Applied Geochemistry 21 (2006) 1570–1592

www.elsevier.com/locate/apgeochem

AppliedGeochemistry

Hydrochemical evidence of the depth of penetrationof anthropogenic recharge in sandstone aquifers underlying

two mature cities in the UK

R.G. Taylor a,*, A.A. Cronin b, D.N. Lerner c, J.H. Tellam d, S.H. Bottrell e,J. Rueedi b, M.H. Barrett b

a Department of Geography, University College London, Gower Street, London WC1E 6BT, UKb Robens Centre for Public and Environmental Health, University of Surrey, Guildford, Surrey GU2 7XH, UK

c Groundwater Protection and Restoration Group, Kroll Research Institute, Sheffield S7 7HQ, UKd Hydrogeology Research Group, School of Geography, Earth and Environmental Sciences, University of Birmingham,

Birmingham B15 2TT, UKe School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK

Received 1 November 2005; accepted 21 June 2006

Available online 23 August 2006Editorial handling by K.G. Taylor

Abstract

Pollution of urban groundwater is routinely reported but the profile of contamination with depth in urban aquifers israrely resolved. This limits understanding of the depth of penetration of urban recharge and contaminants, and use ofurban groundwater. Penetration of anthropogenic solutes (major ions, trace metals) in Permo-Triassic sandstone aquifersunderlying two mature conurbations in the UK was investigated through depth-specific, groundwater sampling of dedi-cated multilevel piezometers. Identification of solute origin and biogeochemical processes (e.g. denitrification, mineral dis-solution) was aided by use of stable isotope ratios (34S/32S, 18O/16O, 15N/14N, 13C/12C) and chemical speciation modelling(PHREEQC). Depth profiles of aquifer hydrochemistry reveal penetration of anthropogenic solutes to depths of between30 and 47 m below ground in the unconfined sandstone and confirm the contributions of faecal and industrial effluents tourban recharge. They also highlight the complexity of solute loading and difficulty resolving solute origin from the range ofpotential sources in urban groundwater. Faecally-derived NO3 is the most pervasive contaminant exceeding drinking-water quality guidelines and is associated with elevated concentrations of B and SO4. Elevated concentrations of Li, B,Cr and Co are observed at depth in groundwater contaminated by long-term industrial land use (metalworking). Observedpenetration of anthropogenic solutes in the unconfined sandstone is consistent with post-development recharge of urbangroundwater (residence times <230 a) indicated by flow modelling, and suggests tentatively that urban abstraction todepths of up to 50 m below ground in the unconfined Permo-Triassic sandstone is required to scavenge contaminatedgroundwater.� 2006 Elsevier Ltd. All rights reserved.

0883-2927/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.apgeochem.2006.06.015

* Corresponding author. Tel.: +44 207 679 0591; fax: +44 207 679 0565.E-mail address: r.taylor@geog.ucl.ac.uk (R.G. Taylor).

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1571

1. Introduction

Mature conurbations underlain by a shallowaquifer commonly share several inter-related char-acteristics: polluted shallow groundwater; a relianceupon water imported to the city; and rising ground-water levels as a result of reduced abstraction andleakage of imported water from pressurised watermains (e.g. Lerner, 1986; Lucas and Robinson,1995; Bruce and McMahon, 1996; Vazquez et al.,1997; Foster et al., 1998; Bonomi, 1999; Riemann,1999). These characteristics are found in a numberof cities in the United Kingdom including Liver-pool, Birmingham and Nottingham that are under-lain, in part, by unconfined sandstone aquifersof Permo-Triassic age (e.g. Rushton et al., 1988;Brassington, 1990; Greswell et al., 1994; Croninand Lerner, 2004). Abstraction of urban groundwa-ter for public and industrial supplies in order tooffset rising groundwater levels and reduce depen-dence upon water imported to the city has beenproposed (Lerner, 1996) but is constrained, in prac-tice, by a lack of information concerning the exis-tence of useable groundwater resources at depthwithin these urban aquifers.

Hydrochemical investigations of urban sand-stone aquifers in the UK have identified both inor-ganic contamination by metals (e.g. Cr) and NO3

(Ford et al., 1992; Ford and Tellam, 1994; Tellam,1994; Rivers et al., 1996; Barrett et al., 1997), andorganic contamination by chlorinated hydrocarbonsolvents, primarily trichloroethene (Rivett et al.,1990; Burston et al., 1993; Barrett et al., 1997; Shep-herd et al., 2006). A limitation of this work is, how-ever, its primary reliance upon sampling fromunlined, industrial boreholes that are open throughlarge vertical sections of the aquifer (i.e. depth-inte-grated sampling). Depth variations in groundwaterquality within urban areas of the Permo-Triassicsandstone remain very poorly resolved but lead tohighly variable well water chemistry that is stronglydependent on the history of pumping (Tellam andThomas, 2002). The dearth of depth-specific hydro-chemical data limits understanding of not only theexistence of useable, urban groundwater but alsothe rate of penetration of urban recharge and,hence, the vulnerability of urban sandstone aquifersto contamination.

Some recent research in the UK has focused uponresolving depth variations in the quality of urbangroundwater within Permo-Triassic sandstone aqui-fers through the application of bundled multilevel

piezometers at dedicated monitoring sites (Tayloret al., 2000, 2003). Previous depth-specific ground-water sampling in the Permo-Triassic sandstone inthe UK has been concerned with point sources ofgroundwater pollution (e.g. Nazari et al., 1993;Thornton et al., 2001) or employed inflatable packerassemblies and porewater sampling (e.g., Brassing-ton and Walthall, 1985; Price and Williams, 1993;Tellam, 1994; Stagg et al., 1998) that do not facilitateroutine monitoring at multiple locations. This papertraces the depth of penetration and origin of anthro-pogenic solutes (i.e. major ions, trace metals) identi-fied through regular monitoring of groundwaterquality at 4 multilevel piezometer installations intwo cities, Birmingham and Nottingham. Previousanalyses of depth-specific observations at theseinstallations have examined the penetration of faecalmicroorganisms (Powell et al., 2003), temporal vari-ations in anionic chemistry (Cronin et al., 2003), andvertical hydraulic gradients compelling advectivetransport (Taylor et al., 2003). This paper analysesdominant water-rock interactions (i.e. naturalcontrols on aquifer hydrochemistry) and origin ofsolutes using stable isotope ratios of C, N, O and Sin aqueous and mineral phases, chemical speciationmodelling in PHREEQC (Parkhurst and Appelo,1999), and molar ratios of solute concentrations.Observations from monitoring stations in the uncon-fined sandstone are contrasted with those within aconfined section of the Permo-Triassic sandstoneand its overlying confining unit of mudstone.

2. Hydrogeological setting

2.1. River Trent basin

Birmingham and Nottingham are situated on out-crops of Permo-Triassic sandstone, known as theSherwood Sandstone Group (SSG), in the RiverTrent Basin of central England (Fig. 1). The citiesfeature comparable, contemporary population den-sities of approximately 3000–4000 people per km2

(1997) and distributions in land use (Table 1). InNottingham, a significant shift from agricultural toresidential land use has occurred over the 20th cen-tury. Coincident with this trend has been a rise inindustrial and commercial/institutional land uses.Discrepancies in land-use distributions between thecities in Table 1 arise, in part, from different analyt-ical methods and the fact that data for Birminghamare restricted to the area underlain by the uncon-fined sandstone aquifer. The hydrogeological setting

Fig. 1. Surface geology of the Trent River Basin, UK (adapted from British Geological Survey, 1981) including (as insets) surface geologyand regional contours of hydraulic head in the Nottingham area (adapted from Charlsley et al., 1990) and Birmingham area (adapted fromJackson and Lloyd, 1983). Note: contours are reported in metres above sea level.

Table 1Distribution of land uses for (1) areas underlain by theunconfined aquifer in Birmingham (Thomas and Tellam, 2006)and (2) the city of Nottingham in 1901 and 1991 (Davison et al.,2002)

Land use/cover Birmingham Nottingham

% Area(111 km2)(1980–2000)

% Area(64.4 km2)(1901)

% Area(82.2 km2)(1991)

Residential 50.8 19.0 52.6Agricultural 2.6 59.5 13.9Recreational 19.2 12.4 13.2Commercial/

Institutional10.1 2.8 7.3

Industrial 2.6 2.1 7.0Waste 0.7 0.1 3.3Transport 12.8 3.5 1.8Water 1.2 0.9 0.8

1572 R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592

in these cities has recently been described (Tayloret al., 2003) and is briefly summarized below. Forcomprehensive reviews, consult Jackson and Lloyd(1983), and Charlsley et al. (1990). The base of theSSG is marked by Permo-Carboniferous deposits(‘‘Coal Measures’’) in Birmingham and carbonaterocks (marl, dolomitic limestone) of Permian agein Nottingham. The SSG comprises a red-bed seriesof fluvial and aeolian sandstones with thin (<1 m)mudstone layers. Fractures increase the transmis-sivity of sandstone beds and facilitate transport ofsolutes and microorganisms by groundwater (e.g.Stagg et al., 1998; Powell et al., 2003). In each city,the sandstone sequence dips to the east where it isconfined by mudstone deposits belonging to theMercia Mudstone Group (MMG). The lithologicaltransition from sandstone to mudstone is considered

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1573

to reflect a shift in deposition by fast-flowing riversto intermittent rivers and ephemeral lakes as a resultof increasing aridity during the Triassic.

