Characterization of groundwater humic substances: influence of sedimentary organic carbon

Applied Geochemistry 15 (2000) 97±116

Characterization of groundwater humic substances: in¯uence of sedimentary organic carbon R. Artinger a,*, G. Buckau a, S. Geyer b, P. Fritz b, M. Wolf c, J.I. Kim a a Forschungszentrum Karlsruhe, Institut fuÈr Nukleare Entsorgungstechnik, 76021 Karlsruhe, Germany UFZ-Umweltforschungszentrum Leipzig-Halle, Sektion Hydrogeologie, 06246 Bad LauchstaÈdt, Germany c GSF-National Research Center for Environment and Health, Institute of Hydrology, 85764 Neuherberg, Germany b

Received 12 June 1998; accepted 4 December 1998 Editorial handling by R. Fuge

Abstract A total of 35 groundwaters from 4 di€erent aquifer systems in Germany are investigated for their physicochemical properties, dissolved organic C (DOC) and humic and fulvic acids. Humic substances are isolated and characterized for their elemental composition, UV/Vis and ¯uorescence spectroscopic properties, size distribution by gel permeation chromatography (GPC) and 14C content. For isolation of sucient quantities of humic substances a mobile sampling system is developed based on a combination of reverse osmosis (RO) and XAD±8 adsorption chromatography. One of the aquifer systems (Gorleben) covers a wide range of hydrogeochemical conditions, whereas the other 3 aquifer systems (Munich, Franconian Albvorland and Fuhrberg) have less diverse properties. One speci®c feature of the Gorleben aquifer system is the presence of a very high DOC, which, in contrast to other aquifer systems, contains considerable amounts of aquatic humic acid. This is attributed to the release of aquatic humic substances originating from sedimentary organic C (SOC) that is abundant in Gorleben sediments. The results show that aquatic humic substances from di€erent aquifer systems have dissimilar properties which di€er from one another. Systematic di€erences are found among humic substances from di€erent regions of the Gorleben aquifer system. Such di€erences are considered to be caused by the mixing of humic substances from the SOC. However, exact quanti®cation of such mixing appears dicult because overlapping e€ects of di€erent geochemical processes feigning a dissolution of SOC cannot be excluded. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Humic substances are ubiquitous in groundwater systems. They do not show a unique chemical structure, but rather represent a group of organic compounds with polyelectrolytic properties and resistance to degradation under aquatic conditions. Due to their large hydrodynamic size and their metal ion interaction

* Corresponding author. E-mail address: [email protected] (R. Artinger)

properties, they appear in natural water as humic colloids that are loaded with metal ions (Kim et al., 1996). Humic substances interact with non-polar organic substances as well. For these reasons humic substances play a crucial role for the geochemical behavior of non-polar organic pollutants, heavy metal ions and a multitude of radionuclides, especially multivalent actinide ions (Manahan, 1989; Kim et al., 1989; Kumke, 1994). Humic substances are divided into two fractions, namely, humic acid (HA) and fulvic acid (FA). Both fractions exhibit hydrophobic properties at low pH,

0883-2927/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 8 8 3 - 2 9 2 7 ( 9 9 ) 0 0 0 2 1 - 9

15±18 17±20 10±13 21±24 6±9

70±73 30±35

65±68 35±38 70±73 134±137 121±125 57±40 83±85 128±130

Gorleben Recharge GoHy±181 GoHy±411 GoHy±421 GoHy±611 GoHy±711

Transition GoHy±182 GoHy±201

Enhanced DOC GoHy±412 GoHy±492 GoHy±572 GoHy±573 GoHy±612 GoHy±851 GoHy±2211 GoHy±2227

Depth (m)

Elster Saale Elster PraÈelster Saale Elster Elster PraÈelster

Elster Weichsel

Weichsel Weichsel Weichsel Weichsel Weichsel

(coarse, sand, clay)

Sediment type

10.7 9.6 12.2 14.2 10.9 10.8 12.1 14.4

102 9.6

9.6 9.4 9.3 8.9 92

7.6 8.0 8.8 8.0 8.4 8.2 8.1 7.8

8.0 7.8

6.4 7.0 6.0 8.4 6.0

Temp. pH (8C)

92 128 17 ÿ32 35 ÿ5 31 ÿ58

ÿ48 121

29 ÿ1 53 72 398

0.36 2.15 0.55 1.63 2.06 2.37 4.99 4.30

0.13 0.17

0.13 0.27 0.35 0.20 0.19

< 0.8 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1

< 0.1 < 0.1

0.1 0.1 0.2 < 0.1 4.5

0.04 0.06 0.08 0.02 0.07

K

2.50 21.7 5.74 18.3 19.8 Z.9 47.0 41.8

0.09 0.08 0.03 0.09 0.08 0.08 0.14 0.33

0.24 0.03 0.33 0.02

0.35 0.47 0.92 0.47 0.45

Eh Conduct. Oxygen Na (mV) (mS/cm) (mg/L)

0.27 0.82 0.53 0.84 0.38

0.11 0.07 0.04 0.10 0.15 0.04 0.09 0.20

0.51 0.32 0.08 0.23 0.56 0.24 0.35 0.53

3.46 11.6 4.74 7.97 13.2 9.85 9.51 7.90

1.04 0.75

0.30 1.18 0.15 1.05 0.15

< 0.01 0.73 0.01 0.02 0.30 < 0.01 0.20 0.32

0.04 0.24

0.34 0.53 0.82 0.33 0.53

2ÿ Ca HCOÿ 3 SO4 (mmol/L)

0.05 0.54 0.09 0.49

0.10 0.16 0.28 0.09 0.28

Mg

0.51 8.39 0.72 10.3 6.59 12.7 37.7 36.7

< 0.2 < 0.2 0.5 < 0.2 0.5 0.4 0.3 < 0.3

7.6 127 14.4 97.2 184 169 93.9 73.4

0.8 0.9

0.5 2.2 1.9 1.4 0.9

1.68 114 9.4 60 171 140 70 58

0.126 0.05

0.009 0.022 ± 0.11 (0.001)

2.6 8.9 3.2 17 9.2 6.7 22 5.9

0.13 0.40

0.14 0.70 0.53 0.46 0.35

H TUa DOC HA FA (mg C/L)

39 21 20 8.0 29

3

0.20 < 0.3 0.29 < 1.1

0.19 0.35 1.04 0.38 0.34

Clÿ

Table 1 Sampled groundwaters and their basic hydrological parameters, chemical composition, DOC concentration and 3H content. nd=not determined (due to oxygen contact during groundwater sampling)

98 R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

10±13 0.5±23 6±18 2±7

Uplift-Mixing GoHy±341

Munich MS±UP±178 MS±U1±630 MS±KP±916

b

a

(sand) below forest below®eld Weichsel Weichsel Weichsel Weichsel Weichsel Weichsel Weichsel

Fuhrberg FF±OW FF±OA FF±T1-4 FF±T3-11 FF±N5-12 FF±N10-4 FF±N10-6 FF±N10-12 FF±N10-14

TU 3H unit (1 TU=0.118 Bq/L). pH corrected according to Runde (1993).

