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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 14 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.
Table 1. Drainage areas of Jackson Creek and tributaries
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DRAINAGE AREA, in sq mi
________________________ PERCENT
BASIN GAGED UNGAGED TOTAL UNGAGED
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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
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Total 14.66 1.94 16.60 11.7
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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 water-quality 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, 1014 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.
Table 2. Annual changes in pond volumes, 1993-95 (Annual change=
sediment accumulation)
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TOTAL SEDIMENT
POND VOLUME, in cubic yards ACCUMULATION
______________________________ ______________
1992 1993 1994 1995 cubic yards tons
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West Pond:
Amount 5,719 5,664 5,618 5,409
Change -55 -46 -209 310 90
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North Pond:
Amount 5,828 5,515 5,130 5,119
Change -313 -385 -11 709 347
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East Pond:
Amount 12,526 11,446 11,261 10,478
Change -1,080 -185 -783 2,048 1,024
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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, low-gradient channel, inhabited by abundant attached vegetation. Such an environment 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.
Table 3. Summary of annual inflow and outflow loads, Jackson Creek
Wetland, 1993-95 [incomplete data for water year 1993; suspended-
sediment loads in tons, nutrient loads in pounds; all rounded to 3
significant figures]
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LOADS CHANGE IN PERCENT
CONSTITUENT INFLOW OUTFLOW WETLAND CHANGE
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February-September, 1993
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Suspended Sediment (tons) 2,480 1,500 -980 -40
Total Phosphorus 11,100 10,100 -1,000 -9
Dissolved Phosphorus 6,080 5,500 -580 -10
Ammonia plus Organic Nitrogen 45,900 40,100 -5,800 -13
Dissolved Ammonia 7,550 8,920 1,370 18
Nitrite plus Nitrate Nitrogen 126,000 133,000 7,000 6
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Water Year 1994
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Suspended Sediment (tons)D 874 246 -628 -72
Total Phosphorus 5,370 3,260 -2,110 -39
Dissolved Phosphorus 1,990 1,750 -240 -12
Ammonia plus Organic Nitrogen 19,300 15,200 -4,100 -21
Dissolved Ammonia 2,660 3,390 730 27
Nitrite plus Nitrate Nitrogen 42,100 37,400 -4,700 -11
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Water Year 1995
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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 4,300 37
Dissolved Ammonia 744 1,060 316 42
Nitrite plus Nitrate Nitrogen 83,700 80,800 -2,900 -3
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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 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).
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.
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.
Layout and illustrations (original): Michelle Greenwood Layout (World Wide Web version): Dan Sullivan Banner graphic: Karen Lonsdorf
District Chief U.S. Geological Survey 8505 Research Way Middleton, WI 53562 (608) 828-9901