2.2. Birmingham

Groundwater flow in Birmingham is partly con-trolled by a regional fault (the Birmingham Fault)that has a SSW strike but variable throw (Fig. 1a).The fault displaces the SSG that outcrops to thewest of the fault whereas, to the east, it is confinedby the overlying MMG. Hydraulic data suggestthat the Birmingham Fault inhibits groundwaterflow (Knipe et al., 1992; Greswell et al., 1994)though there is hydrochemical evidence of flowacross the fault in the vicinity of the Tame Valley(Ford and Tellam, 1994). Birmingham, like Not-tingham, experienced rapid industrial growth dur-ing the latter half of the 19th century. Associatedwith this growth has been increased use of metalsand organic solvents for the manufacture of metalproducts and telecommunications equipment. Useof urban groundwater for public water suppliesdeclined at the beginning of the 20th century afterthe city of Birmingham established water reservoirsin Wales. Nevertheless, intensive abstraction ofurban groundwater for industrial use continuedand reached a peak in the early 1950s (Greswellet al., 1994). A decline in the manufacturing sectorduring the latter half of 20th century correspondedwith decreased abstraction and a subsequent rise inurban groundwater levels (Knipe et al., 1992; Tay-lor et al., 2003) though continued abstraction atsome locations reduces or reverses this trend. Hyd-rochemical (Cl and NO3) and C isotope data showthat recently recharged groundwaters exist withinthe sandstone on the west side of the BirminghamFault whereas older groundwaters exhibiting lowerconcentrations of Cl and NO3 and reduced 14Cactivity occur where the aquifer is confined bythe MMG (Jackson and Lloyd, 1983; Ford andTellam, 1994).

2.3. Nottingham

In Nottingham, groundwater in the SSG flowsfrom outcrop in a southeastern direction towardthe River Trent (Fig. 1b). Considerable uncertaintyexists regarding groundwater flow in the confinedsandstone aquifer and around the River Trent dueprimarily to a paucity of measurements. Similar to

Birmingham, Nottingham features a long historyof groundwater abstraction that extends back tothe late 17th century as well as long-term use of met-als and organic solvents in the manufacture of metalproducts (e.g. bicycles), pharmaceuticals and tele-communications equipment. Analyses of groundwa-ter abstracted from public water supply boreholesfrom 1894 to 1907 (Lamplugh, 1914) indicate thaturban groundwater in the SSG was generally ofgood quality (e.g. total N < 8 mg L�1). In contrast,observed contamination of surface waters by sewage(e.g. Dover Beck) as early as 1869 (Ansted, 1878) ledto the abandonment of surface-water sources infavour of urban groundwater for public water sup-plies (Edwards, 1966). Over time, many public watersupply boreholes have been relocated primarily toareas north and west of the city centre (Croninet al., 2003). The reduction in urban groundwaterabstraction as a result of this transition and a gen-eral decline in manufacturing, led to a rise in urbangroundwater levels. This trend is not, however, uni-form as water levels continue to decline in areaswhere urban groundwater is still abstracted (Tayloret al., 2003). High NO3 concentrations in urbangroundwater frequently exceed drinking waterguidelines (Rivers et al., 1996; Barrett et al., 1997)and provide evidence of recent groundwaterrecharge. Isotopic studies (Rivers et al., 1996; Fuk-ada et al., 2004) indicate that sources of NO3 inurban groundwater include faecal matter (sewage,septic systems, manure) and soil organic N. HighestNO3 concentrations are found NW of the city whichis downgradient from rural Nottinghamshire whereelevated NO3 concentrations in the unconfinedsandstone derive from agricultural inputs (Wilsonet al., 1994).

3. Methodology

Depth-specific sampling of urban groundwaterwas conducted using bundled multilevel piezometersinstalled in dedicated boreholes drilled to depthsranging from 50 to 91 mbgl at two locations in eachof Birmingham (Witton, Bromford) and Notting-ham (Old Basford, Meadows) (Fig. 1). Criteria forsite selection included contrasting hydrogeologicalconditions (e.g. unconfined versus confined) andproximity to (but not located on) industrial pre-mises. In practice, siting of multilevel installationsdepended significantly upon permission from bothpublic and private landowners. A sequential pro-gramme of borehole drilling, geophysical logging,

1574 R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592

packer testing and multilevel piezometer installa-tions was executed between August 1999 and May2000. Details regarding site geology, geophysics,piezometer design, and piezometer constructionare described by Taylor et al. (2003). Each bundledmultilevel piezometer features 11 sampling ports(i.e. screen intervals) that were positioned toaccount for lithological heterogeneities detected bydownhole geophysics.

Hydrochemical sampling was conducted over 3intervals from June 2000 to March 2001. Unliketemporally variable concentrations in microorgan-isms (Cronin et al., 2003; Powell et al., 2003), soluteconcentrations remained highly consistent over thesampling period. Prior to sample collection, individ-ual piezometers were purged using a peristalticpump (Watson Marlow 6035) for a minimum of4 well volumes and until physico-chemical parame-ters including electrical conductivity (EC), pH,temperature (T) and redox potential (Eh) observedat the wellhead using a flowcell (isolated fromthe atmosphere), remained constant. This protocolwas regularly abandoned at one depth interval(9.6–9.8 mbgl) at one site (Meadows, Nottingham)where anoxic hydrochemical conditions failed tostabilise. In addition to measuring wellhead param-eters, including alkalinity, samples were filtered(0.45 lm membrane) and analysed for major ionsand trace metals at each sampling round. Samplesfor cation and metals were preserved through acid-ification (pH < 2) using HNO3. In June 2000,groundwater samples were collected specifically foranalysis of stable isotope ratios of N in NO3 andboth S and O in aqueous SO4. Groundwater sam-ples for C isotope analysis were collected in March2001. To aid the investigation of natural controlson aquifer hydrochemistry, stable isotope ratios ofC and S in mineral phases in selected drill cuttingswere also analysed.

Major ions were determined by ion chromatogra-phy whereas trace metals were analysed by ICP-AES and ICP-MS at the University of Sheffield.Charge balance errors in all analyses are less than10% and predominantly less than 5%. Stable isoto-pic ratios of N in groundwater were analysed atthe University of East Anglia using the method ofFukada et al. (2003). Analysis of stable isotoperatios of C in groundwater and rock cuttings takenduring piezometer construction was conducted bygas chromatography-isotope ratio mass spectrome-try (GC–IRMS) at Queen’s University, Belfastemploying methods outlined by Cronin et al.

(2005). Stable isotope ratios of S and O in aqueousSO4 recovered by precipitation as BaSO4, were ana-lysed at the University of Leeds following proce-dures described by Moncaster et al. (2000). Stableisotope ratios are expressed using the delta per mil(&) notation relative to an international referencestandard: dsample (&) = [(Rsample � Rstandard)/Rstandard] * 1000 where R is the 34S/32S, 18O/16O,15N/14N and 13C/12C ratio of the sample and inter-national reference standard (Canyon Diablo Troi-lite for S, Vienna Standard Mean Ocean Water forO and N, PD Belemnite for C). Chemical speciationmodelling was performed using PHREEQC v.2(Parkhurst and Appelo, 1999). Mineral phasesemployed in these models were based upon com-monly identified detrital minerals and cements inthe SSG and MMG (Edmunds et al., 1982; McKin-ley et al., 2001).

4. Results and discussion

Natural, biogeochemical controls on aquifer hyd-rochemistry observed with depth in Birminghamand Nottingham are first reviewed below. This isfollowed by detailed discussion at each sampling siteof the superimposition of anthropogenic solutes inurban recharge and their depth of penetration.Depth-specific hydrochemical data including well-head parameters, major ions, trace elements andstable isotope ratios of groundwater at each loca-tion are listed in Tables 2 and 3. Stable isotoperatios of C and S in acid-soluble matrix cementsat selected sites and depths are presented in Table 4.

