design of an urban, ground-water-dominated wetland

WETL4NDS, Vol. 16, No. 4, December 1996, pp. 524-531 '~ 1996, The Sociely of Wetland Scientists

DESIGN OF AN URBAN, GROUND-WATER-DOMINATED WETLAND

Richard B. Wins ton Department. of Geology and Geophysk's

Louisiana State Universi O" Baton Rouge, La 70803

Abstract: In the Barney Circle wetland in southeast Washington DC, a proposed site for wetland enhance- ment and creation, ground water supplied over 80% of the flow into the wetland from 25 March to 17 August 1994. In most months, the primary outflow from the wetland occurred as ground-water flow, but in April, almost all the outflow was surface water. Rain, runoff, and changes in storage account for less than a third of the observed surface-water discharge from the wetland to the Anacostia River in April and May. This indicates that ground water inflow was the major source of water. Although it is only a first order basin. the wetland had a small but definite flood-control function. Peak surface-water discharges out of the wetland during a storm event were reduced by 38 to 80% compared to surface water inflow into the wetland. The time during which storm outflow occurred was much longer than the time of storm inflow. Proposed construction of a highway will result in a 13% reduction in the recharge area of the Barney Circle watershed. A calibrated ground-water flow model of the watershed suggests that this will result in a 27@ decrease in baseflow discharge from the wetland. The hydroperiod of the wetland will only be slightly affected, however. By modifying the calibrated watershed model, it was possible to design a modified topography of the wetland that will favor desired plant communities. However, modifying the hydroperiod will require a much greater depth of excavation than would have been predicted based on the average hydroperiod either before or after highway construction. Excavation of the wetland wtll cause ground water in the surrounding uplands to drain more quickly than previously. This change in the hydrology had to be taken into account in the design. This study illustrates how failure to consider the effects of site alteration on local hydrology could result in project failure.

Key Word.w wetland creation, wetland restoration, ground water, Washington, DC

I N T R O D U C T I O N

The Barney Circ le wet land, loca ted near the Sousa Br idge in southeast Washing ton , DC, has been sug- ges ted as a site for we t l and enhancemen t and creat ion. The purpose o f this s tudy was to ana lyze the hydro l - ogy o f the Barney Ci rc le site_ This w o u l d a l low des ign a l te rna t ives to be eva lua ted and would help ensure that the h y d r o p e r i o d o f the site was cons is ten t with the p lant c o m m u n i t i e s p roposed . Because the plants proposed for the site required a longer hyd rope r iod than the wet land now exper iences , part o f the pro jec t in- vo lved de te rmin ing how the hyd rope r iod cou ld be bes t a l tered.

Wet l and des ign was fac i l i ta ted with a sui table g round-wa te r model . A mode l was ca l ib ra ted to cur- rent cond i t ions and then a l tered to reflect the p r o p o s e d future condi t ions . By using a long- te rm wea ther record in the model , it was poss ib le to predic t the va r iab i l i ty in hyd rope r iod and evalua te the su i tab i l i ty o f the proposed design.

G r o u n d - w a t e r - d o m i n a t e d wet lands were r ecogn i z e d by Kusler and Kentu la (1990) as the most diff icult type

o f we t l and to const ruct . They r e c o m m e n d avo id ing the use o f " c o o k b o o k " techniques in des ign ing any wetland, Unfor tunate ly . there is re la t ive ly little gu idance ava i l ab le on how to des ign wet land hydro logy . One o f the few excep t ions is M a r b l e ' s guide (1992) which is based on the Wet land Eva lua t ion Tcchnique ( A d a m u s et at. 1987). The Wet land Eva lua t ion Technique is used to assess the func t ion ing of a we t l and based on certain eas i ly assessed features. Marb l e (1992) r e c o m m e n d s the oppos i t e technique; funct ions are to be c rea ted by es tab l i sh ing those fea tures that A d a m u s et al. ~1987) a s soc ia t ed with those funct ions . Such a technique is appropr ia t e when the features used to recognize that a func t ion is present also cause the funct ion to be pres L ent. M a r b l e ' s t echnique is not appropr ia t e when the fea tures used to assess a funct ion are a result o f the func t ion ra ther than a cause for it. Fo r example , a thick layer o f pea t w o u l d be a g o o d ind ica t ion that we t land h y d r o l o g y is present because th ick pea t depos i t s can on ly lk~rm under very wet condi t ions . That does not mean, however , that we cou ld create we t l and hydro l - ogy by sp read ing a th ick layer o f pea t over the top o f

