Estuatine and Coastal Marine Science (1979) 9,4x-49

Removal of ‘Soluble’ Iron in the Potomac River Estuary

Andrew Eaton Chesapeake Bay Institute, Johns Hopkins University, Baltimore, MD 21218, U.S.A.

Received 20 March 1978 and in revisedform 16 June 1978

Keywords: estuaries; iron; geochemical cycle; Chesapeake Bay; bacteria; coagulation

‘ Soluble’ iron levels in the Potomac River estuary are extremely low, usually less than 0.5 PM at mile 90 (freshwater) and decrease downstream to less than 0.05 us prior to the intrusion of salt water. Removal of ‘soluble’ and fine filterable iron in the freshwater obeys first order kinetics with a rate constant of about 0.1 day-l. Addition of bacteriocides to samples stabilizes iron concentrations, suggesting that in this estuary freshwater colloids are de- stabilized by bacterial polymers. The extensive removal of iron in freshwaters is a different phenomenon from the salinity-dependent removal observed by several other workers in different estuaries.

Introduction

The estuary is the region where fluxes of riverborne constituents to the ocean are determined. Modifications of supplies of metals in this region can ultimately affect mass balances in the open ocean. Iron behavior in estuaries is significant for a number of reasons. It may act as a model for a number of important trace elements which are more difficult to study due to contamination. As it precipitates and coagulates it may scavenge other constituents including trace elements and phosphorus, affecting their mobility in the water and food chain.

It has been shown numerous times that at levels of 3-22 PM iron behavior in estuaries is strongly non-conservative (Coonley et al., 1971; Boyle et al., 1974; Boyle, 1976; Sholkovitz, 1976; Boyle et al., 1977, Murray & Gill, 1978) and that extensive removal occurs rapidly in saline waters. Thus early estimates of the flux of iron to the oceans based on fresh water measurements (Livingstone, 1963) were overestimated.

Preliminary measurements of iron in the Potomac River estuary indicated that freshwater concentrations of ‘soluble’ iron were lower than those observed elsewhere (less than I PM).

None of the previous studies has determined whether measurable removal of iron occurs in the freshwater portion of estuaries. Therefore a comprehensive study was undertaken to determine whether iron removal in the Potomac followed a different pattern from other estuaries and if so to determine the causes for variations in behavior.

Materials and methods

The tidal portion of the Potomac River (Figure I) starts at Washington, DC and flows for a distance of about 95 miles to the Chesapeake Bay. The non-tidal portion of the river extends northwestward into West Virginia. Data in this paper are taken from 4 cruises on the R/V

4= 0302/3524/79/070041+09 $02.00/O @ 1979 Academic press Inc. (London) Ltd.

42 A. Eaton

Figure I. Potomac River Estuary with station locations.

Warfield in 1977. Samples were taken at 5 mile intervals from the mouth of the estuary to the Woodrow Wilson Bridge, approximately 5 miles below Washington DC.

All samples were collected at a depth of I M using a continuous flow hull mounted pumping system. Periodic checks for possible contamination, made by analyzing open bay samples and by occasionally obtaining bottle samples at the same locations as the pumped samples suggest that the inherent sampling contamination is less than 20 nM, the detection limit of the analytical method.

Samples were collected in polyethylene bottles and aliquots were immediately filtered through washed 47 mm 3 pm Nucleopore filters (line filterable iron) and 25 mm Whatman GF/F glass fiber filters with a nominal pore size of 0.7 pm (‘soluble’ iron). Kennedy et al. (1974) demonstrated that ‘soluble’ iron in many rivers is mostly in colloidal form. Con- sequently, measured values of ‘soluble’ iron decrease as successive aliquots are passed through membrane filters as a result of changes in effective pore size.