4.1. Natural biogeochemical controls on groundwater

chemistry in the unconfined SSG

The principal, natural geochemical process indi-cated by the depth-specific hydrochemistry in theunconfined SSG at Witton (Fig. 2), Old Basford(Fig. 3) and Meadows (Fig. 4) is the dissolution ofcarbonate cements. At Witton, molar ratios ofnon-sulphate associated Ca to bicarbonate (mCa-SO4/mHCO3) and d13CDIC signatures (�13.7& to�14.8&) are consistent with calcite dissolution(Eq. (1)) under open-system conditions (Bath et al.,1978) where d13CDIC is controlled by soil CO2 (Eq.(2)). Similarly at Old Basford, congruent dissolutionof dolomite (Eq. (3)) and to a lesser extent calciteunder open-system conditions is indicated in shallowgroundwaters (8.0–30.3 mbgl) by d13CDIC signatures(�12.3& to �15.2&) and molar ratios of Mg to Ca

Tab

le2a

Dep

th-s

pec

ific

chem

istr

y(w

ellh

ead

par

amet

ers,

maj

or

ion

s)o

fgr

ou

nd

wat

erat

mu

ltil

evel

pie

zom

eter

site

sin

Bro

mfo

rd(s

ite

1)an

dW

itto

n(s

ite

2),

Bir

min

gham

Sit

eD

epth

(mb

gl)

EC

(lS

cm�

1)

Eh

(mV

)p

HT (�

C)

HC

O� 3

ðmg

L�

1Þ

Cl�

(mg

L�

1)

NO� 3

ðmg

L�

1Þ

SO� 4

ðmg

L�

1)

Na+

(mg

L�

1)

K+

(mg

L�

1)

Mg+

2

(mg

L�

1)

Ca+

2

(mg

L�

1)

Si

(mg

L�

1)

19.

2–9.

326

20+

592

7.56

11.4

123

12.8

1.5

1720

75.8

5.0

105

511

6.7

112

.4–1

2.5

2670

+66

57.

4410

.911

613

.0<

0.5

1730

59.0

4.5

104

528

6.2

118

.4–1

8.5

2670

+20

07.

4011

.411

613

.4<

0.5

1730

65.6

4.6

105

523

6.3

176

.0–7

6.2

2780

+27

48.

1012

.568

13.8

<0.

520

3077

.23.

611

851

55.

61

81.1

–81.

227

60+

305

8.06

12.2

6113

.8<

0.5

2040

78.8

4.0

120

519

5.8

189

.9–9

0.7

2810

+56

17.

6010

.763

12.9

<0.

518

8091

.93.

511

152

44.

8

29.

3–9.

562

1+

309

6.96

11.7

261

7.0

62.6

36.5

5.0

1.2

3.4

117

6.9

213

.5–1

3.7

613

+32

77.

0510

.725

39.

452

.647

.56.

62.

65.

911

06.

42

16.0

–16.

162

6+

338

6.90

10.6

246

10.6

57.1

50.2

6.4

2.2

6.1

113

6.6

221

.7–2

1.8

920

+34

86.

9410

.728

543

.954

.617

215

.72.

418

.715

67.

02

26.7

–26.

811

86+

340

6.81

10.3

337

68.4

74.1

281

23.2

2.7

14.1

185

7.6

234

.7–3

4.8

1258

+34

76.

8710

.434

878

.077

.528

626

.32.

512

.923

17.

52

38.7

–38.

911

12+

686

6.72

10.4

299

57.1

96.8

216

46.4

2.6

11.1

179

6.7

241

.8–4

2.0

1163

+64

36.

7610

.334

372

.753

.822

857

.43.

146

.613

67.

22

46.8

–47.

010

10+

589

6.88

10.3

293

72.0

51.3

197

50.0

2.7

31.1

138

6.5

251

.9–5

2.0

402

+36

67.

5010

.716

018

.429

.810

.96.

51.

53.

471

.05.

22

59.4

–60.

249

0+

343

7.25

10.7

226

11.3

40.2

24.8

9.1

1.3

3.4

87.5

5.6

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1575

(mMg/mCa) that are slightly less than unity (Fig. 3).Observed enrichment in 13C of DIC (�6.6& to�7.5&) deeper in the sandstone (35.1 and39.3 mbgl) at Old Basford suggests, however, thatcarbonate dissolution has continued under closed-system conditions. Incongruent dissolution of dolo-mite (Eq. (4)) is also indicated by a coincidental risein mMg/mCa to between 1.4 and 1.6

CaCO3ðsÞ þH2CO3ðaqÞ $ Caþ2ðaqÞ þ 2HCO�1

3ðaqÞ ð1ÞCO2ðgÞ þ 2H2OðaqÞ $ H3OþðaqÞ þHCO�1

3ðaqÞ ð2ÞCaMgðCO3Þ2ðsÞ þ 2H2CO3ðaqÞ

$ Caþ2ðaqÞ þMgþ2

ðaqÞ þ 4HCO�13ðaqÞ ð3Þ

CaMgðCO3Þ2ðsÞ þH2CO3ðaqÞ

$ CaCO3ðsÞ þMgþ2ðaqÞ þ 2HCO�1

3ðaqÞ ð4Þ

Relative to observations at Witton and Old Bas-ford, groundwater in the unconfined SSG at theMeadows site is highly mineralised featuring alkalin-ities of >600 mg CaCO3 L�1 and inferred HCO3 con-centrations of between 815 and 1100 mg L�1 (Table3, Fig. 4). As this site underlies a former wetland, oxi-dation of dissolved organic C (DOC) (Eq. (5)) and itssubsequent dissolution (Eq. (2)) are considered toexplain, in part, anomalously high alkalinities. Acomparatively low redox potential (e.g. +176 mV)routinely recorded at the first sampling interval(9.6–9.8 mbgl), suggests that respiration has con-sumed available dissolved O2. Stable isotope ratiosof N in NO3, highly enriched in 15N (d15NNO3 =+42&), clearly indicate that oxidation of DOC hasalso continued via denitrification (Eq. (6)). At thisdepth, an elevated concentration of Mn (0.28 mgL�1) may also derive from the oxidation of organicC through Mn reduction (Eq. (7)). Hydrogen sul-phide was detected by smell at the first sampling inter-val but enrichment in the heavy isotopes of residualSO4 that would result from significant SO4 reduction(Eq. (8)), is not observed (Figs. 4 and 5)

CH2OðaqÞ þO2ðaqÞ ! CO2ðgÞ þH2O ð5Þ5CH2OðaqÞ þ 4NO�3ðaqÞ

! 2N2ðgÞ þ 5HCO�3ðaqÞ þH2OþHOþ3ðaqÞ ð6ÞCHO2ðaqÞ þ 2MnO2ðsÞ þ 3H3OþðaqÞ

! 2Mnþ2ðaqÞ þHCO�3ðaqÞ þ 5H2O ð7Þ

2CHO2ðaqÞ þ SO�24ðaqÞ ! H2SðgÞ þ 2HCO�3ðaqÞ ð8Þ

d13CDIC signatures at the Meadows site vary con-siderably over the depth profile from �4.9& to�15.3& (Fig. 4, Table 3) but are incompatible with

Table 2bDepth-specific chemistry (trace elements, stable isotope ratios) of groundwater at multilevel piezometer sites in Bromford (site 1) and Witton (site 2), Birmingham

Site Depth(mbgl)

Sr(lg L�1)

Ba(lg L�1)

Li(lg L�1)

B(lg L�1)

Fe(lg L�1)

Cr(lg L�1)

Co(lg L�1)

Zn(lg L�1)

d14NNO3

(&)d34SSO4

(&)d18OSO4

(&)d13CDIC

(&)

1 9.2–9.3 6750 21 100 554 <2 <2 <2 39 n.a. +14.9 +11.3 n.a.1 12.4–12.5 7150 18 95 554 <2 <2 <2 25 n.a. +14.9 +11.4 �6.41 18.4–8.5 7500 12 102 630 5 <2 <2 11 n.a. +15.4 +12.4 �7.81 76.0–76.2 9200 5 110 749 9 <2 <2 10 n.a. +15.7 +13.6 �9.21 81.1–81.2 9650 6 109 769 <2 <2 <2 7 n.a. +15.9 +11.7 �9.91 89.9–90.7 10,500 16 120 861 5 <2 <2 34 n.a. +14.9 +12.6 �9.0

2 9.3–9.5 247 76 8 120 <2 <2 5.4 18 +4.7 +3.6 +2.1 �13.72 13.5–13.7 127 67 7 131 5 <2 <2 16 +5.6 +3.1 +3.2 n.a.2 16.0–16.1 119 68 8 125 <2 <2 <2 17 +6.3 +3.6 +3.1 �14.82 21.7–21.8 157 82 13 677 <2 78 5.1 22 +5.2 +2.8 +6.3 n.a.2 26.7–26.8 393 151 16 1750 <2 446 23.1 41 +7.1 +3.2 +7.0 �13.82 34.7–34.8 420 113 16 1620 3 386 20.4 45 +7.9 +2.9 +5.3 �13.82 38.7–38.9 328 121 21 642 <2 39 3.5 24 +10.7 +2.4 +4.5 �14.22 41.8–42.0 243 84 25 1560 <2 189 <2 24 +10.8 +3.6 +7.7 �14.32 46.8–47.0 294 110 24 703 3 52 <2 55 +5.3 +3.4 +5.8 �14.32 51.9–52.0 137 65 7 78 <2 <2 <2 22 +5.6 +2.9 �1.7 �13.82 59.4–60.2 173 61 8 104 <2 <2 9.6 16 +9.0 +1.3 +5.9 �14.3

n.a., not analysed.

1576R

.G.

Ta

ylo

ret

al.

