9
THE RESPONSE OF SPECIFIC CONDUCTANCE TO ENVIRONMENTAL CONDITIONS IN THE EVERGLADES NATIONAL PARK, FLORIDA MARK D. FLORA and P. C. ROSENDAHL National Park Service, South Florida Research Center, Everglades National Park, Homestead, FL 33030, U.S.A. (Received May 19, 1981; Revised July 1, 1981) Abstract. The specific conductance of surface delivery waters to the Shark River Slough (Everglades National Park, Florida) has increased significantly since the completion of Levee 29 (L-29) and the S- 12 water delivery system in the early 1960's. In order to document the response of specific conductance in the slough to changing environmental conditions, it was monitored continuously for a period of 12 mo at a location in central Shark approximately 17 km south of the water delivery site. In addition, variability in isoconductivity patterns throughout the slough was assessed by conducting biweekly surveys at 97 locations. Specific conductance was found to respond to a variety of natural and manmade environmental changes and proved to be an important management tool serving as an early indicator of potentially broad changes in water quality. Factors important in determining overall isoconductivity patterns in Shark Slough include: (1) the ionic composition of surface delivered to Shark Slough via the S-12 delivery structures. (2) the quantitative relationship between the amount of surface water delivered to the slough and the amount of precipitation falling directly upon the slough. (3) the effects ofevapotranspiration and concentration of biological organisms during seasonal periods of extreme drawdown. 1. Introduction The Shark River Slough, located at the southern end of the historic Everglades, comprises the largest freshwater flow system in Everglades National Park (993 km2). Extensive man-made alterations to the natural flow pattern of the Everglades have greatly altered the hydrological regime north of Everglades National Park. The once unregulated sheet flow regime has been transformed into a complex system of levees, canals, water storage impoundments, and water control structures (Figure 1) which influence both the quantity and the quality of water throughouut the Everglades. While many changes had previously altered the flow to Shark Slough, the construction of L-29 and its associated canal in 1962 completely severed the slough from marsh water input via overland sheet flow. The resulting damage to the natural system of Everglades National Park prompted the United States Congress to guarantee a minimum annual surface water delivery equalling 260 000 acre feet yr l (10.19 m 3 s - 1) to the Shark River River Slough (U.S. Senate, 1970). While water quality standards have been developed for these delivery waters (Rosendahl and Rose, 1979), the change from natural sheet flow to waters delivered via canals has caused long term changes in both the specific conductance and ionic composition (Flora and Rosendahl, 1981). Prior to the com- pletion of the L-29 canal, mean wet season specific conductance at station P-33 averaged Water, Air, and Soil Pollution 17 (1982) 51-59. 0049-6979/82/0171-0051501.35. Copyright © 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

The response of specific conductance to environmental conditions in the everglades National Park, Florida

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Page 1: The response of specific conductance to environmental conditions in the everglades National Park, Florida

THE R E S P O N S E OF S P E C I F I C C O N D U C T A N C E TO

E N V I R O N M E N T A L C O N D I T I O N S IN THE E V E R G L A D E S

N A T I O N A L PARK, FLORIDA

M A R K D. F L O R A and P. C. R O S E N D A H L

National Park Service, South Florida Research Center, Everglades National Park, Homestead, FL 33030, U.S.A.

(Received May 19, 1981; Revised July 1, 1981)

Abstract. The specific conductance of surface delivery waters to the Shark River Slough (Everglades National Park, Florida) has increased significantly since the completion of Levee 29 (L-29) and the S- 12 water delivery system in the early 1960's. In order to document the response of specific conductance in the slough to changing environmental conditions, it was monitored continuously for a period of 12 mo at a location in central Shark approximately 17 km south of the water delivery site. In addition, variability in isoconductivity patterns throughout the slough was assessed by conducting biweekly surveys at 97 locations.

Specific conductance was found to respond to a variety of natural and manmade environmental changes and proved to be an important management tool serving as an early indicator of potentially broad changes in water quality.

Factors important in determining overall isoconductivity patterns in Shark Slough include: (1) the ionic composition of surface delivered to Shark Slough via the S-12 delivery structures. (2) the quantitative relationship between the amount of surface water delivered to the slough and the

amount of precipitation falling directly upon the slough. (3) the effects ofevapotranspiration and concentration of biological organisms during seasonal periods

of extreme drawdown.

