Sediment and Nutrient Trapping Efficiency of a Constructed Wetland

Near Delavan Lake, Wisconsin, 1993–1995

by John F. Elder and Gerald L. Goddard

U.S. Department of the Interior—U.S. Geological Survey Fact Sheet FS-232-96

Summary

Jackson Creek Wetland—a 95-acre

shallow prairie marsh containing three sediment retention ponds—was constructed in

1992 to reduce sediment and nutrient inflow to eutrophic Delavan Lake. The function of the wetland as a retention system for

suspended sediments and nutrients (total and

dissolved phosphorus, total ammonia plus organic

nitrogen, dissolved ammonia, and nitrite plus nitrate nitrogen) was studied from February 1993

through September 1995. Input and output load

computations were based on water flow (discharge)

measurements and periodic sampling of suspended

sediments and nutrients at the three inflowing

streams and at the wetland outflow. Results of the

study indicated consistent sediment retention

throughout the year; at times, as much as 80 percent

of the inflow load was retained in the wetland.

Nutrient retention was generally of lesser magnitude and much more variable. Although the annual

budgets confirm net retention for all nutrient forms

except ammonia, data analysis over shorter time

scales show that outflow loads actually can exceed

inflow loads during the late spring and summer

months—the period of greatest likelihood of algal

blooms in the lake. This result demonstrates that

the nutrient-trapping function of the wetland is

variable because of the complexity of the system.

Awareness of such variability can help to maintain

realistic expectations and effective management

practices.

Delavan Lake and Jackson Creek Wetland

Delavan Lake (fig. 1), a highly valued and heavily used sport fishery and

recreational lake in southeastern Wisconsin, has been plagued by waterquality degradation and algal growth during the past 15 years. A severe

blue-green algal bloom occurred in the summer of 1983 despite modifications two years earlier to divert sewage and septic tank effluent out of the

drainage basin of the 2,072-acre eutrophic lake.

The algal growth problems led to intensive, long-term hydrologic and

water-quality studies and a detailed rehabilitation plan (University of

Wisconsin, 1986). Among other recommendations, the plan called for

construction of a wetland at the confluence of Jackson Creek and its two

tributaries, which combine to form the principal inflow to Delavan Lake

(fig. 1). The purpose of this wetland was to reduce sediment and nutrient

inflow to the lake and thus contribute to its long-term protection and

maintenance.

Jackson Creek Wetland was constructed in fall 1992 when a 15-acre

wetland north of Mound Road was enlarged to 95 acres (fig. 2). Water from

the wetland flows into the Delavan Lake Inlet 2.0 miles upstream from

Delavan Lake (figs. 1 and 2A). Nearly all surface-water inflow to the

wetland is through three streams—Jackson Creek, Jackson Creek Tributary

(T1), and Jackson Creek Tributary No. 2 (T2). The combined drainage area

of 16.6 square miles is used principally for agriculture although there has

been some recent change, with increased residential and commercial

development in the Jackson Creek and T1 subbasins. Details of the geology,

soils, and land use are described in a previous report on Delavan Lake (Field

and Duerk, 1988).

In addition to areas of sedge meadow, wet prairie, and shallow-water

marsh, the wetland contains three retention ponds along its upstream edge

at the mouths of the inflowing streams (fig. 2). Jackson Creek, T1, and T2,

respectively, flow into the east, north, and west ponds. Surface areas of these

ponds are about 3.4, 1.2, and 1.2 acres, respectively. Each pond has 1–4

outlet swales which distribute runoff into the wetland.

This fact sheet describes the design, approach, and principal findings of

a study of Jackson Creek Wetland conducted by the U.S. Geological Survey

in cooperation with the Town of Delavan. The primary objective of the

study was to assess the effectiveness of the wetland as a retention system for

suspended sediment and nutrients.

Figure 1. Location of Delavan Lake, Jackson Creek Wetland, three inflowing streams (Jackson Creek

and its tributaries), and outflow of wetland to Delavan Lake Inlet.

