John S. Koreny ',-~, William J. Mitsch j, E. Scott Bait 2, and Xinyuan WW ,4. J Environmental Science ... Redmond, Washington, USA 98052. 4 Present address:.
WETLANDS, Vol. 19, No. 1, March 1999, pp. t82-193 © t999, The Society of Wetland Scientists
R E G I O N A L A N D L O C A L H Y D R O L O G Y OF A C R E A T E D R I P A R I A N WETLAND SYSTEM John S. Koreny ',-~, William J. Mitsch j, E. Scott Bait 2, and Xinyuan WW ,4 J Environmental Science Graduate Program and School of Natural Resources The Ohio State University 210 Kottman Hail, 2021 Coffey Road Columbus, Ohio, USA 43210 2 Department of Geological Sciences The Ohio State University 275 Mendenhall, 125 S. Oval Mall Columbus, Ohio, USA 43210 Present address: GeoEngineers, lnc. 8410-154th Avenue NE Redmond, Washington, USA 98052 4 Present address: Department of Rangeland Ecology and Management Texas A & M University College Station, Texas, USA 77843 Abstract: The hydrology of a created riparian wetland system was characterized for local and regional conditions. Three methods, two laboratory and one in situ, were used to calculate the hydraulic conductivity of the wetland substrate. Hydraulic conductivity values and measured vertical gradients were used to estimate seepage loss to ground water. Surface-water inflow and outflow dominated the hydrologic budget for the wetland. Precipitation, seepage loss, and evapotranspiration were minor components of the hydrologic budget. Water seeping from the wetland into the ground-water-flow system has characteristics of a local groundwater-flow system. Ground-water modeling and particle tracking show that water originating from the wetland seeps into the ground water and flows in the local ground-water system to the southeast portion of the site. Ground-water travel times were estimated to range from 700 to 1,200 days before being discharged to the Olentangy River.
Key Words: wetland hydrology, wetland ground-water hydrology, wetland seepage, wetland modeling, Otentangy River Wetland Research Park
been very successful due to limited understanding o f locations and hydraulic gradient of underground water." Review articles on the hydrology of wetlands emphasize that ground-water and wetland surface-water interactions are often poorly understood and quantified (i.e., Carter 1986,LaBaugh 1986). Many researchers have investigated the hydrology of natural wetland systems (Gosselink and Turner 1978, Mitsch et al. 1979, Winter and Carr 1980, Siegel and Glaser 1987, Siegel 1981, 1983, 1988a, 1988b, Hollands 1987, LaBaugh et al. 1987, Winter and Rosenberry 1995, Lide et al. 1995). Yet, few studies exist that examine the function of constructed wetland sys-
INTRODUCTION Among the greatest information needs on constructed w e t l a n d s are data on h y d r o l o g y , p a r t i c u l a r l y ground-water hydrology (Garbisch 1986, Zedler and Weller 1990, H a m m e r 1992). Hammer (1992) stated, "the long-term success of any wetlands restoration or creation project is, to a very large extent, dependent upon restoring, establishing, or developing and managing the appropriate h y d r o l o g y . . . Our understanding of and ability to estimate subsurface flows (for constructed wetland design) is p o o r . . , attempts to construct wetlands that intercept ground water have not 182
Koreny et al., R E G I O N A L A N D L O C A L H Y D R O L O G Y
183
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tems in a regional hydrologic setting. Hensel and Miller (1991) and Hey et al. (1994) report on a study measuring the post-construction hydrology of four constructed wetland basins on regional ground water at a site along the Des Plaines River in northern Illinois. The authors concluded that the wetlands functioned as a discharge feature, which doubled the baseflow discharge of regional ground water to the Des Plaines River and that the significant water-quality improvements observed at the constructed wetlands could be realized for the entire Des Plaines River watershed if wetlands were constructed on 1 - 2 % of the watershed. This study examines the surface-water and groundwater hydrology and the discharge/recharge functions for two wetland basins constructed adjacent to the Olentangy River and supplied by surface water f r o m the river. Field and modeling methods were used to 1) develop a hydrologic budget for the wetlands, 2) quantify the amount of surface water lost to the ground-
water flow system, and 3) estimate the time of travel and track the t o w p a t h s o f water seeping into ground water and returning back to the Olentangy River. An emphasis was placed on using methods that m a y be e m p l o y e d to investigate and quantify site-specific hydrology at other constructed wetlands. SITE DESCRIPTION The study area is located in Columbus, Ohio,USA and is adjacent to the Olentangy River. The wetland basins are located on The Ohio State University campus at the Olentangy River Wetland Research Park (ORW) and are each one hectare in size (Figure 1). The wetland basins have each been maintained in the same hydrologic conditions as part of a multi-year ecosystem experiment since March 1994 (Wu and Mitsch 1998, Mitsch et al., 1998, Mitsch 1995a, Mitsch 1995b). Water is continuously p u m p e d from the Olen-
184
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Hydrogeologic section through the study area.