1. Introduction

The Shark River Slough, located at the southern end of the historic Everglades, comprises the largest freshwater flow system in Everglades National Park (993 km2). Extensive man-made alterations to the natural flow pattern of the Everglades have greatly altered the hydrological regime north of Everglades National Park. The once unregulated sheet flow regime has been transformed into a complex system of levees, canals, water storage impoundments, and water control structures (Figure 1) which influence both the quantity and the quality of water throughouut the Everglades.

While many changes had previously altered the flow to Shark Slough, the construction of L-29 and its associated canal in 1962 completely severed the slough from marsh water input via overland sheet flow. The resulting damage to the natural system of Everglades National Park prompted the United States Congress to guarantee a minimum annual surface water delivery equalling 260 000 acre feet yr l (10.19 m 3 s - 1) to the Shark River River Slough (U.S. Senate, 1970). While water quality standards have been developed for these delivery waters (Rosendahl and Rose, 1979), the change from natural sheet flow to waters delivered via canals has caused long term changes in both the specific conductance and ionic composition (Flora and Rosendahl, 1981). Prior to the com- pletion of the L-29 canal, mean wet season specific conductance at station P-33 averaged

Water, Air, and Soil Pollution 17 (1982) 51-59. 0049-6979/82/0171-0051501.35. Copyright © 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

Page 2: The response of specific conductance to environmental conditions in the everglades National Park, Florida

5 2 M A R K D. F L O R A A N D P. C. R O S E N D A H L

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eq ~ r 6 e 4 ~

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Page 3: The response of specific conductance to environmental conditions in the everglades National Park, Florida

SHARK SLOUGH CONDUCTIVITY 53

284 gmhos cm i; however, since 1965, ithas increased 76~o to 499 gmhos cm i. These increases are due primarily to a change of water delivery from the overland sheetflow &marsh water whose mean wet season specific conductance averaged 272 gmhos cm i, to canal delivery water with a mean wet season value of 653 gmhos cm- i.

The goals of this paper include: (1) documenting present day isoconductivity patterns found throughout Shark Slough under both wet season and dry season conditions, (2) relating these patterns to quantitative changes in the delivery of canal water to the slough and geographic proximity to the S- 12 delivery site, and (3) assessing the short-term response at a single site (NP-203) to short term environmental changes such as extreme storm events.

2. Methods

For a 12 mo period extending from October 1978 through September 1979, hourly stage and specific conductance were obtained at station NP-203 located in central Shark Slough, approximately 17 km south of the S-12 water delivery structures. Concurrent with this, biweekly stage, specific conductance and rainfall surveys were conducted utilizing 97 stations located throughout the slough. These data allowed for the construc- tion of isoconductivity contour maps documenting biweekly changes in conductivity patterns, water levels and rainfall inputs throughout the study area.

Water inputs to the slough were taken to be the sum of surface water delivery at the S-12 structures and the precipitation occurring over the slough. Rainfall was calculated in biweekly intervals (Lew et al., 1981) utilizing the Thiessen polygon method, and biweekly surface water delivery was derived from discharge records published by the United States Geological Survey (USGS, 1980).

In September 1978, (falling water conditions) and June 1979, (rising water conditions) an analysis of cation composition was completed at delivery site S-12D and at P-36, located in south-central Shark Slough, in order to document changes in specific conduc- tance and cation composition as water travels southward through the slough.

3. Results

A comparative analysis of rainfall input and S-12 water delivery from September 1978 through September 1979 indicates that the study period can be sub-divided into eight hydrologic periods of differing water input character (Figure 2). Of these eight periods, water inputs were greatest in periods I, II, VI, and VIII, and lowest in periods IV and V (Table I). High water inputs in periods VI and VIII were the result of large amounts of precipitation. High precipitation also occurred during period I, but this was coupled by an equally large surface delivery. In period II, the surface delivery exceeded direct precipitation input by 188~o. The periods of lowest rainfall occurred during the dry season (February-April). During this time total water inputs to the slough were low with the contribution of surface water delivery exceeding precipitation by 102 ~o.

