Spatial and Temporal Variations in the Interstitial Water Chemistry of

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Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 28, 2018 | https://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch019

Spatial and Temporal Variations in the Interstitial Water Chemistry of Chesapeake Bay Sediments GERALD MATISOFF, OWENP.BRICKER III, GEORGER.HOLDREN JR., and PETER KAERK Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore,Md.21218 The calculation of chemical mass balance relations in an estuarine environment requires a careful evaluation of the material fluxes within the sediment and across the sediment-water interface. The rapid response of the estuarine environment to variations in temperature, s a l i n i t y and sediment deposition rates makes this type of assessment very d i f f i c u l t . One approach is to examine the integrated results of these effects in terms of the spatial and temporal v a r i a b i l i t y of the concentrations of dissolved species in the sediment. Spatial variations define the limits which may be placed upon the instantaneous concentrations of chemical species as a function of location. Temporal variations interpreted within the framework of the spatial limits may be used to assess the long-term effects of temperature, s a l i n i t y and sediment deposition rates, thus enabling the more accurate calculations of chemical fluxes. Nature of the Study The validity of a temporal study hinges upon the a b i l i t y to accurately relocate the same sampling sites throughout the study period. A r e a l i s t i c evaluation of the navigational capabilities during the period of the study indicates that relocation within a c i r c l e of 500' diameter was possible in open waters. Where stations were close to buoys or other fixed points, accuracy of relocation was substantially improved. Temporal data were collected at station 856-C and 856-E for the period June, 1971 through September, 1973. Most of the data were collected prior to the observations of Bray et a l . (2) on the effects of sample oxidation and consequently, the data are not reliable for any metals, phosphate or Eh. The chloride, s i l i c a t e , sulfate and ammonia concentrations as well as carbonate alkalinity are unaffected by oxidation during squeezing of the sample and these data are presented below. A spatial study was conducted in Chesapeake Bay on April 1416, 1974 to evaluate the v a r i a b i l i t y in i n t e r s t i t i a l water 343

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

MARINE CHEMISTRY

344

chemistry over a s m a l l s p a t i a l range. The mid-bay r e g i o n w i t h a s a l i n i t y approximating h a l f t h a t o f sea water was chosen f o r t h i s study. Three s t a t i o n s forming a t r a n s e c t across the bay at 38° 53 N ( S t a t i o n 853) and a f o u r t h s t a t i o n at 39°03 52"N, 76°19 20"W ( S t a t i o n 904N) were sampled ( F i g . 1 ) . E i t h e r f o u r o r f i v e s e t s o f g r a v i t y cores were obtained i n a T -shaped p a t t e r n at each s t a t i o n , w i t h approximately 100 spacing between each c o r i n g l o c a t i o n . Two cores were taken at each sampling s i t e , one f o r p h y s i c a l d e s c r i p t i o n and one f o r chemical a n a l y s i s . The core used f o r chemical a n a l y s i s was extruded and squeezed at room temperature w i t h a Reeburgh-type squeezer (I) i n a n i t r o g e n atmosphere t o prevent o x i d a t i o n o f metals ( 2 ) . Measurements o f pH, p S and Eh were made by e l e c trode methods i n a c l o s e d c e l l i n the n i t r o g e n - f i l l e d glove box and samples f o r i r o n , phosphate and o t h e r o x y g e n - s e n s i t i v e s p e c i e s were prepared i n the box. !


,f

F

f!