t a n g y R i v e r into the w e t l a n d b a s i n s t h r o u g h two p u m p s u p a n e l e v a t i o n g r a d i e n t to m a i n t a i n a flowt h r o u g h system. T h e two p u m p s , a centrifugal p u m p a n d a " b i o l o g i c a l l y f r i e n d l y " r o t a r y - d i s c p u m p , discharge t h r o u g h s u b s u r f a c e pipes to the w e t l a n d basins.
W a t e r d e p t h i n the b a s i n s r a n g e s f r o m 0.3 m to 0.6 m , a n d the s u r f a c e - w a t e r w e t l a n d stage v a r i e s with p u m p i n g v o l u m e . P u m p i n g rates are a d j u s t e d to m i m ic the c h a n g e in stage a n d flow v o l u m e o f the O l e n t a n g y River. W a t e r flows out o f the w e t l a n d b a s i n s
Table 1. Hydraulic conductivity values for wetland soils, (m/day), [Average -+ Std. Error (No. Samples)]. Laboratory Methods (2.0 × 10 4 to 3.4 x 10-4 m 3 of soil tested)
Wetland 1 Wetland 2 Swale
Field Method (0.10 to 0.17 m e of soil tested)
Orain-Size Method
Permeameter Method
I n s i r u Fie}d Seepage Meter
3.4 - 0.0 × 10 4 (7) 3.4 - 0.0 x 10 * (7) 3.4 --- 0.0 x 10 -4 (2)
49 + 77 x 10 -~ (7) 260 _+ 650 x 10 -" (7) 0.54 --+ 0.45 x 10 -" (2)
0.28 -+ 0.21 (12) 0.19 ± 0.10 (12) not tested
Koreny et aL, R E G I O N A L AND L O C A L H Y D R O L O G Y
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over a weir to a swale and discharges back to the Olentangy River. Because water is pumped into the wetlands, which are higher in elevation than the river stage and local ground-water elevation, ground-water levels are almost always below the wetland stage. Surface water almost always seeps from the wetland into ground water, forming a local ground-water mound. As a result, the wetland generally functions as a ground-water recharge source. The wetland site is topographically isolated from the surrounding uplands to the west by a dry swale that intercepts overland flow and discharges to the north to the Olentangy River. The surface physiography for the region can be characterized as fiat to somewhat rolling. The study area is located within the Olentangy River valley, which runs in a north-to-south orientation. The Olentangy River is the major drainage feature for the region and the only perennial drainage feature within the study area. The river flows from headwaters 134 kin to the north to a confluence with the Scioto River 8 km to the south. The Olentangy River stage is controlled by a dam upstream of the study site, and fluctuations in stage are as much as 3 m during flood events. During normal low-flow conditions, the river
stage drops from 222.0 to 218.5 m over a reach of approximately 3 km across the study area. The Olentangy River valley is composed of a preglacial bedrock valley consisting of the Devonian-age Delaware and Olentangy Limestone. The bedrock valley is filled with glacial or fluvially deposited sand and gravel interbedded, in places, with thin deposits of silty sand or silty clay (Schmidt 1958). The sand and gravel deposits, which yield large quantities of ground water, form the Olentangy River buried-valley aquifer (Figure 2). At the O R W site, the upper 3 to 5 meters of the sand and gravel include silt deposits, while the lower portion is mainly free o f silt or clay. The surface deposits at the site, a thin layer o f glacial till and/or alluvium floodplain, range from 1 to 3 m in thickness and are primarily composed of a matrix o f silty clay with minor components of sand, gravel, and cobbles. To the west of the ORW, silty clay till overlies bedrock, forming uplands that rise in elevation and comprise a local topographic divide. Within the aquifer units examined for this study, ground water flows through the confined sand and gravel and bedrock (limestone) aquifers from the local topographic divide west o f the site to the Olentangy River. Water levels in monitoring wells screened in the
186
W E T L A N D S , Volume 19, No. 1, 1999
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shallow sand and gravel rise above the top of the surface silty-clay alluvium or till. This indicates that the sand and gravel deposits form a confined aquifer. Leakage out of the wetland basins form a local ground-water mound, which was observed in piezometers and monitoring wells installed in the sand and gravel aquifer. The higher heads in this mound increase the downward vertical leakage of ground water into the sand and gravel aquifer. METHODS Wetland Hydrologic Budget Components of the wetland hydrologic budget were measured daily by recording instruments or by O R W