1.3 1.8 4 11 12 4 6 12 14

(sandstone) Keuper Keuper (Lias)-Keuper (Lias)-Keuper Keuper

Franconian Albvorland Laibstadt 76±144 J-S-T III 110±166 Greding I 80±92 Greding II 83±143 Beilngries 80±88

(limegravel) WuÈrm/Riû WuÈrm/Riû WuÈrm/Riû

Weichsel

220±223 PraÈelster 235±238 Elster 216±219 Elster

Channel Brines GoHy±193 GoHy±514 GoHy±653

9.4 10.0 10.0 10.0 10.3 10.0 10.0 10.0 10.0

12.9 18.6 18.4 19.1 15.6

9.3 9.4 9.0

8.6

12.9 16.9 18.9

4.3 4.8 5.8 5.5 5.0 5.3 5.9 5.9 6.0

6.3 6.8 7.3 7.4 7.4

7.2 7.3 7.3

7.0

nd nd nd nd nd nd nd nd nd

594 283 172 185 41

523 634 330

140

6.8/7.1b 137 6.3/7.0b 162 6.5/7.1b 134

0.24 0.42 0.66 0.53 0.33 0.54 0.56 0.45 0.51

0.26 0.57 0.75 0.42 0.52

0.63 0.63 0.67

4.84

110.8 170.9 154.3

nd nd nd nd nd nd nd nd nd

6.0 < 0.1 < 0.1 < 0.1 < 0.1

8.1 8.8 5.6

< 0.1

< 0.1 < 0.1 < 0.1 0.44 1.30

39.6 25.4 82.3 26.5 68.2 23.6

0.50 0.55 0.86 0.45 0.38 0.37 0.35 0.68 0.75

0.08 0.21 0.45 0.19 0.74 0.08 0.46 0.71 0.07 0.42 0.45 0.45 0.27 0.36

0.16 0.51 0.83 0.50 0.78 0.10 0.41 0.58 0.73 0.26 0.47 0.50 0.32 0.34

0.34 0.85 0.96 0.69 0.69 025 0.93 1.81 1.60 0.80 1.78 1.96 1.48 1.34

0.86 1.99 2.26 1.51 2.22

0.17 0.06 0.89 2.13 0.75 < 0.01 0.88 2.29 020 0.06 0.87 2.34

43.0 0.03

2018 6.47 4870 20.3 4014 18.8

0.01 0.02 0.24 0.21 0.10 0.26 0.45 0.55 0.85

1.92 5.10 5.79 4.54 5.08

5.23 5.38 5.75

4.25

3.85 2.09 2.96

0.82 0.63 1.25 2.34 1.02 1.00 1.04 1.70 1.64

0.24 0.92 0.15 0.06 0.57

0.25 0.26 0.25

0.15

29.9 52.9 46.0

0.69 0.67 0.98 0.88 1.01 0.92 1.06 1.49 1.31

0.07 0.04 0.03 0.03 0.06

18 16 17 18 22 15 16 32 36

< 0.4 < 0.7 < 0.8 < 0.7 < 0.5

0.91 37 0.75 34 1.22 36

42.3 0.4

1974 < 0.2 4766 < 0.3 4079 < 0.3

6.1 16.8 11.9 4.4 6.9 11.5 7.9 11.3 9.8

0.5 0.1 0.4 0.3 0.4

0.6 0.9 0.7

64.0

32 1.6 1.3

± ± ± ± ± ± ± ± ±

± ± ± ± ±

± ± ±

41

± ± ±

1.3 4.0 3.0 0.67 4.5 3.2 2.0 1.8 1.8

0.06 0.01 0.03 0.03 0.04

0.12 0.34 0.12

15

0.50 0.20 0.24

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116 99

100

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

which are used for the isolation of aquatic humic substances by chromatographic separation on the nonionic acrylic ester resin XAD±8 (Aiken, 1988; Thurman and Malcolm, 1981). The part of DOC that is not sorbed on XAD±8 at low pH is classi®ed to be made up of hydrophilic compounds. HA and FA show di€erent characteristics with respect to, amongst others, the elemental composition, functional group content, hydrodynamic size distribution and spectroscopic properties (Kim et al., 1990; Stevenson, 1982; Steelink, 1985; MacCarthy and Rice, 1985; Beckett et al., 1987; Pullin and Cabaniss, 1995; Chin et al., 1994). Important aspects for the role of humic substances in transporting pollutants along with a water path are their generation, migration behavior, residence time and stability against degradation. If the stability of aquatic humic substances is suciently long, their migration behavior often follows that of an ideal tracer and, thus, 14C measurements can be used for groundwater dating. Recent investigations demonstrated that radiocarbon dating can contribute to the understanding of their migration behavior and residence time in groundwater systems (Geyer et al., 1993; Geyer, 1993; Geyh and BruÈhl, 1991; Wassenaar et al., 1991; Pettersson and Allard, 1991; Purdy et al., 1992). However, in the presence of 14C free organic deposits this dating method may result in too high groundwater ages, if it is assumed that all the dissolved humic substances originate from the surface (Artinger et al., 1996). Therefore, knowledge of their origin is necessary to reconstruct the migration behavior of humic substances in aquifer systems with organic deposits. In this paper aquatic humic substances from 35 di€erent groundwaters samples of 4 di€erent groundwater systems in Germany are quanti®ed and characterized. Three of the aquifers investigated have relatively well known hydrological and geochemical conditions (Munich, Franconian Albvorland and Fuhrberg). The fourth aquifer (Gorleben) investigated has organic deposits, a very complex geohydrology and a wide range of geochemical conditions. The discussion focuses on the applicability of di€erent characterization methods for the distinction between HA and FA in®ltrated via groundwater recharge and those originating from sedimentary organic C (SOC).

2. Aquifers and investigation Four di€erent aquifer systems in Germany (Munich, Franconian Albvorland, Fuhrberg and Gorleben) are used for the investigation. The sampled groundwaters are shown in Table 1 together with their characteristic

parameters. The aquifer systems are described as follows: Munich. This aquifer is located in the eastern outskirts of the city of Munich in southern Bavaria. It is an uncon®ned lime-gravel aquifer and consists of quaternary ¯uviatile alternating sandy, ®ne and coarse pebbles. The sediment is mainly calcium carbonate and dolomite (>98%) and practically free from organic deposits. The DOC content of the groundwater varies from 0.3 to 1.5 mg C per liter (mg C/L) according to season (Zahn, 1988). From this aquifer 3 samples were taken from the top of the groundwater level. All 3 groundwater samples are saturated with O2 and contain 3H in concentrations of about 35 tritium units (TU), re¯ecting a contamination by atmospheric nuclear testing. This aquifer provides data for humic substances in young recharge groundwater (<40 a) without interference by the in situ generation from sedimentary organic C (SOC). Franconian Albvorland. This aquifer is located in the Franconian Albvorland in northern Bavaria. It is a con®ned sandstone aquifer of the Upper Triassic Keuper sequence and hydrogeologically well investigated (Rietzler, 1979; Eichinger, 1981). The aquifer extends over a distance of about 25 km to the SE and declines moderately with an average water table gradient of 0.8-. The sediments are free from SOC. The groundwater is found to be of Pleistocene origin (>12 000 a) at the end of the ¯owpath (Geyer, 1993; Eichinger, 1981). A mixing with younger water along the groundwater ¯ow is not probable since the aquifer is covered by thick clay layers of the Upper Middle Keuper (Feuerletten). This is also supported by non-detectable 3H concentrations (cf. Table 1). The aquifer is anaerobic beyond 8 km of groundwater recharge and SO4 reducing beyond 18 km. The DOC concentration varies between 0.1 and 0.5 mg C/L. This aquifer allows the characterization of HA and FA in reducing groundwaters with ages exceeding 12 000 a presumably without interference of in situ generation from SOC. Fuhrberg. This aquifer is situated about 30 km N of the city of Hanover in the drinking-water recovery area of Fuhrberg. The aquifer consists of Quaternary ®ne and coarse grained, and partly gravel sands of 20 to 30 m thickness. The free groundwater surface is in sandy podzols with a low humus content. The groundwater samples are all young (<40 a) as seen by the high 3H concentrations (>15 TU) and mass balance calculations (BoÈttcher et al., 1985). In¯ux of DOC with groundwater recharge along the main ¯owpath varies with, amongst others, local vegetation conditions, includ-