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey 


1:100,000 digital data, modified by 


Wisconsin Department of Natural Resources,


Wisconsin Transverse Mercator projection

EXPLANATION

Drainage-basin divide


Subbasin-drainage divide


Streamflow-gaging station 

and automatic water-qualitysampling site


Partial-record station


Elkhorn

Delavan Lake Inlet

D e l a v a n

L a k e

Jackson

Creek

88°30'

42°37'

88°37'30"

42°38'

Jackson Creek


Tributary No. 2 (T2)

Jackson Creek


Tributary (T1) Jackson Creek


Wetland

WISCONSIN

Location of study area


aaaa

in Walworth County
east pond

Jackson Creek

north pond

west pond

Mound Road

Delavan Lake Inlet

Jackson Creek


Tributary (T1)

Jackson Creek


Tributary


No. 2 (T2)

Jackson Creek Wetland

Figure 2. A. Aerial view of Jackson Creek Wetland, showing inflowing and

outflowing streams, and sediment retention ponds (photograph supplied by

Rust Environment and Infrastructure, Inc., Milwaukee, Wis.). B. Partial view

of Jackson Creek Wetland, looking northeast.

Jackson Creek 8.96 1.02 9.98 10.2

Jackson Creek Tributary (T1) 4.34 .22 4.56 4.8

Jackson Creek Tributary No. 2 (T2) 1.36 .70 2.06 34.0

 Total 14.66 1.94 16.60 11.7

 BASIN GAGED UNGAGED TOTAL UNGAGED

DRAINAGE AREA, in square miles PERCENT

Table 1. Drainage areas of Jackson Creek and tributaries Methods of Data Collection, Sampling,

and Analysis

Water flow (discharge) was measured continuously at gaging stations

located upstream from the Jackson Creek Wetland at the Jackson Creek and

T1 sites, and at the wetland outflow weir at Mound Road. Inflow through T2

was also measured periodically. The inflow measurements accounted for 88

percent of the drainage area of the wetland (table 1). The remaining 12 percent

of the area—the intervening areas between the gaging stations and the

wetland inflow points—could not be monitored because of backwater effects

from the ponds. Sediment and nutrient contributions from the ungaged area

within each subbasin were estimated by calculating the load/drainage-area

relation for the gaged area and applying it to the ungaged area.

Samples for the determination of suspended-sediment and nutrient concentrations were collected at least monthly, using the equal-width-increment

(EWI) method described by Guy and Norman (1970). During storms, frequent samples were collected by means of refrigerated automatic waterquality samplers. The water samples were analyzed for concentrations of

suspended sediment, total phosphorus, dissolved orthophosphate, total ammonia plus organic nitrogen (Kjeldahl nitrogen), dissolved ammonia, and

dissolved nitrite plus nitrate nitrogen. Standard colorimetric techniques on an

autoanalyzer (Fishman and Friedman, 1989) were used for analysis of these

constituents.

Suspended-sediment and nutrient loads were computed by use of the

integration method (Porterfield, 1972) at sites for which continuous discharge

data were available. For the T2 site, loads were computed by use of a method

described by Field and Graczyk (1990). The input load to the wetland was

calculated as the sum of the load measured at the gaging station plus the

estimated load from the ungaged area. The time period for load and budget

calculations was the water year—the 12-month period from October 1 to

September 30 (the water year is designated by the calendar year in which it

ends).

The volume of sediment accumulation in the retention ponds was considered to be equivalent to the annual change (decrease) in pond volume. On each

pond, 10–14 parallel cross-sections were established and served as reference

points for four surveys. All the ponds were surveyed initially between

October 1992 and May 1993, and subsequently in October 1993, 1994, and

1995. Cross-section areas at an elevation of 929.00 feet were computed using

the Channel Geometry Analysis Program (Regan and Schaffranek, 1985). By

averaging the areas of adjacent cross-sections, multiplying this average area

by the distance between the cross sections, and summing these section

volumes, the total pond volume was computed.

Six bottom-sediment cores collected with an acrylic gravity corer (3-inch

inside diameter) were used to determine bulk density of the deposited

sediment. The mass of sediment retained in the pond was then calculated as

the product of bulk density and volume of sediment accumulation.