/A

pp

liedG

eoch

emistry

21

(2

00

6)

15

70

–1

59

2

Table 3aDepth-specific chemistry (wellhead parameters, major ions) of groundwater at multilevel piezometer sites in Old Basford (site 3) and Meadows (site 4), Nottingham

Site Depth(mbgl)

EC(lS cm�1)

Eh

(mV)pH T

(�C)HCO�3ðmg L�1Þ

Cl�

(mg L�1)NO�3ðmg L�1Þ

SO�4ðmg L�1Þ

Na+

(mg L�1)K+

(mg L�1)Mg+2

(mg L�1)Ca+2

(mg L�1)Si(mg L�1)

3 8.0–8.1 905 384 7.25 12.9 237 54.6 60.4 116 31.3 5.0 48.7 102 6.13 11.0–11.2 911 384 7.56 14.7 245 52.8 56.9 112 35.7 4.7 48.0 96.0 7.03 15.0–15.1 882 378 7.43 12.4 200 61.1 48.8 102 32.9 4.0 46.0 93.9 4.33 18.0–18.2 903 361 7.71 12.4 267 52.0 37.8 108 128 4.1 25.4 51.8 6.03 22.1–22.2 821 372 7.41 12.0 189 57.5 38.9 102 37.5 3.8 38.1 86.7 5.13 25.9–26.1 678 368 7.97 11.9 190 44.7 42.7 80.5 48.9 3.9 29.1 59.4 4.03 30.2–30.3 522 375 7.90 11.5 143 34.0 35.8 49.3 20.2 1.8 25.3 55.6 3.43 35.1–35.3 525 343 8.56 12.5 184 30.3 28.0 33.1 61.0 4.1 25.4 25.5 3.03 39.1–39.3 493 325 8.04 13.5 154 33.2 28.9 27.0 23.1 3.2 31.5 36.6 3.63 42.2–42.3 1090 295 8.27 12.8 290 150 4.3 98.9 189 5.2 23.0 33.8 n.a.3 47.1–47.9 1480 303 7.65 12.8 254 282 3.7 31.2 205 6.2 37.3 58.5 2.8

4 9.6–9.8 1760 176 7.47 16.0 1100a 36.9 10.6 266 247 14.3 76.7 107 4.64 12.6–12.7 1640 363 6.88 13.1 815a 29.4 24.1 286 140 13.7 100 147 7.24 15.6–15.8 2130 363 6.89 13.0 971a 43.5 13.7 562 306 14.7 115 180 6.94 18.7–18.8 2070 360 6.86 12.0 961a 35.9 5.3 461 214 17.9 133 171 6.74 23.0–23.1 2140 356 6.71 12.7 986a 29.2 9.5 527 112 13.5 166 238 7.54 28.8–28.9 1930 356 6.80 12.6 844a 57.4 10.3 345 132 13.4 112 201 6.44 32.8–33.0 1910 326 6.78 12.6 937a 30.3 10.2 364 165 15.1 118 174 6.04 35.8–36.0 2110 315 6.70 12.7 942a 30.8 9.8 434 144 14.8 135 211 6.44 43.8–44.0 2680 290 6.56 12.8 937a 44.2 <0.5 923 157 19.4 168 350 6.14 47.8–48.5 2450 272 6.70 12.7 847a 69.1 <0.5 720 111 17.2 160 316 5.7

a Bicarbonate concentrations are inferred from alkalinity measurements and, depending upon the presence of organic anions, may be significantly lower.

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Table 3bDepth-specific chemistry (trace elements, stable isotope ratios) of groundwater at multilevel piezometer sites in Old Basford (site 3) and Meadows (site 4), Nottingham

Site Depth(mbgl)

Sr(lg L�1)

Ba(lg L�1)

Li(lg L�1)

B(lg L�1)

Al(lg L�1)

Fe(lg L�1)

Mn(lg L�1)

Cu(lg L�1)

Zn(lg L�1)

d14NNO3a

(&)d34SSO4

(&)d18OSO4

(&)d13CDIC

(&)

3 8.0–8.1 475 52 9 274 120 4 4 915 32 +10.3 +4.1 +6.5 �14.03 11.0–11.2 116 80 6 250 1360 418 7 538 29 +10.5 +3.4 +6.8 n.a.3 15.0–15.1 52 79 4 218 <2 <2 <0.5 5 39 +10.5 +3.2 +5.9 �15.03 18.0–18.2 59 99 3 165 1100 469 7 395 18 +9.9 +3.2 +5.3 �15.23 22.1–22.2 37 80 4 179 575 188 4 424 20 +10.7 +3.5 +5.7 �13.53 25.9–26.1 34 56 4 95 178 75 2 116 17 +9.3 +2.9 +6.8 �14.43 30.2–30.3 25 50 4 53 <2 <2 1 <2 14 +11.4 +2.0 +4.7 �12.33 35.1–35.3 53 426 5 22 506 63 3 340 8 +9.2 +2.1 +1.7 �6.63 39.1–39.3 71 105 3 26 701 128 3 243 30 +11.3 +0.1 +2.4 �7.53 42.2–42.3 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. +1.1 �0.8 n.a.3 47.1–47.9 61 247 5 23 <2 3 1 3 18 n.a. +2.5 +5.7 �6.9

4 9.6–9.8 198 71 6 403 <2 <2 275 <2 26 +42.2 �3.4 +7.0 �12.64 12.6–12.7 429 82 53 751 <2 <2 126 5 64 +24.3 �2.1 +6.5 �15.34 15.6–15.8 611 50 87 1670 <2 <2 323 7 90 +24.6 �2.6 +7.1 �11.94 18.7–18.8 593 57 83 1520 <2 <2 452 7 104 +26.6 +3.2 +7.6 �6.94 23.0–23.1 1020 51 97 2230 786 18 1150 876 138 +25.0 +3.0 +9.4 n.a.4 28.8–28.9 1210 65 73 1600 <2 <2 598 12 68 +34.2 +4.5 +8.5 �4.94 32.8–33.0 1190 56 72 1960 <2 <2 375 9 77 +33.0 +3.1 +7.5 �11.64 35.8–36.0 2580 42 100 2910 <2 <2 358 7 157 +34.9 +3.9 +8.5 �7.74 43.8–44.0 5620 35 137 4000 <2 73 244 6 122 n.a. +7.2 +9.5 �14.64 47.8–48.5 5570 29 142 3330 <2 800 466 5 66 n.a. +6.2 +9.0 �10.6

n.a., not analysed.a Data from Fukada et al. (2004).

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Table 4Stable isotope ratios (d13C, d34S) in solid phase of drill cuttings ofPermo-Triassic sediments in Birmingham and Nottingham

Site Depth(mbgl)

d13C(&)

d34Ssulphate

(&)d34Spyrite

(&)

Bromford 6–9 �0.3 n.a. n.a.16–17 �1.4 +17.4 n.a.24–27 �3.1 +17.0 n.a.42–45 �0.3 +17.0 n.a.87–90 �5.9 +13.3 n.a.90–93 �4.7 +14.6 �20.0

Old Basford 3–6 �10.7 +8.8 n.a.6–9 �3.3 n.a. n.a.12–15 �2.3 +12.0 n.a.18–21 �2.8 +9.9 �26.624–27 �3.3 n.a. �22.927–30 �4.2 n.a. n.a.

Meadows 10–12 �6.7 +14.3 n.a.15–18 �4.4 +13.3 n.a.18–21 �2.8 +7.7 n.a.24–27 �3.1 n.a. n.a.33–36 �4.6 +8.9 n.a.42–45 �4.2 n.a. n.a.48–51 �2.7 +8.9 n.a.

n.a., not analysed.

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1579

oxidation of DOC, which yields CO2 relativelydepleted in 13C (Clark and Fritz, 1997), as the solesource of HCO3 in groundwater. Groundwater issupersaturated with respect to calcite (SIcalcite =+0.25 to +0.44) but concentrations of Ca and Mgat shallow depths (up to 18.8 mbgl) are less than200 and 150 mg L�1, respectively (Table 3). Thehigh alkalinities originating from the oxidation oforganic C are, therefore, considered to result signif-icantly from the presence of organic anions (e.g.acetate) in solution. Sulphate in shallow groundwa-ters (9.6–12.7 mbgl) at the Meadows site is depletedin the heavy isotope (d34SSO4 = �3.4& to �2.1&)as would be expected from the oxidation of pyriteor H2S (Fig. 5). Increased SO4 concentrations below15.6 mbgl are considered to derive, in part, from dis-solution of gypsum. This is suggested by d34SSO4 sig-natures that become more enriched in 34S (Fig. 5,Tables 2 and 4), and coincidental increases in theconcentrations of Ca and Sr (r = 0.93); the latteris a common substitute for Ca in gypsum.

