Fenton et al Tearmann 2009

14 Pages·2010·0.33 MB·English

HUMPHREYS, AARTS, WATSON, WACHENDORF, GALL, TAUBE, PFLIMLIN: Sustainable options for
grassland-based dairy production in the northwest of Europe
Tearmann: Irish journal of agri-environmental research, 7, 129-142, 2009
Variations in travel time for N loading to
groundwaters in four case studies in Ireland:
Implications for policy makers and regulators
Owen Fenton1*, Catherine E. Coxon2, Atul H. Haria1, Brendan
Horan3, James Humphreys3, Paul Johnson2, Paul Murphy3,
Magdalena Necpalova3, Alina Premrov1, 2 and Karl G. Richards1
1Teagasc, Johnstown Castle, Environment Research Centre, Wexford, Ireland
2School of Natural Sciences, Geology, Trinity College, Dublin 2, Ireland
3Teagasc, Moorepark Research Centre, Cork, Ireland
*Author for correspondence: E-mail: [email protected], Tel.: 00 353 (0)53 9171271
Abstract
Mitigation measures to protect waterbodies must be implemented by 2012 to meet the requirements of
the EU Water Framework Directive. The efficacy of these measures will be assessed in 2015.
Whilst diffuse N pathways between source and receptor are generally long and complex, EU legislation
does not account for differences in hydrological travel time distributions that may result in different water
quality response times. The “lag time” between introducing mitigation measures and first improvements
in water quality is likely to be different in different catchments; a process that should be considered by pol-
icy makers and catchment managers. Many examples of travel time variations have been quoted in the lit-
erature but no Irish specific examples are available. Lag times based on initial nutrient breakthrough at four
contrasting sites were estimated to a receptor 500 m away from a source. Vertical travel times were esti-
mated using a combination of depth of infiltration calculations based on effective rainfall and subsoil phys-
ical parameters and existing hydrological tracer data. Horizontal travel times were estimated using a com-
bination of Darcian linear velocity calculations and existing tracer migration data. Total travel times,
assuming no biogeochemical processes, ranged from months to decades between the contrasting sites; the
shortest times occurred under thin soil/subsoil on karst limestone and the longest times through thick low
permeability soils/subsoils over poorly productive aquifers. Policy makers should consider hydrological lag
times when assessing the efficacy of mitigation measures introduced under the Water Framework
Directive. This lag time reflects complete flushing of a particular nutrient from source to receptor. Further
research is required to assess the potential mitigation of nitrate through denitrification along the pathway
from source to receptor.
Key Index Words:Lag time, Water Framework Directive, mitigation, nitrate
IInnttrroodduuccttiioonn ground and surface water pollution (Stark and
Intensification of agriculture poses a challenge Richards, 2008; Humphreys et al.,2009). The
to the sustainable management of soils, water Water Framework Directive (WFD; European
resources and biodiversity. Nitrogen losses Parliament and Council, 2000) attempts to
from agricultural areas can contribute to achieve at least “good ecological status” for all
129
Tearmann, the Irish journal of agri-environmental research, VOL 7, 2009
waterbodies by 2015 with programmes of a ‘groundwater body’ or aquifer, in the context
measures in place by 2012. The Nitrates of the WFD, in Ireland is defined in terms of
Directive (European Council, 1991) enacted the saturated zones in fractured bedrock or in
in the Republic of Ireland (from now on significant bodies of sand and gravel.
referred to as Ireland) in 2006 under Statutory Therefore, groundwater in subsoils, even
Instrument (SI) 101, 2009 is currently the though it is the initial recipient of leached
main legislative mitigation measure in place to nutrients,is not considered part of the aquifer.
achieve the goals of the WFD. The Nitrates In a recent study investigating groundwater
Directive sets limits on stocking rates on farms and surface water contributions to stream flow
in terms of the quantity of N from livestock in Ireland, subsurface soil and subsoil water
manure that can be applied mechanically or (with the exception of sand and gravel) were
directly deposited by grazing livestock on agri- termed interflow, and shallow groundwater
cultural land. A limit of 170 kg N ha-1year-1 were described as shallow bedrock groundwa-
from livestock manure was set. However, the ter where permeability is higher and fracturing
EU Nitrates Committee approved Ireland’s is more dominant (Jennings et al., 2007).