524

Wins ton , U R B A N , G R O U N D - W A T E R - D O M I N A T E D W E T L A N D 525

the soil in an up land area. Th ick peat deposit,,, are a result o f we t l and hyd ro logy but not a cause o f it. Mar- ble 11992) is careful to give the ra t ionales for near ly all o f her r e c o m m e n d a t i o n s lo indicate their wdidi ty . The one place where such ra t ionales are consp i cuous ly absent is in chapte r two, which dea ls with creat ing specific hydroper iods . Because fai lure to create appropriate hydrok~gy is one of the more c o m m o n causes of failure o f we t land-c rea t ion efforts (Mi tsch and Gos- sel ink 1993), des ign cr i ter ia for c rea t ing we t l and hy- d ro logy dese rve c lose scrutiny. This s tudy p rov ided an oppor tun i ty t o eva lua te M a r b l e ' s cr i ter ia with respect to es tab l i sh ing specif ic hyd rope r iods in a g round-wa- t e r -domina ted wet land.

The s tudy also p rov ided an oppor tun i ty to invest i - gate the f lood-peak a t tenuat ion funct ion of the wetland. Based on the hydro~zeomorphic sett ing (Brinson 1993) of the wet land as a headwa te r s t ream, one might nol expec t it to have any effects on f looding.

S E T T I N G

The Barney Ci rc le wet land, with an area o f 0.6 hec- tares, lies be tween a set o f train t racks on the southeas t and a steep ~lope ( app rox ima te ly 45 °) on the nor thwes t (Oigure 1). The sou thwes te rn end of the we t l and is dc l incd by s l ight ly h igher g round near the Sousa Bridge. Surface water f lows toward the nor theast into the Anacos t i a river. The wet land consis ts or" a ser ies o f pools separa ted by a ser ies o f cons t r i c ted zones. The pools are a p p r o x i m a t e l y 10 m wide and up to 50 cm deep. The cons t r ic ted zones are 50 to 100 cm wide with m a x i m u m water depths o f app rox ima te ly 15 cm. Congress iona l C e m e t e r y and Anacos t i a Park on the nor thwes t side o f the Barney Ci rc le wet land are the only substant ia l areas where local , g round water recharge could occ~m They have a c o m b i n e d surface area o f app rox ima te ly 2. I × 10 ~ m-". Beyond the cemetery, near ly the ent ire land surface is i m p e r m e a b l e because of the presence of roads and bui ldings. On the southeast side o f the wet land, there is only a narrow strip of land, a p p m × i m a t c l y 70 m wide, separa t ing the we t land I¥om the Anacos t i a River. With an area of only 3.7 x 10 ~ m:, the potent ia l for g round -w a te r recharge on that side is much less than on the nor thwes t side o f the wet land. Much o f this land on the southeast is lower in e leva t ion than the wet land, further restr ict- ing potent ial g round-wa te r flow into the wet land from this a rea {Figure l ).

The we thmd occurs on top o f up to 3 n! of lead- con tamina ted , d r edged spoils cons i s t ing o f o l ive gray si l ty clay depos i t ed be tween a p p r o x i m a t e l y 1800 and 1920 (l~igure 21. l ,andfi l l depos i t s and cons t ruc t ion de- bris up to 10 m thick over l ie the d r edged mater ia l on the s teep s lope nor thwest of the we t l and (Envi rens

A

B

Washington D.C.

Propose Highway

\qOx'

