An Improved Understanding of
Phosphorus Origin, Fate and Transport within Groundwater and the
Significance for Associated Receptors
Project funders/partners: Scotland & Northern Ireland Forum for
Environmental Research (SNIFFER), Scottish Environmental Protection
Agency (SEPA), Environment Agency, Environment and Heritage Service,
Environmental Protection Agency (EPA).
Background to the research
The United Kingdom Technical Advisory Group Groundwater
Task Team have identified limitations in knowledge relating to
understanding the origin (natural and anthropogenic), fate and
transport of phosphorus within the sub-surface and in groundwater, with
particular regard to the potential impact on dependent surface waters
and terrestrial ecosystems.
Objectives of the research
The purpose of this study was to develop an improved understanding of
the origin (natural and anthropogenic), fate and transport of
phosphorus as it moves through the soil, unsaturated zone, saturated
zone and hyporheic zone to discharge in rivers, lakes or wetlands. The
report aims to answer a number of key questions:
Key findings and
- Are observed phosphorus concentrations in groundwater a
cause for concern in the context of achieving the status objectives for
receiving surface waters and groundwater dependent terrestrial
- Is there evidence that observed phosphorus concentrations
in groundwater are elevated above concentrations which would be
expected to occur naturally?
- Where groundwater P concentrations are elevated, is there
sufficient certainty in our understanding to justify restoration
- What further research or monitoring are likely to be
appropriate to develop the necessary understanding for delivery of WFD
Phosphorus, Groundwater, Risk, Receptors, Thresholds, Baseline
- There are over 49,000 measurements of groundwater P
concentrations available to this project in England and Wales,
Scotland, Northern Ireland and Republic of Ireland, of which about
25,000 are above the Limit of Detection (LOD). They are
generally measured as orthophosphate, as P (dissolved inorganic P), or
Total dissolved phosphorus (dissolved organic and inorganic P).
- Significant areas of the countries have no groundwater P
data- 14% , 37% and 49% by area of groundwater bodies have no
groundwater P observations in England & Wales, Republic of
Irelaand and Scotland, respectively.
- Of the 49,000 measurements of groundwater P, only 76
samples exceed the Drinking Water Standard (2200 μg P /l), of
which all but 5 samples are in England & Wales.
- In all countries, a significant proportion of samples have
concentrations above important ecologically-based P thresholds for
river and lakes under the WFD. For example, more than 24 % of
samples in each of the countries’ datasets (assuming any
samples below the Limit of Detection are at half the LOD) have
Orthophosphate concentrations (as P) in excess of 30 μg P /l
(Chapters 1 and 3). Expressed as a percentage of the area of
groundwater bodies with groundwater P data, 15%, 28% and 9% have median
groundwater P observations in excess of 30 μg P /l in England
& Wales, Republic of Irelaand and Scotland,
respectively. Only 5%, 0.2% and 1 % of the area of
groundwater bodies with groundwater P data have median groundwater P
observations in excess of 60 μg P /l in England & Wales,
Republic of Ireland and Scotland, respectively.
- Literature review (Chapter 2) suggests that anthropogenic
sources which may pose P leaching risks to groundwater include
accumulation of soil P levels as a consequence of long-term
over-fertilization of arable and grassland; manure heaps &
unlined slurry storage facilities; leaking mains water pipes and sewers
and Septic tank discharges.
- Many factors can influence the likelihood of anthropogenic
impacts on groundwater P concentrations to the extent that they result
in ecological impairments in associated receptor systems.
These factors have been synthesised in a generic conceptual
- The sensitivity of the Receptor to groundwater P (Chapter
2) will depend on many factors, especially trophic status, flow regime,
residence time and synchronicity between groundwater P delivery and
biological demand. Season-specific features associated with
many impact responses means that aspects other than just the total P
load should be taken into consideration. The timing of P
transfers in relation to biological demands is an important factor
determining the significance of different sources.
- A comparison of routine monitoring data and BASELINE data
from England and Wales (Chapter 3 and 5) demonstrated the difficulty of
defining natural background concentrations from an analysis of
monitoring data alone.
- Significant correlations between measured groundwater P
concentrations and land use (Chapter 3) and between measured data and
predicted P concentrations arriving at the water table, based on soil P
and groundwater vulnerability (Chapter 4) provide some evidence that
anthropogenic activities may have had an impact on groundwater in
England and Wales. The lack of a significant relationship in
Scotland (Chapter 6) and generally lower concentrations (Chapter 3)
suggests that groundwater concentrations in Scotland are more likely to
reflect natural background levels than diffuse-source transfers
resulting from agricultural activity. The high proportion of
samples below high LODs in Northern Ireland hinders useful
interpretation. For the Republic of Ireland, undertaking a correlation
was not possible due to the unavailability of national datasets such as
the diffuse pollution screening tool or similar.
- On the basis of the evidence presented in this report it is
considered that groundwater P may be a problem for a limited number of
surface water and GWDTE receptors. However, considerable
uncertainties exist over, in particular: defining background levels;
groundwater P concentrations in the many unmonitored groundwater
bodies; the effect of the hyporheic zone in attenuating or mobilizing
P; and the sensitivity of receptors to P delivery from groundwater.
- Significant uncertainty remains in defining natural
background concentrations of phosphorus in groundwater, given the
limitations of the available data on groundwater quality, aquifer
properties (especially mineralogy) and P sources.
- Recommendations for monitoring, data collation and
remediation are made (Chapter 8) where these are considered relevant to
the delivery of WFD objectives. In support of a pragmatic
approach to characterising natural and anthropogenic contributions to
groundwater-P a more detailed knowledge of anthropogenic P sources and
pathways is a major need.
Copies of this report are available from the Foundation, in electronic
format on CDRom at £20.00 + VAT or hard copy at
£35.00, less 20% to FWR members.
The report is available for download from the SNIFFER Website