This document discusses using stable isotopes to trace sources of phosphate in the River Taw catchment area. Phosphate concentrations in the river have been increasing, causing parts of the catchment to fail water quality standards for phosphorus. The document establishes baseline stable isotope signatures for different potential phosphate sources like soil, fertilizer, and manure. Isotope ratios were then measured for phosphate samples collected from sites across the catchment area. Preliminary results suggest the main sources of phosphate in the upper catchment are sewage treatment works, septic tanks, and possibly mineral fertilizers, with septic tanks being a diffuse source that could explain rising concentrations. More sampling is needed to better understand phosphate dynamics and identify other contributing sources

1. Stable Isotopes:
A tool for phosphate apportionment on the
River Taw?
Steve Granger
Taw River Improvement Project
Research funded by the WRT

2. Forms of Phosphorus
North Wyke
Sub-catchments of the Taw are failing
for phosphorus
Particulate P
(>0.45µm)
Soluble P
(<0.45µm):
Organic

3. Forms of Phosphorus
North Wyke
Sub-catchments of the Taw are failing
for phosphorus
Particulate P
(>0.45µm)
Soluble P
(<0.45µm):
Inorganic

4. Phosphate Concentrations at Taw Bridge
Year
90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13
Phosphateconcentration(gPl
-1
)
0
200
400
600
800
Phosphate Concentration Cycles
Environment Agency data

5. Isotope:
atoms of a given element that contain the same number of
protons in their nuclei but differ in the number of neutrons
Stable Radioactive
0.02%
99.98%
Trace
Tracing Phosphate with Stable Isotopes

6. Isotope:
atoms of a given element that contain the same number of
protons in their nuclei but differ in the number of neutrons
Tracing Phosphate with Stable Isotopes
Stable isotopes:
1. Different Mass
2. Same Chemical Reactivity
Measurement:
• Isotope Ratio Mass Spectrometer
• Ratio of the HEAVY to LIGHT
isotope (δ)
• Compare it to an international
standard
• Express the value a ‰

7. Kinetic fractionation: the extra neutron results in slower reactions
Fry. (2006)
Tracing Phosphate with Stable Isotopes

8. Using δ18O as a tracer for phosphate
There is only one stable isotope of P
Most P naturally occurs associated with O, and in its inorganic reactive
forms it is phosphate (PO4). Might the δ18O of the PO4 molecule might
be used?
When PO4 is cycled through biological reactions some O becomes detached and
replaced with water O.
If we know the temperature and the δ18OH2O the value of δ18OPO4 is predictable.
The P-O bond in PO4 is very stable at natural
temperature and pH
So is the δ18OPO4 is different to the predicted value it
may tell us something about the origin of the PO4

9. Sources of PO4
What are the values of δ18OPO4 in different potential PO4 sources?
Are they different and consistent?
(Young et al., 2009).
Establishing the δ18OPO4 of different PO4
sources

10. Tamburini et al
(2010)
• Soil
• Fertilizer
• Manure
Establishing the δ18OPO4 of different PO4
sources

11. Upper Taw Catchment
Source values and variability
Establishing the δ18OPO4 of different PO4
sources

12. Taw Marsh
1 µg P l-1
Sticklepath
4 µg P l-1
Ford Brook
11 µg P l-1
September 2013
N

13. Sticklepath
4 µg P l-1 Taw Green
68 µg P l-1
Wickington
8 µg P l-1
Newlands
66 µg P l-1
Cocktree
7 µg P l-1
deBathe
154 µg P l-1
September 2013
6450
N

14. Newlands
66 µg P l-1
Spires Lake
86 µg P l-1
North Tawton
1611 µg P l-1
Bondleigh
1659 µg P l-1
Taw Bridge
2286 µg P l-1
Ashridge
10 µg P l-1
Bondleigh
Brook
70 µg P l-1
Clapper
Brook
152 µg P l-1
September 2013
9832
Cheese factory: 8100
N

15. Taw Bridge
17.8‰ (16.5)
2286 µg P l-1
September 2013
16.9‰
9832
Cheese factory: 16.5‰ 8100
Taw Green
17.7‰ (16.2)
68 µg P l-1
19.6‰
6450

16. Spires Lake
17.7‰ (16.0)
86 µg P l-1
September 2013
Cocktree
18.8‰ (18.0)
7 µg P l-1
deBathe
17.1‰ (16.8)
154 µg P l-1
Taw Green
17.7‰ (16.2)
68 µg P l-1
Taw Bridge
17.8‰ (16.5)
2286 µg P l-1

17. Upper Taw Catchment
Source values and variability
Establishing the δ18OPO4 of different PO4
sources

18. Taw River Improvement Project
• Within this limited study it is suggested that the main sources of
PO4 in the Upper Taw are sewage treatment works, septic tanks
and possibly mineral fertilisers.
• the P dynamics in the river could be linked quite strongly to septic tanks and could
explain the complicated story of rising concentrations in the lower half of the sub-
catchment. They are a truly diffuse source and difficult to quantify
• These sources are likely to change over time (e.g. during storm
events) hence more work is required and more sources need to
be investigated.

19. Questions?
Taw River Improvement Project
TRIP Research Partnership

20. Taw Marsh
3 µg P l-1
Sticklepath
4 µg P l-1
Ford Brook
5 µg P l-1
March 2013
N

21. Sticklepath
4 µg P l-1 Taw Green
8 µg P l-1
Wickington
4 µg P l-1
Newlands
8 µg P l-1
Cocktree
16 µg P l-1
deBathe
36 µg P l-1
March 2013
479
N

22. Newlands
8 µg P l-1
Spires Lake
24 µg P l-1
North Tawton
33 µg P l-1
Bondleigh
32 µg P l-1
Taw Bridge
25 µg P l-1
Ashridge
9 µg P l-1
Bondleigh
Brook
24 µg P l-1
Clapper
Brook
21 µg P l-1
March 2013
3938
N