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

101

Fig. 1. Schematic description of setup for the enrichment and isolation of aquatic humic substances (from Artinger et al., 1996).

ing variations in agricultural activity. The DOC concentration varies between 17 mg C/L at ®eld sampling sites near the surface and 5 mg C/L at a depth of 11 m near the waterworks well. This aquifer allows the characterization of humic substances from groundwaters under recharge conditions, i.e., young, oxidizing groundwaters with relatively high DOC concentrations. Gorleben. This aquifer system is located at Gorleben in the Northern German Plain (Lower Saxony). The aquifer extends down to 280 m below ground surface and is situated in the rock covering and surrounding a Permian salt dome under exploration for construction of a nuclear waste repository. In the upper part of the aquifer system freshwater is found in ¯uviatile sediments of high permeability. Below this, clayey sediments of low water permeability are found. At higher depths in the vicinity of the salt dome, dissolution of salt results in groundwater with higher salt concentration. Furthermore, the so-called Gorleben chan-

nel is located on top of the salt dome. During the Elster glaciation, the cap rock was removed along this channel, followed by deposition of sand and marl. According to the regional groundwater model, the main recharge area is located S of the salt dome with the Gorleben channel extending towards the NE. For the part of the Gorleben aquifer system discussed in this paper, Pleistocene origin (>12 000 a) is only expected for nearly salt saturated channel brines in the direct vicinity of the salt dome as shown by numerical ¯ow modeling and d2H and d18O isotope signatures (BfS, 1990). Signi®cant 3H concentrations are only found down to 25 m sampling depth Table 1 (cf. also Sonntag and Suckow, 1993). One speci®c characteristic of this aquifer system is the large variety of DOC concentrations, from <1 mg C/L up to more than 200 mg C/L (Buckau, 1991) (cf. also Table 1). The very high DOC concentrations are generally attributed to in situ generation from local Miocene brown coal and

102

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Pleistocene peat deposits (BfS, 1990). Due to the large variation of the DOC concentration and the assumed large contribution of in situ generation from SOC especially in some deep groundwaters, this aquifer o€ers the possibility to study aquatic humic substances from both recharge and SOC as well as mixtures of them. 3. Experimental 3.1. Enrichment and isolation of aquatic humic substances For the characterization of HA or FA, an amount of at least 100 mg of the isolated substance is required. The principle scheme for preparing these HA and FA samples from groundwater consists of (i) enrichment; (ii) isolation; and (iii) separation of HA and FA. For enrichment and isolation of aquatic humic substances a mobile ®eld operation system has been developed. For enrichment the developed system utilizes reverse osmosis (RO) (Wilhelm Werner Co., Germany). Isolation is done by sorption on XAD±8 (Rohm and Haas Co., USA) at low pH (Thurman, 1985). The mobile ®eld operation system for enrichment and isolation of humic substances is described in Fig. 1. This setup works automatically and allows the processing of 5000 L groundwater per day. Simultaneously with the enrichment of DOC by RO, a smaller side stream of DOC concentrate is processed by acidi®cation and sorption of humic substances on XAD±8. Thereby, build-up of the salt concentration in the DOC concentrate is kept within acceptable limits in order not to inhibit the RO process. Depending on the chemical composition and DOC concentration of groundwater, the enrichment and isolation scheme is modi®ed. At low salt concentrations and suciently high DOC content, enrichment is conducted in the ®eld and the resulting 50 L of DOC concentrate is further processed in the laboratory. For high DOC groundwaters enrichment by RO is not applied and for channel brines the high salt concentration does not allow the application of RO. In the latter case, up to 1000 L of groundwater are directly processed in the ®eld using a XAD±8 column of 8 L volume. All groundwaters are pre®ltered at 450 nm pore size to separate the suspended particles. For waters with a high Ca content an ion exchanger (Serdolit CSG, Serva Co., Germany) for water softening is additionally installed upstream of the RO to avoid calcite precipitation at increasing Ca enrichment in the DOC concentrate. Separation and processing of isolated humic substance are accomplished by standard procedures (Buckau, 1991). After elution from XAD±8 with 0.1 M NaOH, HA ¯occulate is separated from FA by cen-

trifugation after acidi®cation to pH 1 (HCl). Isolated HA is dispersed in 0.1 M HCl followed by separation of supernatant via centrifugation. This washing procedure is repeated 3 times. Finally HA is freeze-dried. The FA remainder in the acidic solution after separation of HA is puri®ed twice by sorption on XAD±8, followed by alkaline elution. Finally, FA is puri®ed by cation exchange over BIO REX 70 or AG MP±50 (BIO RAD Co., Germany) in order to remove the excess salt prior to freeze drying. 3.2. Contamination due to sampling Great care is taken with respect to possible contamination during sample enrichment, isolation and handling. This is a critical issue especially in view of the low humic substance concentrations in many of the sampled groundwaters. For estimating the maximum contamination, the impact on the 14C concentration due to the applied humic and fulvic acid sampling methods is tested, using (i) a young 3H containing granitic groundwater with low DOC concentration (<0.1 mg C/L) of high 14C concentration, and (ii) a solution containing humic acid with a very low 14C concentration (Aldrich Co., Germany, 14C concentration approximately 1% modern C (pmc)). For the large ®eld sampling with the XAD±8 column, a small increase in the 14C concentration is found. Assuming a 14 C concentration of 137 pmc for the source of contamination, a total amount of an additional 0.11 mg C during isolation can be calculated. Such a 14C contamination source corresponds to, for example, cellulose that may originate from soxhlet treatment of XAD±8 for its puri®cation. For enrichment and isolation of aquatic humic substances by the ®eld sampling reverse osmosis system coupled with a small XAD±8 column, a small decrease of the 14C concentration is observed. It corresponds to a total contamination of 0.20 mg 14C free C in the isolated humic substance. Using the ion exchanger for water softening of 660 L groundwater, the 14C concentration of isolated humic substance is also reduced, corresponding to an addition of 0.9 mg 14 C free C. Contamination by 14C free C may originate from, for example, tubings, membranes or construction parts of the apparatus of petrochemical origin. The results show that contamination of humic substances during enrichment, isolation and handling are of no great importance for sample amounts of more than 20 mg C. 3.3. Characterization methods DOC is determined using the TOC analyzer Tocor 2 (Maihak Co., Germany). The inorganic C of water samples is removed by degasi®cation of CO2 with Ar after acidi®cation to pH 2.5±3. The DOC concen-

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

103

Table 2 Aquatic humic and fulvic acids isolated from the investigated groundwaters and their 14C concentration, speci®c absorption at 300 (E3) and 400 nm (E4), their ratio E3/E4, ¯uorescence intensity, gel permeation chromatography distribution (in Dalton: distribution maximum, size average Ms and number average Mn), and their size ratio (Ms/Mn). nd=not determined (mainly due to lack of sample amount); : values from di€erent groundwater sampling occasions 14

C (pmc)a

E3 GPC GPC Ms/Mn E4 E3/E4 Fluorescence GPC (Lgÿ1 cmÿ1) (Lgÿ1 cmÿ1) Mn intensity distr. max.b Ms (Dalton) (Dalton) (relative units) (Dalton)