Specific conductance at NP-203 responded to these changes in water input type

Page 4: The response of specific conductance to environmental conditions in the everglades National Park, Florida

54

100-

9 0 -

-- 8 0 - % x • ~ 7O-

r 6 0 - v a~

5 0 -

"~ 404

>0 30"

2 0 -

10-

7 21 SEP

I I J

Fig. 2.

M A R K D, F L O R A A N D P. C. R O S E N D A H L

mu 160,2

I n Rainfall

S-12 De ivery J

Denotes total input for 4 weeks

5 19 2 16 28 14 28 11 25 8 22 8 22 5 19 3 17 31 14 12126 9 23 6 26 OCT NOV OEC JAN FEB MAR APR MAY JUN JUL AUG SEP

19zs I 1979 t I " I ~,i I ,v I v I v, I v,,l v,,, I

HYDROLOGIC PERIOD

Biweekly water contribution to the Shark River Slough via S-12 surface water delivery and rainfall (September 1978-September 1979).

TABLE I

Mean biweekly rainfall and S-12 water input to Shark Slough Aug. 1978-Sept. 1979

Hydro- logical Period

"Dates Mean Mean Mean ~o Biweekly Biweekly Biweekly Rainfall Rainfall S-12 Input Water Input Input (acre-feet) (acre-feet) (acre-feet) × 103 × 103 × 103

Yo S-12 Deliveries Input

I 24 Aug 78 44.7 46.0 90.7 49~ 51~o 5 Oct 78

II 6 Oct 17.3 49.0 66.3 26~o 74Yo 2 Nov 78

III 26 Jan 79 10.3 17.6 27.9 37~o 63~o 25 Jan 79

IV 26 Jan 79 5.2 10.7 15.9 33~o 67~o 08 Mar 79

V 9 Mar 79 1.2 2.2 3.4 35~o 65~o 19 Apr 79

VI 20 Apr 79 105.2 4.4 109.6 96~o 4~o 17 May 79

VII 18 May 79 18.5 13.2 31.7 75~o 25~o 14 Jun 79

VIII 15 Jun 79 45.6 7.6 53.2 86~o 14~o 20 Sep 79

Page 5: The response of specific conductance to environmental conditions in the everglades National Park, Florida

S H A R K S L O U G H C O N D U C T I V I T Y 5 5

800

780

760

740

720

.~ 700

680

Od 660

o 640

.~ 620

::L 6 o o

LU 580

560

,~ 540

520 Z 0 5 0 0

o 480

t~ 46O

440

420

400

380

360

$40

320

300 OCTI NOV I DEC I JAN I FEB I MAR I APR

} 1 9 7 8 I

I ,, I ,,, I ,v I v I v, I v,, I HYDROLOGFC PERJOD

Fig. 3.

- - HOURLY RECORO INTACT

------ DATA GAP

! I !

MAY l JUN l JUL l AUG [ SEP

1 9 7 9

VIII I

Daily specific conductance at NP-203 (October 1978-September 1979).

(Figure 3). During the early dry season hydrologic periods I, II, and III when large amounts of S-12 delivery water entered the slough, specific conductance at NP-203

ranged from 500 to 560 lamhos cm-i . This range of specific conductance is lower than that typically found during this season

in the canal water at the delivery site, but higher than that historically found in the northern portion of Shark Slough prior to the construction of L-29. The conductivity pattern predominant during these periods is determined by canal water input modified via dilution by direct precipitation, which has a specific conductance averaging only 37 gmhos cm-1 (Irwin and Kirkland, 1980). However, because the quantitative input of rainfall is significantly less than surface water delivery during this season, the net effect of the rainfall dilution is less than it is during the wet season.

During periods I through III specific conductance throughout the slough ranged from 400 to more than 700 gmhos cm- 1 with the highest values occurring near surface water delivery sites including the S-12 structures and the Levee 67 extended (L-67 ext) canal (Figure 4a).

Specific conductance at NP-203 decreased slightly in January and February in response to rainfall that was the greatest since mid-November.

