows, which are then released into the groundwater. These results on the origin of the manganese and on its dissolution processes in groundwater make it possible for us to propose an in situ treatment,
179
-..--- ~ 11 METER~ I
WATE_._.RROXIGENATION ~ - - I ------'-"~"
VYRED~-X P~
EXPLOITATION PUMPS
nFC~.S.SINGTANK h
1
~
P2
-P3
Fig. 4. Application of the Vyredox process.
involving the most suitable direct oxygenation of the groundwater appropriate for the water table conditions APPLICATION OF THE VYREDOX PROCESS
Principle This technique consists of eliminating the dissolved iron and manganese before pumping, by creating a highly oxidized zone around the well to be treated by periodic injections of aerated water (Glensvig, 1976; Descroix, 1978; Foliot, 1978 Descroix and Laurent, 1979; Van Beek, 1985; Grombach, 1985; Jechlinger et al., 1985; Seyfried and Olthoff, 1985). With this proposal there is a risk of filling the aquifer pores. This will be discussed in terms of the results on manganese origin and dissolution processes previously obtained. The different stages in groundwater treatment with this process are the following: - - pumping is stopped in the well to be treated - - water from another well is aerated by mixing it with atmospheric air the aerated water is degassed in a tank to eliminate non-dissolved oxygen and other undesirable gases - - t h e oxygen-saturated water is then returned by gravity to the aquifer through the well to be treated and through satellite wells. This operation is continuous and lasts for ~ 20 h - - t h e process is halted for 4 h in order to allow the establishment of a high-potential redox zone in the aquifer after a recharging sequence, water with low concentrations of iron and manganese can be pumped and sent directly into the water mains. It can be used until the dissolved iron and manganese concentrations return to their normal levels (until five to ten times more water has been pumped than has been recharged). The treatment operation is then repeated. In practice, for each site, a preliminary assay is necessary to verify whether -
-
-
-
180 TABLE 6 Main results for P2 after a recharging sequence (Cycle No. 6) (injection of 2120 m3 aerated water and a 18h pause). Pumping flow 135m3h 1 Volume (m3)
Fe
Mn
pH
02
171 467 2530 2972 3538 3983 6007 6533 7103 7549 9658 10105 10676 11123 13235 13682 14253 14699 16810 17257 17828 18275 18381
0.09 0.02 0.02 0:025 0.017 0.04 0.015 0.02 0.02 0.01 0.01 0.03 0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.02 0.02 0.025
0.03 0.03 0.03 0.03 0.04 0.035 0.045 0.045 0.045 0.055 0.065 0.07 0.075 0.08 0.085 0.09 0.12 0.12 0.10 0.134 0.16 0.11 STOP
7.6 7.6 7.6 7.6 7.3 7.3 7.3 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.8 7.8 7.8 7.8 7.8
8.5 7.4 2.5 2.2 1.9 2.0 1.8 1.8 1.7 1.6 1.5 1.4 1.4 1.3 1.3 1.3 1.3 1.3 1.4 1.3 1.3 1.3
t h e p r o c e s s c a n be u s e d a n d to d e t e r m i n e t h e m o d e a n d size of a p e r m a n e n t installation.