F

1

=

A n a l y t i c a l Techniques In g e n e r a l , t e n i n t e r s t i t i a l water samples were squeezed from sediment s e c t i o n s o f each o f the cores. The f i r s t f i v e sample s e c t i o n s were two centimeters i n t h i c k n e s s , and t o g e t h e r comprised the top t e n centimeters o f the core. Three centimeter s e c t i o n s were obtained at about 15-18 cm, and 25-28 cm. The remaining sample s e c t i o n s were 5 cm each, and t h e i r l o c a t i o n s were dependent upon the l e n g t h o f the core. A l l data are p l o t t e d as the midpoint o f each s e c t i o n . Bottom water values were d e t e r mined from the water which was trapped i n the core tube above the top o f the sediment. Since o n l y 20-50 ml o f sample are obtained from each squeezed s e c t i o n , a n a l y t i c a l methods which r e q u i r e o n l y s m a l l a l i q u o t s o f sample must be used. Some o f the a n a l y t i c a l methods have been taken d i r e c t l y from the l i t e r a t u r e , w h i l e others are m o d i f i e d v e r s i o n s o f e x i s t i n g methods. In a l l cases, the analyt i c a l methods were t e s t e d f o r accuracy, p r e c i s i o n , and i n t e r f e r ences. When found necessary a p p r o p r i a t e c o r r e c t i o n s were made. Table I summarizes the a n a l y t i c a l methods which we used. R e s u l t s and D i s c u s s i o n Space l i m i t a t i o n s p r e c l u d e p r e s e n t a t i o n o f a l l the data from each o f the f o u r s t a t i o n s sampled i n the s p a t i a l v a r i a t i o n study. We show here the data f o r s t a t i o n 853-C which i s r e p r e s e n t a t i v e o f the q u a l i t y f o r a l l the s t a t i o n s . A l s o i n c l u d e d are a d d i t i o n a l data from a shallow water s t a t i o n , 853-G. The best agreement between each o f the chemical species and between the p h y s i c a l core d e s c r i p t i o n s i s found i n the data from s t a t i o n 853-E (not shown). Although we present o n l y a p a r t o f our t o t a l r e s u l t s i n t h i s communication, the estimated l i m i t s o f s p a t i a l v a r i a b i l i t y have been determined t a k i n g i n t o account a l l f o u r s t a t i o n s .


19.

MATISOFF


Table I.

ET

AL.

345

Interstitial Water Chemistry

Analytical techniques used in this study

Chemical Parameter

Method

Hh

Electrode

pH

Electrode

pS

Electrode

Chloride

AgN0 Titration

Carbonate Alkalinity

NaOH Back Titration

Sulfate

Sulfate-Phosphate

Reactive Phosphate

Colorimetric

Ferrous Iron

Colorimetric

(8)

Ammonia

3

Reference

(3)

(4) (5),(6),(7)

Colorimetric

(1)

Reactive S i l i c a t e

Colorimetric

(10)

Manganese

Atomic Adsorption

A temporal study was performed at two stations. Station 856-C was sampled 16 times during a two-year period and the data, numbered sequentially, are presented here with only half plotted for the sake of simplicity. Core Descriptions. Visual descriptions of leaf layers, shell debris, or sand layers which result from singular events can provide qualitative information which i s useful i n determining the degree of stratigraphie correlation between the cores. Color banding i s another possible feature which may be used, although the causes of the banding are not well known. Examination of Chesapeake Bay cores shows, however, that none of these features may be used independently since they are often discontinuous from two cores obtained at the same time from the same location. The only features which appear to be reliably consistent over large distances (in fact, over most of the Bay) are 1) a thin brown layer at the sediment-water interface due to oxidation, 2) a greenish-black, water-rich, somewhat s i l t y layer of mud (commonly containing many color bands) beneath the oxic layer, and 3) an olive green, often sandy layer beneath layer 2, which continues to the maximum depth that we have been able to sample by piston coring. (This layer i s not always penetrated by gravity coring.) Contacts separating the layers range from very sharp at many places to gradational at others. We hypothesize that layer 2 i s the result of large quantities of organic debris being brought into the Bay i n the last 100-200 years due to the rapid rise in human population i n the area. We are currently obtaining Pb data to determine the v a l i d i t y of this hypothesis.


