acid-base characteristics of organic carbon in the humex lake skjervatjern

Environment International, Vol. 18, p. 621-629, 1992 0160-4120/92 $5.00 +.00 Printed in the U.S.A. All tights reserved. Copyright ©1992 Pergamon Press Ltd.

ACID-BASE CHARACTERISTICS OF OR- GANIC CARBON IN THE HUMEX LAKE SKJERVATJERN

Pirkko Kortelainen, Mark B. David*, Tuija Roila, and Irma M~kinen National Board of Waters and the Environment, P.O. Box 250, SF-O0101 Helsinki, Finland

EI 9207-166 M (Received 12 July 1992; accepted 3 September 1992)

The Humic Lake Acidification Experiment (HUMEX) was launched in 1988 to study the role of humic substances in the acidification of surface waters and the impacts of acidic deposition on the chemical and biological properties of humic substances. This subproject was designed to determine the contribution of organic acids to the acidity of Lake Skjervatjern (the HUMEX Lake) and the impacts of the acidification on the characteristics of organic carbon. In order to get an empirical measure for organic acidity, dissolved organic carbon (Doe) was fraetionated, isolated, and base-titrated from each half of Lake Skjervatjern. I-Iydrophobic acids were the dominant organic carbon fraction; the total organic acid content was generally greater than 80% of the DOC. The reliability of the fraetionation procedure was tested with synthetic acids and the Nordic Fulvic acid. The D o e fractions did not show high variation over the 1.5-y acidification period. Hydrophilic acids had consistently greater exchange acidities compared to hydrophobic acids, averaging 12.9 l.teq/mg D o e vs. 10.9 bteq/mg DOC, respectively. The dissociation of organic acids during acid-base titrations clearly increased with increasing pH. The high organic anion contribution to the ion balances indicates that humic matter is an important acidity source in Lake Skjervatjern. There are slight signs that the contribution of organic acids to overall lake acidity has decreased since acidification was initiated.

INTRODUCTION

Over the past two decades, acidification has been one of the central issues in environmental research. Extensive national research programmes have been conducted in many European and North American countries. The long-distance transport of pollutants has made acidification also an international problem and resulted in extensive cooperation in both research and environmental policy. In some parts of

*Present address: University of Illinois, Department of Forestry, W-503 Turner Hall, 1102 South Goodwin Av., Urbana, IL 61801 USA.

North America and North Europe, acidic deposition has been found to be responsible for widespread changes in surface water chemistry and biology (Merilehto et al. 1988; Schindler 1988).

Many recent studies have shown that surface waters exist whose chemistry is dominated by organic acidity. Aquatic humie matter is a complex mixture of acids, bases, neutrals, carbohydrates, and hydro- carbons, etc. (Thurman 1985). Due to the presence of carboxyl groups, humic matter in natural waters is typically quite acidic (Perdue et al. 1984).

The contribution of natural organic acidity to the acidity of aquatic ecosystems has been an important subject in acidification research over the past

621

622 E Kor te la inen et al,

decade. However, despite decades of intensive research both in North America and Europe, organic matter in aquatic environments still remains poorly characterised at the molecular level. Consequently, estimation of the acidity sources in humic lakes has proven to be a difficult task.

Because all empirical methods of measuring the organic acidity are laborious, they are not applicable for routine water sample analysis. For example, organic acidity cannot be measured experimentally in extensive surveys or intensive seasonal monitoring. Most of the large-scale studies that have documented the contribution of organic matter to the surface water acidity have used indirect approaches; either ion balance calculations or the model of Oliver et al. (1983). This model was based on organic acid dissociation model (Gamble 1970; Perdue et al. 1980) and requires only dissolved organic carbon (DOC) and pH measurements from the natural water sample.

When a more thorough measure for organic acids is needed, the procedure usually includes the following steps: concentration, fractionation, isolation, purification, and titration. The empirical measurements have been used only in a limited number of mostly methodological studies based on a couple of samples. Since Oliver et al. (1983), there had been no attempts to develop an empirically developed estimate for organic acidity in surface waters until David and Vance (1991) presented empirically measured organic anion for five lakes in Maine.

