New Zealand Journal of Agricultural Research abstracts
Assessment of a technique to remove phosphorus from streamflow
R. W. McDowell
AgResearch Ltd
Invermay Agricultural Centre
Private Bag 50034
Mosgiel 9053
M. Hawke
98b State Highway 33
Rotorua 3074
J. J. McIntosh
Environment Bay of Plenty
PO Box 364
Whakatane 3158
Abstract Decreasing the load of phosphorus (P) in streamflow
and entering P-limited lakes is central to preventing eutrophication.
We trialled a technique to remove P from streamflow of an agricultural
catchment by sorbing P onto a heavy weight steel slag mixture encased
in a mesh: termed a P-sock. This technology allows entrained P to be
removed on demand, and prevents P-rich material from washing downstream
in storm events. One-hundred-and-ninety P-socks were installed in a 200
m reach of streambed in a herringbone fashion, which allowed both
fish passage and mixing with stream water. In water samples and
suspended sediment, pH and P fractions, including dissolved reactive P
(DRP) and total P (TP) were determined. P-socks and bed sediment were
also subsampled, which indicated that after 6 months, only 20% capacity
remained (P sorption maximum 4.5 g kg–1). Overall,
concentrations of DRP and TP decreased on average 35 and 21%,
respectively after the P-socks were installed, while loads decreased 44
and 10%, respectively. While this was an effective removal strategy at
low flows, relatively little P was retained at flow rates >20 litres
s–1. In addition, the technology is expensive compared to
alum dosing. Hence, this approach to P retention is limited to small,
slow flowing waterways and may be more useful in stopping P loss from
sources such as runoff from lane ways where dung is regularly
deposited.
Keywords by-products; retention; slag;
sorption; uptake
A07036; Online publication date 2 November 2007; Received 17 May
2007; accepted 23 September 2007
New Zealand Journal of Agricultural Research, 2007, Vol. 50:
503–510
0028–8233/07/5004–0503 © The Royal Society of New Zealand 2007
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