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"Clinkers" are pieces of slag produced from coal burning ships (see Figure 2). They are particularly abundant i n the sediment just south of Baltimore Harbor. Core descriptions from five cores at station 853-E show that a single "clinker" layer exists i n each core at the following depths : 43 cm, 43 cm, 47 cm, 43 cm, 45 cm. In a l l of the cores these clinkers were found at, or just above the contact between layers 2 and 3. This spatial agreement i s also offered as evidence i n support of the above hypothesis. Chloride. Chloride may be treated as a conservative tracer during mixing of natural waters. When investigating the exchange of water across the sediment-water interface, i t i s therefore important to examine the chloride ion distribution. In Figure 3, spatial data for chloride concentrations at station 853-C are presented. It i s quite obvious that there i s excellent agreement among the five cores obtained at this deep water station. The agreement i s not nearly as good at the shallow water station 853-G (Figure 4). Extremely low chloride values at depth suggest dilution by discharge from a fresh-water aquifer into the Bay environment. Comparison of chloride concentrations between these two stations, both at the sediment surface and at depth i n the sediment, reveals that the values are much higher at the deep water station. This reflects the fact that the salt-water wedge travels farther up the bay i n the main channel of the estuary than i t does along the shallower sides. Data for i n t e r s t i t i a l chloride values which are uncontaminated by ground-water discharge appear to be good to +. 1.5% and reflect the mean s a l i n i t y of the local environment. Temporal data for chloride i s presented i n Figure 5. Near the sediment surface there i s considerable systematic variation with time. Late spring values are lowest due to the high volume of fresh water discharged into the Bay, while late f a l l values are the highest reflecting decreased river discharge and increased évapotranspiration. It appears that the upper 20 cm or so of the sediment responds rapidly to s a l i n i t y changes at the sedimentwater interface, while the sediment below this reflects a longerterm average of the s a l i n i t y i n the overlying water. This i s further supported by examining cores 9, 11, 12, and 13. Core 10, obtained May 25, 1972 (not shown), plots between cores 9 and 11, and reflects a wet spring. Core 11 was obtained 5 days after Hurricane Agnes passed through the Chesapeake Bay area, and the upper part of the core i s even more depleted i n chloride than usual for cores collected during a normal spring. A month later (core 12), chloride i s strikingly absent i n the upper part of the core. Two months after Agnes, the uppermost part of the core had responded to an increase i n chloride resulting from the reestablishment of the s a l i n i t y wedge, although the effects of the storm are s t i l l apparent (core 13). Subsequent data (not


MATisoFF ET AL.



7 6 ° 30'

,

76° 20'

Figure 1. Location of sampling sites in the mid Chesapeake Bay area. Stations 856-C and 856-E were used for the tem poral study. The spatial study was conducted at Stations 853-C, 853-E, 853-G, and 904-N.

Figure 2. Photograph of some "clinkers" ob tained at Station 853-Ε. Upper scale in inches, lower scale in centimeters.


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[Ci]


50

1QQ

15Q

me^ ZOO

E50

300

Depth (cm) 5o

Figure 3. Spatial variation of chloride ion concentration at Station 853-C. Excellent spatial agreement is evident. Water depth at this station is about 80 ft.

300

Depth (cm)

5 0 |

100L

Figure 4. Spatial variation of chloride ion concentration at Station 853-G. Poor spatial agreement and decreasing values at depth suggest the infiltration of low salinity groundwater from a source at depth. Note higher concentration of chloride at all depths at the deeper water station (853-C). This reflects the fact that the salinity wedge infiltrates farther up the bay in the main channel than it does along the shallower sides.


19.

MATISOFF E T A L .

349



presented) show complete recovery from the effects of Hurricane Agnes. This sequence of data indicates the rapidity of response of the upper part of the sediment column to fluctuations i n the s a l i n i t y of the overlying water. Further discussion and a mathematical model for the chloride distribution are presented i n the next paper of this symposium by Holdren, et a l . (11). S i l i c a t e . I n t e r s t i t i a l s i l i c a t e concentrations are typically an order of magnitude greater than the overlying water values. Diatom dissolution i s an important source of s i l i c a (13,14) and clay mineral reactions can either release or take up s i l i c a (12). Both diatom dissolution and clay mineral reactions are complicated functions of temperature, water composition, pH, sediment composition, and particle size (15 and 16), so that a simple explanation of the i n t e r s t i t i a l s i l i c a profiles i s impossible. Figure 6 shows the spatial data for s i l i c a at station 853-C. The data for this station and the three other stations not shown suggest that s i l i c a values are good to ± 10%. Examining figure 7, one can see that the data varies by as much as 450%. Note that data obtained during the winter months exhibit low concentrations of dissolved s i l i c a , and increase with depth, whereas summer data show very high values, and decrease with depth. This observed seasonal variation suggests a strong temperature control on the i n s i t u " dissolved s i l i c a concentrations. Since temperature gradients exist within the cores themselves (18), squeezing samples at " i n s i t u " temperatures would be enormously cumbersome. The error introduced by our squeezing at room temperature (17) would result i n an increase in winter values over the true values, and hence, would minimize the temporal variation i n this data. Thus, this observed temporal variation i s real, and may, i n fact, be even more pronounced than what we have reported here. We are currently working on a mathematical model which w i l l describe this seasonal variation i n terms of temperature fluctuations. Sulfate. Chesapeake Bay sediments are rich i n organic mater i a l , and i t s decomposition plays a major role i n the chemistry of the i n t e r s t i t i a l waters. The anaerobic decomposition of organic matter by sulfate reducing bacteria may be written as (19 and 7) ff