Preliminary studies in October, 1976 using a 0-2 pm Nucleopore filter to define ‘soluble’ iron indicated no measureable (<20 nM) iron even in the freshwater portion of the estuary. This indicated that essentially all the measureable filter passing iron in the Potomac was colloidal in nature. To determine removal mechanisms it is necessary to have precisely determinable concentrations. Experiments with the glass fiber filter indicated that it circum- vented the problems of membrane filters due to its greater capacity and yet it gave re- producible results. Minimum volumes were filtered (25 cm3), a volume found sufficient to rinse the collection bottle, allow for analysis, and produce precise results. This fraction is not

Removal of ‘soluble’ iron in an estuary 43

truly soluble; however, the use of a fine glass fiber filter allows these results to be compared directly with those of Boyle et al. (1977) to determine whether iron behavior in the Potomac is significantly different from other East Coast estuaries.

Iron was determined calorimetrically in all filtrates using a modification of the method of Stookey (1970). IOO pl of 2% hydroxylamine hydrochloride-3 M-HC%-O-P M-Ferrozine solu- tion was added to 5 ml of sample and allowed to react for 45 min to an hour at room tempera- ture. The sample was then buffered at pH 4.5 with IOO ~1 of 4 M-NaAc and the absorbance measured at 562 nm in a IO cm cell. Precision of the analysis is j-3% or 40 no, whichever is greater.

Samples were obtained in March, June, and October, 1977 under progressively lower flow conditions and determinations were made on board ship immediately following collection. In December 1977 samples were collected following heavy rainfall (high flow) and filtrates acidified for 3 days prior to determinations of Fe. Analysis of acidified filtrates from September and December after an additional 180-270 days of storage gave no measureable differences from original concentrations.

0.6 Aa 0.5 - 9 March 1977

.

A Q=488m3d

0.4- .

0.3 - .

0.2 -

1 S*O?5%

0.1 - .A .

. I 1 I I I I f I I

'j; 0.2 s=o.4%a 28 9eptambef I977 3 1 A . 1 . Q=24m36’ ILQ 04 A A A .

8 Decrmbrr 1977 Q=780m3a'

2.0

I.5

i

.

I.0 A

1 S=O25%0

0.5 .

II I I I .*&A , , , 90 80 70 '60' 50 40'30 20 IO

Miles from mouth

Figure 2. ‘ Soluble’ iron versus distance downstream in the Potomac Estuary.

44 A. Eaton

Results

(a) ‘Soluble’ iron

The distribution of ‘soluble’ iron in the Potomac River estuary is shown in Figure 2. Maximum measured values at the freshwater end were usually less than 0.5 PM, except in December when they were as high as 3.4 PM. Iron values decreased rapidly to concentrations of less than 0.1 FM going downstream, reaching the minimum values prior to the detection of measureable salt. This is somewhat different from the observations of Boyle et al. (1977) in other estuaries, who observed constant iron values in the low salinity portions of many estuaries and who found upon mixing freshwater and seawater that iron removal was minimal at low salinities and increased markedly at high salinities. Iron values within the saline portion of the estuaq were generally below detection limits and it is therefore impossible to say whether further removal was occurring in that region. The lack of measureable ‘soluble’ iron in the saline portion of the estuary is consistent with the observations of Murray & Gill (1978) who found no measureable 0.8 urn filter passing iron in the saline waters of Puget Sound.

As river flow decreases over the course of the year, the apparent initial freshwater iron value also decreases, being less than 0.1 PM in September 1977. The net effect of decreased river flow is an increase in water residence time for any given portion of the estuary. Thus in situ removal processes such as sorption, precipitation, or biological uptake will have more time to

8 March 1977

0 = 486 m3 s-l

20

t 21 June 1977

Q= 76m3 s-’

28 September 1977

Q=24m3 s-’

. . . .A. . .

t- 1 IIII 90 80 70 60 50 40 30 20 IO

Moles from mouth

Figure 3. Fine filterable iron versus distance downstream in the Potomac Estuary.

Removal of ’ soluble’ iron in an estuary 4.5

affect concentrations of biogeochemically active elements. Conversely processes which supply soluble iron to the estuary, such as diffusion from sediment interstitial waters, will also have more time to occur, thereby increasing soluble iron concentrations. In this case removal processes apparently dominate iron distributions since concentrations decrease with de- creasing river flow.

