Methods• Measured the inlet and outlet sample locations for background lithium and phosphorus concentrations• Added 500 g /5L of LiCl salt tracer solution at the inlet sample location on June 26, 2012• Programed the ISCO auto-sampler at the outlet sample location to pump 500mL of available marsh water

into 24 1L plastic bottles at 6 hour intervals beginning on June 26, 2012 at 1:35 PM• Measured the lithium concentration using a Flame Atomic absorption spectrometer (FLAA) [3]

• Measured the orthophosphate concentrations using the Hach 8048 method on the Hach DR 5000TM UV- Vis spectrophotometer [4]

• Measured the water level elevation (H) with the use of two level loggers that were placed in stilling wells at the inlet and outlet sample sites

• A third shuttle was placed at the outlet to measure the barometric pressure• Calculated flow in cubic meters per hour using two empirical formulas

• Inlet 90 V-notch weir⁰• =m3/hr

• Outlet 0.5m rectangular weir without end contractions• = m3/hr

Results

4/1/12 4/11/12 4/21/12 5/1/12 5/11/12 5/21/12 5/31/12 6/10/12 6/20/12 6/30/12 7/10/120

20

40

60

80

100

120

140

160PWS Hydrograph for Prairie Wolf Slough over Time

Flow through Inlet

Flow through Out-let

Date and Time

Flow

(cub

ic m

eter

s per

hou

r)

Storage Evapotranspiration

Graph1 shows the flow through the inlet and outlet from the beginning of April to the end of June. Prairie Wolf Slough experienced a storage stage from the end of April to the middle of May. Evapotranspiration was experienced from the middle of May to the middle of June as the flow was greater in the inlet than the outlet.

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150-2.77555756156289E-17

0.00499999999999998

0.00999999999999998

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

Li Concentration in PWS Outlet over Time

July 2012

November 2011

Time (Hrs since Input)

Lith

ium

Con

cent

ratio

n (m

g/L)

Graph 4 shows the total phosphorus concentration as well as the orthophosphate (SRP) concentrations collected from the outlet on June 26, 2012 through July 2, 2012.

0

1

2

3

4

5

6

7SRP Concentration during Different Flow Conditions

Flow Conditions

SRP

Con

cent

ratio

n (m

g/L)

No Flow – July 2012 Flow – November 2011

Graph 2 shows the orthophosphate levels at the outlet during different flow conditions.

6/26/2012 6/27/2012 6/28/2012 6/29/2012 6/30/2012 7/1/2012 7/2/2012 7/3/20120

5

10

15

20

25

30

35

Phosphorus Concentration in PWS Outlet over Time

TP Analysis

SRP Analysis

Time (Days)

Phos

phor

ous C

once

ntra

tion

(mg/

L)

Graph 3 shows the lithium concentration over time to determine the retention time. One lithium study was conducted in November 2011 where there was flow; thus, the retention time is determined by the peak. The second study was conducted in July 2012 where there was no flow. As a result, no peak was experienced.

Site DescriptionPrairie Wolf Slough Demonstration Project (PWS) is a

constructed wetland located in unincorporated Lake County, Illinois (T43N, R12E, Sec 17). It is owned and managed by

the Lake County Forest Preserve District. The wetland drains 98 ha (242 ac) into the Middle Fork of the North-Branch

Chicago River. The site is approximately 14.1 ha (35 ac) in total area and was drained for farming in the early 1900s. In 1994, 10.1 ha (25 ac) of the site was restored from farmland

to wetland and the remaining 4 ha (10 ac) was left as a woodland. The wetland was hand-planted with 61,000 plugs

(≈ 1 plug/m2) and approximately 3 ha was open water.[2]

Aerial photograph of PWSPrairie Wolf Slough at the Middle Fork

Overarching ProblemPWS serves as a point source of phosphorus to the Middle

Fork-North Branch Chicago River. More phosphorus is leaving the wetland than is entering it.