The Humic Lake Acidif icat ion Experiment (HUMEX) was launched in 1988 to study the role of humic substances for acidification of surface waters and the role that acidification plays for the chemical and biological properties of the humic substances. Lake Skjervatjern is located in an area which receives little or no acidic deposition. To achieve a proper reference, the lake was divided into two halves (A: experimental; B: reference) by a plastic cur- tain. Half A and the corresponding catchment area have been acidified since October 1990 to examine the response of both lake water chemistry and biological activity (Gjessing 1991). This experimental approach allows a detailed study of changes in lake water DOC as a result of acidification.

The objectives of this project are to: 1) determine the contribution of organic acids to the acidity of Lake Skjervatjern; and 2) examine the impact of acidification on the composition and character of organic carbon. In order to get an experimental measure for organic acidity, hydrophobic and hydrophilic acids were isolated from Lake Skjervatjern.

The isolated acids were base-titrated to obtain charge density as a function of pH. Organic anions were calculated by analysing titration results of the isolated acid fractions. This empirical estimate for organic anion was compared to organic anion estimated by ion balance calculations. This paper summarizes the fractionations, isolations, and titrations of DOC conducted during a 1.5-y period after the acidification was started. The results from September 1990, before acidification was initiated, have been presented in David et al. (1991).

MATERIAL AND METHODS

Sampfing Bulk samples for organic acid isolation studies

were collected from the A and B halves in September 1990 before acidification was started, and in September 1991, one year after treatment. At each time, depth profile samples from both lake halves for fractionation studies were also collected. In addition, samples for fractionation studies have been taken from both outlets every three months since September 1991. This sampling program: 1) organic acid isolations from bulk samples in September, and 2) fractionations from both outlets every three months, will be carried out during the next years.

Analytical methods

In order to study the acidity characteristics of DOC before and during acidification of the other lake half, the dissolved organic carbon was fractionated and organic acids were isolated, purificated, con- centrated, and titrated.

A modified procedure of Leenheer (1981) based on XAD-8, anion-exchange (Duolite A-7), and cation- exchange (AG-MP-50) resins was used to fractionate dissolved organic carbon into hydrophobic acids and neutrals, and hydrophilic acids, bases, and neutrals. The fractionation procedure was tested with synthetic acids. In addition, since November 1991, the Nordic Fulvic acid has been fractionated frequently to test the repeatability of the procedure.

Organic acids (the most important part of DOC) were isolated using the modified procedure of Leen- heer (1981). Samples were passed through cation-exchange resin, lowered to pH 2.0 and pumped through columns of XAD-8 and anion-exchange resins that were connected in sequence. Hydrophobic acids (HPO-A) were eluted from the XAD-8 column using NaOH and protonated by passing through cation-exchange resin. Hydrophilic acids (HPI-A) were eluted from the anion-exchange column with NaOH, passed

Organic carbon characteristics in the HUMEX Lake 623

through a cat ion-exchange column with NaOH, acidif ied to pH 2.0, and readsorbed on XAD-8. Adsorbed hydrophilic acids were then removed following the procedure outlined for collection of hydrophobic acids. This isolation procedure is similar to the procedure used to isolate the Nordic Fulvic acid standard during summer 1987. A detailed descrip- tion of the fractionation and isolation methods is given in David and Vance (1991).

The above isolation procedure was carried out in September 1990 and in September 1991. In addition, another slightly different isolation procedure was used in September 1991. Hydrophobic and hydrophilic acids from both lake halves were isolated using the fo l l owing p rocedure : Samples were passed through cation-exchange resin, lowered to pH 2.0, and pumped through columns of XAD-8 and XAD-4 that were connected in sequence. Hydro- phobic acids were eluted from the XAD-8 column and hydrophilic acids from the XAD-4 column respect ively , using NaOH. Both hydrophilic and hydrophobic acids were protonated by passing through a cation-exchange resin. Ronald Malcolm from U.S. Geological Survey, Denver, Colorado used this isolation procedure when isolating humic and fulvic acids from 1200 L of water from both outlets in

September 1990 and September 1991. In September 1991, samples were collected by the U.S. Geological Survey at the same time as our samples (Malcolm 1992).