staff. The volume of water pumped into the wetlands was recorded by two m-line flow meters. Wetland stage elevations were measured in both wetland basins using continuous stage recorders, and wetland outflow was determined by calculating the amount of water flowing over the two v-notch weirs at various wetland stages. A meteorologic station was maintained between the two wetland basins to measure precipitation and evapotranspiration. Seepage loss from the wetlands to ground water was initially calculated using the Darcy equation (Todd 1980). Values for wetland sediment hydraulic conductivity used in the seepage calculation were determined by three methods: Hazen grain-size method (Kruesman and de Ridder 1991), laboratory permeameter method ( A S T M 1990), and in situ field per-
Koreny et al., R E G I O N A L A N D L O C A L H Y D R O L O G Y
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uw-7 Ground-waterpotentiometrk; 22"t.8elevation in monitoring well Surface-water stage in wetlands ranged from 220.90 to 220_95 AII elevations in ro arnsl ~w.r
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Ground-water potentiometric surface at the ORW site on August 4, 1994.
m e a m e t e r method (Lee and Cherry 1978). The hydraulic gradient used in the seepage calculation was obtained by direct m e a s u r e m e n t at nested p i e z o m e t e r s installed at two locations in each wetland basin. Wang et al. (1997) later used a m o r e accurate method to estimate seepage involving direct observation of the decline of the wetland surface-water stage over a period when inflow, outflow, and evapotranspiration were zero.
River H y d r o l o g y Olentangy River stage levels were recorded directly upstream and downstream from the O R W site using continuous stage recorders. A rating curve was developed for the Olentangy River by staff from the U.S. Geological Survey office in Columbus, Ohio by gauging the river flow at the Clinton Park D a m (directly upstream from the wetlands) at various stages using
188 standard methods (Carter and Davidian 1968, Kennedy 1984). River sediments were sampled at nine locations, and grain-size analyses data were used to calculate the riverbed hydraulic conductivity according to the Hazen method. Streambed elevations were measured at 26 locations to develop a riverbed-elevation profile. Ground-water Hydrology The ground-water hydrology of the study area was characterized by reviewing existing well logs and establishing a monitoring well network consisting of 18 new monitoring wells and 21 existing wells. Information on lithology for the area was compiled from reviewing local well and boring logs and from data collected during subsurface drilling for the project. Soil borings were drilled with 10.8-cm, hollow-stem augers, and soil samples were collected continuously during drilling with a 5.1-cm-diameter, split-spoon sampler. Soil samples were analyzed for grain-size (ASTM 1972) and vertical hydraulic conductivity (ASTM 1990). Ground-water-monitoring wells were constructed of PVC materials including a 5.1 cm × 1.5 m well screen with 0.025-cm slots and 5.1-cm well casing. A sand filter pack was placed around the well screens, a bentonite seal was placed above the filter pack, and the remaining weU annulus was sealed with a cement-bentonite grout. The monitoring well network consisted o f wells located throughout the study area and completed in shallow silty-sand deposits 3 m below grade surface (bgs), lower sand and gravel deposits up to 20 m bgs, and in limestone up to 50 In bgs. On the ORW site, wells were positioned to measure the horizontal and vertical hydraulic gradients. Two sets of nested wells were constructed with each set containing three wells completed at 4 m bgs, 10 m bgs, and 20 m bgs to measure vertical gradients. Two sets of two nested piezometers were located within each wetland basin to determine vertical gradients between wetland surface water and ground water. These piezometers were constructed by driving 0.08-cm PVC tubing approximately 1.5 and 3.0 m below the bottom of the wetland sediments. The end of each PVC tube was sealed with flexible plastic screen to prevent sediments from entering the tube. Sand was poured around each piezometer screen, and the annular space above the screen of each piezometer was sealed with bentonite to prevent leakage along the piezometer annulus. The vertical gradient between the wetland surface water and ground water was measured using a monometer as described in Lee and Cherry (1978). Ground-water levels were measured manually beginning in June 1992, two years prior to active pumping o f water through the wetland system. Ground-water levels were measured continuously at 5 wells, using
