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Decreasing boron concentrations in UK rivers: Insights into reductions in detergent

formulations since the 1990s and within-catchment storage issues

Colin Neal a,⁎, R ichard J. Williams a, Michael J. Bowes a, Michael C. Harrass b, Margaret Neal a,Philip Rowland c, Heather Wickham a, Sarah Thacker c, Sarah Harman a, Colin Vincent c, Helen, P Jarvie a

a Centre For Ecology and Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, OXON OX10 8BB, United Kingdomb Rio Tinto Minerals, 2500 W. Higgins Road, Suite 1000, Hoffman Estates, Illinois 60169, USAc Centre For Ecology and Hydrology, Lancaster Environment Centre, Library avenue, Bailrigg, Lancaster, Lancashire, LA1 4AP, United Kingdom

a b s t r a c ta r t i c l e i n f o

Article history:

Received 5 May 2009

Received in revised form 26 October 2009

Accepted 27 October 2009

Available online 22 November 2009

Keywords:

Thames

Lois

Humber

Ribble

Boron

Sodium

Sewage

Detergent

The changing patterns of riverine boron concentration are examined for the Thames catchment in southern/

southeastern England using data from 1997 to 2007. Boron concentrations are related to an independent

marker for sewage ef fluent, sodium. The results show that boron concentrations in the main river channels

have declined with time especially under baseflow conditions when sewage ef fluent dilution potential is at

its lowest. While boron concentrations have reduced, especially under low-flow conditions, this does not

fully translate to a corresponding reduction in boron flux and it seems that the “within-catchment” supplies

of boron to the river are contaminated by urban sources. The estimated boron reduction in the ef fluent input

to the river based on the changes in river chemistry is typically around 60% and this figure matches with an

initial survey of more limited data for the industrial north of England. Data for ef fluent concentrations at

eight sewage treatment works within the Kennet also indicate substantial reductions in boron

concentrations: 80% reduction occurred between 2001 and 2008. For the more contaminated rivers there

are issues of localised rather than catchment-wide sources and uncertainties over the extent and nature of

water/boron stores. Atmospheric sources average around 32 to 61% for the cleaner and 4 to 14% for the more

polluted parts.

The substantial decreases in the boron concentrations correspond extremely well with the timing and extentof European wide trends for reductions in the industrial and domestic usage of boron-bearing compounds. It

clearly indicates that such reductions have translated into lower average and peak concentrations of boron in

the river although the full extent of these reductions has probably not yet occurred due to localised stores

that are still to deplete.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

While boron is of relatively low abundance in the earth, it is

commonly found within river waters where there are urban/

industrial sources or drainage from borate deposits (Neal et al 1998;

Cöl and Cöl, 2003; Chetelat and Gaillardet, 2005; Wyness et al., 2003).

The major industrial applications are glass/ceramic manufacture

(largest market share of 56% global borate demand) and detergents,

soaps and cleaning products (17% of global borate demand); other

industry sectors include chemical and fertiliser minerals, paper

production, pharmaceuticals, wood products, metallurgy and insula-

tion (RPA, 2008). Other sources include drainage from disused

coalmines, leaching from tips and atmospheric sources such as fly-

ash and marine aerosols (Neal et al., 1998; Wyness et al., 2003). The

major stores/reservoirs and fluxes of boron have been assessed and

clearly there is a large atmospheric circulation linked to generation of

aerosols from the oceans and anthropogenic perturbation of the

global boron cycle has more than doubled the mobilisation of boron

from the crust, contributing significantly to the B transport in rivers

(Argust, 1998; Park and Schlesinger, 2002).

Boron is enriched in rivers (Neal, 2000) largely due to a com-

bination of sourceand its ability to form stableand highlysoluble oxy/

hydroxyl anions (borate in particular). There may be environmental

issueswith regardto boron in waters butthereis uncertainty over this

(Drinking Water Directive 98/93/EC; Howe, 1998; WHO, 1998;

Weinthal et al., 2005; RPA, 2008) and there are also reports of

boron concentrations reducing in municipal waste-water in recent

years (Metzner et al., 1999). This reduction coincides with the

adoption in Europe of low-temperature washing, which allows the

use of percarbonates as a bleaching agent: boron in the form of

perborate was better suited to hot-water washing, so the market shift

was most directly seen as a reductionin formulations using perborate.

Science of the Total Environment 408 (2010) 1374–1385

⁎ Corresponding author.

E-mail address: [email protected] (C. Neal).

0048-9697/$ – see front matter © 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.scitotenv.2009.10.074

Contents lists available at ScienceDirect

Science of the Total Environment

j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

mailto:[email protected]

http://dx.doi.org/10.1016/j.scitotenv.2009.10.074

http://www.sciencedirect.com/science/journal/00489697

http://www.sciencedirect.com/science/journal/00489697

http://dx.doi.org/10.1016/j.scitotenv.2009.10.074

mailto:[email protected]



Sales of sodium perborates in Europe fell from 421.6 kt/yr in 1997 to

57 kt/yr in 2007 (RPA, 2008). Other industry figures indicate that UK

imports of borates decreasedfrom1997 on,althoughnot as dramatically

as perborate (RIS, 2006). Boron concentrations would therefore be

expected to fall in surface waters in line with such reductions.

In this paper,data on boron concentration andfluxes are examined

for a majorUK river isused to examine ifboron has reduced inef fluent

impacted riversover thepast decadein line with trade reductions. The

river examined is the Thames in southern/southeastern England,where there are data for the past ten years ( Neal et al., 2000; Jarvie

et al., 2002). This basin is of particular importance as there are major

issues of environmental management associated with the nutrients

(phosphorus in particular) that are critically linked to river eutrophi-

cation and the Water Framework Directive. Boron is also used as a

marker for sewage ef fluents and this is of importance in relation to

assessing sources and impacts of not only phosphorus, but surfactants

as well (Neal et al., 2000, 2005a; Jarvie et al., 2006; Fox et al., 2000;

Holt et al., 2003). Further, for the Thames region there are growing

issues of population growth, water consumption and climate change

leading to a reduced potential for ef fluent dilution (Rodda, 2007).