Sediment outflow from each pond was calculated as the difference

between inflow and accumulation in the pond. The combined outflow from

the three ponds was compared to the measured wetland outflow to determine

the amount of deposition within the wetland downstream from the ponds.

Nutrient and Sediment Retention

in the Wetland

Changes in Retention Ponds

Sediment accumulated in the ponds each year. Total accumulations ranged

from 310 to 2048 yd3

 (cubic yards) during the study period (table 2). In the

north and west ponds, most sediment was deposited at their downstream ends.

In the east pond, however, 58 percent of the deposition was at the upstream

end; this was probably due to the larger size and the non-circular shape of the

pond, in addition to redirection of much of the flow through the three outlet

swales near the middle of the pond.

Mean bulk densities of the deposited sediment in the north and east ponds

were very similar— 0.49 and 0.50 ton/yd3

 (0.58–0.60 g/cm3). By contrast, the

mean sediment bulk density in the west pond was only 0.29 ton/yd3 (0.35 g/

cm3

). The feeder stream for the west pond, T2, is predominantly a wide, lowgradient channel, inhabited by abundant attached vegetation. Such an enviA

B
ronment favors settling of dense sediment particles in the channel before they

can reach the pond.

A suspended-sediment budget for Jackson Creek Wetland in water years

1994–95 is summarized in fig. 3. The west, north, and east ponds retained 34,

47, and 74 percent of sediment inflow, respectively. The combined retention

of 752 tons (58 percent of inflow) leaves an estimated total sediment outflow

from the ponds of 536 tons. Nearly all this amount (532 tons) was measured

in the wetland outflow at Mound Road. The residual 4 tons of sediment was

presumably trapped in the wetland between the ponds and Mound Road.

releases during low-flow periods. In fact, virtually all retention

in 1994 occurred during a single month: February. Nearly all

other months were actually periods of net phosphorus release,

reflected by the large positive changes during the third and

fourth quarters. Phosphorus mobilization during spring and

summer might be due to increased solubility in anaerobic

conditions (Mitsch and Gosselink, 1993, p. 141) which could, in

turn, be caused by higher rates of microbial respiration in

warmer temperatures.

Are these seasonal variations only a curiosity or could they

have important implications for downstream aquatic ecosystems? Although no direct evidence of their significance is

available, the fact that phosphorus releases tended to occur in

late spring and summer makes them coincident with the likely

timing of algal blooms downstream in Delavan Lake Inlet and

Delavan Lake. Further indication of a potential problem is the

fact that the proportional release of dissolved orthophosphate—

the fraction that is available for algal uptake—was even greater

than that of total phosphorus.

The lack of coupling between suspended sediment and

nutrient transport dynamics is also notable. The suspended

material presumably is retained because of the reduced velocity of water flow,

which allows the material to settle. One might expect that the nutrient loads

would be largely associated with oxidized suspended sediments and, therefore, would be entrained together with the sediments. However, this was

clearly not the case; net release of nutrients often coincided with substantial

retention of sediments. This result indicates that biogeochemical processes

mobilized the sediment-associated nutrients and thus prevented their effective retention in the wetland, at least periodically.

Climate is an additional factor that could have important effects on

nutrient load characteristics. Precipitation at Lake Geneva National Weather

Service Station, 7 miles southeast of Jackson Creek Wetland, was 3.59 inches

above normal during the 1993 study period (February–September). It was

12.19 inches below normal in water year 1994 and 4.74 inches below normal

in water year 1995. The high water flows in 1993 produced heavy nutrient

loading into and out of the wetland, which is evident in the results given in

table 3 and fig. 4. The abnormally high flow volumes and velocities that year

may also have reduced the nutrient retention effectiveness of the wetland, as

indicated by the relatively low net retention numbers in 1993 (table 3).

Figure 3. Suspended-sediment budget for Jackson Creek Wetland, water years 1994–95.