The isolated hydrophobic and hydrophilic acids were base-titrated to obtain organic acid dissociation as a function of pH. Titrations were conducted on an Orion 960 Autochemistry unit using 0.1 N KOH. Ionic strength was adjusted to 0.01 using KCl and samples were bubbled with N2 gas before and during the base titrations.

A first derivative method was applied to the titration data to compute exchange acidity and charge density. Exchange acidity is defined in this paper as the total acidity in the sample titrated to pH 7 and charge density as the dissociated organic anion content per mg DOC at a given pH. When calculating exchange acidity or charge density of organic carbon, the residual anions and cations of the isolated acids were included.

Charge density (~teq/mg DOC) = (Residual cations -

Residual anions + k ÷ + H ÷ - OH) /DOC (1)

Table I. Titration results of the isolated organic acids from Lake Skjervatjern (A: experimental half; B: reference half). In September 1991, hydrophilic acids were isolated also by XAD-4 resin (see text),

C, exchange acidity; a, b, and c parameter values in equation (2); HPO-A hydrophobic acids; HPI-A hydrophilic acids.

Sample C t pK. a b c

September 1990

HPO-A 10.3 4.19 -8.72 4.37

HPI-A 13.4 4.03 -13.0 6.60

-0 .252

-0 .423

September 1991

A HP0-A ii.0 4.13 -10.5 5.22

A HPO-A i0.6 4.08 -9.72 4.87

B HPO-A 11.3 4.13 -i0.I 5.06

B HPO-A i0.6 4.16 -9.06 4.56

A HPI-A 13.9 4.07 -15.8 7.56

A HPI-A (XAD-4) 10.7 3.83 -i0.0 5.34

B HPI-A 14.9 4.04 -16.1 7.82

B HPI-A (XAD-4) 12.1 3.85 -10.5 5.69

-0.318

-0.292

-0.303

-0.266

-0.488

-O.348

-0.499

-0.362

624 P. Kortelainen et al,

where charge density is the unit charge at each pH, k + the base added, and the denominator is mg of DOC in solution titrated.

Using titration results, the dissociation or organic acids at different pH values can be calculated. The second order equations for charge density as a function of pH (Table 1 and Fig. 3) can be used to determine the charge density of organic acids at any pH within pH 4 and 7.

An empirical measure for organic anion was developed using titration results of isolated hydrophobic and hydrophilic acids (second order equations as a function of pH) combined with fractionation results (Table 1).

Organic anion = d*DOC(mg/L)(a + bpH + c(pH) 2) +

e*DOC(mg/L)(a' + b'pH + c'(pH) 2) (2)

where d and e are the percentages of hydrophobic and hydrophilic acids, respectively.

This empirical organic acidity estimate was compared to organic anion estimated by ion balance calculations as a difference between g Cations and Y- Inorganic anions.

In September 1990, the fractionation, isolation, and titration studies were carried out at the University of Illinois, U.S.A. Since September 1991, this experimental work has been conducted at the Na- tional Board of Waters and the Environment, Finland using similar equipment and techniques. Inorganic chemical measurements are frequently carried out at the Norwegian Institute of Water Research. From some samples, complete inorganic chemistry was also measured at the University of Illinois and at the National Board of Waters and the Environment.

Outlet 1 m 2 m

7060 j Sept.-90

50-

4o-

2 0

10

0 A

~ Sept.-g1 60

o O a

#

B Outlet 1 m 3m m 7 m 9rn

v A Outlet 1 rn 2 m 3 m

Sept.-90

B Outlet l m 3 m 5rn 7 m 8 m

I HPO-A HPO-N HPI-B HPI-A HPi-N

Location

Fig. I. Dissolved organic carbon fractions as a percentage of DOC for Lake Skjervatjern by half and depth in September 1990 (David et al. 1991) and 1991. A and B are composite samples by depth of experimental and reference halves, respectively. HPO-A: hydrophobic acids; HPO-N: hydrophobic neutrals; HPI-B: hydrophilic bases; HPI-A: hydrophilic acids; HPI-N: hydrophilic neutrals.