W E T L A N D S , Volume 19, No. 1, 1999 pressure transducers and data loggers, beginning in June 1994. Aquifer Characterization Aquifer tests consisting o f falling head permeability (slug) tests and single and multiple well pumping tests were performed at the site. Falling-head slug tests were conducted on 21 shallow wells completed within the upper portion of the sand and gravel aquifer, and the data were analyzed to obtain hydraulic conductivity values using the Bouwer and Rice method (1989). An aquifer pumping test was conducted on a 15-cm-diaineter, partially-penetrating well, and water-level response data were collected from four monitoring wells completed in the sand and gravel confined aquifer. Because the sand and gravel aquifer underlying the surficial silty-clay materials shows leaky confining behavior, the pumping test data were analyzed using the Hantush (1960) method for leaky confined aquifers. Hydraulic conductivity values were obtained for the bedrock aquifer by analyzing data from well-performance tests conducted on seven deep bedrock wells using the method described by Bradbury and Rothschild (1985). Modeling A three-dimensional, finite-difference, ground-water model was developed to examine the discharge/recharge function of the constructed wetland basins. The ground-water-flow program M O D F L O W (McDonald and Harbaugh 1988) and the ground-water particletracking program MODPATH (Pollock 1991) were used for modeling scenarios based on collected field data. A five-layer, block-centered, finite-difference grid was oriented in a north-south and east-west direction consisting of 78 columns and 83 rows. The grid density is variable and increases to a higher resolution throughout the ORW site. No-flow boundaries were established along the northern and southern boundaries of the model to simulate the field-measured, groundwater-flow gradient. The eastern and western boundaries of the model were established as specified head boundaries using measured ground-water elevations in these areas. The Olentangy River was simulated as a head-dependent flux boundary using the measured riverbed elevation and the calculated riverbed conductivity. Because the wetlands function as a ground-water recharge source, the wetland basins were simulated as a specified flux from the wetlands into the upper layer of the model. Since ground-water elevations were above the top of the overlying confining units, all of the model layers were simulated as confined. The model convergence criterion was set at 0.003 m (An-
Koreny et aL, R E G I O N A L AND L O C A L H Y D R O L O G Y derson and Woessner 1992). Full details on the layer elevations and aquifer parameters incorporated in the model are found in Koreny (1996). The model was calibrated to ground-water elevation data collected on June 21, t995. These values were generally reflective of average ground-water conditions at the site. The program M O D F L O W P (Hill 1992) was used to aid in calibration. Seventy model runs were performed, and model parameters were adjusted with each successive model run to provide a better fit between the simulated and measured heads. At the end of each model run, calibration statistics were calculated to evaluate the model error. Calibration was performed until all simulated model heads matched the measured head targets within ~- 0.5 m for model layers 2 through 5; no calibration targets were available for layer 1. A sensitivity analyses determined that recharge, hydraulic conductivity, river stage and riverbed conductance were the most sensitive model parameters and constant-head values and wetland seepage values were the least sensitive model parameters. To delineate how ground water flows at the site and within the regional ground-water-flow system, hypothetical particles were placed upgradient o f the