~ Barney Circle ~o=-'- Wetland

~. . , . . . ._~cost ia Park Congressional

C _ _ ~ cemetery

~~~acostia park~-'-~ TWeirs arney irce e an - - - - Railroad .... '3'00 m

g so cells x 37 cells

N / '~ TBarney I No Flow cells Circle Wetland

Constant Bead Cells 150 rn

Figure 1, (A) Location map for the Barney Circle wetland. (B) Watershed of the Barney Circle welland IC) Location~ of wells, piezometers, and weirs. (D) Boundary ctmditions a n d h y d r a u l i c c o n d u c l i v i t y /t~ne,. in the" g rcmru[ ,.~,'aler mt~de[

of the Barney Circle wetland.

526 WETLANDS, Volume 16, No. 4. 1996

20. Congressional Cemetery

g nacostia Park

C o n s t r ~ ~ . . . . . .

~ ,~ , , ~ \ g,"Lf'7 y U dfi, o po it. e i;nd ~ ~ ~ River o O ~ : ~ : i , . ~ , , , '

300 m V.E. = lOx

Figure 2. Cross section showing the stratigraphy of the Barney Circle wetland. Line of cross section (A-A') shown on Figure 1.

1993, K. Troensegaard, personal communica t ion 1994). Congressional Cemetery is underlain by natural sediments (poorly sorted sand and gravel) rather than landfill material (King Troensegaard, personal com- munication, 1994; Figure 2).

METHODS

Water in wetlands comes from three sources, precipitation, surface water, and ground water. Water is lost from an area as evapotranspiration, surface water, and ground water. Any imbalance between the water sources and sinks will be reflected in a change in stored water. The water balance is thus

(P + S m + G~,) - (E + Soo, + Goo,) = + AStorage (1)

P = precipitation, S,, = surface water inflow, G,, = ground-water inflow, E = evapotranspiration, So, ' = surface water outflow, Go= = ground-water outflow.

l determined monthly water budgets for the period 3/25/94 to 8/17/94 by using weather and hydrologic data to estimate each of these terms except ground- water outflow. Ground-water outflow was taken as the residual of all the other terms. Ultimately, ground water turned out to be the major source of water for the wetland. Thus, a ground-water flow model was used to determine the impact of the proposed highway construction on the wetland and to estimate how the topography of the wetland would need to be altered to achieve desired hydroperiods.

Surface water inflow and outflow to and from the wetland was measured with 90 ° v-notch weirs sealed with plastic sheeting and sandbags. Daily precipitation and rainfall data from Washington National Airport 6 km away were supplemented by a non-recording rain gauge accurate to 1.3 mm read during each site visit. Rainfall, streamflow, and water levels in wells and piezometers were monitored at least once each week and more frequently during some rain events.

I assumed that evapotranspiration occurred at the

potential rate throughout the growing season as estimated with the Blaney-Criddle method (Dunne and Leopold 1978). Crop coefficients were those given for pasture grass by Dunne and Leopold (1978).

1 estimated surface-water runoff into the wetland with the Soil Conservation Service (SCS) runoff equation as outlined in McCuen (1989) assuming a type C soil. This is appropriate because the soil in the watershed is primarily silty clay.

Water levels were measured in 36 wells and piezometers installed in and near the wetland (Figure I ). Four- teen of the wells were constructed of 5.08-era-diameter PVC pipe with a 0.76-m-long slotted well screen. Twenty- two additional wells and minipiezometers were constructed of 1.3-cm PVC or CPVC pipe. Fif- teen of these were a set of nested piezometers, each with screened intervals 10 cm long in the wetland at depths ranging from 0.4 to 2.6 m. The remaining wells were installed around the periphery of the wetland at locations where they would aid in detecting horizontal hydraulic gradients. The elevations of the tops of all the piezometers and wells were surveyed. Hydraulic conductivity was estimated based on Bouwer and Rice slug tests (Bouwer and Rice 1976, Bouwer 1989). Spe- cific yield was estimated based on changes in head in response to rainfall (Dolan et al. 1984). (Specific yield is the freely available water in an unconfined aquifer. It is defined as the ratio of the volume of water re- leased by draining a portion of the aquifer to the volume of the aquifer which was drained.)