72.8 21.1 76.0 21.1 45.2 23.1

4.9 5.4 6.5

0.59 0.73 0.88

Fulvic acids 8.2 0.49 7.3 0.61 7.4 0.74

2900 3100 3750

2350 2880 3690

690 830 1080

3.4 3.5 3.4

Franconian Albvorland Laibstadt 79.8 21.0 J-S-T III 29.3 21.9 Greding I 15.6 21.3 Greding II 25.4 22.0 Beilngries 13.9 25.6

3.5 nd nd nd 3.4

0.38 nd nd nd 0.57

9.1 nd 8.0 6.7 6.0

0.33 nd nd nd 0.51

2450 3000 3400 4850 2950

1900 1780 3710 2510 3110

560 540 860 640 760

3.4 3.3 4.3 3.9 4.1

Fuhrberg FF-OW FF-OA FF-T1-4 FF-T3-11 FF-N5-12 FF-N10-4 FF-N10-6 FF-N10-12 FF-N10-14

74.5 21.1 85.0 21.2 79.9 20.6 117.322.3 85.3 21.3 80.3 21.1 81.5 21.1 138.423.9 119.222.1

6.2 8.6 8.1 6.4 8.1 9.9 7.1 9.9 10.4

1.1 1.5 1.4 1.0 1.4 1.8 1.3 1.8 1.9

5.8 5.8 5.9 6.6 5.7 5.5 5.7 5.4 5.3

0.60 1.34 0.87 nd 0.68 0.90 0.17 0.75 1.05

3750 5450 5450 4100 5450 4800 3400 6950 5800

3700 4670 4030 3940 4050 4180 2910 5730 5440

1040 1530 1120 1220 970 1290 830 1290 1670

3.6 3.1 3.6 3.2 4.2 3.2 3.5 4.4 3.3

Gorleben recharge GoHy-181 GoHy-411 GoHy-421 GoHy-611 GoHy-711

75.9 21.2 109.721.4 92.5 21.5 38.2 25.4 100.921.5

6.6 5.7 6.4 11.0 4.4

1.2 0.80 1.0 2.4 0.57

5.8 7.2 6.2 4.6 7.6

0.43 0.70 0.56 1.05 0.34

4500 4300 3300 6250 2450

3490 3680 2750 5000 2060

790 940 770 1380 570

4.4 3.9 3.6 3.6 3.6

Transition GoHy-182 GoHy-201

46.1 21.2 58.6 22.4

7.0 7.9

1.4 1.6

6.0 4.9

1.01 0.70

7050 6350

5130 4810

1230 1230

4.2 3.9

Enhanced DOC GoHy-412 GoHy-492 GoHy-572 GoHy-573 GoHy-612 GoHy-851 GoHy-2211 GoHy-2227

8.12 2.2 4.72 0.2 13.5 20.4/23.621.8 7.02 3.2/ < 3.6 26.7 22.3 11.3 20.4 13.4 20.4 10.6 23.3

10.6 10.4 10.4 14.9 12.1 12.0 17.5 16.4

2.0 2.1 2.1 3.6 2.2 2.7 4.5 3.7

5.3 5.2 4.9 4.2 6.2 4.5 3.9 4.4

1.64 1.50 1.34 1.89 1.97 1.35 1.84 1.82

8200 8250 8100 9600 8100 9450 9350 9550

7170 7090 7740 8120 7120 8330 8340 8080

2090 3400 2430 3110 2800 3610 3310 3290

3.4 2.1 3.2 2.6 2.5 2.3 2.5 2.5

Channel brines GoHy-193 GoHy-514 GoHy-653

46.8 21.1 20.3 21.3 40.0 21.3

4.2 5.8 3.9

0.77 0.87 0.55

6.6 6.7 7.2

0.65 1.12 0.70

7100 6550 6350

8220 7390 7280

2440 1940 2530

3.4 3.8 2.9

Sample

Munich MS-UP-178 MS-U1-630 MS-KP-916

(continued on next page)

104

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Table 2 (continued ) Sample

Uplift-Mixing GoHy-341

14 C (pmc)a

E3 E4 E3/E4 Fluorescence GPC GPC GPC Ms/Mn (Lgÿ1 cmÿ1) (Lgÿ1 cmÿ1) intensity distr. max.b Ms Mn (relative units) (Dalton) (Dalton) (Dalton)

56.3 20.8

10.4

2.3

4.5

nd

nd

nd

nd

nd

Gorleben recharge GoHy-181 GoHy-411 GoHy-611

Humic Acids 15.4 21.3 26.3 21.2 13.8 20.6

19.9 22.1 nd

6.1 6.9 nd

3.3 3.2 nd

0.22 0.30 nd

8750 8800 nd

±c ±c nd

±c ±c nd

±c ±c nd

Transition GoHy-182

3.42 1.4

23.9

7,3

3.3

035

9000

±c

±c

±c

Enhanced DOC GoHy-412 GoHy-492 GoHy-572 GoHy-573 GoHy-612 GoHy-851 GoHy-2211 GoHy-2227

3.12 1.2 < 1.0 2.12 1.7 < 1.3 1.72 1.1 < 1.7 1.02 0.9 1.72 1.1

nd 32.1 23.4 27.3 28.0 21.1 25.5 28.3

nd 12.3 8.9 9.5 10.6 8.6 9.4 9.8

nd 2.6 2.6 2.9 2.6 2.5 2.7 2.9

0.26 0.21 0.15 0.28 0.20 0.15 0.12 0.38

10100 30700 27900 22800 40300 31000 26100 21700

±c ±c ±c ±c ±c ±c ±c ±c

±c ±c ±c ±c ±c ±c ±c ±c

±c ±c ±c ±c ±c ±c ±c ±c

Uplift-Mixing GoHy-341

9.52 0.1

27.3

10.5

2.6

0.20

24550

±c

±c

±c

pmc percent modern C (1 pmc=0.226 Bq 14C per g C). GPC peak at exclusion limit not accounted for (cf. Fig. 6 and Fig. 7). c Not evaluated due to an unknown fraction at the exclusion limit of the gel column (cf. Fig. 6).

a b

tration is measured by IR detection of CO2 from catalytical thermal oxidation of organic C. Elemental composition (C, H, N, O and S) is determined by the ``Analytische Laboratorien Malissa und Reuter'' (Gummersbach, Germany) or ``Mikroanalytisches Labor des Anorganisch-chemischen Instituts der Technischen UniversitaÈt MuÈnchen'' (Munich, Germany). Carbon, H and N are quanti®ed by heat conductivity detection after sample combustion and gas chromatographic separation of CO2, H2O and N2. Oxygen is pyrolized with C at 11508C in a He atmosphere to CO. After oxidation with CuO, CO2 is detected by heat conductivity. Sulfur is determined as SO2ÿ by relative conductimetry in 3 H2O2/H2SO4, following sample combustion with O2 at 13008C. UV/Vis spectroscopy is performed with a Cary 5 spectrophotometer (Cary Co., USA). UV/Vis spectra of humic substances are dependent on pH, ionic strength, and the extent of complexation with higher valent metal ions (MacCarthy and Rice, 1985; Buckau, 1991). Consequently, a comparison of results requires controlled conditions. The following medium is used: 0.1 M NaCl, 10ÿ3 M tris(hydroxomethyl)amino-

methane, and 10ÿ3 M EDTA at pH 8.5. EDTA was added to avoid a possible in¯uence of higher valent metal ions by complexation with humic substances. FA and HA concentrations are about 20 mg/L. Gel permeation chromatography (GPC) separates entities mainly with respect to their hydrodynamic size. Nevertheless, due to charge exclusion or sorption on the gel matrix, a deviation of the hydrodynamic size may occur. Although a tailing is observed, especially on humic and fulvic acids from the Gorleben recharge groundwaters, the results are presented as a mean hydrodynamic size (in Dalton) in this paper. This should not suggest that the given hydrodynamic sizes always describe the real hydrodynamic size correctly, but rather that this is a convenient way to compare results from GPC under consistent experimental conditions. Fractogel TSK HW±50 (Merck Co., Germany) is used as gel. Under the given experimental conditions it is found to allow the fractionation of substances with hydrodynamic sizes up to around 130 000 Dalton. The hydrodynamic size of humic and fulvic acids varies with pH, ionic strength and the degree of complexation with metal ions (Shaw et al., 1994, Buckau, 1991). To ensure reproducible and intercomparable results, an