For this paper, the hypothesis tested is that boron concentrations

have declined in the ef fluent from sewage treatment works (STWs)

leading to marked reductions in the concentrations of boron within

the river. In order to assess this, a second tracer (sodium) that is

enriched in ef fluents (Neal et al., 2005a,b) is used as an independent

marker of ef fluent inputs within the context of the mixing of water

endmembers (c.f. End-Member Mixing Analysis, EMMA, Christopher-

sen et al., 1990). For thestudy, supplementary information on ef fluent

chemistry is examined to see if there is direct evidence for change. For

this, previously published data for the Kennet catchment (mainly for

2001) is examined and is supplemented with new data for 2008 for

three STWs.

2. Study area, monitoring programme and approach

The Thames is the major river draining southern/southeastern

England, to its estuary in the southeast at the metropolis of London

(about 346 km in length and over 10,000 km2 in area). For our study,data are available for the upper half of the basin and there are three

rivers studied here where there is a suf ficient record to examine the

changing patterns in boron concentrations over the past decade.

Firstly, the main stem of the river Thames was monitored at Howbery

Park, ∼2 km upstream of the town of Wallingford (catchment area

3482 km2). Here the catchment is largely rural but there are urban/

light industries at towns such as Oxford, Aylesbury and Thame. The

bedrock is mainly of permeable Chalk and low permeability clays and

the river is largely supplied from groundwater sources. Mean annual

rainfall is 715 mm, runoff is 216 mm and the baseflow index is 0.64

(Marsh and Hannaford, 2008). Secondly, there is a tributary of the

Thames, the Thame, and its catchment is rural/agricultural with the

main populations being at the towns of Aylesbury and Thame. The

geology comprises mainly clays and sandstones with some limestone/ Chalk. The Thame was monitored at Wheatley (catchment area

534 km2). Mean annual rainfall is 655 mm, runoff is 230 mm and the

baseflow index is 0.59 (Marsh and Hannaford, 2008). Thirdly, there is

another tributary of the Thames, the Kennet (catchment area of about

1200 km2). This study focuses on the upper reaches of the river at two

sites, Clatford and Mildenhall, around 2 km upstream and down-

stream, respectively, of the market town of Marlborough and its

sewage ef fluent (Marlborough STW) input that discharges directly to

the river. The catchment area at Marlborough is around 42 km2 and

the water in the river is primarily sourced from the underlying Chalk

aquifer. Mean annual rainfall at Marlborough is 828 mm, runoff is

195 mm and the baseflowindexis 0.94(Marsh and Hannaford, 2008).

Thelength of monitoring variedand forthe Thamesand theThame

there is a data gap between 2002 and 2006: Table 1 summarises the

sampling years andthe numberof samples collectedeach year. Forthe

Kennet sites, there was no sodium data for 1997. Details of the

catchment, sampling location, analytical methodology and general

water quality are provided by Neal et al. (Thames 2000, Thame 2006

and Kennet 2008).

STW final ef fluents have been sampled intermittently and at

different locations across the Kennet catchment and data from the

various studies are collated with our new data to assess for the first

time directly if boron concentrations have actually declined. Threesets of information were examined. Firstly, there is ef fluent data for

monthly sampling at six small STWs during 2001: East Grafton,

Froxfield, Great Bedwyn, Shalbourne, Wilton and Hungerford, with

respective population equivalents of 290, 300, 1240, 470, 130 and

5800 (Neal et al., 2005b). Secondly, there is weekly ef fluent data

(2003 to 2005) for Marlborough STW of population equivalent 9250

(Neal et al., 2005a, 2008). Thirdly, for our new data, ef fluents were

monitored weekly in 2008 at Hungerford, Marlborough and Newbury

STWs, the latter with a population equivalent of 63,600.

The river and ef fluent samples were analysed for major, minor and

trace elements with filtering to 0.45 μ m in the field. Boron and sodium

concentrations were determined by inductively coupled plasma optical

emission spectroscopy (ICPOES) on acidified samples. Calibration was

undertaken with our own standards and these were cross-checked

against international certified standards. The laboratories participated

Table 1

The Kennet, Thame and Thames monitoring period and flow/boron record.

N Flow (cumecs) Boron conc (μ g/l) Boron

flux

(tonne)Avg Min Max Avg Fw-

avg

Min Max Base Storm

Thames at Howbery Park

1997 38 14 2.26 87 269 210 108 414 338 131 107

1998 51 36.1 3.15 151 166 114 69 354 324 79 149

1999 51 34 3.67 156 167 125 61 304 278 74 154

2000 51 47.1 4.52 195 146 98 60 300 262 68 167

2001 51 38 4.08 162 138 96 61 252 230 69 132

2006 48 24.8 3.05 128 99 82 62 148 123 68 74

Thame at Wheatley

1997 29 1.5 0.73 11.6 NA NA NA NA NA NA NA

1998 50 5 0.92 32.8 255 174 78 467 436 100 31.6

1999 35 4.1 0.74 23 283 208 77 565 524 97 30.9

2000 36 6.31 1.04 44.9 212 143 84 398 362 90 32.7

2001 Na 6.09 1.07 33.4 178 125 69 274 256 85 27.6

2006 48 2.66 0.55 17.9 133 113 86 166 159 98 10.9

Kennet at Clatford

1997 43 0.11 0.01 0.34 9 9 0 17 11 11 0.04

1998 52 1.07 0.24 2.5 20 21 11 30 13 22 0.81

1999 51 1.1 0.25 4.47 23 24 15 33 16 25 0.96

2000 51 1.75 0.44 8.23 24 25 20 29 21 26 1.59

2001 31 1.45 0.23 5.16 21 24 16 31 17 26 1.26

2002 50 1.23 0.26 7.01 21 23 16 26 18 24 1.03

2003 50 1.1 0.07 15.6 18 20 11 23 15 22 0.80

2004 51 0.54 0.13 1.3 21 22 16 25 18 22 0.432005 38 0.4 0.1 0.99 21 22 16 34 19 22 0.32