4.2. Natural biogeochemical controls on groundwater

chemistry in the MMG and confined SSG –Bromford, Birmingham

Dissolution of gypsum is the dominant geochem-ical control on groundwater chemistry observed

within the MMG (9.2–76.2 mbgl) and underlyingconfined Bromsgrove Sandstone Formation of theSSG (81.1–90.7 mbgl) at Bromford (Fig. 1, Table2). Groundwater is brackish and saturated withrespect to gypsum (SIgypsum = +0.01 to �0.11). Sta-ble isotope ratios of S and O in aqueousSO4(d34SSO4, d18OSO4) are enriched in heavy iso-topes (34S, 18O) (Fig. 5, Tables 2 and 4) and thusclearly identify gypsum as the source of aqueousSO4. Shallow groundwater (9.1–18.5 mbgl) in thefissured MMG is supersaturated with respect to cal-cite (SIcalcite = +0.31 to +0.14) and saturated withrespect to dolomite (SIdolomite = +0.10 to �0.26).d13CDIC of dissolved inorganic C (DIC) ranges from�6.4& to �9.9& (Table 2) and is enriched in 13Crelative to that (�14&) predicted for groundwaterin contact with soil CO2. As a result, dissolutionof calcite and dolomite (d13C = �0.3 to �5.9&) isconsidered to have evolved under closed-systemconditions. High Ca concentrations from gypsumdissolution favour displacement of exchangeableNa (i.e. cation exchange), as suggested by highmolar ratios of Na to Cl (mNa/mCl P 7.0), andrestrict HCO3 to relatively low concentrations(<125 mg L�1) due to the solubility of calcite.

Similar to observations of Ford and Tellam(1994), NO3 is detected in very low amounts(<2 mg L�1) in the shallow fissured MMG andundetected in the underlying SSG. Chloride concen-trations are also low (12.8–13.8 mg L�1) throughoutthe hydrochemical profile and consistent with con-centrations (<20 mg L�1) in uncontaminatedgroundwater in the SSG derived from atmosphericdeposition in Birmingham (Jackson and Lloyd,1983). The depth profile of solute chemistry atBromford indicates neither recent recharge nor con-tamination within the MMG and SSG. Advectivetransport from the surface is certainly inhibited byupward vertical hydraulic gradients (iv) that areobserved within the MMG (Taylor et al., 2003).

4.3. Penetration of anthropogenic solutes in urban

recharge

4.3.1. Witton, Birmingham

The multilevel piezometer in Witton is adjacentto playing fields within the Tame Valley, an areaof long-term industrial landuse (Ford and Tellam,1994), and samples groundwater at 11 intervalsbetween 9.3 and 60.2 mbgl. The piezometer is casedthrough a thin layer of Pleistocene-Holocene fluvio-lacustrine deposits and installed in the Kidderminster

2.0 3.0

34SSO4(‰)

50 150 250 500 1000

sulphate34S/32S

trace elements( -1)

C-1)

nitrate-1)

EC( -1)

sulphate-1)

1500

boronchromium

<1 10 30 40

6.0 8.0 10.0

15NNO3(‰)

40 60 80

nitrate15N/14N

hetero-geneities*

lacu

strin

ede

posi

tsfil

l0

5

10

15

20

25

30

35

40

45

50

55

60

depth(mbgl)

She

rwoo

d S

ands

tone

Gro

up (

Kid

derm

inst

er fo

rmat

ion)

6.8 7.0 7.2 500 1100 0-0.5-1.0

pH SI molar ratios molar ratios

calcitedolomite

7.4 700 900 -1.5

* symbols: fissure mudstone band coarse horizon

mMg/mHCO3

mCa-SO4/mHCO3

0.50.30.1

mMg/mCa

mNa/mCl

1.00.60.2

TTC: thermotolerant coliforms

Fig. 2. Depth-specific hydrochemistry of the Sherwood Sandstone Group at Witton, Birmingham (No ber, 2000). Thermoto nt coliform (TTC) bacteria counts derive fromPowell et al. (2003). Lithological heterogeneities were identified by geophysical logging (Taylor et al., 3).

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vem200

TT

20

lera

sulphate34S/32S

0.5 1.5 2.5 3.5

boron(μg·L-1)

40 60 80 100 120 100 200

δ34SSO4 (‰)

sulphate(mg·L-1)

< 40 μg·L-1

(atmosphericdeposition)

sewage-contaminatedrecharge

nitrate15N/14N

9.0 10.0 11.0

30 40 50 60

hetero-geneities*

depth(mbgl)

* symbols: fissure mudstone band coarse horizon

Sher

woo

d Sa

ndst

one

Gro

up (N

ottin

gham

Cas

tle /

Lent

on fo

rmat

ions

)Pe

rmia

n

0

5

10

15

20

25

30

35

40

45

50

7.4 7.8 8.2 800 1200 0 0.5 1.0

pH SI

calcitedolomite

δ13CDIC(‰)-14 -12 -10 -8

molar ratios1.0 2.0 3.0

mMg/mCamNa/mCl

δ15NNO3(‰)

nitrate(mg·L-1)

EC(μS·cm-1)

4.3 mg·L-1

3.7 mg·L-1

<1 1 2 3 4

TTC(cfu·100mL-1)

TTC: thermotolerant coliforms

Fig. 3. Depth-specific hydrochemistry of the Sherwood Sandstone Group at Old Basford, Nottingham (June 2000). Thermotolerant coliform (TTC) bacteria counts derive from Powellet al. (2003). Lithological heterogeneities were identified by geophysical logging (Taylor et al., 2003).

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25002000

lithologicalheterogeneities*

depth(mbgl)

7.26.8

fill

fluvi

algr

avel

sN

ottin

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Cas

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orm

atio

n

0

5

10

15

20

25

30

35

40

45

50

pH600400 800

-2 +2 +6

21 3 54

sulphate34S/32S

strontiumboronmanganese

δ34SSO4 (‰)

sulphate(mg·L-1)

trace elements(mg·L-1)

nitrate(mg·L-1)

TTC(cfu·100mL-1)

bicarbonate(mg·L-1)

EC(μS·cm-1)

dissolutionof evaporticgypsum

10 20

+10 +20 +30 +40

<0.5 mg·L-1

<0.5 mg·L-1

nitrate15N/14N

δ15NNO3(‰)

<1 5 10 15 20

bicarbonate13C/12C

1000900 1100

-12 -8

δ13CDIC(‰)TTC: thermotolerant coliforms* symbols: fissure mudstone band

Fig. 4. Depth-specific hydrochemistry of the Sherwood Sandstone Group at the Meadows, Nottingham (June, 2000). Bicarbonate concentrations are inferred from alkalinitymeasurements and, depending upon the presence of organic anions, may be significantly lower. Thermotolerant coliform (TTC) bacteria counts derive from Powell et al. (2003).Lithological heterogeneities were identified by geophysical logging (Taylor et al., 2003).

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δ34S S

O4

(per

mille

)

δ18OSO4 (per mille)

evaporitic gypsum

influenced by gypsumdissolution and sulphatereduction

atmospheric/urban

oxidation of reduced S compounds

soil mineralisation

shaded symbols denotean anthropogenic origin

Witton

Bromford

Old Basford

Meadows

-2.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

-4.0

4.0

8.0

12.0

16.0

Fig. 5. d34SSO4 (CDT) versus d18OSO4 (V-SMOW) in groundwater samples from multilevel piezometer sites in Birmingham andNottingham.

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1583

Formation of the unconfined SSG (Figs. 1 and 2).The shallowest sampling horizon occurs approxi-mately 1.8 m below the top of the sandstone and8.0 m below the water table (Fig. 2). Local abstrac-tion has induced a downward iv in the sandstone of0.09–0.01, measured in November 2000 to a depthof 47 mbgl. An upward iv of 0.1 exists between59.8 and 52.0 mbgl and coincides with a more trans-missive lithology (Fig. 2). The deepest piezometer(59.8 mbgl) terminates approximately 30 m abovethe base of the aquifer according to local boreholelogs collated by Ford (1990).

Nitrate in concentrations exceeding the drink-ing-water quality guideline value of 50 mg L�1

(WHO, 1993; EC, 1998) is observed from 9.3 to47.0 mbgl. Stable isotope ratios in aqueous nitrate(d15NNO3) at shallow depths (9.3–16.1 mbgl) rangefrom +4.7& to +6.3& and reflect N derived fromsoil mineralisation and application of inorganicfertilizers (Rivers et al., 1996; Kendall, 1998).Below this, trends of rising NO3 concentrationsand d15NNO3 values (+5.2& to +10.8&) to adepth of 38.9 mbgl indicate increased faecal con-tamination as enrichment in the heavy isotope(15N) results from the volatilisation of isotopicallylight ammonia (NH3) from faecal matter. The

potential contribution of acid wastes associatedwith local metalworking industries to NO3 concen-trations is unclear but recent analyses (McQuillan,2004) indicate that industrial nitric acids are lessenriched in 15N (d15NNO3 = �38& to +3.4&;N = 23) than NO3 observed in groundwaters atWitton. A faecal origin of NO3 is supported bydetection of thermotolerant coliforms (TTCs)(Fig. 2) and other bacterial indicators of faecalpollution (i.e., faecal streptococci, sulphite-reduc-ing clostridia) throughout the hydrochemical pro-file (Powell et al., 2003).