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

elution medium is chosen consisting of 0.1 M NaCl, 0.05 M Na2HPO4, 10ÿ3 M EDTA at pH 8.5 and 10 volume % of methanol. Samples of 20 to 100 mL are injected with humic or fulvic acid concentrations between 0.7 and 4.0 g/L. The elution pro®les are determined by absorption at 254 nm (Hewlett-Packard 8452A diode array spectrophotometer). For calibration of the GPC column, the exclusion volume is determined with Dextrane blue and the total volume with sodium azide. In the working range the column is calibrated with globular proteins, namely, Albumin (67 000 Dalton), Ovalbumin (43 000 Dalton), Ribonuclease (13 700 Dalton) and Cytochrome C (12 400 Dalton). The outcome of the GPC is characterized by general properties of the elution pro®les, the hydrodynamic size at maximum absorption, and the size average (MS) and number average hydrodynamic size (Mn). The latter two are evaluated in accordance with Yau et al. (1979). Reproducibility is found to be approximately 2500 Dalton for sizes around 10 000 Dalton and approximately 22000 Dalton in the size range around 30 000 Dalton (cf. GPC distribution maximum of around 2500 to 10 000 Dalton for fulvic acids and about 10 000 to 30 000 Dalton for humic acids (cf. Table 2)). Fluorescence spectroscopy is carried out in addition to the absorption detection of the GPC elution pro®les. The excitation wavelength for the ¯uorescence measurement is 340 nm. The detection of the emitted ¯uorescence light is made via optical ®ber using a diode array spectrophotometer IRY 700 GR (Spectroscopy Instruments Co., USA) at 425 nm. The ¯uorescence yield of humic substances is determined by integration over the ¯uorescence signal of the whole GPC elution pro®le. 14 C measurement on a sample amount of 0.5 g C or more is made by conventional liquid scintillation counting after combustion to CO2 followed by synthesis of benzene (Eichinger et al., 1980). For the combustion method used, the maximum C contamination is determined to be equivalent to 0.24 mg C with a 14C concentration of 115 pmc. For smaller sample amounts, accelerator mass spectrometry is used (Isotrace Laboratory, University of Toronto) (Beukens, 1992). The 14C concentration is given in pmc (% modern C) referred to 94.9% of the activity concentration of the NBS oxalic acid standard in 1950. All given values are corrected with respect to the impacts of sample handling. 4. Results and discussion The results have been obtained for individual groundwaters of di€erent aquifer systems. Groundwaters from the Gorleben aquifer are divided

105

into di€erent groups/areas based on their hydrological and geochemical characteristics, namely, (i) recharge area with shallow groundwaters having 3H concentrations of 8 TU or more; (ii) transition area with deep groundwaters with no signi®cant 3H concentrations (<1 TU); (iii) high salinity brines from the Gorleben channel; (iv) deep groundwaters with enhanced DOC concentrations (between 7.6 and 169 mg C/L contrasting with <3.2 mg C/L in non-enhanced DOC groundwaters; and (v) one near surface groundwater with enhanced DOC (GoHy±341) where young recharge water is mixed with up-lifting water from the depth (`uplift-mixing'). Re¯ecting the important hydrological and geochemical di€erences, this classi®cation of Gorleben groundwaters is applied throughout this paper. 4.1. DOC, humic acid, fulvic acid and

14

C concentration

The concentration and composition of DOC in the sampled groundwaters is shown in Table 1 and Fig. 2. Besides the Gorleben aquifer, DOC in most aquifers is found to be composed of 10 to 40% fulvic acid but, no detectable amount of humic acid, while the rest is made up of hydrophilic compounds. The relatively high DOC concentrations in the Fuhrberg groundwaters, as compared to the values in other recharge and transition groundwaters, must be attributed to di€erences in soil and vegetation conditions, including agricultural activities, because the 14C concentration of Fuhrberg fulvic acids are comparable to those of recharge fulvic acids from the Gorleben aquifer (cf. Table 2). This fact infers that the SOC contribution to the aquatic fulvic acid is not signi®cant. In Gorleben groundwaters varying concentrations of humic acid are found, with its fraction increasing with the DOC concentration. Very small concentrations of humic acid are found in recharge, transition and channel brine groundwaters, whereas in enhanced DOC and uplift-mixing groundwaters humic acid is a dominant part of DOC. The very high humic and fulvic acid concentrations in some deep Gorleben groundwaters (enhanced DOC and uplift-mixing groundwaters) can be explained by the presence of SOC, especially lignite deposits. Analysis of drill cores from the GoHy±573 and ±2227 wells show lignite fractions of more than 20% in some layers (Kim et al., 1996). Very low humic acid concentrations are found in the recharge areas and thus the humic acid in enhanced DOC groundwaters may be considered as resulting solely from SOC. Also an enhanced concentration of fulvic acid is found in enhanced DOC groundwaters, although the increase is signi®cantly lower than that of humic acid. Therefore, a less pronounced contribution of fulvic acids from SOC can be concluded (cf. Table 1).

106

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Fig. 2. DOC concentration and composition of investigated groundwaters.

In Fig. 3 the 14C concentration of fulvic acids is represented versus the fraction of humic acid of DOC that is taken as an indicator for the in situ generation of aquatic humic substances originating from SOC. In the deep enhanced DOC groundwaters, 14C concentrations of fulvic acid are found to be typically below 20 pmc. These groundwaters are of Holocene origin (Artinger et al., 1996) and, therefore, between their recharge and present time, a maximum 14C decay of approximately 2 hal¯ives is possible. Considering that recharge fulvic acid has 14C concentrations of around 80 pmc (Artinger et al., 1996), a considerable contribution of fulvic acid from SOC seems reasonable. The release of fulvic acid from SOC may be considerable not only in deep groundwaters but also in young 3H containing recharge groundwaters. This may be the case in the recharge groundwater GoHy±611 which shows a fulvic acid 14C concentration of 38 pmc as well as a signi®cant contribution of humic acid to the DOC (8%). In Fig. 3, the channel brine fulvic acids show decreasing 14C concentrations although no humic acids

are found. The concentrations and characteristic properties of the channel brine fulvic acids (cf. below) resemble those of recharge fulvic acids and the 14C concentrations re¯ect the expected groundwater age (Artinger et al., 1996). Therefore, an absence of fulvic acid from SOC can be assumed in the channel brines and a decrease in the 14C concentration may be related to radioactive decay. In the uplift-mixing groundwater GoHy±341, the speci®c hydrogeological and geochemical conditions result in a humic acid concentration similar to the average value found in enhanced DOC groundwaters, but the 14C concentration is rather similar to the values in recharge and transition groundwaters. To sum up, DOC, humic acid and fulvic acid concentrations as well as their 14C contents provide important information concerning the origin of aquatic humic and fulvic acids in di€erent groundwaters. In the Munich, Fuhrberg and Franconian Albvorland aquifers, aquatic fulvic acid originates predominantly from the humus horizon with no detectable amount of aquatic humic acid. In Gorleben, and especially in

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Fig. 3.