Objective Our objective was to measure water retention time in the

marsh as well as phosphorus concentration as a function of varying rainfall intensity. Lithium chloride was used as a

tracer to measure water flow and, hence, retention time, from the inlet to the outlet since lithium does not readily absorb to

clay particles and is not easily absorbed by plants.

IntroductionPrairie Wolf Slough is a restored wetland, and has been

studied since 1998. An excessive concentration of phosphorus causes eutrophication, resulting in dead zones. Dead zones are an area of water that has low oxygen levels and are not able to support living organisms.[1] Lithium is often used in

flow tracer studies in wetlands to assess retention time, which is determined by the peak of highest concentration of lithium

concentration [mg/L] as it flows through the exit of the marsh.

Abstract In this study we measured the hyrdrologic retention time in Prairie

Wolf Slough wetland using lithium chloride (LiCl) tracer. LiCl was selected because Li is a conservative element that it is not readily absorbed by plants. Retention time is a measure of the

amount of time that water flowing into the wetland before exiting the wetland. The longer the retention time, the more likely that

sediment in the water settles out and that plants remove nutrients such as nitrogen and phosphorous. In this study the results were inconclusive due to the lack of rain that was experienced during June and July 2012. Retention time is an important property to

consider when developing wetland restoration plans.

Discussion Prairie Wolf Slough experienced piston flow during the rain event on April 15, 2012. The piston flow carried water and phosphorus through the outlet to the Chicago River. Prairie Wolf Slough experienced storage as the flow through the inlet was greater than the flow through the outlet from April 20, 2012 to May 15, 2012. The lack of rain from May 16, 2012 through June 20, 2012 may have contributed to high rates of evapotranspiration. The marsh is located in an open area which allowed the little water that was in the marsh to evaporate into the atmosphere. The plants, such as cattails, located in the marsh absorbed water and transpired water vapor, resulting in high humidity. The retention time of the marsh was detected by the lithium peak at about 16 hours November 2011 when there was flow through the outlet (Graph 2). The data collected for July 2012 was a period where rain events were rare. With the lack of rain and water flow the lithium was not able to travel quickly through the marsh. Thus, there was no sharp peak, and our data is inconclusive for the retention time in July 2012. The total phosphorus (TP) concentration is greater than the orthophosphate concentration (SRP) at the PWS outlet location over time (Graph 3). This is due to the fact that SRP analysis only tests for inorganic phosphorus, and the TP analysis tests for inorganic and organic phosphorus. We compared SRP concentrations in samples collected by the ISCO sampler at the outlet during rain events in November 2011 and July 2012. There was a significantly higher concentration of orthophosphate [6.31mg/L] when there was no flow through the outlet in July 2012 in comparison to the outlet samples from November 2011 [0.20 mg/L] when there was flow at the outlet. The higher concentration of orthophosphate in July 2012 was due to the fact that there was no flow and high evaporation, thus preferentially concentrating both SRP and TP.

References[1] Mitsch, William; Gosselink, James G. Wetlands: The Phosphorus Cycle. John Wiley & Sons: Ohio, 2007.[2] Montgomery, James; Eames, J. Marshall. Prairie Wolf Slough Wetlands Demonstration Project: A Case Study Illustrating the Need for Incorporating Soil and Water Quality Assessment in Wetland Restoration Planning, Design and Monitoring. Restoration Ecology 2008, 16.4, 618-628.[3] Kinard, W. Frank. Chemistry 221 Laboratory: Quantitative Analysis, Atomic Emission Spectroscopic Determination of Lithium. http://kinardf.people.cofc.edu/221LabCHEM/ CHEM221L%20Atomic%20Emission%20Determination%20of%20Lithium.htm (accessed July 16, 2012).[4] Phosphorus, Reactive (Orthophosphate)-PhosVer 3 (Ascorbic Acid) Method 8048, Spectrophotometer Powder Pillows or AccuVac® Ampuls . Hach Company. 2012.

Figure 1 is the marsh at PWS in June Figure 2 is the marsh at PWS in July

The Great Phosphorus Mystery at Prairie Wolf Slough Wetland:Water Retention Time and Phosphorus Concentration

Darcy Velazquez and Matthew Ross