Table 3. Summary of annual inflow and outflow loads, Jackson Creek

Wetland, 1993–95 [Incomplete data for water year 1993; suspendedsediment loads in tons, nutrient loads in pounds; all rounded to 3 significant

figures]

 February–September, 1993

Suspended Sediment (tons) 2,480 1,500 -980 -40

Total Phosphorus 11,100 10,100 -1000 -9

Dissolved Phosphorus 6,080 5,500 -580 -10

Ammonia plus Organic Nitrogen 45,900 40,100 -5800 -13

Dissolved Ammonia 7,550 8,920 1370 18

Nitrite plus Nitrate Nitrogen 126,000 133,000 7000 6

 Water Year 1994

Suspended Sediment (tons) 874 246 -628 -72

Total Phosphorus 5,370 3,260 -2110 -39

Dissolved Phosphorus 1,990 1,750 -240 -12

Ammonia plus Organic Nitrogen 19,300 15,200 -4100 -21

Dissolved Ammonia 2,660 3,390 730 27

Nitrite plus Nitrate Nitrogen 42,100 37,400 -4700 -11

 Water Year 1995

Suspended Sediment (tons) 413 286 -127 -31

Total Phosphorus 2,700 2,370 -330 -12

Dissolved Phosphorus 1,060 925 -135 -13

Ammonia plus Organic Nitrogen 11,500 15,800 4300 37

Dissolved Ammonia 744 1,060 316 42

Nitrite plus Nitrate Nitrogen 83,700 80,800 -2900 -3

CONSTITUENT INFLOW OUTFLOW

CHANGE IN

WETLAND

PERCENT

CHANGE

LOADS

Table 2. Annual changes in pond volumes, 1993–95 (Annual change =

sediment accumulation)

Sediment accumulation, yd3

Mean bulk density, ton/yd3

Sediment trapped in pond, tons


Pond sediment ouflow, tons


Total pond sediment outflow, tons


Additional wetland deposition, tons


Sediment ouflow at Mound Road, tons


223


536


4


532


145
 168


Total pond sediment inflow, tons 219 417 652

Stream

Pond

Jackson Creek


Tributary No. 2

Jackson Creek


Tributary Jackson Creek

254


x 0.29


= 74


West Pond

396


x 0.49


= 194


North Pond

968


x 0.50


= 484


East Pond

West Pond:

Amount 5,719 5,664 5,618 5,409

Change -55 -46 -209 310 90

North Pond:

Amount 5,828 5,515 5,130 5,119

Change -313 -385 -11 709 347

East Pond:

Amount 12,526 11,446 11,261 10,478

Change -1,080 -185 -783 2,048 1024

 1992 1993 1994 1995 cubic yards tons

POND VOLUME, in cubic yards

TOTAL SEDIMENT

ACCUMULATION

Nutrient and Sediment Trapping Function

Comparisons of inputs and outputs of suspended sediments and nutrients

(table 3 and fig. 4) show the variable effectiveness of the Jackson Creek

Wetland as a filter for these substances. As previously indicated, suspended

sediments were consistently retained in the wetland. The retention was

greater than 20 percent at all times except the winter/early spring period, and

during the growing season, retention was frequently greater than 80 percent.

For nutrients, however, the trapping effectiveness of the wetland was much

less consistent. With the exception of ammonia, total nutrient loads generally

decreased substantially between the inflows and the outflow on an annual

time scale (table 3). The inflow:outflow ratio, however, was highly variable

seasonally (fig. 4) and was frequently less than 1, an indication of net release

instead of retention. The annual budget by itself thus provides incomplete

information about the dynamics of nutrient transport through the system.

An example of the limitations of the annual-scale nutrient budget is

evident in the 1994 phosphorus data (fig. 4). The season of greatest transport

during that year was, by a large margin, the winter/early spring period. That

period was also the time of greatest fractional retention of total and dissolved

phosphorus. This combination of high transport and uptake accounted for

nearly all of the annual net retention; it overshadowed later small-volume
For more information, please contact:

District Chief

U.S. Geological Survey

6417 Normandy Lane

Madison, WI 53719

(608) 274-3535

www: http://wwwdwimdn.er.usgs.gov

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS-232-96

Layout and illustrations: Michelle Greenwood

Banner graphic: Karen Lonsdorf

Implications of Study Results for

Wetland Management

The Jackson Creek Wetland retained suspended sediment effectively and

consistently, trapping 46 percent of the input load during the study period.