It is possible to use the observed correlation between hydraulic gradient and baseflow discharge (Fig- ure 3) to estimate the rates of ground-water flow in the Barney-Circle wetland. If all the ground water that entered the wetland left as surface flow, one would see a close correlation between the hydraulic gradient toward the wetland and the discharge. (The hydraulic gradient, together with hydraulic conductivity and aquifer geometry, controls ground-water flow.) A graph of baseflow discharge vs. hydraulic gradient would show a straight line passing through the origin. If there was a constant rate of ground-water flow out of the wetland and other fluxes were negligible, one would expect to see a similar relationship except that the line would be shifted down so that the y-intercept passed below the origin. The distance between the y-intercept and the origin would be the rate at which ground water was flowing out of the wetland. A near- constant ground-water discharge out of the wetland is reasonable in the Barney-Circle wetland because the hydraulic gradient between the wetland and the Ana- costia River varied little. The amount of ground-water inflow could be estimated as the product of the hydraulic gradient and the slope of the line relating bas- eftow discharge to hydraulic gradient (Figure 3).

Wins ton , U R B A N , G R O U N D - W A T E R - D O M I N A T E D W E T L A N D 527

E 0.008

m 0.004 ¢-

o

0.000 o

-o.oo4

-0.01 0.00 0.01 0.02 0.03 0.04

Hydraulic Gradient Figure 3, Relationship of baseflow, surface-water discharge to hydraulic gradient. If all the ground water flowing into the wetland left the wetland as surface water, one would expect that all the points would lie on a straight line passing through the origin. The fact that they do not lie on such a straight line indicates that a substantial portion of the ground water that enters the wetland leaves the w'etland as ground water rather than as surface water. The negative of the yqn- tercept of the line relating baseflow discharge to hydraulic gradient is an estimate of the amount of ground-water discharge out of the wetland that occurs when the water level in the wetland is at its maximum in spring.

R E S U L T S

1 mon i to r ed the r e sponse o f the we t l and to ra infal l dur ing s torms on June 16 and June 27. Rainfa l l amounts r eco rded at Wash ing ton Nat iona l A i rpo r t were 3.0 and 1.3 m m respect ive ly . No ra infal l da ta were ava i lab le f rom the Ba rney Ci rc le we t l and because o f fai lure of the rain gauge. Dur ing both rain events , peak su r face -wate r d i scha rge rates were subs tan t ia l ly grea ter at the inlets to the we t l and than at the out le t (F igure 4). F l o w at the inlets ceased soon af ter ra infal l ended. However , s to rm flow f rom the out le t con t inued for several hours. The vo lume o f d i scharge f rom the out le t was grea ter than the amoun t that en te red the we t l and f rom the inlets. On June 16, d i scharge into the we t l and to ta led 1.8 m 3 and out le t d i scharge was 11.6 m ~. One June 27, d i scharge into the we t l and was 12.8 m-' and out le t d i scharge was 19.1 m ~. The d i sc repan- cies in vo lume reflect runof f genera t ion by saturat ion, ove r l and flow in the wet land, and genera t ion o f s to rm- flow by throughf low or sha l low g round-wa te r flow (F igure 4).

Slug tests r evea led cons ide rab le var ia t ion in hydraulic conduc t iv i ty within the we t l and sediments . A m o n g 11 s lug tests, the a r i thmet ic mean was 2.9 × 10 -6 m/s, with a s tandard dev ia t ion o f 1.7 × 10 o m/s. Tile geo- metr ic mean was 3.4 × 10 7 m/s. In one zone, recov- e ry was too rapid to be m e a s u r e d with ava i l ab le equip- ment. By using the i ndependen t es t imate o f g round- water d i scharge out o f the we t l and in spr ing (5.6 ×

0.0010

2 fi

0.0005

0.0000 15:00

June 16

16:00 17:00

1

a .

18:00 19:00 20:00

0.003

g 0.002

0.001 a

0.000

, , June 27 ,~ - - - Outflow ~, - - - Inflow #1

. . . . . Inflow #2

11:00 12:00 13:00 14:00 15:00 16:00

Figure 4, Stream hydrographs for storms of June 16 and June 27. The peak discharge rate out of the wetland is re duced considerably compared with the inflow indicating that the wetland has a flood control function. However, the total volume of discharge out of the wetland is much greater than the surface water discharge into the wetland.

10 -6 m3/s , Figu re 3) and site g e o m e t r y , the hydrau l i c conduc t iv i ty o f this zone was e s t ima ted at 1.2 × 10 -~ m/see. I f this h igh -conduc t iv i t y zone is inc luded, the a r i thmet ic mean hydrau l i c conduc t iv i ty is 1.8 × I0 4 rots. This va lue c o m p a r e s f avorab ly with the va lue ob- ta ined f rom a ca l ib ra ted g round -w a te r model as will be d i scussed later.

Es t imates o f specif ic y ie ld o f the we t land sed iments var ied f rom 4 .2% to 16.7%, with a mean o f 8.3%. Since p i e z o m e t e r s 2 3 - 3 0 were all at the same locat ion, es t imates f rom those p i ezome te r s are not i ndependen t o f one another, If those va lues a re exc luded , the me- d ian specif ic y ie ld is 7.1. In the water budget , 1 used a specif ic y ie ld o f 7 .5% to ca lcu la te the vo lume of wate r en ter ing or l eav ing s torage when the wate r table was beneath the soil surface. The specif ic y ie ld data were used in the g r o u n d - w a t e r m o d e l to be d i scussed later.

