Uranium occurrence and behaviour
in British groundwater
This report describes the concentrations and distributions of uranium
(U) in groundwater from aquifers in Great Britain and discusses the
most likely sources and controls on U mobility. The report also reviews
the ranges of U observed in groundwater worldwide in order to place the
British data within a wider context. Groundwater-chemistry results are
presented from 116 samples of raw groundwater taken from 101
operational boreholes and springs across England & Wales. The
samples are from a selection of public and private water sources.
Twelve of the sources were sampled twice, once in the spring of 2005
and once in autumn 2005 in order to provide some limited assessment of
the temporal variation in U concentrations. The report also describes
the results from 1556 analyses of groundwater U collated from the BGS
groundwater-chemistry database and various published accounts. This
provides a basis for assessing the implications to the water industry
and regulators of U in groundwaters in England & Wales in the
event that a new European drinking-water limit for the element is
introduced in the coming years.
The mobility of U in water is controlled by a number of factors, among
the most important being pH, redox status and concentrations of
coexisting dissolved ions. Uranium is a redox-sensitive heavy metal
that occurs in water principally under oxic conditions in its
hexavalent (U(VI)) form. It is usually complexed in solution,
especially with carbonate ligands, but also less significantly with
phosphate, fluoride or sulphate depending on their respective dissolved
concentrations and ambient pH. Under anoxic conditions, U is reduced to
its tetravalent U(IV) state and its concentration in water is usually
low as a result of stabilisation of the sparingly soluble mineral,
Groundwaters often have higher concentrations of U than surface waters
because of the large solid/solution ratios in aquifers and the greater
influence of water-rock interactions. Uranium occurs as a major
constituent of minerals such as uraninite, coffinite and autunite.
These can be significant localised sources of U in some groundwaters,
especially those in mineralised areas and some granitic terrains.
Uranium is also closely associated with iron oxides, phosphates, clays
and organic matter and these minerals can be important sources, as well
as sinks, of U. The abundance of phosphates in aquifers is usually
limited but iron oxides and clays are common rock-forming minerals and
are particularly important in iron-rich and argillaceous sediments and
metasediments. The concentrations of U in abundant silicate minerals
such as quartz and feldspar and carbonate minerals are usually low.
Results from the 101 groundwater sources analysed in England &
Wales indicate a range in U concentrations of
<0.02–48.0 μg L–1
(median 0.39 μg L–1). The vast
majority of samples had U concentrations well below the WHO provisional
guideline value for U in drinking water of 15 μg L–1,
with concentrations in only two samples exceeding it. Both these were
from private supplies. The observed range compares reasonably with that
from the collated database of 1556 groundwater samples from Great
Britain, which was <0.01–67.2 μg L–1
(median 0.29 μg L–1). Of the
latter dataset, 0.71% (11 samples) exceeded the WHO provisional
guideline value for U in drinking water of 15 μg L–1,
while 0.45% (7 samples) exceeded 20 μg L–1
(the Canadian standard) and 0.26% (4 samples) exceeded 30 μg L–1
(the US-EPA maximum contaminant level). A large majority, 78.1% (1216
samples), had U concentrations less than 1 μg L–1.
This indicates that most British groundwaters have concentrations well
below those that would become problematic if a European drinking-water
limit comparable to the WHO provisional guideline value or current
American regulations were to be imposed.
The distribution of U in the groundwaters has strong links with aquifer
geology. Highest concentrations in the collated groundwater dataset
(1556 samples) occur in borehole sources in the Old Red Sandstone and
Permo-Triassic Sandstone aquifers (up to 48 μg L–1
and 67 μg L–1 respectively). A
single borehole sample from the Torridonian Sandstone of Scotland also
had a relatively high concentration (6.6 μg L–1).
These aquifers are all red-bed sandstones, their most characteristic
unifying feature being the abundance of Fe(III) oxides which occur as
grain coatings and cements. The dissolved U in these aquifers is
thought to be derived principally by desorption from iron oxides,
facilitated by complexation with dissolved carbonate species at
alkaline pH. The high U concentrations tend to be limited to the
unconfined portions of these aquifers where oxidising conditions
prevail, allowing the predominance of the oxidised U(VI)-carbonate
complexes. In reducing confined aquifers, groundwater U concentrations
tend to be low (<1 μg L–1).
Concentrations were variable and occasionally high in groundwater from
other aquifers, although none exceeded 15 μg L–1.
Most carbonate aquifers had low groundwater U concentrations, with
median values typically of 0.2–0.3 μg L–1
although values ranged up to 7.8 μg L–1
in the Carboniferous Limestone and 7.6 μg L–1
in the Chalk. These occasional high values may be linked to local
U-mineralisation, and in the case of the Chalk to interaction of
groundwater with phosphate horizons.
Concentrations in groundwaters from granites of south-west England were
generally low (up to 3.6 μg L–1)
in our study, despite the known U mineralisation in the rocks of the
region and the observation of groundwater U concentrations up to 11.6
μg L–1 by other researchers.
High-U groundwaters appear not to be a widespread feature of the
granites of the region, perhaps because of the short residence times of
the groundwaters and their slightly acidic, low-alkalinity
compositions. The sporadic nature of the U mineralisation may also be a
Of the 12 sources that were sampled and analysed more than once during
the study period, all but four had differences in dissolved U
concentration of less than 15%. The variations in the remainder are
difficult to interpret from the limited numbers of samples,
particularly in the most extreme case which had concentrations varying
between 3.48 μg L–1 and 48
μg L–1. However, the results
suggest that at least a few sites can experience significant time
variations in groundwater U concentrations. The causes are unknown but
seasonal variations in groundwater level and pumping rates leading to
differing flow patterns are possibilities.
The observed concentration ranges of U in British groundwater are
relatively narrow compared to those in groundwaters from other parts of
the world where concentrations can span some six orders of magnitude
(<0.01–8000 μg L–1).
The higher concentrations tend to be found in U-mineralised areas and
U-rich granitic terrains (e.g. western USA, Scandinavia), which are of
relatively limited extent in Britain.
Copies of this report may be available as an Acrobat pdf download under the 'Post 2000 Reports' heading of the Research Page on the DWI website.