7 0 -

5 0 ¸-

, 0

30--

2 0 -

10

0

A

R 4

B

m HPO-A r'-~ m,o-,

HPI-II

[~" HPI-A

Sept.-90 Sept.-91 Dec.-91 M arch -92 Ju ly -92 Sept.-90 Sept.-91 Dec.-91 M a r c h - 9 2 Ju ly -92

Fig. 2. Dissolved organic carbon fractions as a percentage of DOC in outlet samples for both lake halves (A: experimental; B: reference). See Fig. 1 for fraction identification.

RESULTS AND DISCUSSION

The annual precipitation in the research area is high, about 2 500 mm. The theoretical water retention time is low (0.13 y in Half A and 0.37 y in Half B) (Gjessing 1992). Large variations in some water quality parameters during a short period have been found. Before acidification was started, large variations (e.g., in outlet TOC, Na, and C1 concentrations) have been found (Gjessing 1991).

Fractionations of DOC

In samples used for fractionation or isolation studies, DOC ranged from 4.8 mg/L to 14.1 mg/L. In every sample, the proportion of organic acids was overwhelming (generally >80%) compared to neutral and basic components. The proportions of acid, neutral, and basic components in Lake Skjervatjern were comparable to the proportions in Maine lakes (David and Vance 1991). Before acidification was initiated, samples from all depths and both outlets had uniform DOC fractions, with hydrophobic and hydrophilic acids averaging 55% and 30%, respectively (David et al. 1991). The DOC fractions did not show high variations over the 1.5 y acidification period (Figs. 1 and 2). In September 1991, the total hydrophobic/hydrophilic ratio was lower in the experimental half compared to the reference half (1.1 vs. 1.5). However, during winter 1992 similar ratios were found in both lake outlets.

The variation of TOC concentrations in Lake Skjervatjern is high. This may cause some variation in the proportions of different fractions in both lake halves. Low DOC lakes have been found to have smaller proportions of their DOC as hydrophobic acids, and greater hydrophilic acids and neutrals compared with higher DOC lakes (David et al. 1991).

Some variation can also be due to seasonal variation. A decrease in the ratio of hydrophobic to hydrophilic acids during summer and fall months has been found (Vance and David 1991b). Experimental acidification of soil samples has resulted in an increasing proportion of hydrophilic acids and decreasing proportion of hydrophobic acids. However, clear changes were not found until at pH values much below four (David et al. 1989; Vance and David 1991b).

Duplicate fractionations were made on several samples, with variation between replicate analysis <5%. Comparison of the fractionation and isolation data indicates that the two methods gave results within 9% for hydrophobic acids, hydrophilic acids and hydrophilic neutrals.

Model compounds included oxalic and benzoic acids and potassium phtalate. Of the oxalic acid, 99% was recovered as hydrophilic acids, whereas 98% of the benzoic acid was recovered as hydrophobic acids. For potassium phtalate, 69% of DOC was recovered as hydrophobic acids and 30% as hydrophilic acids.

The average Nordic Fulvic acid composition was hydrophobic acids 85% (range 79-89%); hydrophobic neutrals 4%; hydrophilic bases 2%; hydrophilic acids 8%; and hydrophilic neutrals 1%.

Titrations of isolated organic acids

Organic acids are mostly weak acids which are never totally dissociated and never totally protonated in natural waters. The titration curves of the isolated acids can be used to determine the dissociation of organic acids at any pH between four and seven (Fig. 3).

In September 1991, one year after the acidification was started, the dissociation of hydrophobic acids as

626 P. Kortelainen et al

1 6

1

Hydrophoblc acids

0 ~ 1 0 ~ 6

4

2 i t I t I

4 . 0 4 . 5 5 . 0 5 . 5 6 . 0 6 . 5

pH

Hydrophlllc acids

B S e p t . - 9 1 - • A , ~ p t . - 9 1 ( X A D - 4 )

a - B S e p t . - 9 1 (XAD-4) O l i v e r

7 . 0 4 . 0

I I I I I

4.5 5.0 5.5 6.0 6.5 7.0

pH Fig. 3. Dissociation of isolated hydrophobic and hydrophilic acids as a function of pH for both lake halves (A: experimental; B: reference) during September 1990 and 1991. In September 1991, two isolation methods, anion exchange resin and XAD-4 resin, were used.