application for a derogation of this limit to Saturated subsoils (interflow) and an
allow grassland-based (mostly dairy) farmers underlying groundwater body (shallow and
to operate at up to 250 kg N ha1 year1 from deep groundwater) may often form a
livestock manures, under the understanding hydraulic continuum, but usually do not share
that this derogation will not impinge on meet- the same hydrogeological characteristics. This
ing the requirements of the Nitrates Directive. defining framework has clear implications for
The current average stocking density on dairy nutrient migration pathways and their man-
farms is 1.81 livestock units (LU) ha-1 agement. It is therefore important to investi-
(Humphreys, 2008). gate all scenarios that could contribute to
Nitrate (NO-) leaching pathways between water quality status. The components of lag
3
soils, groundwaters and rivers are generally time for nitrates are: vertical travel time, hori-
long and complex (Collins and McGonigle, zontal travel time, flushing of an aquifer to
2008) and such pathways vary depending on below a threshold value and travel time
soil/subsoil type, bedrock geolo- through the hyperoic zone. In this paper, ver-
gy/hydrogeology and climatic factors such as tical and horizontal travel times will only be
rainfall. The lag time between introducing considered. This travel time represents initial
protection measures and first improvements in or fastest breakthrough of nitrate to a surface
water quality is therefore likely to be different waterbody. Total lag time may be much longer.
in different catchments comprising different
soils and geologies and should be considered FFoouurr ccoonnttrraassttiinngg ccaassee ssttuuddiieess –– ssiittee cchhaarraacctteerr--
by policy makers and catchment managers iissttiiccss
(Kronvang et al., 2008). Such hydrological The objective of this paper was to compare the
travel times will be used in this study to indi- potential vertical and horizontal travel times of
catethe nutrient breakthrough response times nitrate to a virtual surface water receptor on
under different geologies and to categorise four contrasting study sites in Ireland. These
each site as high risk or otherwise. hydrological travel times were then used to
The saturated zone beneath a water table is indicate the likely first hydrological response
technically defined as groundwater, which is a times to be expected in a virtual receptor 500
principle receptor of water and leached nutri- m from the site. Groundwater vulnerability
ents from the unsaturated soil/subsoil. This categorisation is based on the thickness and
definition applies to groundwater protection permeability of sub-soil overlying bedrock
schemes. However, groundwater recognised as aquifers and the thickness of the unsaturated
130
FENTON, COXON, HARIA, HORAN, HUMPHREYS, JOHNSON, MURPHY, NECPALOVA, PREMROW, RICHARDS:
Variations in travel time for N loading to groundwaters in four case studies in Ireland:
Implications for policy makers and regulators
zone in sand and gravel aquifers (Misstear and prises 1.69% of the land area of Ireland, and
Brown, 2008). Each site was assigned a vul- soils with similar vulnerability extend to 4.6%
nerability category: of the country (Ryan et al., 2006). The
Oak Park, Co. Carlow – high vulnerabili- Teagasc research farm at Curtins, Moorepark
ty (unsaturated zone sand and gravel thickness is located within a lowland limestone area.
> 3 m); Moorepark, Co. Cork – extreme vul- The farm presently evaluates alternative man-
nerability (moderate permeability soils agement systems for spring calving pasture-
0.5–4.0 m thick, karst); Solohead, Co. based milk production. Well-drained soils are
Tipperary – moderate vulnerability (low per- present to a depth of 4.5 m. Due to the under-
meability soils, 5–10 m thick); Johnstown lying karstified limestone aquifer, groundwa-
Castle, Co. Wexford – moderate vulnerability ter flow direction is not uniform on site.
(low permeability soils, 5–10 m thick). Depth to bedrock approximately is 2.5 m and
depth to groundwater is approximately 29 m.
Sand and gravel aquifer,Oak Park, Co. Carlow
This case study was based on hydrogeological Poorly productive aquifer, Solohead, Co.
investigations performed by Premrov et al. Tipperary.
-
(2007 and 2008), and on a bromide (Br ) The Teagasc research facility at Solohead is
tracer study initiated by Hooker (2005). located on apoorly drained till to a depth of 5
Hydrogeological investigations were carried m, overlying a poorly productive Devonian
out on a 10 ha area, underlain by a shallow sandstone and mudstone aquifer >10 m
sand and gravel aquifer with inter-bedded depth. The site is undulating with occasional
silt/clay lenses (Premrov et al., 2007 and emergence of sand and gravel at the surface.