2006 48 0.65 0.16 2.31 21 23 17 26 18 25 0.54

Kennet at Mildenhall

1997 43 0.11 0.01 0.34 68 61 38 90 80 52 0.24

1998 52 1.07 0.24 2.5 30 27 15 52 47 23 1.05

1999 51 1.1 0.25 4.47 37 31 22 52 48 26 1.24

2000 51 1.75 0.44 8.23 30 28 24 38 37 29 1.78

2001 31 1.45 0.23 5.16 30 27 20 36 35 27 1.42

2002 50 1.23 0.26 7.01 25 24 21 32 30 23 1.07

2003 50 1.1 0.07 15.6 24 22 15 44 30 22 0.88

2004 51 0.54 0.13 1.3 26 25 20 32 30 22 0.49

2005 38 0.4 0.1 0.99 26 25 23 32 30 25 0.36

2006 48 0.65 0.16 2.31 24 24 20 28 26 24 0.57

Base = baseflow, Storm= stormflow. N = numberof samples. “Weekly” and “Regress”

represents the flux as estimated using the weekly spot data and integrated estimates

using the concentration-flow relationship and the daily flows through the year.

1375C. Neal et al. / Science of the Total Environment 408 (2010) 1374–1385



in external analytical quality control protocols. Analysis involved three

replicates and for every ten samples there was a set of our own

standards followed by another set of standards run “as samples” for

quality control purposes (if the standards run as samples differed from

the true values by more than 5% then the analysis was repeated. The

standards were used to correct for any instrument drift. The lowest

quotable value was 1.5 μ g/l for boron and 10 μ g/l for sodium.

Flow data were used from long-term gauges near the sampling

points: the Thames at Days Lock, The Thame at Wheatley and theKennet at Marlborough (Marsh and Hannaford, 2008: data supplied

courtesy of the National River Flow Archive: http://www.ceh.ac.uk/

data/nrfa/index). Corrections have not been made to allow for exact

representation of flows at the monitoring points (as opposed to the

flow gauging points) as this is of secondary importance and the

differences will be relatively small. In the case of the Kennet at

Clatford and Mildenhall, it is assumed that the additional volumetric

input of ef fluent is not significant.

With regard to ef fluent discharges, there are variable numbers of

inputsand ef fluent flowsfor these rivers. Forthe Thames at Days Lock:

there are 123 STWs upstream of this sampling point, with a dry

weather flow totalling 2.32 cumecs. Particularly notable are the

ef fluent discharges from Swindon, Oxford and Abingdon (population

equivalent of 200,000, 186,000 and 38,400, respectively). For the

Thame at Wheatley: there are 24 STWs with dry weather flows

totalling 0.48 cumecs. For this river, the most significant STW services

Aylesbury and has a population equivalent of 94,400. For the Kennet

at Clatford, there are two small rural STWs, the nearest often

discharges to a dry stream bed and the dry weather flows total

0.014 cumecs. For the Kennet at Mildenhall, including Marlborough

STW(populationequivalent of 11,000), there arethree STWs with dry

weather flows totalling 0.089 cumecs. N.B. the dry weather flows

represent the flow corresponding to times when it has not rained for

at least a week: the data presented here are based on estimates by the

water companies — they have not been required to monitor flows and

report them until very recently.

2.1. Tracing ef fluent sources of Boron

Boron has low reactivity within the catchment and river (i.e. it is

chemically conservative). Hence, changes in concentration in the river

represent a mixing of the ef fluent with water supplied from the

catchment from groundwater/hyporheic and riparian storage and

near surface runoff during rainfall events. Chemically conservative

water quality determinands such as sodium, potassium, magnesium,

chloride and sulphate are also enriched in ef fluents (Neal et al., 2005a,

2008). Hence plots of boron against these other markers would be

expected to exhibit linear relationships. Initial examination of the

relationships for our study sites indicates straight-line relationships,

but there was less scatter for sodium as the other markers have other

sources such as fertilisers that complicate the picture. Hence for the

present study sodium is used for the comparison.

The extremities of these linear plots are defined by an “ef fluentendmember” (or an integrated set of “ef fluent endmembers” when

there are a series of ef fluent inputs) and a “within-catchment

endmember” that makes up the remainder of the river flow. When

boron declines within the ef fluent, the gradient of the line changes with

greatest difference occurring when the ef fluent component is the

greatest and the mixing lines should converge to a point corresponding

to the within-catchment endmember (assuming that the endmember

remains constant). Correspondingly, for the “ef fluent endmember” the

linesdiverge andthe composition of thatendmember lies on theline but

at higherconcentrations. Analysis of themixing relationshipsprovidesa

way of assessing the nature of the mixing proportions and the

constancy/change in endmember compositions.

The mixingapproachis extended in this paper by examiningfluxes

following the approach described by Neal et al. (2010). In brief,

consider the situation where ef fluent mixes within the river with

water representative of other sources. If the annual flow in the river

downstream of the ef fluent input is F river and the annual flow of the

ef fluent is F ef fluent while the corresponding flow weighted boron

concentrations are [Briver] and [Bef fluent], with fluxes B fluxriver and

B fluxef fluent, respectively, a flux balance can be formulated, assuming

that the within-catchment storage is constant on an annual basis.