Results from 0liver et al. (1983) are also shown for comparison purposes.

a function of pH was close to the results from Sep- tember 1990. The duplicate isolations from both lake halves in September 1991 gave similar titration results (Table 1 and Fig. 3). The dissociation of the hydrophobic acids and exchange acidity in Lake Skjervat- jern were similar to the results of Oliver et al. (1983). Oliver et al. (1983) was based on the humic/fulvic acid isolation procedure, in which the isolated acids would be classified as hydrophobic acids.

Hydrophilic acids are not isolated if the humic/fulvic acid isolation procedure is followed. In September 1991, the hydrophilic acids were isolated using two different techniques: anionexchange vs. XAD-4 resin. At pH 4.5 (normal pH in Lake Skjervatjern), the difference in charge density between these methods is <1.5 Ixeq/mg DOC. However, it increases to almost 3 ~teq/mg DOC at pH 7. Compared to the results from September 1990, the anion exchange resin in 1991 gave higher exchange acidities, whereas XAD-4 isolation results were lower. The differences between anion exchange and XAD-4 charge densities at high pH may be due to: 1) differences in DOC extracted by the resins (hydrophilic neutrals were 25% and 10% using XAD-4 and anion exchange resins, respectively, indicating greater DOC retention with the anion exchange resin); and 2) variable alterations in hydrophilic acids by the resins (the anion exchange resin retains DOC more strongly than XAD-4 and requires a stronger base to elute compared to XAD-4).

Before acidification, titration results showed no differences between the A and B halves, for either hydrophobic or hydrophilic acids. Exchange acidity averaged 10.3 and 13.4 ~teq /rag DOC for hydrophobic and hydrophilic acids, respectively (David et al. 1991). In September 1991, exchange acidity for hydrophob ic and hydroph i l i c acids in Half A averaged 10.8 and 12.3 lxeq/mg DOC, respectively, and in Half B 11.0 and 13.5 lxeq/mg DOC, respectively.

The results from Lake Skjervatjern as well as the studies of McKnight et al. (1985), David and Vance (1991), and David et al. (1991) show clearly that hydrophilic acids are the strong acidity associated with the DOC, which is reflected in higher charge density values for hydrophilic acids at a given pH compared to hydrophobic acids. The FT-IR and 13C-NMR spectra suggest that hydrophobic and hydrophilic acids are primarily carboxylic acids. Both potentiometric titration results and 13C-NMR spectra data suggest greater content of carboxylic functional groups per C in hydrophilic acids compared to hydrophobic acids (Vance and David 1991a).

Organic anion and ion balances

The various acids in surface waters can be iden- tified by measuring the corresponding anions. Using solution cation-anion balances, an estimate of the organic anion contribution to the acidity can be definied by the anion deficit. This approach also allows the calculation of charge density of DOC at


!i e,m

:1 v

3(

2£

1C

C - ! 0

ti e,

A tie, e, e,

e, e, e, e,ti e,

e, e, ~A e,e, e, e,

4 b ~ tAe` A ~ ~ ti - # e,

\ : , , ° e, A

e,

~ e ` e`

I L f I I I I I I I - 10 0 10 20 30 40 50 60 70 80

A n i o n d e f i c i t (~eq/L)

Fig. 4. Organic anion based on empirical measurements (A') as a function of anion deficit (Z Cations - Z Inorganic anions). The data s e t was collected before acidification was initiated.

the sample pH ((X cations - Z inorganic anions)/DOC). A more quantitative method for calculating organic anion and organic charge contribution uses the potentiometric titration data of isolated organic solutes. The empirical organic anion measurements for lakes have been used in Oliver et al. (1983), in David and Vance (1991) and in David et al. (1991). In this study, organic anion and charge density were measured empirically. Equation (2) based on the titration results of the isolated acids as well as the DOC fractionation results was used to calculate organic anion. Empiri- cally measured organic anions were compared to the anion deficits in ion balances.