Fine filterable iron The tine filterable iron fraction shows somewhat greater variability than the ‘soluble’ iron, but is generally similar in trends (Figure 3). In March and June 1977 there is extensive re- moval in freshwaters. Under the lowest flow conditions there is no apparent removal in that region. In the low salinity regions, around miles 5570 there is a slight increase in the fine filterable iron levels, probably reflecting the higher suspended load in the turbidity maximum of the Potomac. Downstream of that region values decrease in a generally conservative manner. Concentrations range from a high of 2 pM at the upstream end in March 1977 to less than O-I FM for the downstream end in June 1977.

The decrease of soluble and fine filterable iron in the freshwater portion of the Potomac indicates that iron colloids are destabilized much more readily in this estuary than in the estuaries studied by Boyle et al. (1977).

Discussion

In previous studies of soluble iron behavior in estuaries, extensive removal has been noted in mid salinity regimes (5--15%J when initial iron concentrations ranged from 3 to 22 PM (Boyle et al., 1977). Under the conditions observed in the Potomac River estuary, with initial con- centrations normally less than o-5 PM, a different removal pattern has been observed on three separate occasions. All of the apparent removal of soluble iron occurs in freshwater. Such

2 4 6 6 IO 12 14

Days &.wweam from P9D

Figure 4. Removal Rate of iron in the Potomac Estuary.

46 A. Eaton

removal is not readily explicable using the model of Boyle et al. (1977). Therefore another explanation must be sought.

Using discharge values for the two weeks preceding our sampling periods and estuarine volumes over 5 mile intervals (Cronin, 1971) one can estimate travel times for a parcel of water in the estuary. Figure 4 is a semi-log plot of the March and June iron concentrations versus estimated travel times. Slopes of the calculated regression lines average 0.1 day-l with no significant differences among the three lines. In September initial values were too low to calculate any removal rates. It appears that the distribution of ‘soluble’ and fine filterable iron in the estuary obeys first order kinetics. A theoretical laboratory removal rate for oxidation of ferrous iron would be 675 day-l assuming 5 mg I -l oz and a pH of 7.5 at 2oT (Stumm & Morgan, 1970). These laboratory rates have been verified in the field by Lewis (1976). The observed rate of iron removal is much slower than the predicted rate for precipita- tion of ferrous iron. Either the precipitating iron is being stabilized by complexes or the observed removal is not a precipitation reaction. Several lines of evidence suggest the latter hypothesis as the more likely explanation. (I) The same removal rate is calculated for both ‘soluble’ and fine filterable iron. The observed removal mechanism must be able to affect both particulate and ‘soluble’ iron equally. (2) ‘Soluble’ iron concentrations measured without the addition of the reducing agent were very low, suggesting that most of the iron observed is ferric. (3) For ferrous iron oxidation and precipitation the reaction is highly pH dependent, whereas there is no apparent rate change in spite of variable pH.

The alternative explanation to a precipitation phenomenon is a coagulation process (Stumm & O’Melia, 1968; Hahn & Stumm, 1970; Stumm & Morgan, 1970; Edzwald et al., 1974).

Hahn & Stumm (1970) have discussed coagulation in natural waters and noted that under low ionic strength conditions (freshwaters) colloids are relatively stable. They noted three mechanisms for destabilization. (I) Coagulation induced by an increase in electrolyte con- centration, (2) adsorption of oppositely charged species on surfaces, changing the surface potential, (3) formation of molecular bridges, such as bacterial polymers.

The increase in eletrolyte concentration is an unlikely explanation for these data since removal of iron is observed in a constant chemical medium of low ionic strength. These data alone do not allow one to differentiate between the latter two mechanisms since the observed decrease in iron concentrations could well be reflecting coagulation of clays as a result of sorption of iron hydroxides on the surfaces.