Thus

Bfluxriver = Bfluxeffluent + Bfluxcatchment ð1Þ

and hence

Bfluxriver = ½Bcatchment⁎F river + ð½Beffluent–½BcatchmentÞ⁎F effluent: ð2Þ

Thus a plot of B fluxriver against F river should give a straight-line

relationship with a slope (δB fluxriver/ δF river) equal to Bcatchment. Under

the condition F river=F ef fluent, i.e. all the river flow is made up of

ef fluent, the dry weather flow, F DW, then B fluxriver=B fluxef fluent and

under this limiting condition

Bfluxeffluent = F DW = ½Beffluent: ð3Þ

Within this paper annual fluxes are estimated, but obtaininghighly accurate values are dif ficult owing to issues of sampling

frequency and the possibility of extremes in high flows being missed

(Littlewood et al., 1998). For the present study a commonly used and

respected approach is taken, but the uncertainty must be borne in

mind. The equation used is:

F = a⁎ f annual⁎∑i = 1 t o nc i⁎ f i =∑i = 1 t o n f i ð4Þ

where “F ” is thechemical flux, “i” is the sample numberin a given year

with concentration with a corresponding concentration and flow of c iand f i, respectively, while f annual is the annual flow in that year and “a”

is a scaling factor to give the desired units.

With regard to the relationship between concentration and flow,

here the results are examined using a simple power relationship with

flow across all the study sites:

½ = a⁎F b

ð5Þ

where [] is the concentration of boron or sodium: the “a” and “b”

terms are constants for each year and “F ” is the volumetric flow of the

river in m3/s. The “a” term can be considered as representing the

general concentrations of level of boron/sodium in the water while

the “b” term is more linked to therate of change of concentration with

flow. The “b” term is dimensionless, the “a” term has complex units

and for simplicity, no units are given with the data presented. This

form of equation has been found for other study areas with regard to

boron (Neal et al., 1998; Chetelat and Gaillardet, 2005).

Within the study, baseflow and stormflow concentrations are

compared andfor this, we have estimatedthe baseflow andstormflowaverages using the five lowest and the top five flows within a given

year.

3. Results

3.1. General patterns

The time series for boron (and sodium) show large seasonal and

year-to-year variations (Fig. 1). For boron, the averages and ranges

increased substantially in the order Kennet at ClatfordbKennet at

Mildenhall≪Thame≈Thames (Table 1). Apart from the Kennet at

Clatford, baseflow concentrations were higher than stormflow

concentrations by up to a factor of around five in the case of the

Thames and Thame: for Clatford, boron concentrations were higher in

1376 C. Neal et al. / Science of the Total Environment 408 (2010) 1374–1385

http://www.ceh.ac.uk/data/nrfa/index






stormflow compared to baseflow and the differences were less than a

factor of two (Table 2).

Boron concentrations declined with time across all the sites apart

from the Kennet at Clatford (Table 1). The largest difference

(particularly under baseflow) was for the first year of study (1997)

in the case of the Kennet and the Thames when conditions were very

dry and summer flows were particularly low. In part, this initial

change may be explained by increased dilution rather than a change

in ef fluent composition and this inference is backed up by high

sodium concentrations for 1997 to 1998. However, after 1997, the

decline for boron continued and it was not backed up by similar

declines in for sodium. The declines we refer to here are for averages,

but the same features were observed for flow weighted averages. The

annual averages and flow-weighted averages were highly linearly

correlated and the intercepts were insignificantly different from zero.

Thegradientto theregressionline (with zero intercept) indicatedthat

the flow weighted averages were lower than the average apart from

the Kennet at Clatford where the gradient was close to unity: 1.07±

0.91, 0.91±0.03, 0.74±0.04 and 0.71±0.05 for the Kennet at

Clatford, the Kennet at Mildenhall, the Thames and the Thame.

Normalising the boron data to sodium revealed a clear decline in

the ratio for the period 1997–

2001 with 2006 (Table 2). For the

Fig. 1. Boron and sodium time series.




Kennet (Mildenhall), Thame, Thames, the decline in the averageamounted to 52, 48 and 43%, respectively. However, under stormflow

conditions there is little change in the ratio.

3.2. Concentration relationships with flow

Boron concentrations showed an initial steep decline with

levelling off as flow further increased for all the sites other than at

the Kennet at Clatford where there was an increase and a subsequent

levelling off as flow increased (Fig. 2).

For boron, its concentration relationship with flow changed over

time while there was no corresponding change for sodium. The main

changes occurred for baseflow andaverage valueswhilethe high-flow

component showed little variation. This provides strong evidence fora declining boron concentration in the ef fluent and near constant

within-catchment sources. Note, however, that the stormflow boron

and sodium concentrations were higher in the order Kennet at

ClatfordbKennet at MildenhallbThame≈Thames and the greater the

ef fluent load, the greater the “contamination” even at high flows

(Tables 3 and 4). Using dry weather flow information coupled to the

concentration-flow relationships established above, the reductions in

boron concentrations in ef fluents were estimated to be 50% for the

Kennet at Mildenhall, 69% for the Thame and 63% for the Thames. For

theThameand theThames, there were no data for theperiod between

2001 and 2006 and comment cannot be made as to when the declines

took place between these years. However, for the Kennet at

Mildenhall, the main change occurred around 2001/02 when there

was a step change.