Prec ip i ta t ion was a b o v e normal th roughou t the win- ter p reced ing the s tudy. In March prec ip i ta t ion was near ly 2.5 t imes normal and was the m a x i m u m on record (F igure 5A). Prec ip i ta t ion in Apr i l , May, and June was a p p r o x i m a t e l y 50% be low normal . Prec ip i ta t ion was near normal for J u l y a n d wa.~ 5 0 % a b o v e n o r m M for the first hal f o f Augus t . Prec ip i ta t ion was only a smal l part o f the wa te r budge t o f the we t land (F igure 6).

S t r eam flow through the out le t dec reased f rom 0.02

528 WETLANDS, Volume 16, No. 4. 1996

BO

._=

t r 4 0

.>_

-~ 20 E

0.03-

g a.o2-

0.01 -

0 .00-

-0.01

~ " 0.02. g

o.oi

r~

0.00 2.3-

2.2

2.1,

m 2.0- - r

1 .g i 1.84

A

Weekly

B

J

* Modeled L

C

°

Artifioially constrained: rnodelc~d heads could not exceed the channel base.

1/1 3/2 5/1 6/'30 8,/29

Figure 5. (A) Cumulative Rainfall. (B) Observed and modeled hydraulic gradients toward the Barney Circle wetland. (C) Observed and modeled surface-water discharge from Barney Circle wetland. (D) Observed and modeled head in the Barney Circle wetland.

mVs in March 27 to 0 in late July (Figure 5C). Surface- water flow into the wetland occurred frequently at the beginning of the study. However, the surface-water inflow to the wetland was trivial compared to surface- water outflow from the wetland. Surface-water inflow to the wetland was a negligible part of the water budget in all months except April (Figure 6). Surface-water outflow from the wetland was the largest outflow in the water budget in April but had decreased to a negligible amount by June (Figure 6)

Estimated ground-water inflow into the Barney Cir- cle wetland exceeded 3.4 × l0 t m ~ between May 25 and April 30, whereas ground-water outflow (calculated by difference) was only 2 × t04 mL For July, the inflow had dropped to approximately 1.0 x 104 m 3 and ground-water outflow to 9.6 x 103 m 3 (Figure 6). This makes ground water the most important component of the water budget (t,igure 6).

Runoff into the wetland varied greatly among months. Monthly volumes estimated with the SCS runoff equation ranged from 5.7 × 1 04 m J for late March and April to 0 in May and June (Figure 6). Inflow

volumes were negligible in comparison with other components in the water budget. In the monthly water budgets, ground water always accounted for more than 70% of the inflow into the wetland, In three months, May. June, and July, it accounted lor more than 90% of the inflow. Ground water also accounted for a major part of the outflow from the wetland, In every month except April, it accounted for more than 70% of outflow. In April, it accounted for almost 50%. Stream flow • accounted for almost 50% of the outflow in April, 23% in May, and less than 10% in June, July, and August . Evapo t ransp i ra t ion accounted for sl ightly more than 10% of the outflow in June, July, and Au- gust. Changes in stored water were always less than 5% of the monthly water budgets.

The monthly water budgets reveal that ground-water flow is a key component in the hydrology of the Bar- ney Circle wetland. Other components of the water budget are much smaller in comparison. In addition, inflow from rain and overland flow do little to main- tain high water levels as shown by the rapid return to baseflow conditions following storms. Thus, a ground- water flow model would be the best way to estimate the effects of highway construction on the wetland and predicting how an altered topography would affect the hydroperiod of the wetland.

H Y D R O L O G I C M O D E L I N G AND DISCUSSION

Numerical hydrologic modeling can be used to better understand the processes at work in the Barney Circle wetland and to help predict the hydrologic con- sequences of var ious poss ib le design decisions• Ground-water flow is controlled by Darcy 's Law, the properties and boundaries of the medium through which the ground water flows, and the locations, amounts, and timing of recharge, evapotranspiration, and discharge• The ground-water flow model must in- corporate all these factors to be successful. In numerical models, ground-water flow is simulated by break- ing down the ground-water flow region into a large number of small blocks. Darcy's law is applied to flow between each pair of adjoining blocks to assess flow throughout the region of interest. The major limitation of ground-water flow models is that the properties of the ground-water flow medium and recharge to and discharge from ground water are never precisely known and must be estimated. In this study, I have used M O D F L O W (McDonald and Harbaugh 1988), a widely accepted numerical ground-water flow model- lng program, with modi~cations documented in Prudic (1989), Hill (1990), and McDonald et al. (1991).