However, the wetland apparently is not continuously effective as a site for

nutrient retention, even in its current status as a relatively young and highly

productive ecosystem. This finding, although illustrating some limitation of

the wetland as a nutrient sink, does not necessarily conflict with the general

concept of nutrient retention in the wetland. As observed in other wetlands

(Elder, 1988; Richardson, 1988), the variability reflects the capacity of the

system to function not only as a sink but also, periodically, as a facilitator of

nutrient transformation and transport. The results of this study thus illustrate

the complexities of biogeochemical cycles in any ecosystem, wetlands

included. Understanding these complexities may help us to avoid placing

unrealistic expectations on natural or constructed wetlands as systems with

unlimited filtering capacity.

Field, S.J., and Graczyk, D.J., 1990, Hydrology, aquatic macrophytes, and water

quality of Black Earth Creek and its tributaries, Dane County, Wisconsin, 1985-

86: U.S. Geological Survey Water-Resources Investigations Report 89-4089,

38 p.

Fishman, M.J., and Friedman, L.C., eds., 1989. Methods for determination of

inorganic substances in water and fluvial sediments: U. S. Geological Survey,

Techniques of Water-Resources Investigations, book 5, chap. A1, 545 p.

Guy, H.P., and Norman, V.W., 1970, Field methods for measurement of fluvial

sediment: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. C2, 59 p.

Mitsch, W.J., and Gosselink, J.G., 1993, Wetlands: New York, Van Nostrand

Reinhold, (2nd Ed.), 722 p.

Porterfield, George, 1972, Computation of fluvial sediment discharge: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap. C3,

66 p.

Regan, R.S., and Schaffranek, R., 1985, A computer program for analyzing channel

geometry: U.S. Geological Survey Water-Resources Investigations Report 85-

4335, 49 p.

Richardson, C.J., 1988, Freshwater wetlands: transformers, filters, or sinks?: Forem

(Duke University), v. 11, p. 3–9.

University of Wisconsin, Institute for Environmental Studies, 1986, Delavan Lake:

a recovery and management study: 332 p.

Figure 4. Average daily loads of suspended sediment and nutrients at inflow (total of all stream inflows) and outflow of Jackson Creek Wetland, and percent

change in wetland. Negative percent change indicates retention; positive percent change indicates release. Data shown by quarter, water years 1993–1995;

quarter 1 = October–December, quarter 2 = January–April, quarter 3 = May–June, and quarter 4 = July–September. Second quarter of 1993 = February–April

(no data for January).

References

Elder, J.F., 1988, Factors affecting wetland retention of nutrients, metals, and organic

materials. in: Kusler, J.A., and Brooks, G., eds. Wetland Hydrology, Chicago, IL,

Association of State Wetland Managers, Inc., p. 178–184.

Field, S.J., and Duerk, M.D., 1988, Hydrology and water quality of Delavan Lake in

southeastern Wisconsin: U.S. Geological Survey Water-Resources Investigations

Report 87-4168, 61 p.

SUSPENDED SEDIMENT

TOTAL PHOSPHORUS

DISSOLVED ORTHOPHOSPHATE

INFLOW

OUTFLOW

20

16

12

8

4

0

100

80

60

40

20

0

60

40

20

0

QUARTER: 2

WATER YEAR: 1993 1994 1995

3 41 23 4 23 4 1

200

100

0

-100

200

100

0

-100

200

100

0

-100

pounds pounds tons

AVERAGE DAILY LOAD

PERCENT CHANGE IN WETLAND

QUARTER: 2

WATER YEAR: 1993 1994 1995

3 41 23 4 23 4 1

400

300

200

100

0

100

80

60

40

20

0

1000

800

600

400

200

0

AVERAGE DAILY LOAD, pounds

200

100

0

-100

200

100

0

-100

200

100

0

-100

PERCENT CHANGE IN WETLAND

TOTAL AMMONIA + ORGANIC NITROGEN

DISSOLVED AMMONIA

DISSOLVED NITRITE + NITRATE

INFLOW

OUTFLOW