The two mechanisms can be tested through additional experimental data. Samples of un- filtered water were obtained from the shoreline just south of station P90. The samples were stored in polyethylene bottles with different preservatives at room temperature and the 0.7 pm filter passing iron determined after varying periods of time (Table I). In October 1977 samples were stored in light and dark bottles to determine whether phytoplankton could alter iron levels and with HgCl, added as a biocide. There was no difference in the iron removal rate for the light and dark bottles whereas the HgCI, stabilized iron concentrations. Since addition of HgCI, could stabilize colloidal iron as a result of physical and chemical properties in addition to its role as a biocide the experiment was repeated in April 1978

using phenol, formaldehyde, and azide as preservatives. In April, iron was determined by carbon rod atomization atomic absorption spectrometry since it was not known how the preservatives would affect calorimetric determinations.

Both sets of data show that addition of preservatives with bacteriocidal properties results in a stabilization of iron levels. This suggests that the production of bacterial polymers is the most likely explanation for the destabilization of the iron colloids in the freshwater portion of the Potomac estuary.

Removal of ‘soluble’ iron in an estuary 47

TABLE I. Iron removal with time

October rg77

‘ Soluble’ Fe (PM)

Time (h) Light bottle Dark bottle 40 parts/xo’ HgCls

0 I ‘0’ i; 0.29 0.39

185 0.18

I .o’ 0.32 0’39

0.18

April 1978

‘ Soluble’ Fe (PM)

I ‘on 0.6 0.6

0.8

Time (h) Light bottle 0.4% Phenol 0.5% Formaldehyde 0.2% Na Azide

0 3’4 2.6 3’0 2.8

I9 2.8 2’8 3’3 3’2 43 1.6 2.9 3-I 2.8

70 I-3 3’0 3’2 2’7

“The higher value relative to the September 1977 cruise probably reflects the greatly increased average flow in the interval (Q- IOO MS s-l in October 1977).

(I) ‘Soluble’ iron levels in the Potomac river estuary are extremely low, usually less than 0.5 pM at the freshwater end and less than 0.05 pM within saline portions of the estuary.

(2) All of the ‘soluble’ iron and much of the fine filterable iron is removed within the wholly fresh water portion of the estuary.

(3) Removal of ‘soluble’ and fint filterable iron follows first order kinetics, with a rate constant of 0.1 day-‘.

(4) Experiments involving the addition of a bacteriocide suggest that destabilization of colloidal iron in the Potomac occurs as a result of bacterial activity.

Partially supported by Department of Energy contract No. E76-So2-3292 (Document no. Coo-3292-034) and EPA contract ICPRB. CBI Contribution No. 258. Discussion with Owen Bricker and Eric Hartwig was helpful. The paper benefited greatly from reviews by E. Boyle and E. Callender.

References

Boyle, E. 1976 PhD Thesis. MIT-WHOI. Boyle, E. A., Collier, R., Dengler, A. T., Edmond, J. M., Ng A. C. & Stallard R. F. 1974 On the

geochcmical mass balance in estuaries. Geochimica et Comochimica Acta 38, 17Ig--I@. Boyle, E., Edmond, J. & Sholkovitz, E. 1977 The mechanism of iron removal in estuaries. Geochimica et

Cosmochimica /ktcJ 41, 1313-1324. Coonley, L. S., Jr., Baker, E. B. & Holland, H. D. rg7r Iron in the Mullica River and Great Bay, New

Jersey. Chemical Geology 7, 51-63. Cronin, W. rg7r Volumetric area1 and tidal statistics of the Chesapeake Bay and its tributaries. CBI Spec.

Rept. 20. Edzwald, J., Upchurch, J. & O’Melia, C. 1974 Coagulation in Estuaries. Environmental Science and

Technology 8, 58-63.

48 A. Eaton

Hahn, H. & Stumm, W. 1970 The Role of Coagulation in Natural Waters. AmericanJournal of Science 268, 354-368.

Kennedy, V. C., Zellweger, G. W. & Jones, B. F. 1974 Filter pore-size effects on the analysis of Al, Fe, Mn and Ti in water. Water Resources Research IO, 785-790.

Lewis, D. 1976 The Geochemistry of Mn, Fe, U, PO-2x0 and major ions in the Susquehanna River. PhD Thesis. Yale University.