The recharge area for the Barney Circle wetland seems to be Congressional Cemetery and Anacostia Park. Thus, I excluded areas beyond Congressional

Winston, URBAN, G R O U N D - W A T E R - D O M I N A T E D W E T L A N D 529

5 0 i

~" 40- "5

30- O

20- ¢.,

- t 0 -

50-

E -6 40-

o 30- O

(,9 o ~ 20-

Inflows I I Declinein S~orage

Rain Runoff Ground water

Outflows F ~ Increase in Storage

Evapotranspiration Stream Flow Ground wmer

0 Apr May Jun Jul Aug

Figure 6. Monthly water budgets for the Barney Circle wetland. Ground water is the major source of water for the wetland but large amounts of water leave the wetland via ~ound water as well as entering it.

Cemetery from the model. This is an artificial con- straint because ground-water flow could cross the park boundaries even if no recharge was occurring.

Thc modeled region was broken into square grid cells 15 m on a side (Figure 1D). Recharge was assumed to occur uniformly across the watershed. Monthly average recharge rates were set equal to precipitation minus runoff and potential evapotranspiration. Potential evapotranspiration was based on the Blaney-Criddle Method (Dunne and Leopold 1978). Precipitation and other weather data came from 30 years of monthly data between 1964 and 1993 at Washington National Airport (NOAA 1993) and from daily published values f rom Washington National Air- port in the Washington Post during 1994. I assumed that runoff was 1% of precipitation if monthly rainfall was less than 150 cm and 30% of any rainfall in a month above and beyond 150 cm. This relationship was based on the amounts of runoff predicted during 1994 with daily rainfall data by using the SCS runoff equation. Use of the daily data for the full 30 years would result in an improved estimate of runoff.

1 assumed that all ground-water flow occurred in the sediment above an elevation of -0 .45 m. (This elevation was chosen based on slug-test results that showed a distinct decline in hydraulic conductivity with depth.) 1 simulated the wetland with the drain module of MODFLOW. The convergence criterion for each time step was set at 0.0006 m. Initial conditions

were based on a steady state flow model and average annual values of recharge and evapotranspiration fol- lowed by eight modeled years with average monthly values of recharge and evapotranspiration. I used fif- teen equal t ime steps per month.

I assumed that hydraulic conductivity varied across the watershed. Three zones with different hydraulic conduetivities were used (Figure 1D). Zone I was for the region far away from the wetland where natural sediment occurred. Zone 2 was within the wetland and the adjoining landfill and dredged material. Zone 3 was for the sediment between the wetland and the An- acostia River.

I varied hydraulic conductivity in each of the three zones to reach four goals. (1) Modeled surface water discharge out o f the wetland from March to August 1994 should closely match baseflow discharge out of the wetland (Figure 5C). (2) The hydraulic gradient toward the wetland should closely match the observed gradient between piezometers 11 and 16 during peri- ods of baseflow (Figure 5B). (3) Head in Congres- sional Cemetery should be sufficiently high to account for the spring observed in the cemetery. (4) Head in the wetland during the model period should be mod- erately close to the actual heads (Figure 5D). Because I used monthly average rate of precipitation and evapotranspiration rather than daily rates, I did not expect to match the observed heads closely. The drain module of M O D F L O W does not simulate ponded water, so the heads in the first two months were artifi- cially constrained to be lower than the actual water levels in the wetland.

The location of the boundary between the natural sediment in zone 1 and the landfill material in zone 2 had little effect on discharge out of the wetland. With any reasonable boundary between zones 1 and 2, it was possible to calibrate a model with an acceptable water table in the cemetery. Thus, the precise nature of the flow in Congressional cemetery ultimately had little influence on the rate of discharge out of the wetland- - the pr imary goal of the model, and the boundary between zone 1 and 2 was arbitrary.