Aqueous SO4 is relatively depleted in 18O (i.e.d18OSO4 < +4&) at shallow depths (Fig. 5, Table2) and is, therefore, considered to trace SO4 froman isotopically lighter source of O (e.g. reducedinorganic S, soil) than SO4 derived from atmo-spheric deposition (Krouse and Mayer, 2000). Dee-per in the aquifer between 21.7 and 47.0 mbgl,d34SSO4 and d18OSO4 signatures (Figs. 2 and 5) sug-gest that elevated SO4 concentrations derive primar-ily from anthropogenic sources that potentiallyinclude atmospheric deposition affected by fossil-fuel combustion (Bottrell et al., 2000; Moncasteret al., 2000), sewage (van Dover et al., 1992), andindustrial acids (Hughes et al., 1999; Pilcher, 2005)

1584 R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592

rather than mineral sources (+14& to +22&) suchas gypsum (Table 4; Clark and Fritz, 1997; Zhaoet al., 1998; Bottrell et al., 2000). At depths of51.9–60.2 mbgl, concentrations SO4 of decline dra-matically to <25 mg L�1 along with Cl concentra-tions (<20 mg L�1) that are consistent with aprincipally meteoric origin (Jackson and Lloyd,1983; Moss and Edmunds, 1992).

Coincidental increases in the concentrations ofSO4 and trace elements between 21.7 and 26.7 mbglimply that the shallow zones (9.3–16.1 mbgl) receiverecharge from a less polluted, local source (playingfield) than deeper zones (21.7–47.0 mbgl) that areinfluenced by one or more plumes of contaminatedrecharge from surrounding industrial areas (Fig. 2).Boron, Cr, Co and Zn are detected in concentrationsof up to 1.7, 0.45, 0.023 and 0.045 mg L�1 respec-tively (Table 2), that exceed drinking-water qualityguideline values for B and Cr (WHO, 1993; EC,1998). Strong correlations (r > 0.95) among the con-centrations of these trace elements between 21.7 and38.9 mbgl are consistent with dilution from a singlepollutant source. Cr, Co and Zn are commonlyapplied metals in non-ferrous metalworking and B,in concentrations of >1.5 mg L�1 (Fig. 2), is consid-ered to derive significantly from use of boric acid inthe local metalworking industry (Ford and Tellam,1994). Boron concentrations arising from sewageeffluent are generally observed to be less than1.5 mg L�1 (Barrett et al., 1999; Morris et al.,2005). Correlations between the concentrations oftrace metals and both SO4 (r > 0.90) and Cl(r > 0.82) are slightly weaker, due in part to alterna-tive sources of these anions in urban groundwater,but suggest that H2SO4 and HCl are also compo-nents of the effluent from local, non-ferrous metal-working. The absence of a correlation betweenconcentrations of trace metals and NO3 (r < 0.03)suggest, similar to stable-isotope data, that local met-alworking is not a significant source of NO3 withdepth.

At depths of between 41.8 and 47.0 mbgl at Wit-ton, Co concentrations are below the detection limit(2 lg L�1) and the strength of correlations betweenconcentrations of trace elements (Cr, B) and SO4

slightly decreases (r > 0.86). It is unclear whetherthe differences represent two distinct plumes with acommon source (metalworking industry) or simplycontrasting pathways and pore volumes throughwhich contaminated recharge has passed. The originof Li in concentrations of up to 0.025 mg L�1

remains unclear. Although Li has been detected in

shallow (<10 m) piezometers in concentrationsranging from 0.2 to 0.4 mg L�1 in association witheffluent from non-ferrous metalworking industriesin the Tame Valley (Ford and Tellam, 1994), it ispoorly correlated with other trace-element concen-trations between 21.7 and 38.9 mbgl at Witton(r < 0.39). Lithium may alternatively derive fromsewage in which it has been observed in concentra-tions of between 0.01 and 0.02 mg L�1 (Morriset al., 2005). Increased concentrations of Cr relativeto other alloy elements such as Zn (Table 2) may, asproposed by Ford and Tellam (1994), stem from theincreased mobility of Cr at near-neutral pH in itsanionic form as chromate. The coincidental rise inthe anions of acid wastes (SO4, Cl, B) and trace met-als is consistent with the assertion of Ford et al.(1992) that metals may be mobilised by acidicrecharge though only a minor reduction in pH isobserved. A rise in pH at 51.9 mbgl coincides withdecreases in EC and the concentrations of trace met-als and anthropogenic anions (Fig. 2, Table 2).

4.3.2. Old Basford, Nottingham

In Old Basford, the monitoring site is locatedwithin a park surrounded by areas of long-termindustrial activity and long-term residential landuse.The multilevel piezometer penetrates the entirethickness of the SSG and extends 7 m into theunderlying Permian marl (Figs. 1 and 3). The shal-lowest sampling interval occurs at 8.0 mbgl, approx-imately 7.5 m below the top of the sandstone and2 m below the water table recorded in March2001. Water levels recorded from May, 2000 toMarch, 2001 indicate a slight downward iv (0.008)at shallow depths (8.1–18.1 mbgl), the latter is influ-enced by localised, shallow groundwater abstraction(Taylor et al., 2003). Below 22.1 mbgl, the ivreverses and is consistent with a groundwater dis-charge area. Vertical hydraulic gradients betweenthe sandstone and underlying marl are downward(0.01–0.04).

Nitrate concentrations exceed the drinking-waterquality guideline value of 50 mg L�1 (WHO, 1993;EC, 1998) at shallow depths (Fig. 3) and graduallydecrease with depth to 28.9 mg L�1 at the base ofthe SSG (Table 3). d15NNO3 ranges from +9.2&

to +11.3& and traces a faecal source of NO3. Thisis corroborated by the coincidental detection ofTTCs (Fig. 3) and other faecal microorganisms(Powell et al., 2003) throughout the hydrochemicalprofile. The observed decrease in NO3 concentra-tions with depth is considered to reflect historical

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1585

patterns of faecal loading either directly to soil orvia septics and sewers as the absence of both reduc-ing conditions in the SSG and enrichment in d15N ofaqueous NO3 is incompatible with significant deni-trification. Indeed, low concentrations of NO3

(�4 mg L�1) in the underlying Permian marl (Table3) are consistent with pre-industrial recharge to theSSG in the Nottingham area (Edmunds and Smed-ley, 2000).

Sulphate concentrations at Old Basford rangefrom 116 to 81 mg L�1 and correlate well (r = 0.82)with trends observed for NO3. At shallow depths(8.0–26.1 mbgl), stable isotope ratios of S and O inaqueous SO4 (d34SSO4 = +4.1& to +2.9&;d18OSO4 = +6.8& to +5.3&) (Figs. 3 and 5, Table3) reflect a predominantly anthropogenic originincluding one or more of atmospheric depositionfrom fossil-fuel combustion, sewage, and industrialacids (Hughes et al., 1999). A substantial decreasein SO4 concentrations with depth (30.3–39.3 mbgl)corresponds with a shift toward an isotopicallylighter (d34SSO4 = +2.1& to +0.1&; d18OSO4 =+2.4& to �0.8&), natural source of SO4 (Fig. 5).The oxidising conditions in groundwater (Table 3)and observed decline in d18OSO4 (Fig. 5, Table 3)refute the possibility that bacterial SO4 reduction(BSR) is responsible for the observed drop in aque-ous concentrations (Krouse and Mayer, 2000) sinced18OSO4 is enriched in 18O in residual sulphate duringBSR (Strebel et al., 1990).

The depth profile of B closely follows observedtrends in anthropogenic NO3 (r = 0.91) and SO4

(r = 0.94) concentrations in the SSG. B concentra-tions decrease by an order of magnitude from0.27 mg L�1 in shallow groundwaters to <0.03 mgL�1 toward the base of the sandstone. There is noobvious geochemical source of B in the SSG but Bconcentrations at the base of the SSG are consistentwith those derived from atmospheric deposition inNottingham (Edmunds et al., 1982). Strong correla-tions with SO4 and NO3 suggested that elevatedconcentrations of B in shallow groundwaters origi-nate from the same anthropogenic sources (i.e.,sewage, metalworking industries). As such, B con-centrations are consistent with the observed depth(30.2–30.3 mbgl) to which anthropogenic N and Shave penetrated the SSG at this site. The possiblecontribution of effluent from metalworking indus-tries to B concentrations is supported by elevatedconcentrations of Cu, Fe and Al exceeding drink-ing-water guidelines in shallow groundwaters(WHO, 1993; EC, 1998) that also decrease with

depth. The observed mobility of Al and Cu at nearneutral pH remains unclear but may arise fromorganic or inorganic complexation. In Birmingham,concentrations of Al complexed with F, have beenobserved in excess of 10 mg L�1 in industrially con-taminated groundwater discharging into the RiverTame (Ellis, 2003).