3.3. Boron–sodium mixing relationships

There were strong linear relationships between boron and sodium

across the sites (Fig. 3). For the Kennet at Clatford, there was a limited

variation in boron and sodium concentration range and while the

relationship was linear there was marked scatter. Further, there was

no clear year-to-year variation. For the other sites, there are much

stronger linear relationships, but these linear relationships vary from

year-to-year ( pN

99%). At low boron and low sodium concentrations,the lines for the various years converge and this corresponds with

high flows and the non-sewage endmember (i.e. the background

boron input from the catchment). The values were similar for the

Thames (11 mg/l Na, 60 μ g/l B) and Thame (12 mg/l Na, 70 μ g/l B) but

lower for the Kennet at Mildenhall (8 mg/l Na, 20 μ g/l B). Over time,

the gradient declined (Fig. 4). In the case of the Thames the gradient

was ∼8 μ g/mg for the years 1997 to 2001 but by 2006 the gradient

declined over three fold to 2.2 μ g/mg. For the Thame, there was also a

marked decline similar to that for the Thames with a gradient of

7.8 μ g/mg for the period 1998 to 2001 and 1.4 μ g/mg by 2006, a

decline of a factor of 5.6. For the Kennet at Mildenhall, the gradient

declined from ∼6.5 μ g/mg for 1998/99 to 3.6 μ g/mg by 2000 and to

0.7 μ g/mg by 2006: a reduction by a factor of 9.3. The Kennet at

Mildenhall indicates a marked decline in gradient in 2000 and the

data provides infilling information compared to the Thames and

Thame as it includes the years 2002 to 2005. It seems that the decline

in gradient occurs a year or more before that for the Thame and

Thames (Fig. 4).

Translating the information into an estimate of the change in

boron concentration in the ef fluent requires extrapolation of the

mixing line to the ef fluent endmember. This involved (1) estimating

the sodium concentration in the ef fluent using the sodium-flow

relationship and the dry weather flow information and (2) extrap-

olating the mixing line to the estimated ef fluent endmember

concentration for sodium and the corresponding value for boron

was taken as the boron ef fluent concentration. This analysis indicates

a boron reduction in the ef fluent of 45, 65 and 57% for the Kennet at

Mildenhall, the Thame and the Thames.

3.4. Annual flux variations

Annual boron and sodium fluxes varied between years and across

sites (Tables 1 and 2). In the case of the Thames and the Kennet, the

annual fluxes were relatively low in 1997 as this wasa particularlydry

year. In the case of the Thames and the Thame, for the earlier years of

record, the fluxes were similar for both boron and sodium concentra-

tions, but in both cases the fluxes were lower for the last year of

record, 2006 where boron fluxes reduced by around 50 to 60% and

sodium concentrations reduced by around 25 to 50%. For the Kennet

at both Clatford and Mildenhall, there was a similar pattern except

that the fluxes peaked in 2000/2001 and the subsequent declines

were around a factor of three to five times. Some of these reductions

occurred simply because of the differences in flow from year-to-year(Fig. 5). However, while the sodium and water fluxes were linearly

correlated throughout the study period across the sites, the boron line

diverged around 2000, with lower boron fluxes after 2000 for the

Kennet at Mildenhall, the Thame and the Thames apart from the

Kennet at Clatford there was no clear separation before and after

2000. The change in the boron flux up to 2000 and post 2000 was

assessed for the Kennet at Mildenhall, the Thame and the Thames. To

do this, the boron flux/water flux relationship for 1997–2000 was

determined by linear regression and the annual flux of boron was

estimated. Then the annual flows post 2000 were used to estimate

what the boron load would have been had no boron reductions taken

place and these estimates were then compared with the actual loads

post 2000 to assess the boron losses that had accrued. The results

indicated a loss of around 11% for both the Thames and Thame and no

Table 2

Water quality statistics for sodium and the boron to sodium ratio for the Thames at

Howbery Park, the Thame at Wheatley and the Kennet at Clatford and Mildenhall.

Sodium (conc — mg/l, flux t onne /yr ) B/ Na (μ g/mg)

Avg Fw-Avg Base Storm Flux Avg Base Storm Flux

Thames at Howbery Park

1997 43.9 35.9 53.9 25.7 14,693 6.13 6.26 5.11 5.68

1998 28.5 22.0 48.2 17.5 24,106 5.82 6.71 4.54 5.09

1999 27.6 23.6 40.0 16.8 24,903 6.05 6.96 4.37 5.19

2000 25.0 18.6 42.0 14.3 27,826 5.86 6.24 4.75 5.29

2001 25.2 19.9 36.7 15.6 23,793 5.47 6.27 4.42 4.87

2006 31.1 23.0 43.6 17.2 17,937 3.19 2.81 3.94 3.42

Thame at Wheatley

1998 39.0 29.1 60.4 19.5 4243 6.54 7.23 5.15 5.83

1999 40.5 32.9 62.1 20.1 3703 6.99 8.45 4.82 6.03

2000 33.7 24.0 53.3 15.2 4826 6.28 6.79 5.91 5.94

2001 31.8 24.8 41.3 19.6 4603 5.60 6.20 4.34 5.07

2006 43.4 31.3 59.5 21.2 2529 3.06 2.67 4.64 3.48

Kennet at Clatford

1998 7.3 7.6 6.9 8.0 260 2.71 1.94 2.76 2.73

1999 7.3 7.7 6.8 8.1 266 3.09 2.31 3.05 3.19

2000 7.6 7.7 7.3 7.6 426 3.20 2.81 3.43 3.38

2001 7.3 7.9 6.8 8.1 361 2.83 2.56 3.24 3.10

2002 7.1 7.4 6.7 8.0 287 3.01 2.63 2.95 3.10

2003 7.2 7.9 6.3 8.7 272 2.49 2.37 2.49 2.752004 7.1 7.3 6.5 7.4 124 2.93 2.73 3.02 3.03

2005 7.6 7.6 7.2 7.9 92 2.82 2.61 2.80 2.89

2006 7.2 7.7 6.6 8.1 155 2.90 2.75 3.07 2.89

Kennet at Mildenhall

1998 10.0 9.5 11.6 9.1 306 3.04 4.02 2.56 2.97

1999 9.8 9.1 11.4 8.7 315 3.84 4.20 2.98 3.43

2000 9.2 8.8 10.7 8.4 482 3.26 3.46 3.43 3.23

2001 10.5 8.9 12.6 8.3 401 2.87 2.77 3.22 3.09

2002 9.8 9.0 11.7 8.5 343 2.60 2.59 2.72 2.70

2003 11.7 9.3 16.6 8.8 302 2.07 1.78 2.47 2.38

2004 11.1 10.0 11.9 9.2 166 2.39 2.49 2.37 2.54

2005 11.6 11.1 12.9 10.9 133 2.26 2.34 2.27 2.43

2006 10.5 9.8 12.2 9.1 200 2.31 2.17 2.67 2.45

1378 C. Neal et al. / Science of the Total Environment 408 (2010) 1374 –1385



discernable loss for the Kennet at Mildenhall for 2001. However, by

2006, the estimated losses were much greater especially for the

Thames and Thame (42 and 51%, respectively: 22% for the Kennet).