-
2008). The Br experiment was carried out on This is a heavy textured clay loam soil (25%
an adjacent field approximately 200 to 300 m sand and 42% clay). Heavy textured soils clas-
south-east of the case study site (Hooker, sified as clay, clay loam or silty clay loam rep-
2005). Sand and gravel aquifers underlie resent 32% of agricultural soils in Ireland
approximately 2% of Ireland and are the only (Humphreys et al., 2008). The boundary
aquifers with solely intergranular permeability. between the till and underlying bedrock is not
Such aquifers are usually unconfined and are uniform and the sand and gravel aquifer is not
generally thin; typically between 5 and 15 m wholly confined. Shallow groundwater flow
saturated thickness. Premrov et al. (2007) direction in the till may deviate from general
found mean concentrations at the start of the flow direction in the deeper confined aquifer
experiment in November 2006 in the unsatu- (Daly and Teillard, 2001). Concentrations of
rated zone, at 0.9 m below ground level (bgl), NO -N in shallow groundwater (1 m bgl,
3
of 59 mg NO -N L-1. Mean concentrations of below ground level) under four dairy grassland
3
35 mg NO -N L-1were observed in the satu- based systems from 2001–2002 were exam-
3
rated zone during the same period. ined by Humphreys et al.(2008). Losses of
NO -N to shallow (1 m bgl) groundwaters
3
Karst limestone aquifer, Curtins farm, Fermoy, were low; mean groundwater NO -N concen-
3
Co. Cork trations were <3.0 mg L-1. NO -N losses were
3
Atypically in the EU, Ireland has predomi- largely independent of N inputs, N surpluses,
nantly unconfined bedrock aquifers with fis- deposition of excreta-N at the soil surface and
sure permeability only. Carboniferous lime- residual mineral N in the soil at the start of
stone is the primary aquifer and occurs as out- drainage.
crop or shallow bedrock covering approxi-
mately 50% of Ireland. The free draining acid
brown earth, which dominates the site, com-
131
149 The Teagasc research facility at Johnstown Castle, located on heterogeneous glacial
150 deposits on a 4.2 ha gently sloping (2%) field, comprised six non-grazed plots on a
151 beef farm. The overburden, morainic in nature, varies in thickness from 1–20 m. This
152 is underlain by Pre-Cambrian greywacke, schist and massive schistose quartzites that
153 have been subjected to low grade metamorphism (Diamond, 1988). The surface
154 elevation increases sharply at the head of the plots (>71 m Above Ordnance Datum)
155 forming a hill (Sandhill) comprising both sand and fine loamy till. Some of this sand
156 may have been soliflucted, resulting in stratification between sand and underlying low
157 permeability fine till. The Sandhill is well to excessively drained, whereas the plots
158 are moderately to poorly drained. Shallow groundwater NO-N concentrations ranged
3
159 from background levels <1 mg NO-N L-1 to >11.3 mg NO-N L-1 (Fenton et al.,
3 3
160 2009).
161
162 Materials and methods
163 Calculation of vertical pathway travel time in soil/subsoil
164 Vertical water flow and leaching of nutrients through unsaturated soils (Fig. 1) are
Tearmann, the Irish journal of agri-env1i6r5o nminefnluteanlc erde sbeya rncegha,t i vVeO Lpo 7re, 2w0a0te9r pressure. Flow is facilitated by matrix and
166 gravitational potential gradients. Depending on the matrix potential, which is related
Poorly productive aquifer, Johnstown Castle, tribution or soil/subsoil type, and water con-
167 to the pore size distribution or soil/subsoil type, and water content in the unsaturated
Co. Wexford tent in the unsaturated zone, the hydraulic
The Teagasc research facility at Johnstow1n68 coznondeu, cthtiev hityyd rawuliilcl ccohndauncgtiev.i tyF woril l vcehratnigcea. lF otrr avveertlical travel time estimation, depth
Castle, located on heterogeneous glacial time estimation, depth of infiltration calcula-
169 of infiltration calculations based on effective drainage (from January 2005 - January
deposits on a 4.2 ha gently sloping (2%) field, tions based on effective rainfall (from January
comprised six non-grazed plots on a beef farm. 2005 - January 2008) and subsoil physical
170 2008) and subsoil physical parameters versus watertable depth were used as follows
The overburden, morainic in nature, varies in parameters versus watertable depth were used