4.3.3. Meadows, Nottingham

The Meadows multilevel piezometer is located ina residential area that is 1 km south of Nottinghamcity centre and 200 m north of the River Trent(Fig. 1). The piezometer is cased through 4.0 m offill and 5.1 m of fluvial gravels, and installed intothe Nottingham Castle Formation of the SSG (Figs.1 and 4). Despite the proximity of the site to theTrent River, the iv through the sandstone is slightlydownward (0.003) and, hence, inconsistent with theexpectation of groundwater discharge to the TrentRiver. A fault that is located between the monitor-ing site and river, acts as a barrier to groundwaterflow (Taylor et al., 2003).

d15NNO3 signatures are highly enriched in 15N(d15NNO3 = +42&) in shallow groundwaters of theSSG (9.6–9.8 mbgl) through denitrification. Belowthis, a significant rise in the redox potential (Table3) is considered to result from mixing of oxic localrecharge and horizontal groundwater flow. This issupported by lower HCO3 concentrations andd15N signatures (d15NNO3 = +24& to +35&), lessenriched in 15N, that suggest NO3 has been sub-jected to significantly less denitrification than shal-lower groundwater in the sandstone. Fukada et al.(2004) applied stable isotopes of N and O in NO3

and the Rayleigh equation to confirm that isotopicenrichment from denitrification at the Meadowsoccurs from an isotopically heavy source of N(d15NNO3 = +13.7&) consistent with a faecal ori-gin. Detection of TTCs at this site (Fig. 4; Powellet al., 2003) supports this deduction. A progressivedecline in redox potential with depth starting at28.9 mbgl coincides with evidence of denitrificationfrom both d15NNO3 signatures that are relativelyenriched in 15N (+33& to +35&), and increasedHCO3 (dissolved inorganic C) concentrations. Atthe base of the hydrochemical profile between 43.8and 48.5 mbgl, NO3 is undetected (<0.5 mg L�1)but there is a doubling in SO4 concentrations (Table3) relative to overlying groundwater.

High concentrations of some trace elements (e.g.B, Li) reflect anthropogenic influences on ground-water chemistry deduced for N. Concentrations of

1586 R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592

B (0.4–4.0 mg L�1) exceed those that are expected toderive from leaky sewers alone (e.g. Fig. 3; Barrettet al., 1999; Morris et al., 2005). Over the hydro-chemical profile, B concentrations are well corre-lated with both Li (r = 0.93) and Sr (r = 0.90) andconsequently indicate a common source. Ford andTellam (1994) recorded high B, Li and Sr concentra-tions (1.8, 0.39, and 1.3 mg L�1, respectively) in ashallow piezometer (SB10) in the vicinity of a for-mer chemical storage area. The possibility that thegroundwater chemistry has been influenced, in part,by industrial effluent is supported by a sharp rise inmetal concentrations including Cu, Mn and Al at adepth of 23.0 mbgl (Table 3). It is unclear whetherthe mobility of these metals can be attributed tocomplexation or a fissure at this depth (Fig. 4) lim-iting sorption to Fe oxides in the SSG. A similardepth-specific plume of metal pollutants has, how-ever, been observed in association with fissure flowin the Daybrook area of Nottingham (Stagg et al.,1998; Taylor et al., 2000).

4.4. Depth of penetration of anthropogenic recharge

in urban groundwater

Groundwater sampled at specific depths frommultilevel piezometers in Birmingham and Notting-ham represents the flow or urban recharge throughdifferent pore volumes over a range of time periods.In both cities, borehole hydrographs (Taylor et al.,2003) and flow modelling studies (Knipe et al.,1992; Greswell et al., 1994; Trowsdale and Lerner,2003) indicate that natural flow regimes have beensignificantly disrupted by urban abstraction. InNottingham, Trowsdale and Lerner (2003) estimatefor the period of 1981–1990 that 85% of the urbanrecharge is captured by pumping wells and ground-water residence times in the unconfined aquifer donot exceed initial development of the city (i.e.<230 a). Depth profiles of aquifer hydrochemistry(Figs. 2–4) clearly show the influence of anthropo-genic recharge on urban groundwater and arebroadly consistent with residence times indicatedby flow modelling.

The observed depth-specific hydrochemistryunderlying Birmingham and Nottingham exposesthe complexity of solute loading to urban aquifersfrom multiple sources over a variety of timescales,and highlights a range of different pathways in theSSG. Anthropogenic SO4 and B are clearly detectedin groundwater to depths of 30.2 and 47.0 mbgl inOld Basford (Nottingham) and Witton (Birming-

ham), respectively (Figs. 2 and 3). Chloride concen-trations below these depths (Tables 2 and 3) areindicative of recharge derived primarily from atmo-spheric fallout in the SSG (Edmunds et al., 1982;Jackson and Lloyd, 1983; Edmunds and Smedley,2000). Deeper penetration of NO3 in concentrationsof 28.9 mg L�1 at the base of the SSG at Old Bas-ford (39.3 mbgl) and 40.2 mg L�1 at the deepestsampling interval (60.2 mbgl) at Witton, is consid-ered to stem from an earlier history of loading(Alves and Tellam, 2002) and thus represent longergroundwater residence times and flowlines. Faecalmicroorganisms (i.e., bacteria and viruses), detectedin low concentrations with depth in the SSG, traceconsiderably more rapid aquifer penetration ratesalong preferential pathways (e.g., fissures) (Tayloret al., 2004) than those indicated by depth profilesof anthropogenic solutes.

A cross-sectional summary of the dominantrecharge sources and their depth of penetration inthe unconfined SSG at each multilevel piezometersite is provided in Fig. 6. Localised infiltration ofanthropogenic NO3 is evident from the profile ofcontamination and stable isotope tracers at OldBasford (Fig. 6). Strong correlations among theconcentrations of NO3, B and SO4 with depth areconsidered to reflect loading from a common source(sewage, metalworking). At Witton (Fig. 6), thesuperimposition of a plume of recharge contami-nated by urban land use (e.g. sewerage, metalwork-ing) on aquifer hydrochemistry is evident from largecoincidental increases in pollutant concentrationsbelow a depth of 16.1 mbgl (15 m below the watertable). Discharges from local metalworking activi-ties via quenching pits, soakaways and sewers areconsidered to be a contributing source since concen-trations of Cr, Co, Zn (Fig. 3, Table 2) and conju-gate bases of acids (sulphuric, boric), commonlyused in this industry, rise concomitantly. At theMeadows, a spike in metal concentrations (Cu, Al)derived from industrial effluent is detected at23.0 mbgl and faecally-derived NO3, attenuated bydenitrification, has reached a depth of 36.0 mbgl(Fig. 6).

Evidence of recent urban recharge in the MMGand confined SSG at Bromford (Birmingham) isnot observed. The low permeability and upwardvertical hydraulic gradients (0.02–0.1) at this loca-tion inhibit transmission of recharge. Brackishgroundwater in the MMG and SSG is saturatedwith respect to gypsum and dominated by disso-lution of this cement. In Nottingham, numerical

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deeper penetration offaecal effluent (A)

Fig. 6. A cross-sectional summary of the dominant solute sources in urban recharge, their depth of penetration, and biogeochemical processes in urban areas of the unconfinedSherwood Sandstone aquifer traced at three locations: Witton (Birmingham), Old Basford (Nottingham), and The Meadows (Nottingham).

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1588 R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592

modelling of groundwater flow in confined areas ofthe SSG shows extended groundwater residencetimes that are in the order of 103–104 a (Trowsdaleand Lerner, 2003). Similar groundwater residencetimes in the confined SSG north of Nottinghamhave been estimated from geochemical data (Edm-unds and Smedley, 2000). At the Meadows site inNottingham, an area of reclaimed swamplandaround the River Trent, groundwater is stronglyinfluenced by the oxidation of organic C via a rangeof processes (Eqs. (5)–(8)) including denitrification.Observed reducing conditions and high DIC con-centrations (Fig. 4) are similar to groundwater con-taminated by landfill leachate (Fetter, 1999; Taylorand Allen, 2006). At this site, local faulting inhibitsgroundwater flow.

4.5. Tracing natural and anthropogenic solutes in

urban groundwater

Definitive classifications of the origin of solutesfrom stable isotope ratios are problematic in urbanaquifers where a wide range of potential sourcesexist and can share similar isotopic signatures. ForSO4, overlap in the d34SSO4 and d18OSO4 signaturesarises, in part, from the common source (e.g. Trias-sic gypsum) of many anthropogenic S-bearing com-pounds in the UK (Hughes et al., 1999) and inhibitsdiscrimination of these sources (e.g., fossil-fuel com-bustion, industrial acid wastes, fertilizers) based onstable-isotope ratios alone. Although slight overlapcan occur in the stable isotope ratios of N thatderive from soil organic N and faecal matter (Ken-dall, 1998), microbial tracers of faecal pollution (e.g.thermotolerant coliforms, enteroviruses) confirmthat contamination of recharge from faecal sourceshas occurred at each site.