14

107

C concentration of Gorleben fulvic acids plotted versus the fraction of DOC consisting of humic acid.

some deep groundwaters, the release of aquatic humic and fulvic acids from SOC takes place in addition to aquatic fulvic acid from the soil horizon. The discussion below is focused on the possibility of identifying the origin of aquatic humic and fulvic acids by their characteristic properties. Here, it should be borne in mind that the extended residence time of aquatic humic substances in a given groundwater may change their characteristic properties due to the prevailing physico-chemical conditions of the groundwater irrespective of their origin.

4.2. Elemental composition of humic and fulvic acids The elemental composition (C, H, O, N and S) of the humic and fulvic acids investigated are shown in Table 3. The individual values are normalized to the sum of the elements analyzed and referred to dried substances. The results re¯ect the generally found di€erences between humic and fulvic acids, namely, the higher C and lower O content of humic acids as compared to fulvic acids (Steelink, 1985; Pettersson et al., 1994). The element ratios of H/C and O/C are lower in

Table 3 Elemental composition as well as H/C and O/C ratios of isolated aquatic humic and fulvic acids from di€erent aquifers. Samples from the Gorleben aquifer are additionally arranged according to hydrogeological/geochemical aspects. The numbers of samples studied are given in brackets Aquifer

C (%)

H (%)

O (%)

N (%)

S (%)

H/C

O/C

Munich FA (3) Franconian Alb. FA (4) Fuhrberg FA (9) Gorleben FA Recharge area (5) Transition area (2) Enhanced DOC (8) Channel brines (3) Gorleben HA Recharge area (1) Transition area (2) Enhanced DOC (8) Uplift-Mixing (1)

52.22 0.6 56.32 2.1 52.42 4.0

4.420.2 6.120.5 4.520.5

37.721.2 35.323.2 38.724.0

1.220.1 0.720.2 1.020.5

4.520.8 2.021.0 3.521.6

1.01 20.06 1.31 20.09 1.02 20.09

0.54 20.03 0.47 20.06 0.56 20.11

54.62 2.8 53.02 0.1 53.02 2.2 51.32 1.9

5.020.6 4.620.8 4.320.5 5.320.2

38.321.6 38.124.1 39.922.4 42.721.7

0.520.1 1.020.3 1.620.5 0.520.2

±

1.10 20.09 1.04 20.17 0.98 20.12 1.23 20.01

0.53 20.04 0.59 20.06 0.57 20.05 0.63 20.05

55.0 56.82 1.0 58.62 1.6 57.4

4.4 4.420.3 4.820.4 5.1

36.1 34.421.1 33.120.8 34.8

1.7 2.520.8 2.320.9 1.12

0.96 0.94 20.09 0.98 20.09 1.07

0.49 0.45 20.01 0.42 20.02 0.45

±

: large scatter in data including results below detection limit.

±

2.322.0 ±

2.9 1.921.0 1.220.7 1.6

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R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

aquatic humic acids than in aquatic fulvic acids. This is due to the enhanced aromatic entities of humic acid (Steelink, 1985; Pettersson et al., 1994). On the other hand, humic acid from humus is reported to have a higher O/C ratio than humic acid from peat/lignite (Steelink, 1985). Fulvic acids from young recharge groundwaters with or without a minor in¯uence of SOC (Munich, Fuhrberg and the Gorleben recharge area) and from the Gorleben transition area show very similar H/C and O/C ratios. Fulvic acids from the Franconian Albvorland aquifer with progressively reducing conditions along with the main ¯owpath (groundwater ages exceeding 12 000 a), show a lower O/C ratio and a higher H/C ratio. This may be the result of advanced chemical or microbiological degradation, leading to a reduced O content (De Haan, 1983). Fulvic acids from the channel brines exhibit relatively high H/C and O/C ratios. Whether or not this result re¯ects a selective ionic strength driven precipitation of fulvic acids with less O-containing hydrophilic functional groups and a higher content of aromatic groups cannot be ascertained. For the Gorleben enhanced DOC groundwaters the H/C and O/C ratios do not deviate signi®cantly from the values of recharge and transition waters. Regarding humic acids the data are available only for the enhanced DOC groundwaters and thus no conclusion can be drawn from the elemental composition. To sum up, the elemental composition does not provide information on the presence of humic and fulvic acids which might originate from SOC. The di€erences observed may rather re¯ect the geochemical conditions, such as the ionic strength driven selective precipitation or impact of redox variations over extended periods of time. 4.3. UV/Vis spectroscopy UV/Vis spectroscopy is commonly used for the characterization of humic substances (Schnitzer, 1978; Stevenson, 1982; MacCarthy and Rice, 1985). The spectra of humic substances show an increasing absorption with decreasing wavelength without characteristic features. The characterization is made by either the speci®c absorption at one wavelength (Petterson et al., 1994; Kukkonen, 1992; Pennanen, 1972) or the absorption ratio at two wavelengths for a qualitative description of the curvature of the absorption continuum (Chin et al., 1994; Kukkonen, 1992; De Haan, 1983; Stevenson, 1982). In this work, absorptions at 300 and 400 nm (E3 and E4, respectively) are used as well as their ratio (E3/E4). These wavelengths are selected for their low impact of background compensation and, consequently, good reproducibility. Due to the high molecular weight and colloidal form of the dissolved humic substances, the absorption spectrum

Fig. 4. Speci®c absorption at 300 nm (E3) and 400 nm (E4) plotted versus their ratio (E3/E4) for investigated humic and fulvic acids.

includes the e€ect of light scattering (Power and Langford, 1988; Wylie and Lai, 1986; Pennanen, 1972). The results of UV/Vis spectroscopy are shown in Table 2. In Fig. 4 the speci®c absorption (in L/gÿ1 cmÿ1) of humic and fulvic acids at 300 nm and 400 nm is plotted versus the absorption ratio E3/E4. These two properties vary over a wide range and show the following tendency: the higher the speci®c absorption, the lower is the E3/E4 absorption ratio for humic and fulvic acids. Following the published results (Chin et al., 1994; Kukkonen, 1992; Traina et al., 1990; Gauthier et al., 1987), an increase in the speci®c absorption and a decrease in the absorption ratio are indicative of an increasing humi®cation, aromaticity and molecular weight of humic substances. A fundamental di€erence is not observed between the absorption ratio of 465 to 665 nm, often cited in the literature, and the E3/E4 ratio used. In Fig. 5 the average speci®c absorption at 300 nm (E3) is plotted versus the average absorption ratio of 300 and 400 nm (E3/E4) for humic and fulvic acids separated into di€erent groups of groundwaters. A signi®cant di€erence is observed to exist between humic and fulvic acids as well as among the humic or fulvic acids. As a general trend for both humic and fulvic acids, the speci®c absorption is increasing and the E3/E4 ratio is decreasing in the order of recharge, transition, enhanced DOC. Fulvic acids from Gorleben channel brines as well as from Munich and the Franconian Albvorland aquifers resemble the Gorleben recharge fulvic acids. In contrast to this, Fuhrberg fulvic acids

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

109

Fig. 5. Speci®c absorption at 300 nm (E3) plotted versus the absorption ratio E3/E4 of isolated humic and fulvic acids arranged according to the di€erent groups of investigated groundwaters. The fulvic acid of the recharge groundwater GoHy-611 is shown separately.