The hydraulic conductivity of the sediment between the wetland and the river was the most important factor in determining the rate of surface-water discharge from the wetland. The higher its hydraulic conductivity, the more water leaves the wetland as ground-water flow and the less is left for surface-water flow. The hydraulic conductivity in the wetland itself was the most important factor in determining the hydraulic gradient toward the wetland; higher hydraulic conductivities resulted in lower gradients.

Ultimately, the hydraulic conductivities that resulted in acceptable results were 4.6 × 10 ~ rn/s beneath Con- gressional Cemetery, 1.2 x 10 -~ rrds in the wetland,

530 WETLANDS, Volume 16, No. 4, 1996

and 1.5 >4 10 4 m/s between the wetland and the river. The latter two values are similar to one another, as they should be since the sediments in both areas are similar. They also are close 10 the average hydraulic conductivity calculated previously---2.0 × 10 a m/see. If the measured hydraulic conductivity had been greatly different from the value obtained by calibrating the model, it would have been cause for concern, ttowevm; similar values, in and of themselves, do not prove that the model is correct.

Once a calibrated model had been constructed, it could be used to predict the effects of future hurnan activities. Planned highway construction, for example, would reduce recharge and thus affect ground-water discharge from the recharge area to the wetland. The model results suggest that the planned 13% reduction in recharge area would result in a 27% reduction in basetlow discharge. The timing of this discharge, however, would change very little from the present timing (Figure 71. The timing of the basellow discharge is controlled primarily by the timing of precipitation. Precipitation timing would not be affected by highway construction. Of course, if enough of the watershed were paved, no infiltration would occur and the wetland could dry out. Highway construction might also affect water quality, but such effects are beyond the scope of this study. Over the long term, all water that enters the wetland must also leavc it by one route or another. The amount that can leave by ground-water discharge is limited by the hydraulic gradient and the aquifer propcrties. If ground-water discharge out of the wethmd is insuflicient to ntatch the inflow to the wetland, the remaining water leaves the wetland as surface water. At present, surface-water and ground-water discharge out of the wetland are nearly equal. Howevel; if the recharge were reduced by 50%, bascflow discharge would cease and all discharge out of the wetland would be as ground water. The site would probably cease to be a wetland,

One goal of this study was to design a new topography for the wetland that would result in a longer hydroperiod. This proved difficult to achieve. To create a seasonal ly f looded hydroper iod in a non-tidal , oround-watcr-dominatcd wetland. Marble (1992) suggests excavating to a depth between tile seasonal high water table and the normal water table. I used the calibrated model to choose that elevation throughout the wetland. However, when the model was altered to reflect the new topography, there was virtually no change in the hydroperiod. Instead, water levels tl~rougllout the wuterslled were lowered. Lowering the elewttion of the wetland had provided better drainage but had not altered the hydroperiod, To achieve the design goals, the topography had to be lowered by an additional 3 0 - 6 0 cm. In most locations, the final de-

Aug J

o c Jun 2s

Apr

o t

A u g { -

:~ Jun

Apr

B Feb i ~ /

1965 1970 1975 1980 1985 1990

Year

Discharge (m3/s) o.oo5 - - 0 . 0 0 6 0 . 0 0 2 - 0 . 0 0 a

m i n i 0 .004 -- o.oo5 o.oo -- o.oo2 0.003 -- 0.004 0.000 -- 0.001

Figure 7. Modeled baseflow discharge from the Barney Circle wetland with (B} and without IA) including the el]i:ClS of highway construction. Although baseflow discharge ix re duced approximately 27% it the highway is constructed, the liming of discharge is altered very little.

sign topography was below the lowest ground-water elevation observed at those locations, These results suggest that design of the topography of a ground- water-dominated wetland is more difficult than it would seem baked on Marble's cri|erion, I[ the effects of site alteration on the hydrology had been ignored, the project probably would have failed.

There are at least Iwo potenlia] criticisms of lhis study, (1) The wetland in question has not yet been altered to change the hydroperiod, so it is impossible to judge whether tile proposed topography results in the desired hydroperiod. (2) The structure or the matl]- ematical model may not correctly represent the true hydrology of the wedand, so any conclusJoll drawn from such a model must be uncertain. Both these criticisms have merit, yet they do not alter the conclusion

Winston. URBAN, GROUND-WATER-DOMINATED WETLAND 531