thickness from 1–20 m. This is underlain b1y71 asf ofor lallol wcass ef ostru dailels :c ase studies:
Pre-Cambrian greywacke, schist and massive
ED
172 RF [1]
schistose quartzites that have been subjected = NED [1]
to low grade metamorphism (Diamond, RF
173 PV RF [2]
1988). The surface elevation increases sharpl1y73 PV = [ 2 ] [2]
= n
at the head of the plots (>71 m Above
PV
Ordnance Datum) forming a hill (Sandhill1)74 TD PV /1000 [ 3 ] [3]
174 TD= NED/1000 [3]
comprising both sand and fine loamy till. =
Some of this sand may have been soliflucted11,7755 whwwehhreeerr eeR RRFFF isii ss rerreeccchhhaaarrrgggeee fffllluuuxxx (((mmmmmm dd aadyya--11y))-,,1 )EE,DD E Diiss eeiffsffeeccttiivvee ddrraaiinnaaggee ((rraaiinnffaallll--aaccttuuaall
resulting in stratification between sand and effective rainfall (rainfall-actual evapotranspi-
176 evapotranspiration) for the study period (mm), NED is number of days recharge
underlying low permeability fine till. Th1e76 raetivoanpo)t rfaonrsp itrhatei onst) ufdoyr tphee rsitouddy (pmermio)d, (NmmE)D, NiEsD is number of days recharge
Sandhill is well to excessively drained, where1-77 nuomccburesr i no fth disa pyesr iroedc h(daarygse), oPVcc iusr pso irne vtehliosc iptye r(miomd day), n is effective porosity and
177 occurs in this period (days), PV is pore velocity (mm day), n is effective porosity and
593a s thFe ipgulortes Caraep mtioondse rately to poorly drained. (days), PV is pore velocity (mm day), nis
178 TD is total depth of infiltration during the study period (m). Depending on antecedent
Shallow groundwater NO -N concentration1s78 effTeDct iisv teo taplo dreopsthit yof ainnfidlt raTtiDon dius ritnogt tahle dsteupdyth p eroiofd (m). Depending on antecedent
3
594r ange d from background levels <1 mg NO -N179 insfoilitl rcaotnidointio nds,u erfifnecgt ivteh dera instauged my ayp oerr imoady n(omt o)c.cur and can be determined daily
3 179 soil conditions, effective drainage may or may not occur and can be determined daily
L-1 to >11.3 mg NO -N L-1 (Fenton et al., Depending on antecedent soil conditions,
5952 009F).igure 1. Generi3c diagram of vertica11l88 00a nedf fbbehyyco aatri visszppeoee ccrniiafftiiiaccnl fmm apoolaldd teemhll,,w aee..yagg y.. oSSsr cc thhomuu llttaaeey ee vttn iaarollt..tu ((a22ol00c 00cr55eu))c rff eooaprr ntggodrraarss ssllaanndd ssyysstteemmss iinn IIrreellaanndd uunnddeerr
181 cadni ffbeere ndte dtrearimnaignee cdo nddaitiiloyn sb (yM ao osrpepeacrikf,i cS omloohedaedl ,and Johnstown Castle). Premrov
596 500 m away. 181 different drainage conditions (Moorepark, Solohead and Johnstown Castle). Premrov
MMaatteerriiaallss aanndd mmeetthhooddss e.g. Schulte et al.(2005)for grassland systems
182 et al. (2009) has developed an extension of this model for tillage systems, based on
Calculation of vertical pathway travel time in182 ine It raell.a (n2d00 u9)n hdaesr ddeivfefleorpeendt adn reaxinteangsieo nc oonf dthiitsi omnosdel for tillage systems, based on
597
soil/subsoil 183 (Mreosuolrtse pfraormk ,t hSeo lOoahke Paadr ka ncads eJ osthundys tsoitwe.n M Cetaesotrloel)o.gical data was collected on sites
183 results from the Oak Park case study site. Meteorological data was collected on sites
Vertical water flow and leaching of nutrients Premrov et al.(2009) has developed an exten-
598 184 in Oak Park, Moorepark and Johnstown Castle. Solohead weather data was compiled
through unsaturated soils (Fig. 1) are influ1-84 sioinn Ooafk t hPaisrk m, Modooerle fpoarrk t ialnlad gJeo hsnyssttoewmn sC, absatlsee. dS oolnohead weather data was compiled
599e nced by negative pore water pressure. Flow i1s85 resuusilntgs sfirtoe msp ecthifeic Oraiankfa llP daarkta acnads et hset uredmya insiinteg. data from the Moorepark site.
185 using site specific rainfall data and the remaining data from the Moorepark site.
facilitated by matrix and gravitational poten- Meteorological data was collected on sites in
186 Where available, tracer experiment breakthrough data in shallow ceramic
600t ial Fgirgaudriee n1.t s . Depending on the matri1x86 OWakh ePrea rka,v Mailaoboler,e ptarrakce ra nedx pJeorhimnesntot wbnre aCkathsrtoleu.gh data in shallow ceramic
potential, which is related to the pore size dis1-87 Socluophs/epaidez owmeeattehrse rw dasa atals ow uasse dc o(Omapk iPleadrk uasnidn Mg osiotreepark).