Assay A t t h e B e a u c a i r e s i t e a n e x p e r i m e n t w a s c a r r i e d o u t u s i n g P1 a n d P2 w e l l s (Fig. 4). T h e w a t e r w a s p u m p e d f r o m P1, a e r a t e d , d e g a s s e d a n d i n j e c t e d i n t o P2. A f t e r a t e m p o r a r y i n t e r r u p t i o n , t h e o p e r a t i o n w a s r e v e r s e d a n d P2 w a s p u m p e d a n d w a t e r w a s i n j e c t e d i n t o P1. D u r i n g t h e p r e l i m i n a r y tests, t h i s s c h e m e m a d e it p o s s i b l e to t r e a t t h e t w o w e l l s s i m u l t a n e o u s l y w i t h o u t b o r i n g a n y s a t e l l i t e wells. T h e t r e a t m e n t d e v i c e w a s s i t u a t e d a t a n e q u a l d i s t a n c e f r o m t h e t w o w e l l s (Fig. 4), t h e o x y g e n a t i o n s t a t i o n w a s f u l l size, w i t h a flow s i m i l a r to t h a t of t h e w o r k s ( 1 5 0 m 3 h 1). T h e a e r a t e d w a t e r v o l u m e to be i n j e c t e d w a s 2500 m 3 for e a c h 24-h cycle, w h i c h w a s d i v i d e d as follows: - - p u m p i n g f r o m P1 a n d i n j e c t i o n i n t o P2 for 1 8 h - - i n t e r r u p t i o n of p u m p i n g a n d p a u s e for P2 (6 h) D u r i n g t h e n e x t c y c l e t h e o p e r a t i o n w a s r e v e r s e d , a n d so on. E v o l u t i o n of dissolved oxygen, i r o n a n d m a n g a n e s e c o n c e n t r a t i o n s was
181 followed in order to specify the pumped volume beyond which the manganese concentration would be above the limit for drinking water. From one cycle to another we noticed the regular decrease of iron and manganese concentrations owing to injections of recharged water that was less and less loaded with these elements and to the creation of a zone enriched in dissolved oxygen around the treated wells. Normal manganese concentrations for drinking water were reached after four injection cycles. During a given cycle, pH and dissolved oxygen concentration decreased in the pumped water. The return of the system to its initial state accompanied a progressive increase in the iron and manganese concentrations. Long lasting pumping (cycle 6) (Table 6) showed that iron and manganese concentrations first increased slowly, while the pH and dissolved oxygen decreased. The latter stabilized at the low value of 1.3 mg 1 1 characteristic of the aquifer. The concentration of manganese then increased more quickly. The ratio of pumped water volume (concentrations ~< manganese limit for drinking water) to injected aerated water volume was 3.5.
Discussion According to the theoretical model proposed by different authors, the Vyredox system results in precipitation at the limit of the enriched oxygen zone; the secondary effect would be to fill the pores of the aquifer. Conversely, according to this model, the injection of aerated water helps to maintain a population of oxidizing bacteria, which contributes to the precipitation of the metals and, by its activity, maintains a high pH and redox potential. Even for this hypothesis, a theoretical estimate (Descroix, 1978; Foliot, 1978) shows that for a 5 mg 1 1 iron concentration, the life of the aquifer would be greater than 100 years, despite the risk of its pores becoming progressively filled. However, our studies on the origin and the experimental evidence of manganese dissolution by bacteria make us believe that this risk is non-existent or minimal, and therefore we propose another model. In natural conditions, the release of iron and especially of manganese stems from bacterial activity owing to oxygen deficiency. In this case, the facultative anaerobic bacteria of the aquifer use the metallic oxides as final acceptors of electrons, leading to the dissolution of iron and manganese. The oxygen supplied by the Vyredox process stops the release of Mn, both by a redox increase and through a modification of bacterial respiration: the bacteria readily use the dissolved oxygen molecules. It is likely that the bacterial and chemical processes are superposed. The metallic oxides would remain on their substrate (pebbles) in the aquifer, which eliminates any risk of pores becoming filled, at least in the oxidizing zone. We also note that the pores in aquifers in Finland, Sweden, and in the Federal Republic of Germany, where Vyredox stations have been working for more than 10 years, show no sign of becoming filled.
182 CONCLUSION
An understanding of the processes involved in groundwater contamination by manganese reveals the usefulness of determining its speciation on aquifer sediments. This information is necessary both for preventive and curative treatment. If the aquifer is drilled and a chemical analysis reveals the presence of manganese on the sediments, it would be advisable to adopt the necessary recommendations for aquifer protection. To prevent manganese release into the groundwater and then into the main water supply, it is necessary to avoid enrichment of the aquifer by organic matter, which should induce a reducing environment. When it is too late for prevention, as in the case of autogenous manganese, an inexpensive in situ treatment can be carried out. The success we have had must be added to the numerous other successes involving permanent Vyredox stations around the world (in 1984 more than 50 in eight European countries and in the U.S.A.). Therefore, although some critics of this method (Mouchet and Rault, 1979) doubt the reliability of the process, it seems that its suitability for solving the problem of contamination of groundwater by autogenous manganese and iron has been proven. The use of this technique requires an initial study of the origin of these metals, as we have done in the case described here. When the manganese and (or) iron origin is autogenous, the artificial oxygenation of the aquifer allows the metallic oxides to remain on the components of the strata forming the aquifer, which eliminates any risk of the pores becoming filled at least at the periphery of the wells.
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