Livingstone, D. A. 1963 Chemical composition of rivers and lakes. USGS Professional Paper 440-G. Murray, J. & Gill, G. 1978 The geochemistry of iron in Puget Sound. Geochimicu et Cosmochimica Acta

42, 9-20. Sholkovitz, E. R. 1976 Flocculation of dissolved organic and inorganic matter during the mixing of

river water and seawater. Geochimica et Cosmochimica Acta 40, 831-845. Stookey, L. L. 1970 Ferrozine: a new spectrophotometric reagent for iron. Ads of Chemistry 42, 779

781. Stumm, W. & Morgan, J. J. 1970 Aquatic Chemistry. Wiley-Interscience. Stumm, W. & O’Melia, C. 1968 Stoichiometry of Coagulation. journal of the .4merican Water Works

Association 60, 514-539.

Appendix-Raw data for the Potomac Estuary

Estimated days Salinity <I i 0 In vivo Fe (p&f)

downstream Mile (x0) pH Transmission fluorescence <o,7um <3pm

0 90 1’0 85 2’3 80

4’3 75 8.2

12.3 :;

16.5 60

5.5 50 45 40 35 30 25 20

15

60.7 SQ; X4.8 80 27.8 75

:; 60

55 50 45 40 3.5 30 2.5 20

15 10

5 0

Potomac River Estuary 8-9 March 1977 0’11 7’33 60 22

0'12 7’4 61 26

0.13 7’5 63 39 0.14 7’5 66 41 0.16 7.6 74 83 0.19 8.2 52 131 0'22 8.5 50 ‘7’ O’S 56 ‘44 2’5 8.4 64 12.5 3.9 8.4 70 180 6

8.6 0’02,O’O j 0’32

11.6

Potomac River Estuary 21-22 June 1977

0’1 7’3 0’1 7’3 0’1 7’5 0’1

0.13 ;:z

0.6 8.3

“7 3’0 4’5 6.3 7’9 8.5 9.0 9’9 9’9

r1.3

83

76 70 72 78 70

2:

i: 51 36

76 92

57

0.57 2.05

0.54 1.64

0'43 r-64

0.48 1'25

0'34 "39 0.23 o-66

0'14 0.62

0'12 0-80

0'07

0'12 0.64

0~30,0*25 I.09 0'20 1'00

0~09,0~05 0.36

0'02 0.32

0'12 0.32

to.02 0.27

0.46

0'25

0.26

0.32

0'25

0.25

1'02

o-07

<0'02

<0'02

<0'02

<o.oz

<0.02

Removal of ‘soluble’ iron in an estuary 49

Potomac River Estuary 28 September 1977

Estimated days

downstream Mile Salinity

CXJ Fe(P)

co.7lu-n <3Pm

0 0.18 21 i; 0.17 46 80 0.4

75 0.7 70 2’5 65 2’7 60 3’9 55 5.6 50 7.6 45 9.8 40 IO.7 35 12’5

30 13’4 25 13’4 20 14.6

15 15-6 IO 16.2

5 16.5 0 16.6

0.13 0’13 0'11 0'11 0.08 0.07

0.03 0.14 0’02

0’02

0.80 0.86 0.95 0.84 0.87 I ‘25 1’00

0.73 o-68 0.25 0.41 0.29

0’25

0’21

0.44 0.27 0.17 0’20

0.13

Potomac River Estuary 8 December 1977

Estimated days

downstream Mile Salinity

cxo) PH In vivo

fluorescence

0 90 0'1 7'97 II 0.99 0.6 85 0’1 7’92 II 3’4 I.4 80 0'1 7'95 II 2.16 2.7 75 0’1 7’95 II 2’45 5’0

i; 0.14 7.83 13 1’39

7.6 0.15 7.78 14 0.07 10’2 60 0’25 7.80 15 0.27

55 1’9 7.88 II 0’10

50 3’3 i-98 IO <0.04 45 4’5 8.01 18 <OS04 40 54 8.10 16 <0.04 35 8.5 8.34 16 <0.04 30 10’2 8.35 57 <OS04 25 10.8 8.39 <0.04 20 11.5 8.46 :: to.04