A rapid increase began in early March (NP-203 = 480 gmhos cm- 1) and continued until mid-April (NP-203 = 780 gmhos cm- i). The reason for this increase is a seasonal dry down, occurring annually as early as January and as late as April, during which water

Page 6: The response of specific conductance to environmental conditions in the everglades National Park, Florida

56 M A R K D . F L O R A A N D P . C . R O S E N D A H L

) I . • • k . . F - A l l . ~ "

I I ~ " . i l l / / ,1 i d" • / i

, , / . r ~ . /

/ . • . /

T-~- -~ - - __~ I if / I " ". (0 / / / •

FALLING WATER / i " / / I FALLING WATER IIEARLY DRYSEASON / "" / i I LATER DRY SEASON L . . . . . . . / j ~ ¢ . " ~ t o / i J _

l <J / // - - i /

~ , ~ . . IL 19 APR 79 ~ , . ,, 3 MAY 79 I 121 L p._ l h i:~ L l i i I • ~ l k +_ILl I i • ~ l k t 11

I . ~ / • . I - :l / ~'

~. , a: ~1 I ~ t 3 1 1 M • " " FI" O~ll l i f o A

i i / 1 " . ' 1 IJ" i i I ~ I I . . • " - . . . . . ,. / i . ' , c . / A . /" _ - - - ¢ - ' . ," / . . .

. m . ~ ? . . . . / / / + . x

0 "" i I "" I 4 Ix \ : I I EXTREME DRY SEASON : / i i

, ; y ~ . ~ .. --.--.~ ~ j , .. / f , WET SEASON . _ . . . . . i l I . . . . . . . • . " ~ , i / I

/ "PARK BOUNDARIES • " / / / / ------SLOUGH BOUNDARIES / / / / /

/ / SPECIFIC CONDUCTANCE / • SAMPLING LOCATION

Specific conductance contours (llmhos cm-1 x 100 @ 25 ° C) for the Shark River Slough during dry season and wet season conditions.

Fig. 4.

outflow and evapotranspiration exceed water input. The rapid increases as observed at NP-203, occur when areas are severed from continuous sheet flow and the Everglades becomes a series of shallow pools. These pools typically have higher ionic concentrations due to both evaporation and the concentration of biological organisms in and around the pool and remain until the beginning of the wet season. This seasonal increase in conductivity is demonstrated throughout the slough. In early March it exceeded 600 gmhos cm- 1 in all areas of Shark Slough (Figure 4b). By mid-April, specific conduc- tance in Shark Slough had reached its seasonal maximum, with values in the northern slough exceeding 800 gmhos cm-1 and in the southern slough being greater than 1200 gmhos cm- 1 (Figure 4c). In extremely dryyears (1963, 1967, 1968, 1971, and 1974) values exceeding 15 000 gmhos cm-1 have been recorded at station P-35 in southern Shark Slough indicating the northward advance of estuarine waters during the peak dry season.

In water year 1979, the dry season ended abruptly when an intensive storm event occurred on April 24 to 25, 1979. During this storm more than 16 inches of rain fell on Miami with between 4 and 8 inches falling directly on Slark Slough (South Florida Water Management District, 1979). The effects of this event on isoconductivity patterns were

Page 7: The response of specific conductance to environmental conditions in the everglades National Park, Florida

SHARK SLOUGH CONDUCTIVITY 57

immediate. At NP-203 the value dropped from almost 800 gmhos cm 1 before the storm to approximately 400 gmhos cm 1 afterwards. Conductivity throughout the slough also decreased (Figure 4d) so that by 3 May values throughout most of the slough was less than 600 gmhos cm-i.

In the period following this storm event, wet season conditions were predominant. Water inputs were large, with rainfall accounting for more than 80 ~o of the total water input. Specific conductances at NP-203 during the wet season were the lowest of those encountered in this study ranging from 310 to 500 gmhos cm- 1, with rapid fluctuations occurring in response to both storm events and changes in water deliveries.