resemble Gorleben transition fulvic acids. Fulvic acids from the recharge groundwater GoHy±611 and the uplift-mixing groundwater GoHy±341 resemble the enhanced DOC fulvic acids. In the Franconian Albvorland the E3/E4 ratio shows a decrease with increasing distance from recharge along the main ¯ow direction to the most distant downstream well of Beilngries. According to the 14C dating of dissolved inorganic C in the Franconian Abvorland aquifer, the groundwater in the Beilngries well is of Pleistocence origin (Geyer et al., 1993). Furthermore, the redox potential changes from 594 mV to ÿ41 mV from the recharge to Beilngries. This could be an indication for an impact of the residence time on the spectroscopic properties or changes with the geochemical environment. The range of E3/E4 ratios, as observed for the Franconian Albvorland water, is comparable with the recharge waters of Gorleben and Munich. Furthermore, fulvic acids from the young near-surface Fuhrberg groundwaters with no reducing conditions are found to have lower E3/E4 ratios than those from other recharge groundwaters and from the reducing Franconian Albvorland aquifer. In the Gorleben young (<40 a) recharge groundwater GoHy±611 containing 3H, the 14C concentration of fulvic acid and E3/E4 ratio are signi®cantly lower than in other young recharge groundwaters. The 14C concentration is about half to one third of the concen-

tration found in fulvic acids from other recharge waters, while the UV/Vis absorption properties resemble fulvic acids from enhanced DOC groundwaters. In this case, it may be assumed that the origin, i.e., release of fulvic acid from SOC, is the governing factor for the spectroscopic properties rather than local geochemical conditions. Therefore, it may be concluded that the UV/Vis absorption properties of aquatic humic and fulvic acids depend on both the origin, including di€erences in recharge conditions, and to a lesser extent on the prevailing geochemical conditions in the groundwater. Consequently, the UV/Vis spectroscopic properties of aquatic humic substances may be useful to identify their origin. 4.4. Gel-permeation chromatography (GPC) Some representative GPC chromatograms of Gorleben groundwaters are shown in Fig. 6. In all cases, some degree of tailing is observed, which is more pronounced for humic and fulvic acids from recharge and transition groundwaters and, thus, caution is necessary when interpreting the hydrodynamic size. With respect to humic acids from di€erent groups of Gorleben groundwaters, di€erences occur in (i) the overall elution peak area; (ii) the position of the elution pro®le in the working range of GPC; and (iii)

110

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Fig. 6. GPC elution pro®les of selected humic and fulvic acids from Gorleben groundwater.

the presence of the second peak at the exclusion limit of the column. IR studies on di€erent GPC fractions of humic acid have shown that the material eluted with the peak at the exclusion limit is distinctively di€erent from the rest. In this fraction the typical IR bands for proton exchanging carboxyl groups are absent (Kim et al., 1990). The elution peak of this kind is found for humic acids from enhanced DOC groundwaters, whereas humic acids from recharge and transition water show very weak indications only in this range. GPC elution pro®les for Gorleben fulvic acids reveal

that (i) the overall elution peak area re¯ects di€erences for fulvic acids from di€erent regions, (ii) the elution pro®les are shifted towards smaller hydrodynamic size for fulvic acids from enhanced DOC, via transition to recharge, and (iii) no peak occurs at the exclusion limit of the GPC column. In Table 2 the GPC results are given by the hydrodynamic size at the elution pro®le maximum (peak at exclusion limit is not considered), the size average and the number average of the hydrodynamic size. As expected from the relatively symmetric GPC elution pro®les, the size average and the distribution maximum show comparable results, the number averages, however, are smaller. The ratio of size average and number average (Ms/Mn) re¯ects the symmetry of the GPC elution pro®les (Yau et al., 1979). In general, somewhat lower values are found for fulvic acids from Gorleben enhanced DOC groundwaters than from others. The fulvic acid from GoHy±611 resembles those from transition groundwaters, but not from recharge groundwaters. The humic acid from GoHy± 412 resembles recharge and transition humic acids. This can be understood from Fig. 2, where it is shown that the humic acid fraction of DOC in this groundwater is low compared to that from other enhanced DOC groundwaters. This may suggest that the groundwater GoHy±412 is only moderately in¯uenced by processes leading to release of DOC from SOC. In Fig. 7 the GPC results are summarized by plotting the speci®c absorption at 300 nm versus the hydrodynamic size of the elution pro®le maximum. The humic acids from recharge and transition belong to one group, the uplift-mixing GoHy±341 and enhanced DOC groundwaters to another group. The GPC elution maxima of fulvic acids from Gorleben increase from recharge via transition to enhanced DOC. Fulvic acids from the channel brines and GoHy±611 are found to be in the range of transition groundwaters. Fulvic acids from the Franconian Albvorland and Munich resemble those of the Gorleben recharge area, whereas Fuhrberg samples fall in the range between recharge and transition fulvic acids. To sum up, GPC with UV/Vis detection of humic and fulvic acids reveals some important di€erences between di€erent aquifer systems as well as within the Gorleben aquifer. The presence of a peak at the GPC exclusion limit represents a distinctive di€erence between the humic acids from enhanced DOC and uplift-mixing groundwaters and between the humic acids from recharge and transition groundwaters. The humic acid from GoHy±412 with a moderately enhanced DOC concentration is found to resemble humic acids from recharge and transition, rather than humic acids from other enhanced DOC groundwaters, with respect to the absence of a peak at the exclusion limit and the position of the distribution maximum.

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

111

Fig. 7. Speci®c absorption at 300 nm (E3) plotted versus the GPC elusion pro®le maximum of isolated humic and fulvic acids arranged according to the di€erent groups of investigated groundwaters. The humic acid peak at the exclusion limit of the GPC column is not accounted for. The fulvic acid of the recharge groundwater GoHy-611 is shown separately.

4.5. Fluorescence spectroscopy The results obtained from the characterization by ¯uorescence spectroscopy are given in Table 2. Some examples of ¯uorescence spectra are shown in Fig. 8. The spectra possess the general characteristics of aquatic humic substances (Hayase and Tsubota, 1985; Miano et al., 1988; Visser, 1983). For ¯uorescence spectra of humic acids from recharge via transition to enhanced DOC groundwaters, a decrease of the shoulder at the higher wavelength of the spectra can be observed. The fulvic acids show similar spectra for the Franconian Albvorland, Munich and Gorleben channel brine and recharge groundwaters, whereas the spectra for FA from transition and enhanced DOC groundwaters with a higher hydrodynamic size are slightly shifted towards a longer wavelength. These results are in accordance with the bathochromic shift observed for HA and FA fractions of di€erent hydrodynamic size, as obvious for GoHy±411 in Fig. 9. The same behavior has been found by Jones and Indig (1996) for di€erent fractions of Aldrich humic acid (Aldrich Co.). The correlation between the ¯uorescence spectra and the hydrodynamic size of fulvic acids indicates that the ¯uorescence spectra may also provide some information about the origin of humic substances. Somewhat more distinct di€erences can be seen in

the ¯uorescence intensity (see Fig. 10). In general, the ¯uorescence intensity of humic acid is lower than of fulvic acid. While humic acids from di€erent types of Gorleben groundwaters cannot be distinguished from one another, fulvic acids from enhanced DOC groundwaters have a considerably higher ¯uorescence intensity than other fulvic acids. The ¯uorescence intensities of fulvic acids from Fuhrberg, Munich and Gorleben transition and channel brine groundwaters are comparable with the values for Gorleben recharge and Franconian Albvorland fulvic acids. As expected from the above discussions concerning the fulvic acid GoHy±611, the ¯uorescence behavior indicates an admixture of fulvic acids originating from SOC, as this is done by the DOC composition, 14C concentration, UV/Vis absorption and the hydrodynamic size. Therefore, ¯uorescence spectroscopy could be an additional tool for determining the origin of fulvic acids. 4.6. In¯uence of physico-chemical/geochemical conditions The above discussion has shown that a characterization may be useful to distinguish humic and fulvic acids from di€erent aquifers, but also from di€erent sources within an aquifer system, such as Gorleben. The characteristic properties of aquatic humic substances may re¯ect the source, the generation mechan-