ORW on the western and eastern edge of the model boundary in upland areas and flow pathlines were tracked throughout the ground-water-flow system until discharged to the Olentangy River. The seepage of water out of the ORW wetlands into the ground-water-flow system was simulated by placing twelve particles at the top of the upper model layer within the area of the wetland basins. Pathlines of surface water seeping from the wetlands into ground water were tracked from the wetlands through the ground-water-flow system until discharged to the Olentangy River. RESULTS A N D DISCUSSION Wetland Hydrology The hydrologic budget of the ORW wetlands included surface inflow components (pumping and direct precipitation), and outflow components (surface-flow, evapotranspiration, and seepage). The surface inflow (up to approximately 62,730 m3/month) and surface outflow (up to approximately 61,980 m3/month) dominated the hydrologic budget. During this study, seepage loss was initially calculated to range up to 300 m3/ month based on estimates of the hydraulic conductivity of wetland substrate. Seepage estimates for the basins were later revised by Wang et al. (1997) using additional data and a more accurate seepage estimate method. The revised seepage estimates ranged from 3,274 to 6,498 m3/month, approximately 5% to 10%
189 of the total surface inflow. Seepage was a relatively minor portion of the overall hydrologic budget due to the low hydraulic conductivity of the silty clay wetland substrate. Two laboratory methods and one in sire field method were used to estimate the hydraulic conductivity of the wetland substrate. Average hydraulic conductivity estimates for the three methods employed are summarized in Table 1. The grain-size analyses methods used the smallest sample size, and mean hydraulic conductivity was 0.00034 m/day. The laboratory permeameter method used a larger sample size, and mean hydraulic conductivity was 0.016 m/day. In situ permeameter methods used the largest sample size, and mean hydraulic conductivity was 0.24 m/day. Table 1 shows that the hydraulic conductivity values were dependent upon the sample size used for each method, similar to the findings of Bradbury and Muldoon (1990). Olentangy River Based on stage measurements and gauging completed on the Olentangy River at the Clinton Park dam, stage ranged from 220.5 to 221.5 m amsl, and flow volume averaged 35 mVsec for the period from January 1994 to April 1995. The river stage drops approximately 2 m at two low-head dams adjacent to the O R W site: the Clinton Park dam to the north of the site and the Dodridge dam to the south. These dams control the river stage adjacent to the O R W and influence ground-water elevations across the site. The fluctuations in river stage observed at the Clinton Park dam are shown on Figure 3. Conceptual Hydrologic System The ORW was conceptualized as a riparian wetland system with local riparian hydrologic functions existing within the influences of the regional hydrologic system. Water is pumped into the wetlands from the adjacent Olentangy River and discharges through the outflow swale back to the river. A minor amount of water is lost to evapotranspiration and seepage. Figure 3 shows that surface-water elevation in the wetlands is generally maintained higher than the local potentiometric level in the underlying sand and gravel aquifer, and consequently, the wetlands function as a source of ground-water recharge via leakage across the silty clay immediately underlying the wetlands. As a result, surface water generally seeps from the wetlands into the ground-water-flow system. For short periods during river flood stage, when the hydraulic gradient is reversed, the wetlands function as a ground-water-discharge sink, and ground water flows into the wetlands