Before acidification was initiated, an overall charge balance for both lake halves (two samples) showed agreement between the anion deficit and empirically determined organic anion (David et al. 1991). The equations for organic anion based on empirical measurements were applied to all chemical samples collected from Lake Skjervatjern before acidification (Gjessing 1991). Anion deficits were generally 5-20 kteq/L lower than calculated organic anions (Fig. 4). Using the ion balance approach, the mean organic anion content and charge density in Lake Skjervatjern were 18 ~eq/L and 2.8 tteq/mg TOC, respectively. For some samples with low TOC con-

centration, the ion balance approach gave negative organic anion concentrations.

The charge density values from Lake Skjervatjern based on ion balance calculations are comparable to the results from a 1000-lake survey in Norway (Hen- riksen et al. 1988a; Henriksen et al. 1988b) as well as the results from the RAIN Project (Wright et al. 1988; Wright 1989).

When organic anion based on the experimental approach was included in ion balances in September 1991 (one year after acidification was initiated), the sum of anions was higher than the sum of cations (Fig. 5). The difference between inorganic anions and cations was 27-36 tteq/L (<18% of the total amount of anions). The agreement between the two methods was slightly better for XAD-4 compared to anion exchange resin isolations. Reasons for the systematic differences between the anion deficit and organic anion based on empirical measurements are unknown and need further study.

For Maine lakes, empirical equation vs. ion balance calculations have given good agreement (David et al. 1991). The organic anion was measured using the same empirical methods as in this study. The characteristics of organic carbon, and consequently charge densities, can be expected to vary regionally, tem- porally, or as a function of water residence time or

628 P. Kortelainen et al

~" 250 1

"" 200 a" o

v

¢,,,

o 150 , ~

m

c-

o 100 c 0 o t -

O 50

A-

Lab i l e m o n o m e r i c A I : * Mg =* Ca" NH(*

c r NO~"

[~-K'~-7 F" Na"

~ H " SO, 2-

A B A B

Anion exchange XAD-4 Fig. 5. Lake Skjervatjern charge balance for both lake halves (A: experimental; B: reference) in September 1991. Organic anion was

measured experimentally with two different methods (anion exchange vs. XAD-4).

other limnological factors. Many Norwegian watersheds can be characterized by having thin organic soil horizons and considerable bedrock exposure, as well as stoop topography. Precipitation amounts and runoff are high in Norway resulting in rather low water residence time. In addition, there may be some changes in the complexation ability of organic carbon in response to long-term sulfate deposition.

Sulfate concentrations in Half A have increased during the acidification period. A slight decrease in pH has also been found as well as some signs of decreased organic matter concentration in the treated half (Gjessing 1992). In September 1990 before acidification was initiated, the experimentally measured organic anion concentrations were equal in both lake halves. In September 1991, measured organic anion concentrations wore lower in the experimental half compared to the reference half. This was demonstra- ted by both isolation techniques, anion exchange method and XAD-4 method (Fig. 5). This suggests that the contribution of organic acids in the overall lake acidity has decreased after the acidification was started.

CONCLUSIONS

Fractionation of Lake Skjervatjern samples indicates that most of the DOC is composed of organic

acids (generally >80%). Hydrophobic acids are the dominant form of DOC. Organic acids arc weak acids which dissociate across a wide range of pH values, but arc not completely dissociated in natural waters. The dissociation of isolated acids during acid-base titrations clearly increases with increasing pH. Hyd- rophilic acids consistently have greater exchange acidities and charge densities compared to hydrophobic acids.

No major differences in DOC quality were found between Lake Skjervatjern basins A and B before acidification was initiated (David et al. 1991). The 1.5-y acidification period of the other lake half with the corresponding catchment area has not caused dramatic changes in lake water pH and TOC. No clear differences in the composition of TOC have been found either.

Organic anions are dissociation products of organic acids. The high organic anion contribution to the ion balances indicates that humic matter is an important acidity source in Lake Skjervatjern. There are slight signs that the contribution of organic acids in the overall lake acidity has decreased after acidification was started. This is probably due to the observed increase in sulfate concentrations as well as a slight decrease in pH and organic matter concentration in the experimental half (Gjessing 1992).