that to design the hydrology of a wetland one must understand the processes that control the hydrology. Although a model is necessarily an imperfect repre- sentation of reality, it incorporates a better understanding of the system than does the " ro le-of- thumb" approach that Marble advocates (Marble 1992). The rule- of-thumb approach requires no real understanding of the processes controlling the hydrology. A model of a particular site may be incorrect to a greater or lesser degree. However, with a model, if the design fails, it may be possible to determine what was wrong and thus correct the problem. With a rule-of-thumb approach, this is impossible; failure can lead only to con- fus ion- -no t insight and improvement. At the Barney Circle wetland, the model shows that two critical variables determine the hydrology of the wetland: the infiltration rate in the neighboring recharge area and the transmissivity of the sediments between the wetland and the Anacostia River. In future studies, refined values for those variables might significantly improve the understanding of the hydrology of the wetland.

This study illustrates one way in which wetland- creation efforts based on ground-water sources can fail. When the topography is altered, the hydrology of the site may also be altered in ways that do not favor wetland creation. In this case, lowering the topography would result in increased drainage of the upland but relatively little change in hydroperiod. Such effects must be understood for wetland creation efforts to suc- ceed.

ACKNOWLEDGMENTS I thank Michael O'Connell, King Troensegaard,

Chris McQuale, and The Environmental Company, Inc. for their advice and assistance during this project. I also thank Michael S. Hollins and Envirens, Inc. for funding for this project.

LITERATURE CITED Adamus, RR., E.J. C[arain, Jr., R.D. Smtth, and R.E. Young. 1987.

Wetland evaluation technique (WET) Volume II: Methodology.

U S. Army Corps of Engineers Waterways Experiment Station. Vicksburg, MS, USA. Operational Draft Technical Rcpor~ Y 87 and Federal Highway Administration (FHWA-IP-88-029).

Bouwcr, H. 1989. The Bouwer and Riec slug test--an update. Ground Water 27:304-309.

Bouwer, H. and R.C, Rice. 1976. A slug test for determining hydraulic conductivity 0f unconfined aquifers with completely or partially penetrating ",veils. Water Resources Re~earch 12:423- 428.

Brins~n, M.M. 1993. A h,c'drogeomorph~c classification for wetlands, U,S. Army Corp~ of Engineers, Waterways Experiment Sta- tion. Wasfiington, DC, USA. Wetland Research Program Technical Rcport WRP-DE-,I,

Dolan, T,J,, A.J. Hermann. S.E. Bayley. and J. Zoltek. 1984. Evapo transpiration of a Florida. U.S,A., freshwater wetland. Journal of Hydrology 74:355-371.

Dunne, T. and L.B. Leopold, 1978. Water in Environmental Plan- ning. Freeman and Company, New York. NY, USA.

Enviren~, Inc. 1993. Barney Circle wetland mitigation preliminary plan. Envirens, Freeland, MD, USA~

Hill, M.C. 1990. Preconditioned conjugate gradient 2, A computer program for solving ground-water flow equations. U.S. Geological Survey Water Resources Investigations Report 90 4048,

Kusler, J.A. and M.E. Kentula. 1990. Executivc summary, p. xvii- xxv. In J.A. Kusler and M.E. Kentula (ed.) Wetland Creation and Restoration: The Status of the Science. Island Press, Washington. DC, USA.

Marble, A.D. 1992. A Guide to Wetland Functional Design. Lewis Publishers, Boca Raton, FL, USA.

McCuen, R.H 1989, Hydrologic Analysis and Design. Prenticc Hall, Englewood Cliffs, N J, USA,

McDonald. M.G. and A.W. Harbaugh. 1988. A modular three-di- mensional finite-difference ground-water flow model. Techniques of Water-Resources Investigations of the United States Geologic-'d Survey, Book 6, Chapter A1.

McDonald, M.G., A.W. Harbaugh, BR. Orr, and D.J. Ackerman_ 1991. A method of converting no-flow cells to variable head cells for the U.S. Geological Survey modular finite-difference ground- water flow model. U.S. Geological Survey. Open-File Report 91-536.

Mitsch, W.J. and J.G. Gosselink. 1993. Wetlands, Second Edition. Van Nostrand Reinhold Company, New York, NY, USA.

NOAA (National Oceanic and Atmospheric Administration). 1993. Local climatological data, monthly summary, Washington D.C. National Airport. National Oceanic and Atmospheric Adminislra lion, Washington, DC, USA. C 55.286/6-09:003/13.

Prudie, D.E. 1989. Documentation of a computer program to sim~ ulate stream-aquifer relations using a modular, finite-difference, ground-water flow model. U.S. Ge,,logical Survey. Open File Re- port 88 729.

Manuscript received 23 February 1996: revision received 8 July 1996; accepted 19 August 1996.

design of an urban, ground-water-dominated wetland

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