Multiple solute sources in urban aquifers impairuse of proposed chemical indicators of groundwaterresidence time (Li) and sewage contamination (B)in the SSG. Elevated Li concentrations observedin association with other anthropogenic solutes indepth-specific groundwater samples from Wittonand the Meadows violate the presumption of asolely geological origin in the SSG that has beenassumed over long flow systems on millennial time-scales in rural areas (Edmunds and Smedley, 2000).Caution is therefore, required in the application ofLi as a residence-time indicator in urban groundwa-ter and rural areas where urban groundwater mayhave migrated. Boron, recommended as a tracer ofsewage contributions to urban recharge (Barrett

et al., 1999), was detected in high concentrationsin association with not only domestic sewage (e.g.Old Basford) but also contamination from metal-working activities at both Witton and Meadowssites.

Ford and Tellam (1994) suggest that barite(BaSO4) saturation indices can act as a guide to ver-tical stratification of groundwater quality in the Bir-mingham aquifer. This is supported by observationsat Witton where a shift from saturated (SIbarite =+0.81) to unsaturated (SIbarite = �0.40) conditionswith respect to barite corresponds with the decreasefrom relatively high anthropogenic concentrationsof SO4 to naturally low concentrations (Fig. 2).The SIbarite ‘guide’ depends, in part, upon the avail-ability of lithogenic barite, the solubility of which isconsidered to control Ba concentrations. In the SSGat Old Basford, barite saturation indices are unableto show clear trends in groundwater quality becausea decrease to naturally low SO4 concentrationsbelow 30.3 mbgl (Table 3) coincides with an increasein Ba concentrations and groundwaters remain sat-urated with respect to barite (SIbarite > +0.20).

4.6. Development of urban groundwater in the

unconfined SSG

Depth-specific observations of aquifer hydro-chemistry in the Permo-Triassic sandstone are lim-ited to 4 locations in two cities so inferencesregarding the development of urban groundwaterare tentative and should be regarded with caution.Nevertheless, the hydrochemical profiles derivedfrom dedicated multilevel piezometers provide a pre-liminary indication of the depth of penetration andsources of polluted urban recharge. Faecal matterand industrial effluent are identified as the mostprominent sources of contamination to urbangroundwater in the unconfined SSG. Trace elements(B, Cr) were detected in concentrations that exceeddrinking-water quality guidelines at depths of upto 47 mbgl in areas of long-term industrial land use(metalworking). Faecally-derived NO3 is, however,the most pervasive risk to water quality and wasdetected in elevated concentrations (>45 mg L�1)to depths of between 30 and 47 mbgl. This findingis consistent with contemporary studies of urbanrecharge to the SSG that show sewer exfiltration rep-resent 5% of the total recharge flux (211 mm a�1) inNottingham (Yang et al., 1999) and between 10%and 25% of the total recharge flux (150–200 mm a�1)in Doncaster (J. Rueedi, personal communication).

R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592 1589

As the residence time of NO3 at depth in the aquiferremains unclear, the contribution of other faecalsources of N such as septic tank discharges andurban animals (Alves and Tellam, 2002) predatingurban sewerage must be considered. Seasonal corre-lations between faecal viruses (Noroviruses, Coxsac-kievirus B4) discharged to sewers (i.e. shed fromhuman population) and those detected at depth ingroundwater at Old Basford in Nottingham (Powellet al., 2003) confirm, nevertheless, that leaky sewerscurrently contribute to urban recharge.

Hydrochemical profiles in the SSG support deduc-tions of Ford and Tellam (1994) in Birmingham thatdeeper groundwaters dilute contaminated ground-water entering open boreholes at shallow depths.These data provide, furthermore, an initial indicationof the depths (i.e. between 30 and 47 mbgl) whereurban groundwater relatively uncontaminated byanthropogenic solutes in the SSG may be found. Asanthropogenic solutes reach greater depths in theSSG, well discharges, particularly from shallow bore-holes (<50 m in depth), are increasingly expected toyield groundwater of unacceptable chemical qualityfor drinking water. Nevertheless, abstraction of shal-low urban groundwater, blended over large depthintervals (e.g. 10–50 mbgl), may serve industry orbe put to other uses such as augmentation of river-flow (e.g. River Trent). This would serve to offset ris-ing water levels and restrict drawdown of chemicalcontaminants deeper into the aquifer. Alternatively,abstraction of relatively uncontaminated ground-water at depth (e.g. >50 mbgl) could offset risinggroundwater levels by inducing downward flow ofshallow groundwater. Such a strategy that reliesupon uncertain natural attenuation (e.g. denitrifica-tion) in the SSG of key chemical (NO3, Cr) andmicrobiological (enteroviruses) pollutants, carriesthe risk of drawing anthropogenic contaminationdeeper into urban aquifers.

5. Conclusions

Depth profiles of the hydrochemistry of ground-water sampled from multilevel piezometers underly-ing the mature conurbations of Birmingham andNottingham (UK) reveal penetration of anthropo-genic solutes to depths of between 30 and 47 mbglin the unconfined Sherwood Sandstone aquifer. Thedepth-specific aquifer hydrochemistry also exposesthe complexity of contaminant loading to urbangroundwater that results from multiple sources and

plumes that have migrated through different porevolumes over a range of time periods. The depth ofpenetration of anthropogenic solutes is consistentwith post-development recharge of groundwater (res-idence times <230 a) indicated by regional flow mod-eling in Nottingham. Penetration of a confining unitof mudstone (Mercia Mudstone Group) and underly-ing confined Sherwood Sandstone Group (SSG) byanthropogenic solutes in urban recharge is, however,not detected.

Stable isotope ratios of N, S, O and C constrainthe origin of solutes from a wide range of sourcesthat can occur in urban aquifers. Stable isotoperatios of N in NO3 trace the primary source ofNO3 in concentrations that commonly exceed thedrinking-water guideline limit of 50 mg L�1, to sew-age. Stable isotope ratios of S and O in SO4 indicatea range of possible anthropogenic sources includingsewage, atmospheric fall-out from fossil-fuel com-bustion, and industrial H2SO4 waste. Definitiveattribution (i.e. ‘fingerprinting’) of solute sourcesand resolution of their relative contributions toobserved solute concentrations, based on stable-iso-tope ratios, are hindered by the limited availabilityof stable-isotope measurements of anthropogeniceffluents (e.g. acid wastes, sewage) and common,ultimate origin of many anthropogenic compounds(e.g. Triassic gypsum). For NO3, regular detectionof additional tracers (faecal indicator bacteria, ente-roviruses) confirms the contribution of faecal wastesto urban recharge. Stable isotope ratios in SO4, dis-solved inorganic C and NO3 also indicate naturalgeochemical processes in the SSG including dissolu-tion of carbonate cements (calcite, dolomite) andgypsum, oxidation of reduced S (pyrite) and denitri-fication. Similar to NO3 and SO4, elevated concen-trations of trace elements (B, Cr, Co, Li) to depthsof between 30 and 47 mbgl in the SSG derive fromanthropogenic sources (e.g. metalworking, sewerleakage). Industrial sources of B undermine its spec-ificity as a proposed tracer of sewer leakage in urbangroundwater (Barrett et al., 1999).

Depth-specific measurements of solute and iso-tope chemistry confirm that faecally contaminatedwater is a significant component of urban rechargeto the unconfined SSG and highlights the impactof local industrial land-use on groundwater quality.Although this is consistent with recent water bal-ance studies that suggest leaky sewers contributeto urban recharge, the residence time of NO3 withdepth in the SSG and, hence, the relative contribu-tion with depth of contemporary (i.e. leaky sewers)

1590 R.G. Taylor et al. / Applied Geochemistry 21 (2006) 1570–1592

and historical (septic systems, animal wastes) dis-charges remain unclear. Coincidental detection offaecal microorganisms indicates considerably morerapid flowpaths within the SSG than solutes butconfirms faecal contamination in urban rechargeresults from present-day sewer leakage. The inter-face between recharge contaminated by anthropo-genic solutes and those primarily derived fromnatural geochemical or meteoric sources (i.e. pre-industrial) is observed at multilevel installations tooccur to depths of up to 50 mbgl. These observa-tions provide a preliminary estimation of the mini-mum depth at which urban abstraction would berequired to scavenge chemically contaminatedgroundwater in these aquifers.

Acknowledgements

Research was funded by the National Environ-ment Research Council (UK) under the URGENTprogramme (Grant No. GST021986). Numerousindividuals aided field research including OwenBaines (University of Bradford), Roger Livesey(University of Birmingham), Drs. Kathy Pond andKaren Powell (University of Surrey), and Drs.Sam Trowsdale and Ruth Davison (University ofSheffield). Research activities were facilitatedthrough cooperation with the UK EnvironmentAgency (Rob Harper, Phil Humble), Severn-TrentWater Plc (Rik Rodgers, Matt Hudson) as well asthe city councils of Birmingham and Nottingham.Support in the form of a Canada–UK MillenniumFellowship from the Natural Sciences and Engineer-ing Research Council of Canada to R. Taylor isacknowledged. We are grateful for the critical com-ments of an anonymous reviewer and Dr. AdrianBath that improved the manuscript.

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