Examining the changes in the boron to sodiumratio for theequivalent

period gave reductions of 9% for 2001 and 55% for 2006 in the case of

the Thames, 17% and 70% for the Thame and 0% and 27% for the

Kennet at Mildenhall.

The application of Eq. (2) indicates that the regional concentra-

tions are 61, 91 and 27 μ g/l for the Thames, Thame and Kennet at

Mildenhall, respectively, for boron and 13, 19 and 8 mg/l for sodium.

3.5. Within year flux variations

Following the approach outlined with Eqs. (1)–(3), boron and

sodiumfluxes were plotted againstflow for the individual data points

in a given year, it was clear that boron and sodium fluxes showed

strong positive relationships withflow eachyear(Fig.6).In the caseof

the Kennet, there were single straight lines for individual years and in

the case of Clatford, the intercept was always negative and at low

flowstherewas curvatureof the line. Correspondingly, forthe Thames

and the Thame, there was curvilinear rather than a straight line

relationship for some years.

Given the issues of negative intercept for Clatford and high scatter

for Mildenhall, only the Thames and Thame were considered and for

this a power relationship akin to Eq. (5) for the Thames and Thame

were employed to estimate the ef fluent concentrations of boron and

sodiumusingthe dryweatherflows(Table 5).In the caseof boron,the

results indicated marked reductions in ef fluent flux for the Thames

and Thame (66 and 76% respectively). In contrast, sodium showed no

changes in ef fluent flux through the years. Nonetheless, for the

Thames and Thame, the curvilinear relationship indicated behaviour

more complex than simple two component mixing. In terms of

regional concentrations, there are little year-to-year differences

(apart from 1997).

3.6. Boron in sewage

The ef fluent concentrations of boron and sodium are provided in

Table 6. For the earliest records (2001) both boron and sodium

concentrations were relatively high (average concentrations vary

between 384 and 1034 μ g/l for boron and 64 and 121 mg/l for sodium.

For the intermediate period (2003 to 2005), the ef fluent of

Marlborough STW was monitored and there were lower boron and

sodium concentrations (range in average 82 to 99 μ g/l and 39.2 to

47.4 mg/l, respectively). For the latest monitoring (2008), Hungerford

STW had the highest concentrations for both boron and sodium

(range 70 to 188 μ g/l and 38 to 85 mg/l, respectively). For Marlbor-

ough STW, boron concentrations were around a half of those for

Fig. 2. An illustration of therelationshipbetweenflowwithsodium and boronconcentration. Datafor theThamesand the Kennetat Clatford provide the limitingcases: data for1998

and 2006 are plotted to show the extent of boron input differences at low flows.




rainfallconcentration forboron andsodium in thearea is 3.25 μ g/l and

1.94 mg/l respectively (Neal et al., 2004a). With an evaporative

concentration that averages around a factor of 2.5 for the area, this

corresponds to a predicted stream water concentration of 8.1 μ g/l and

4.85 mg/l, respectively, compared with corresponding flow weighted

averages observed in the Kennet at Clatford of 24 μ g/l and 7.6 mg/

l while the regional estimate using flux relationships gives values of

13 to 25 μ g/l and around 9 mg/l respectively. This corresponds to a

rainfall contribution of 32 to 61% for the boron load and around 54%

for sodium.

In the case of the Kennet at Clatford, ef fluent inputs were largely

discharged to the unsaturated zone via septic tanks or local discharge

from small STW outfalls. At Clatford, unlike all the other sites, boron

concentrations increased with increasing flow. This represented

flushing of the stored boron during rainfall events when the un-

saturated zone wetted up and water wastransferredto the river (Neal

et al., 2004b). For this site, there was no clear change in boron

concentration apart from the initial dry year when water wasprobably supplied from the aquifer and would bypass the ef fluent

inputs. The data for this site is unusual in that the estimated boron

concentrations in the ef fluent are (unacceptably) negative. At low

flows, groundwater sources that maintain flows and these ground-

waters are low in boron (average 14 μ g/l: Neal et al., 2002). Analysis

of the same groundwater data for sodium provided values of 6 to

7 mg/l which is similar to the mean for the river. For the Kennet at

Mildenhall, there is clearly evidence for a point source dilution effect

from the Marlborough STW discharging upstream, with additional

evidence for a reduction in the boron supplied to this STW.

For the Thames and the Thame, with the much greater STW inputs

and lower aquifer storage than the Kennet, the changes observed

were much greater in terms of boron concentration andflux and these

sites provide the clearest representation of the boron reductions that

Fig. 3. Plots ofsodium against boronfor four sitesin theThamesbasin.To simplifymatters,the data wasseparatelyplottedforthe years1997to 1999, 2000 to2003and 2007 to2006.

Fig. 4. The variation in mixing gradient over time for the Kennet at Mildenhall, the

Thame and the Thames.




have taken place. Our results for the Thames and Thame show five

critical features:

1. Stormflow concentrations of boron and sodium were higher for the

more impacted catchments. Thesamewas true forestimates of boron

concentrations in the ef fluent and in the regional background.