REFERENCES

David, M. B.; Vance, G. F.; Rissing, J. M.; Stevenson F. J. Organic carbon fractions in extracts of O and B horizons from a New England spodosol: Effects of acid treatment. J. Environ. Qual. 18: 212-217; 1989.

David, M. B.; Vance, G. F. Chemical character and origin of organic acids in streams and seepage lakes of central Maine. Biogeochemistry 12: 17-41; 1991.

David, M. B.; Vance, G. F.; Kortelainen, P. Organic acidity in Maine (U.S.A.) lakes and in HUMEX Lake Skjervatjern (Nor- way). Finnish Humus News 3: 189-194; 1991.

Gamble, D. S. Titration curves of fulvic acid: The analytical chemistry of a weak acid polyelectrolyte. Can. J. Chem. 48: 2662-2669; 1970.

Gjessing, E. Humic lake acidification experiment, Status Septem- ber 1990, one week before start of the treatment, Report No. 1/91 from HUMEX. Norwegian Institute for Water Research, Oslo; 1991.

Gjessing, E. The water chemistry of Lake Skjervatjern's separated basins two years before and one year after start of treatment. HUMOR/HUMEX Newsletter (Norwegian Institute for Water Research, Oslo) I: 17-19; 1992.

Gjessing, E. The Humex project: Experimental acidification and its humic lake. Environ. Int. 18: 535-543; 1992.

Henriksen, A.; Brakke, D. F.; Norton, S. Total organic carbon concentrations in acidic lakes in southern Norway. Environ. Sci. Technol. 22: II03-II05; 1988a.

Henriksen, A.; Lien, L.; Traaen, T. S.; Sevaldrud I. S.; Brakk¢ D. F. Lake acidification in Norway - Present and predicted chemical status. Amble 17: 259-266; 1988b.

Leenheer, J. A. Comprehensive approach to preparative isolation and fractionation of dissolved organic carbon from natural waters and wastewaters. Environ. Sci. Tochnol. 15: 578-587; 1981.

Malcolm., R.L. Quantitative evaluation of XAD-8 and XAD-4 rosins used in tandem for removing organic solutes from water. Environ. Int. 18: 597-607; 1992.

McKnight, D. M.; Thurman, E. M.; Worshaw R. L.; Hemond, H. Biogeochemistry of aquatic humic substances in Thoreau's Bog, Concord, Massachusetts. Ecology 66: 1339-1352; 1985.

Merilehto, K.; Kenttamies, K.; Kamari, J. Surface water acidification in the ECE region. Nordic Council of Ministers, Copenhagen, Nord; 1988: 89.

Oliver, B. G.; Thurman, E. M.; Maicolm, R. L. The contribution of humic substances to the acidity of colored natural waters. Geochim. Cosmochim. Acta 47: 2031-2035; 1983.

Perdue, E. M.; Router, J. H.; Ghosal, M. The operational nature of acidic functional group analyses and its impact on mathemati- cal descriptions of acid-base equilibria in humic substances. Geochim. Cosmochim. Acta 44: 1841-1851; 1980.

Perdue, E. M.; Router, J. H.; Parrish R. S. A statistical model of proton binding by humus. Geochim. Cosmochim. Acta 48:1257- 1263; 1984.

Schindler, D. W. Effects of acid rain on freshwater ecosystems. Science 239: 149-157; 1988.

Thurman, E. A. Organic geochemistry of natural waters. Boston, MA: Martinus Nijhoff; 1985.

Vance, G. F.; David, M. B. Chemical characteristics and acidity of soluble organic substances from a northern hardwood forest floor, central Maine, USA. Geochim. Cosmochim. Acta 55: 3611-3625; 1991a.

Vance, G. F.; David, M. B. Forest soil response to acid and salt additions of sulfate: IH. Solubilization and composition of dissolved organic carbon. Soft Sci. 151: 297-305; 1991b.

Wright R. F. Rain Project: Role of organic acids in moderating pH change following reduction in acid deposition. Water Air Soil Pollut. 46(1-4): 251-259; 1989.

Wright, R. F.; Lotse, E.; Scrub, A. Reversibility of acidification shown by whole-catchment experiments. Nature 334: 670-675; 1988.

acid-base characteristics of organic carbon in the humex lake skjervatjern

Documents