In-stream Phosphorus Injection |
A solution of K2HPO4 ( 2000 mg P L-1)
and KBr (50,000 mg Br L-1) was injected at a constant rate into
Lateral 22A (L22A) and S1 Coulee (S1). After 1.5 hr of injection, (previously
determined as the time required for Br concentrations to plateau) water
samples were collected at 1 upstream location and 4 to 5 down stream locations
every 10 to 20 min for 1.5 hr. Samples were immediately filtered, returned
to the lab, and analyzed for dissolved P and Br. Select samples were kept
and analyzed for pH, electrical conductivity (EC), and total suspended solids
(TSS). Automated samplers set up at a minimum of two down-stream locations
were used to confirm plateau concentrations. Bottom sediments were collected
for determination of equilibrium P concentration (EPCo). |
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Calculation of P Uptake Length |
Phosphorus uptake
length is a measure of the average distance an ortho-phosphate molecule
travels prior to biotic or abiotic uptake. Phosphorus uptake length was
calculated based on the longitudinal decrease in DP downstream from the
steady-state in-stream P injection. A simplified model linking P transformations
to down-stream transport during steady-state P injection can be written
as: |
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Equation
1 |
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where Cx is the P concentration
(mg L-1) at some distance x (m) down stream, Cb
is the background P concentration, Co is the P concentration
at x=0, kc is the uptake rate coefficient (s-1),
and u is the stream velocity (m s-1). Equation 1 can
be re-arranged to yield: |
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Equation 2 |
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Phosphorus uptake length (Sw)
can be calculated as: Sw=u/kc. In
accordance with Eq. 2, a plot of ln[(Cx–Cb)/(Co–Cb)]
will give a straight line with slope (a) equal to –kc/u,
therefore, Sw= –1/a. |
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