2. Stormflow concentrations of boron and sodium were broadly similar

throughout the study period and so were the regional concentra-

tions. However, in detail, as the average annual boron concentration

declined with time, there was a modest decline in the stormflow

averages — a decline much smaller than the baseflow counterpart.

3. The boron and sodium fluxes varied from year-to-year in line with

water flux (plus some deviations associated with boron reductions

in the ef fluent) but the relationship was curvilinear.

4. A major decline in boron concentration in baseflow and ef fluent

over time was observed.

5. The boron to sodium ratio became higher in stormflow than

baseflowforthefinal year of record (2006), the reverse of previous

years.

If all the boron and sodium came from ef fluent sources and these

sources were constant, then there should have been a near constant

flux. This does not occur. Clearly there were within-catchment

sources of boron and sodium, linked to population density and

urban development that did not rapidly deplete (over time, storm

flow concentrations did not show any decline). In the Thames and

Thame catchments that receive relatively high levels of STWef fluents,

the boron load increased more rapidly at low to intermediate flow.

Fig. 5. Plots of boron and sodium flux against water flux for the Kennet at Clatford and Mildenhall, the Thame and the Thames.

1382 C. Neal et al. / Science of the Total Environment 408 (2010) 1374 –1385



This suggests that these stores arein the lower parts of thecatchment,

that arewell connectedto theriver systemand where the main towns

and associated STWs are located. Even at high flows there were

elevated boron and sodium concentrations for the Thames and

Thame, and the regional boron concentrations were also relatively

high. For example, the regional boron concentrations ranged from 48

to 59 μ g/l for the Thames and 54 to 77 μ g/l for the Thame and this

compares with 8 μ g/l for rainfall and 14 μ g/l for Chalk groundwater.

Correspondingly, the regional sodium concentrations ranged from 11

Fig. 6. Flow versus boron flux for the Kennet, Thames and Thame, covering the early (1998 –1999), mid (2000–2003) and late (2004–2006) parts of the record.

Table 5

Regression statistics for boron and sodium flux versus water flux for the Thames and

the Thame using the equation boron/sodium flux =aFlowb where “a” and “b” are

constants.

a b load Eff Reg a b load Eff Reg

Thames boron Thames sodium

1997 539 0.637 922 397 80 1997 75.9 0.714 139 59.7 19

1998 590 0.554 940 405 55 1998 71.5 0.685 127 54.9 14

1999 608 0.566 979 422 36 1999 58 0.76 110 47.4 12

2000 523 0.595 863 372 50 2000 66 0.695 119 51.1 11

2001 420 0.624 710 306 48 2001 54.1 0.748 101 43.7 11

2006 162 0.807 320 138 59 2006 66 0.704 119 51.5 13

Thame boron Thame sodium

1998 407 0.473 288 600 58 1998 56.8 0.598 36.6 76.3 13

1999 441 0.417 325 677 47 1999 54.6 0.629 34.4 71.7 16

2000 369 0.546 247 515 63 2000 55.1 0.608 35.3 73.5 9

2001 276 0.599 178 370 54 2001 44.3 0.707 26.3 54.9 6

2006 140 0.856 75 155 77 2006 47.5 0.679 28.8 60.1 14

Eff = ef fluent concentration and Reg = regional concentration: units μ g/lfor boronand

mg/l for sodium. In all cases pb

0.0001.

Table 6

Boron and sodium statistics for sewage treatment works in the Kennet catchment.

Site Year Na (mg/l) B (µg/l) B/Na (µg/mg)

Avg Min Max Avg Min M ax Avg M in M ax

East Grafton 2001 120.7 52.5 163.3 1034 333 1427 8.5 5.9 12.0

Froxfield 2001 106.0 36.2 155.2 588 177 1 072 5.3 3.3 6.9

Great Bedwyn 2001 110.0 36.2 155.2 755 177 1543 6.5 3.3 10.9

Hungerford 2001 63.8 14.5 93.5 500 85 816 7.5 5.9 9.0

Shalbourne 2001 71.1 8.1 161.8 409 36 934 5.4 4.4 6.8

Wilton 2001 66.0 27.6 133.6 384 92 769 5.6 3.3 7.6

Marlborough 2003 47.4 37.5 56.0 99 71 113 2.1 1.8 2.5

Marlborough 2004 39.2 29.7 51.3 92 66 127 2.4 1.8 3.0

Marlborough 2005 40.3 24.6 50.5 82 53 106 2.1 1.6 2.6

Hungerford 2008 73.6 38.3 85.3 101 70 188 1.4 1.1 2.3

Marlborough 2008 31.5 22.4 46.3 46 33 61 1.5 0.9 1.9

Newbury 2008 47.5 22.4 85.1 69 33 115 1.5 0.9 1.9

The Hungerford data are highlighted in bold — this is the only location where there is

data for the early (2001) and late (2009) part of the record.




to 14 mg/l for the Thames and 6 to 14 mg/l for the Thame and this

compares with 4.85 mg/l for rainfall and 6 to 7 mg/l for Chalk

groundwater. Using the flow weighted river data for the Thames and

the Thame (and excluding the dry year data for 1997, which is

atypical), then the rainfall component for boron was estimated to be 4

to 14% for the Thames and 4 to 10% for the Thame: the corresponding

figures for sodium are 21 to 26% and 23 to 32%. Clearly for these more

polluted rivers, the rainfall is less significant than for the rural ones.

However, the percentage was higher for sodium relative to boronbecause of the greater significance of anthropogenic boron inputs as

opposed to other sources for sodium including the atmosphere.