(1) Thus, an interdependency exists among sulfate, carbonate alkal i n i t y , ammonia, phosphate, pH, and Eh. In addition, the presence of large quantities of iron and manganese i n the sediment results in other dependencies due to precipitation of iron and manganese sulfides, phosphates, and carbonates. The system i s s t i l l affected, however, by the same physical processes that control



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MARINE

CHEMISTRY

Depth (cm) 50I40?o-

Station 8S&-C 1 30-VI-71 2 11 -VIJ1-71 3 27-X-71 5 24-XI-71 ? 2Z-II-7Z

< \ 28-1V-7t

11 27-VI-72

8o-

Figure 5. Temporal variation of chloride ion concentration at Station 856-C. Dates are given as day— month (in Roman numerals)-year. Note excellent agreement at depth, but seasonal variation in the upper 20 cm.

13 Z2-VII1-72

90100-

100

200

goo

600

Figure 6. Spatial variation of dissolved silica at Station 853-C



19.

MATisoFF ET AL.


351

the chloride distribution. A changing water composition at the sediment-water interface w i l l , therefore, have an important influence on the sulfate distribution. Temperature fluctuations affect the rate of organic decomposition and, therefore, the rate of reduction of sulfate and production of the species i n equation 1. One would thus expect a rough seasonal correlation among these species, but as an added complexity the composition of the i n t e r s t i t i a l waters may also be constrained by mineral equilibrium reactions. Although the spatial data for sulfate i s unavailable, i t seems reasonable that i t s reproducibility should be good to at least ^ 10%. Temporal data i s presented i n Figure 8. It i s quite clear that the temporal variation for sulfate far exceeds these estimated spatial limits. This variation shows a seasonal trend. During the warmer months when biological processes are most active, almost a l l of the sulfate i s reduced to sulfide, but during the colder months when these, processes slow down, not a l l of the sulfate that has diffused into the sediment i s reduced. The effect of Hurricane Agnes can be seen by examining core 12. Sulfate has been substantially reduced i n the upper 15-20 cm of the core. This i s i n agreement with chloride data. A month later, essentially a l l of this sulfate i s gone. Carbonate Alkalinity. At the pH of i n t e r s t i t i a l waters, the primary component of the carbonate alkalinity i s the bicarbonate ion. It i s produced not only from the oxidation of organic matter by sulfate reducing bacteria as described by equation (1), but also from any "reverse weathering" reactions (20) which might be taking place. I f reverse weathering i s taking place, i t s effect on the carbonate a l k a l i n i t y i s probably quite small with respect to bacterial activity. I f the alkalinity i s produced primarily by bacterially mediated oxidation of organic matter by sulfate, an inverse relationship should exist between sulfate and carbonate (21). Troup (22) and Bray (7) have shown that such a relationship exists for tnese species i n the Chesapeake Bay. Figures 9 and 10 give the spatial data for 853-C and 853-G, respectively. The data appear to be accurate to ± 10%. The shallow-water station exhibits considerably more variation and lower values than the deeper-water station. Comparison of the carbonate alkalinity data with the chloride data for station 853-G suggests that the variation i n the carbonate alkalinity reflects a variation i n chloride, and hence, a variation i n the amount of sulfate available for organic decomposition. Similarly, the magnitude of the carbonate alkalinity at stations 853-G and 853-C reflects the chloride concentration at the two stations. Thus, by equation (1), the same effects should be seen i n the ammonia and phosphate profiles at the two stations. Examination of Figures 3 § 4, 9 § 10, 12 δ 13, and 15 6 16 for these effects shows excellent agreement with prediction.


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too

ZOO

300

4QQ 500

6po

loo 800 Soo

Figure 7. Temporal variation of dissolved silica at Station 856-C. Note strong seasonal variation.

10

15

20

25

30

Depth (CM

>

50|

Station 856-C 1 30-VI-71 2 11 - V I I I - 7 1 3 27-X-71 5 2H-XI-71 7 2Z-II-72 9 28-IV-72

11 27-VI-72

12 1

Spatial and Temporal Variations in the Interstitial Water Chemistry of

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