Changes in cation composition are also noted as water flows south through the slough. Generally, these changes are the result of dilution during periods of high rainfall and concentration during period of low rainfall. In both September 1978 and June 1979, specific conductance and cation composition at the water delivery site (S-12D) were similar (Figure 5). As the water flowed south the specific conductance decreased during

3 -

e E

o o 1 o

u

S-12 D S - 1 2 D 13 Sept '78 14 June '79

, Sp. C o n d . = 6 0 0 ~ u L m h o s crn -~ 3 - Sp. Cond. = 700,~uLmhos crn-~

32-

.o I

I

I . Ca'++ N a + Mg-H- K + Ca ~ " Na+ M g + ~ K+

3 -

g2 E

o

a 1 ~J

P - 3 6 13 Sept '78 Sp. Cond . = 4 2 0 ~ mhos

h Ca++ Na+ Mg-H- K +

3 -

" ~ 2 - o E

q,J

P - 3 6 14 June '79 Sp. Cond . = 6 2 0 ~ m h o s

Ca ++ N~0" M g ++ K +

Fig. 5. Cation concentration (meq 1-1) during falling water and rising water hydrological conditions at S-12D and P-36.

Page 8: The response of specific conductance to environmental conditions in the everglades National Park, Florida

58 MARK D. FLORA AND P. C. ROSENDAHL

a period of high rainfall (September 1978), but remained unaltered during a period of lower rainfall (June 1979), demonstrating that alterations to these parameters while in

the slough, is largely related to the quantitative relationship between surface water delivery and direct precipitation.

4. Conclusions

Factors important in determining isoconductivity patterns of Shark Slough include the

quality and quantity of surface water delivered to the slough, the quantitative relationship

between the amount of surface water delivery and the amount of direct precipitation, and the effects of interrupted sheet flow within the slough during seasonal drydown periods.

The temporal trend from year-to-year is primarily dependent upon the quality of the

surface water delivered to Shark Slough, and can be expected to continue to increase

should the specific conductance and cation concentration of the delivery water continue to increase.

Minimum and maximum extremes in specific conductance are likely to vary annually,

dependent upon regional rainfall and hydrologic conditions. Seasonal lows are found

generally during the wet season with seasonal high conditions occurring during the dry season. Prolonged drought, extreme rainfall events, and extensive releases of water from

the conservation areas for purposes of flood control can all change the specific conduc- tance regime dramatically in a short time.

In a system as complex and impacted as the Everglades, it is important to maintain a continuing monitoring program to both detect any change in water quality and to

differentiate between those changes occurring in response to natural conditions and

those produced by man-made changes. Specific conductance has been shown to be a valuable mangement tool in this respect. While it cannot be used as a broad based

measure of overall water quality, it can be utilized as an early indicator of changes in water quality. Since specific conductance responds to so many different environmental

cues, a significant change often signals the need for a more complete chemical analysis of the overall quality of the water.

References

Flora, M. D. and Rosendahl, P. C.: 1981, 'Specific Conductance and Ionic Characteristics of the Shark River Slough, Everglades National Park, Florida', South Florida Research Center Technical Report T-615, National Park Service, Everglades National Park, Homestead, Florida, 55 pp.

Irwin, G. A. and Kirkland, R. T.: 1980, 'Chemical and Physical Characteristics of Precipitation at Selected Sites in Florida', U.S. Geological Survey Water Resource Investigations 80-81, Tallahassee, Florida, 70 pp.

Lew, R. M., Flora, M. D., and Rosendahl, P. C.: 1981, 'An analysis of rainfall input into the Shark River Slough, Everglades National Park, Florida', South Florida Research Center Technical Report, National Park Service, Everglades National Park, Homestead, Florida.

Rosendahl, P. C. and Rose, P. W.: 1979, Environmental Management 3, 483.

Page 9: The response of specific conductance to environmental conditions in the everglades National Park, Florida

SHARK SLOUGH CONDUCTIVITY 59

South Florida Water Management District: 1979, 'Preliminary Report on the Severe Storm of April 24--25, 1979', 37 pp.

United States Geological Survey: 1980, 'Water Resources Data for Florida, 1979', Volume 2A. South Florida Surface Water, 688 pp.

U.S. Senate: 1970, 'River Basin Monetary Authorizations and Miscellaneous Civil Works Amendments', 91st Congress, Second Session, Senate Report No. 91-845; 25 pp.