EVALUATION OF EQUILIBRIUM PHOSPHORUS CONCENTRATION TECHNIQUES (EPC 0) USING
DIFFERENT BASE SOLUTIONS
Gina M. Lucci1,2, Richard W. McDowell1, Leo M. Condron2
1AgResearch,
2Agriculture
Invermay Agricultural Centre, Private Bag 50034 Mosgiel, New Zealand
and Life Sciences, Lincoln University, P.O. Box 84 Lincoln 7647, New Zealand
Corresponding author’s email: Gina.Lucci@agresearch.co.nz
Phosphorus (P) and nitrogen (N) are the two main limiting nutrients for algal growth in freshwater
ecosystems. Phosphorus entering waterways may originate from diffuse sources such as agriculture,
and point sources such as waste water treatment facilities. In-stream physical, chemical and biological
processes further affect P concentrations. Bed sediments are central to in-stream processes that
affect P concentrations in streamflow. Sediments are made up of local parent material as well as
deposited fine particulate material resulting from erosion. Equilibrium phosphorus concentration
(EPC0) measurements of stream water and sediments are used to identify the sediments as sinks or
sources of P. When dissolved (<0.45 µm) reactive P (DRP) in overlying water is greater than the EPC 0
of the sediment, the sediment will remove DRP from the water column, and the opposite is true when
DRP<EPC0. It is therefore important that the EPC0 is accurately determined so that in-stream fluxes of
DRP to and from sediment can be better understood. EPC 0 is calculated by incubating a known mass
of sediment with base solutions containing graduated concentrations of P (most often as KH2PO4) for
6-24 hours depending on method (Green et al., 1978; McCallister & Logan, 1978; Nair et al., 1984).
Base solutions contain salts like CaCl2 or KCl in varying concentrations to imitate water’s natural ionic
strength. Klotz (1988) demonstrated a wide range in EPC0 values measured for the same sediment
using base solutions with different Ca concentrations (0-400 mg Ca L-1). CaCl2 concentrations used in
the literature range from 0.0005M to 0.01M, which are said to be a reflection of the water’s natural
chemistry, although few authors supply actual measurements of the sample water’s chemistry.
To investigate the best method of establishing EPC0, six different base solutions (stream water, 0.01M
KCl, 0.01M CaCl2, 0.0005M CaCl2, distilled and tap water) were used to determine EPC 0 in sediments
taken during baseflow conditions in late summer from a number of streams draining different land
uses (native forest, urban and agricultural). If the main criterion for selection is to have an ionic
strength that mimics stream water then base solutions using the stream water is most appropriate.
However, stream water may not be best if ionic strength varies greatly, or there is potential for EPC 0 to
be influenced by microbes or suspended material in stream water. There may also be difficulty in
transporting sufficient amounts of stream water back to the lab.
Data showed that EPC0 measured using stream water ranged from 0.004 to 2.64 mg P L -1 which
illustrates the variety of sites sampled. Two sites where stream water EPC 0 values were > 0.5 mg P L-1
also had the lowest stream water concentrations of Ca (3-4 mg Ca L-1). The base solution which
produced similar EPC0 at both these sites was distilled water. For EPC0 values < 0.5 mg P L-1,
0.0005M CaCl2 showed the closest correlation with stream EPC0 values (y = 1.0504x + 0.0055; R² =
0.68) followed by tap water (y = 1.1072x + 0.0203; R² = 0.63).The tap water and dilute CaCl 2 had
similar Ca concentrations, 18 and 20 mg Ca L-1 respectively, and were close to the average stream
water Ca concentrations of 21 mg Ca L-1. Using a standardized 0.01MCaCl2 solution, as proposed by
Nair et al.(1984) and used by many others, resulted in EPC0 much lower than stream water EPC0.
This would have overestimated the sediment’s capacity to remove P from overlying water. Therefore it
should become standard practice to measure stream water Ca concentration before deciding on an
appropriate base solution from which to derive EPC0.
Keywords: Phosphorus, sediment, erosion, equilibrium P concentration
References:
Green, D. B., Logan, T. J. & Smeck, N. E. 1978. Phosphate Adsorption-Desorption Characteristics of
Suspended Sediments in the Maumee River Basin of Ohio. Journal of Environmental Quality,
7, 208-212.
Klotz, R. L. 1988. Sediment control of soluble reactive phosphorus in Hoxie Gorge Creek, New York.
Canadian Journal of Fisheries and Aquatic Science, 45, 2026-2034.
McCallister, D. L. & Logan, T. J. 1978. Phosphate Adsorption-Desorption Characteristics of Soils and
Bottom Sediments in the Maumee River Basin of Ohio. Journal of Environmental Quality, 7,
87-92.
Nair, P. S., Logan, T. J., Sharpley, A. N., Sommers, L. E., Tabatabai, M. A. & Yuan, T. L. 1984.
Interlaboratory Comparison of a Standardized Phosphorus Adsorption Procedure. Journal of
Environmental Quality, 13, 591-595.