With regard to the atmospheric component, a term for dry depo-

sition of boron and sodium is missing and there are uncertainties over

what the value might be.However,the dry deposition must have been

less than the difference between the groundwater and the evaporated

rainfall values, and probably much less given ef fluent (septic tank)

inputs to the catchment (i.e. much less than the 43% and 31% dif-

ference for boron and sodium, respectively). For comparative

purposes, the rainfall percentages estimated for boron were lower

for the Thames and Thame and higher for the Kennet by around a

factor of two compared with the average found for a isotopic study of

the Seine (Chetelat and Gaillardet, 2005) where the rainwater

contribution averaged around 25%.

In hydrologically well connected and polluted urban areas, boron

(and sodium) sources might include material trapped within the

drainage system (e.g. within/near-channel storage in the hyporheic/

riparian zones) or contaminating the urban lands that enter the river

during wetting up periods. Combined sewage treatment works

receive runoff from paved areas and boron fluxes could increase at

such times but this flux increase would be masked when examining

concentration dynamics because boron concentrations decline as flow

increases. Within-channel storage, perhaps from recharge to the

underlying aquifer at times of drought, could be re-mobilised as flow

levels increase. Whatever the process, while perhaps changing in

extent, there seem to be additional boron inputs across theflow range

and certainly leaky sewers can contaminate groundwaters in urban

areas of boron and sodium as well as several other pollutants (Held

et al., 2006).Boron has been used previously as a tracer for water and pollution

and point source inputs (Neal et al., 1998, 2005a; Jarvie et al., 2006).

With regard to understanding water fluxes within river basins, boron

may well remain a suitable tracer but there is a need to establish a

more complete understanding of boron sources and water sinks.

Further, the value of sodium as a tracer is becoming plainer with this

study.

The combination of water-mixing and flux inventories seem to

provide complimentary approaches that are strengthened in their

union. The inconsistency between the simple modelling approach for

flux and the field data point to issues of storage and to the naivety of

considering the stream water as being a mix of just two components

ef fluent and a regional background. While the mixing models might

show simple relationships, the issues of volumetric storage introducean added complexity. The present study provides a start, but to take

things forward there is the need for a more “full blooded” dynamic

modelling approach. This dynamic approach needs to consider the

within-catchment sources as being comprised of a range of terms

which consider the general catchment, near stream and urban areas

and potentially groundwater storage terms in a more explicit way

than is commonly used in many such models.

Finally, with regard to the most significant finding for the study, a

marked reduction in boron within the Thames over time and its

inferred linkages with declines in the ef fluent corresponding to a

reduction in boron usage, there remains the issue of the true national

significance even though the Thames is a major UK river. We have

undertaken an initial survey of some of our other data where we have

information for boron and sodium concentrations in rivers of (1)

northeastern and central England and the Scottish borders for the

1990s (as collected as part of the Land Ocean Interaction Study; Neal

and Robson, 2000; Neal and Davies, 2003) and (2) the industrial

north-west of England for 2008/9 (the Ribble and Wyre basins;

Scholefield et al., 2008). This data represented several years of weekly

monitoring (17 sites) in the case of the LOIS and one year of

fortnightly monitoring (24 sites) for the Ribble/Wyre and both sets

covered a mix of rural, agricultural and industrial settings. In both

cases, straight-line relationships ( pb

0.0001) were observed betweensodium and boron, but there were marked differences in gradient

(δB/ δNa): 3.82±0.07 for the 1990s and 1.63±0.07 for 2008/9. Al-

though the datasets cover two distinct regions and time periods, it

seems that there has been a marked lowering in gradient. Given that

these straight-line relationships have intercepts close to zero then it

seems that there has been a reduction in boron within the ef fluent of

57%. This number matches well our observations and it clearly

indicates that the reduction is of national significance.

5. Conclusion

There are three fundamental conclusions to this study. Firstly,

there have been substantial decreases in the boron concentrations of

ef fluent discharges from STWs, and these decreases are in line withreductions in the industrial and domestic usage of boron-bearing

compounds. This is indicatedboth for our study of the Thames and the

data of the Humber/Ribble/Wyre. Secondly, the reduction in annual

boron flux is smaller. This accrues because of indirect inputs of boron

from the catchment themselves and these inputs have not reduced

boron concentrations. Thirdly, despite this, these indirect inputs

simply cannot be related to a regional diffuse input as there will be

locally contaminated areas and regions of recharge/discharge espe-

cially near to the river.

However, there is no direct measurement of changes in household

products containing boron and no data on perborates that are

believed to be the main component of change. Rather, the work

strongly indicates a reduction of point source boron based not only on

the river data but also from direct measurements of the ef fl

uents.Indeed, there may well be other sources of boron from the urban

environment that have not yet reduced and this is indicated by higher

stormflow and regional estimates for the more urban impacted

systems relative to the rural catchments and by the non-linear

relationships between born/sodiumflux and flow for individual years.

In the case of ef fluents discharging to the unsaturated zone, the

recovery period will be longer due to increased within-catchment

storage. Thefindingsfrom this study arelikely to be observed in rivers

in the UK, across Europe and wider, wherever boron content

reductions in household detergent products are taking place. The

boron and sodium data throw up issues of water and contaminant

storage and it seems that both are of value as hydrological tracers.

Clearly a high quality inventory of boron sources and sinks is required

and this includes information on atmospheric deposition and storage

in urban areas that the study flags.

Acknowledgements

We thank the two reviewers whose critical comments and

suggestions helped produce a much better end-product. The study has

been supported over the years by various funding, in particular with

regard to the Natural Environment Research Council and the Environ-

ment Agency of England and Wales, the latter also providing field

support in the case of the Kennet for some of the years of monitoring.

The present paper was also stimulated from the Catchment Hydrology,

Resources, Economics and Management (ChREAM) project, funded

under the joint ESRC, BBSRC and NERC Rural Economy and Land Use

(RELU) programme.

1384 C. Neal et al. / Science of the Total Environment 408 (2010) 1374–1385



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