190 from the riparian area between the wetlands and the Olentangy River. The regional ground-water-flow system is primarily a through-flow system receiving low amounts o f recharge (precipitation) as leakage through the surficial silty-clay deposits. Ground-water-elevation data collected from piezometers were used to construct a regional potentiometric surface map of the sand and gravel aquifer, shown on Figure 4. Ground water flows from a topographic divide to the west of the ORW to the Olentangy River to the east through the sand and gravel and bedrock (limestone) aquifers. Ground-water contributions to the Olentangy River within the study area were estimated at approximately 0.1 m3/sec (less than 1% of river flow), based on modeling analyses (discussed later). Away from the O R W site, the regional ground-water-flow system is influenced by seasonal fluctuation causing seasonal increases and decreases in groundwater elevation. Near the Olentangy River and in the vicinity of the ORW site, the ground-water-flow system is heavily influenced by 1) a local ground-water mound caused by the seepage o f surface water to ground water from the wetland basins, and 2) the change in river-stage elevation at two low-head dams (Figure 5). Although the river is gaining (receiving ground-water discharge) throughout most o f the study area, the presence of the two low-head dams causes minor drainage reversals that change the Olentangy River from a gaining stream to a losing stream for a short stretch above each dam. The stage of the Clinton Park dam north o f the wetlands is approximately 2 m higher than the Dodridge Dam. These two dams affect the hydraulic gradient on the O R W site by creating a high ground-water elevation on the northern portion of the site and a low elevation on the southeastern portion. This is expressed in the local groundwater-flow system as a gradient o f approximately 0.004 between the two dams. The hydraulic gradient causes ground water to flow from the northwest near the wetlands to the southeast area below the Dodridge Dam. The observed gradient from the wetlands in a direction to the southeast was confirmed during modeling analyses. Aquifer Parameters Slug tests conducted in the upper portion of the sand and gravel aquifer indicated that hydraulic conductivity ranged from 0.2 to 4-. 1 m/day. The multi-well aquifer test conducted in the lower portion o f the sand and gravel indicated that hydraulic conductivity ranged from 24 to 97 m/day and aquifer storage ranged from 4.4 × 10 4 to 9.7 × 10 -4 (Table 2). The values of hydraulic conductivity for the underlying limestone
W E T L A N D S , Volume 19, No. 1, 1999 Table 2. Hydraulic parameters for sand and gravel aquifer underlying ORW site determined by pumping test.
Monitoring Well b
Storage (S) x 10 -4
Hydraulic Transmissivity Conductivity (T) (K) (m-'/day) (m/day)
MW-D 1 " " MW-D2 5.5 923 38 MW-D3 3.9 582 24 MW-C l 9.7 2,260 93 MW-C34 4.4 2,366 97 MW-E ~ ~ Data calculated using the Hantush (1960) leaky confined method = Monitoring well is located within 1.5 times the saturated aquifer thickness and was not analyzed. b = Monitoring well locations are shown on Figure 1. bedrock aquifers calculated from data collected from the well performance tests ranged from 1.9 to 11 m/ day. Modeling Analyses Ground-water-flow modeling and particle tracking were used to determine the direction and time of travel for water seeping from the wetland basins into the ground-water-flow system. The results o f the modeling analyses indicate that surface water enters the ground-water system through vertical leakage and flows southeast toward the Olentangy River along a local ground-water-flow path. The local ground-water-flow path is primarily controlled by the gradient caused by the difference in stage elevation at the Olentangy River dams above and below the wetland site (Figure 6). The flow o f ground water in the greater study area as predicted from particle tracking represents a regional flow system (Toth 1962, Freeze and Witherspoon 1966). Recharge to the regional ground-water-flow system is from uplands to the west of the wetlands, and ground-water flow from these upland areas is downward, following the regional hydraulic gradient into the Columbus Limestone bedrock aquifer (Figure 7). Near the Olentangy River, the regional hydraulic gradient is upward, and ground water discharges into the river. The relationship between the ground-water-flow system created by wetland seepage to ground water and the regional ground-water-flow system is illustrated in Figure 7. The wetland causes a ground-water mound and a downward vertical gradient near the wetlands, which changes to an upward vertical gradient closer to the Olentangy River. The seepage of surface water from the wetlands into ground water represents a local flow system. The water seeping from the wet-
Koreny et al., REGIONAL AND
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