1. Groundwater Nutrient Flux Meter Application for Determination ofGroundwater Nutrient Flux Meter Application for Determination of
Phosphorus Flux: Application in Lake Okeechobee Basin, FLPhosphorus Flux: Application in Lake Okeechobee Basin, FL
M. Kelly Hamilton, Jaehyun Cho, James W. Jawitz, Michael D. Annable and Kirk Hatfield
Soil and Water Science Department, University of Florida, Gainesville, FL 32611 USA
IINTRODUCTIONNTRODUCTION
Water quality in Lake Okeechobee has steadily declined over the
past fifty years as a result of excess phosphorus (a limiting nutrient)
entering the lake. Phosphorus loading is a recurring problem in agricultural
systems as excess phosphorus concentrations drive P-limited systems to a
eutrophic state. Florida has numerous wetlands throughout its landscape
that impact the fate of nutrients. One of the methods proposed to control P
is through the use of isolated wetlands. It is estimated that approximately
16% of the Okeechobee Basin consists of wetlands with over half existing
as isolated wetland. As part of a larger study of phosphorus (P) movement
within cow-calf agricultural systems, nutrient flux meters (NFMs) were
deployed around several isolated wetlands to monitor and determine P flux.
The (NFM) is a cylindrical, permeable unit of variable length containing a
sorbent that fits tightly into a well casing and was designed to use a variety
of sorbents depending on application. A strongly basic anion exchange
resin was used to retain phosphate present in groundwater while a separate
sorbent was used to hold a tracer to determine water flux. Lab experiments
were conducted for the selection of an appropriate sorbent that was suited
for low concentrations of P and other parameters. The NFM was deployed
in several monitoring wells to determine the mass flux of P present in the
groundwater resulting from cow-calf operations at the Larson Ranch in
Okeechobee, Florida. Several NFMs were deployed in groundwater
monitoring wells surrounding the isolated wetlands and in the artificial
drain from the wetlands. Phosphorus fluxes will be determined by
extraction from the anion exchange resin. From the deployment of the
NFM for a pre-determined amount of time, P mass flux is expected to be
higher in the wells that were upgradient of the wetland and reduced at the
exit point. Groundwater flux is also expected to be toward the wetland
based on the observed gradients.
••SSITE DESCRIPTIONITE DESCRIPTION
• Located within the Lake Okeechobee watershed in Okeechobee County,
Florida
• Two drained, isolated wetlands located on Larson Ranch.
• Subtorpical Florida rainy season (May-October) and a dry season
(November-April) with approximately 110cm of precipitation and 122cm
evapotranspiration annually.
• Wetlands at this site are an expression of the surficial aquifer which
exists as ponded water overlying the clayey Hawthorn Formation.
• NFM installed at the perimeter of the wetland at the upland area to
intercept flowing groundwater to the lower laying wetland area.
Fig. 1. Okeechobee County, Florida. Red rectangle shows the location of
the Larson Ranch field sites. All geographic data were taken from the
Florida Geographic Data Library (GeoPlan). Stream, major road, and
wetland data provided by the USGS, Florida DOT, and U.S. Fish and
Wildlife service, respectively.
Fig2a) Aerial photograph of the
Larson Ranch (study wetlands
circled in black).
Fig2b) Larson Wetland #1 and
Wetland#2.
Fig2c) Wetland area
RRESULTSESULTS
FFUTURE INVESTIGATIONUTURE INVESTIGATION
• Mixed media
• Other nutrients/contaminants
• Tracers will be added to the second
sorbtive matrix to identify fluid flux.
These tracers will be leached from the
sorbent at rates proportional to the fluid
flux (cumulative or time-averaged
ground water flux).
• Collection of data from NFM currently
in field.
DDESIGNESIGN
What is a Nutrient Flux Meter (NFM)?What is a Nutrient Flux Meter (NFM)?
•Measure water and contaminant fluxes[M/L2T]
•Self contained permeable unit, variable length, able to accommodate
different types of mixed media
•Contaminant mass is intercepted, sorbed, and used to quantify cumulative
mass flux.
Threaded
bolt holding
unit together
Well
Casing
Crinoline
sock
Rubber
washers to
separate
sections
End nut
Sections
containing
resin
• Mc is the mass of contaminant sorbed
• L is the length of the sorbent matrix or the vertical thickness of aquifer interval
interrogated;
• Rdc is the retardation of contaminant on the sorbent,
• MRC is the relative mass of a hypothetical resident tracer retained after time period
t where that tracer has the same retardation is Rdc.
• r is the radius of the PFM cylinder
• θ is the water content in the PFM
• α characterizes the convergence or divergence of groundwater flow in the vicinity
of the PFM
• Ω represents the mass fraction of initial tracer remaining on the sortive matrix after
exposting the NFM to groundwater flow for period (t).
**For high values of Rdc, or alternatively, short installation duration, The flux can
be calculated using equation (3)
• Sorptive matrix selected based on ability to adsorb phosphorus.
- A benefit to using Lewatit was its ability for it to inhibit microbial
consumption of phosphorus
• Isotherm result confirms Lewatit anion exchange resin as good candidate
for sorptive matrix of NFM (Fig 4a).
• Hydrus 2D flow simulations run to simulate the movement of phosphorus
onto anion exchange resin (Fig 4b).
• This simulation (Fig 4b) along with previous equations assumes all mass
entering the well is retained by the PFM sorbent.
Why use a Nutrient Flux Meter (NFM)?Why use a Nutrient Flux Meter (NFM)?
• In situ measurements of nutrient flux generate critical data needed to
optimize the design and assess performance of proposed nutrient sources and
groundwater remedial systems.
• These fluxes, when integrated over a source area, produce estimates of
nutrient source strength and nutrient mass loads to groundwater and surface
water.
• Using nutrient fluxes measured down-gradient of on-going remediation
activities, it is feasible to verify the performance of existing technologies, assess
cumulative benefits, and estimate prevailing environmental risks.
590 0 590295 Meters
´
Fig 2a
Fig 2b
Fig 2c
Fig 3a
Fig 3b NFM
Flow field simulation or groundwater
convergence to well containing NFM
Fig 3c
Fig 4a
Fig 5
α
• P flux vs depth has been
established (Fig 5) as a result of
preliminary experiments within the
wetland area
References
Hatfield, A., Annable, M., Cho, J., Rao, P.S.C., Klammer, H. 2003. A Direct Passive Method for Measuring Water
and Contaminant Fluxes in Porous Media. Journal of Contaminant Hydrology, Vol. 75, No. 3-4, 2004, pp.
155-181.
In the simpliest case, assuming linear behavior,
time averaged, specific discharge through the
NFM can be estimated
t
Rr
q dR
D
θ)1(67.1 Ω−
=
Fig 4b: P sorption
t=0 hrs
t=4680 hrs
t=2200
hrs
Water
flow
Water Flux
Contaminant Flux
dcRC
c
c
RMLr
qM
J
θαπ )1(2
−
=
(2)
rLt
M
J c
c
πα2
=
(1)
(1)
Larson Wetland 1
Larson Wetland 2
flume
flume
11/26/04