112

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Fig. 8. Fluorescence spectra of humic and fulvic acids (excitation at 340 nm). The spectra are recorded at the hydrodynamic size fraction with the highest ¯uorescence intensity. For reasons of comparability, the spectra are depicted with the same height.

ism and the geochemical conditions (involving also microbial mediated reactions) at the place of their origin. For example, humic substances originating from the soil zone will generally re¯ect the oxidizing conditions, whereas humic substances originating from SOC will re¯ect the reducing conditions. Furthermore, aquatic fulvic acids from the recharge zone may also show variations, thus re¯ecting di€erent soil/vegetation conditions. The physico-chemical conditions of Fuhrberg groundwaters can be described as oxidizing with pH values around 4 to 6. Groundwaters from the other investigated sites have pH values above 6 and redox conditions which vary from oxidizing to reducing Eh values. The characteristic properties of fulvic acids from Fuhrberg, however, do not correspond to those of the Gorleben recharge groundwaters, but to the transition groundwaters with reducing conditions and pH values around 8. Therefore, the characteristic prop-

erties of fulvic acids to a large extent seem to depend on their origin and not on the prevailing local groundwater conditions. Despite these results, the in¯uence of changing geochemical conditions on the characteristics of aquatic humic substances during the transport along the groundwater ¯owpath cannot be excluded (cf. Fig. 11 and Fig. 12). As obvious from Fig. 11, the absorption ratio E3/E4 of fulvic acids appears to decrease with decreasing redox potential along the ¯owpath in the sandstone aquifer of the Franconian Albvorland. This is not to imply, however, that the redox potential determines the UV/Vis absorption behavior of fulvic acids, but rather that the redox potential may be an indicator of processes in¯uencing the absorption behavior. In Fig. 12 di€erences in the GPC distribution maximum of fulvic acids from Gorleben, Munich and the Franconian Albvorland are plotted versus the pH of groundwater. Although the values scatter considerably, a correlation of the hydrodynamic size with the pH of the groundwater can be recognized. As regards the molecular size of fulvic acids in the Franconian Albvorland aquifer the change is not ascribed to a di€erent source (sediments presumably free from SOC). Therefore, the characteristic properties of fulvic acids in the Franconian Albvorland may indeed be in¯uenced by the geochemical condition of the groundwater. However, due to the large contribution of fulvic acids originating from SOC, this conclusion is not directly applicable to the Gorleben aquifer. In this case, the pH of the groundwater re¯ects the physicochemical condition associated with the release process of humic substances from SOC. In the special case of the saline Gorleben groundwaters, a signi®cant in¯uence of the ionic strength on the characteristic properties of the humic substances is expected. It is conspicuous that in none of the highsaline groundwater signi®cant fractions of humic acids are found (this work and Buckau, 1991), although overlaying groundwaters contain humic acids. On the other hand, the fulvic acids show a signi®cantly higher 14 C concentration than those in¯uenced by the SOC. This strongly suggests that the groundwater entering the saline Gorleben channel has not been in contact with organic deposits or that the fulvic acids found re¯ect a selective ionic strength driven precipitation of humic acids and part of the fulvic acids. From the latter process, it might be expected that the especially humic substances originating from DOC with high molecular sizes are precipitated preferentially. The results of this study suggest the prevalence of the latter process. To sum up, the origin (including source, geochemical conditions and processes of generation) appears to dominate the characteristic properties of aquatic humic substances. This results in di€erent properties of

R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

113

Fig. 9. Fluorescence spectra of di€erent hydrodynamic size fractions of fulvic and humic acid from GoHy-411 (excitation at 340 nm). For reasons of comparability, the spectra are depicted with the same height.

aquatic fulvic acids from di€erent aquifer systems as well as di€erent sources (soil zone versus peat/brown coal deposits) within the Gorleben aquifer. However,

under special geochemical conditions like the saline Gorleben groundwaters or changes in geochemical conditions over extended time periods, as in the

Fig. 10. Speci®c absorption at 300 nm (E3) plotted versus the ¯uorescence intensity of isolated humic and fulvic acids arranged according to the di€erent groups of investigated groundwaters. The fulvic acid of the recharge groundwater GoHy-611 is shown separately.

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R. Artinger et al. / Applied Geochemistry 15 (2000) 97±116

Fig. 11. Absorption ratio E3/E4 of fulvic acids from the Franconian Albvorland aquifer plotted versus the redoxpotential of the groundwaters.

Franconian Albvorland, the prevailing geochemical conditions also need to be taken into account. 5. Summary and conclusions Aquatic humic and fulvic acids from di€erent aquifers and di€erent locations within an aquifer show di€erent characteristic properties. These di€erences appear to depend on the source of the aquatic humic and fulvic acids as well as on the physico-chemical/ geochemical conditions of the groundwater. In the Gorleben aquifer system high DOC concentrations in some deep groundwaters can be attributed to the admixture of SOC. The same is true for a groundwater at low depth (10±13 m) that, in view of its mineral composition and in agreement with the regional groundwater model, originates from deep layers. Within this aquifer system the following characteristic properties of aquatic fulvic and humic acids may be attributable to a predominant contribution of SOC: (i) a higher fraction of humic acid in DOC, (ii) a higher speci®c absorption and ¯uorescence intensity of fulvic acids, (iii) a lower E3/E4 absorption ratio of fulvic acids, (iv) a higher hydrodynamic size of fulvic and humic acids. An example of those indicators is the appearance of a large peak at the exclusion limit of the applied GPC system for humic acids from enhanced DOC groundwaters in contrast to those from recharge and transition groundwaters. Another example may be the fulvic acid in the young recharge groundwater GoHy±611, where the characteristic properties listed

Fig. 12. Hydrodynamic size at the GPC elution pro®le maximum of the Gorleben, Munich and Franconian Albvorland fulvic acids plotted versus the pH values of the groundwaters.

above resemble Gorleben transition or enhanced DOC groundwaters, which contain humic substances originating from SOC. The characteristic properties of aquatic humic and fulvic acids from di€erent groundwaters within an aquifer system may also be ascribed to geochemical properties. The applied characterization methods of aquatic humic and fulvic acids may be summarized as a useful tool in identifying the origin of aquatic humic and fulvic acids. In special cases, a possible in¯uence of changing geochemical conditions and processes needs to be taken into consideration. In conclusion, the characterization of aquatic humic substances in the presence of organic deposits is an essential tool for investigations of the role of humics in transporting pollutants. The characterization methods described above not only provide information about the origin, but also about the mobility and the longterm stability of humic substances. Furthermore, these investigations are very helpful for the determination of groundwater residence times using radiocarbon dating on humic substances. In addition, a better understanding of the fate of aquatic humic substances allows conclusions to be drawn with regard to the geochemical evolution of groundwater systems. Acknowledgements The authors would like to acknowledge the ®nancial support of the Bundesministerium fuÈr Bildung, Wissenschaft, Forschung und Technologie. The authors also thank H. Siela€ and D. Wesselow of the DBE at the Gorleben site for their assistance in

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groundwater sampling, C. Kardinal and H. Halder for assistance in the DOC sample preparation and characterization, and F. Scherbaum for assistance in the ¯uorescence measurement. The isotope measurements were carried out under the supervision of W. Rauert and P. Trimborn.

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