187 cups/piezometers was also used (Oak Park and Moorepark).
188
Figure 1:Generic diagram of vertical and hor1i8z8o nta l pathways to a virtual receptor 500 m away.
500 m 189 Calculation of horizontal travel time in shallow groundwater
189 Calculation of horizontal travel time in shallow groundwater
190 Flows in the saturated zone (Fig 1.) are a function of the saturated hydraulic
Shallow/perched 190 Flows in the saturated zone (Fig 1.) are a function of the saturated hydraulic
groundwater Vertical pathway
191 conductivity (K ) and the po(tveanrtiiaabll eg rdaedpitehn)t, which in most cases is gravitational. In
191 conductivity (Ksat) and the potential gradient, which in most cases is gravitational. In
sat Horizontal pathway
192 the saturated zone, K remains constant at a particular location but varies spatially
192 tRhiev esraturated zone, Ksat remains constant at a particular location but varies spatially
sat Watertable
193 due to the heterogeneity of the aquifer and between different aquifers and geological
Bedrock aquifer 193 due to the heterogeneity of theS uabqsuoifile br eadnrodc bk etween different aquifers and geological
interface
e.g.11 L99im44 estouunnne iiattqss..u ifKKersat mmaayy aallssoo vvaarryy dduuee ttoo aanniissoottrrooppiieess iinn tthhee aaqquuiiffeerr.. HHoorriizzoonnttaall ttrraavveell ttiimmee
sat
195 estimation was calculated by effective Darcian linear velocity, v (m day-1). This can
195 estimation was calculated by effective Darcian linear velocity, v (m day-1). This can
196 be calculated from:
601 196 be calculated from:
6021 32
603
FENTON, COXON, HARIA, HORAN, HUMPHREYS, JOHNSON, MURPHY, NECPALOVA, PREMROW, RICHARDS:
Variations in travel time for N loading to groundwaters in four case studies in Ireland:
Implications for policy makers and regulators
specific rainfall data and the remaining data RReessuullttss
from the Moorepark site. Where available, The vertical and horizontal travel time results
tracer experiment breakthrough data in shal- from the individual case studies are presented
low ceramic cups/piezometers was also used below.
(Oak Park and Moorepark).
SSaanndd aanndd ggrraavveell aaqquuiiffeerr,, OOaakk PPaarrkk,, CCoo..
Calculation of horizontal travel time in shallow CCaarrllooww
groundwater Vertical travel time
Flows in the saturated zone (Fig 1.) are a func- Mean precipitation during the 2005–2008
tion of the saturated hydraulic conductivity period was 857.5 mm. The estimated vertical
(K ) and the potential gradient, which in matrix travel time of approximately 1 to 2
sat
most cases is gravitational. In the saturated years corresponded with mean total effective
zone, K remains constant at a particular rainfall of 350.2 mm. Effective drainage was
sat
location but varies spatially due to the hetero- estimated to have occurred on 89 days year-1
geneity of the aquifer and between different (mean value for 2005 to 2008), which equates
aquifers and geological units. K may also to an average vertical pore velocity of 15.7
sat
vary due to anisotropies in the aquifer. mm day-1 (n=25%) through the unsaturated
e
Horizontal travel time estimation was calcu- zone and depth of infiltration in this period of
lated by effective Darcian linear velocity, v (m 1.4 m. In the tracer experiment initiated by
day-1). This can be calculated from: Hooker in 2004, the breakthrough of Br-trac-
er occurred in suction cups at 0.9 m depth
1 dh
197 v K [ 4 ] ( n = 8 ) f r o m 2 7 t o 4 2 days after surface tracer
=− satn dx
e application (Hooker, 2005). This gives daily
198 w[4h] e re Ksatis estimated from slug injection vertical travel times of 0.02 to 0.03 m day-1
tests (Bouwer and Rice, 1976), hydraulic gra- (Hooker, 2005). In the saturated zone the
199 where K is estimated from slug injection tests (Bouwer and Rice, 1976), hydraulic
dient is staht e change in the watertable head over time of first occurrence of the tracer was after
200 tghraed idenist tiasn tchee cbheantwgee einn t htew woa tewrtealbllse ihne adt hoev erf itehled dist2an2c7e bedtwayese,n twwoh wileel lst ihne peak concentration
(dh/dx) and n is effective porosity calculated occurred 345 days after tracer application
201 the field (dh/dx)e and n is effective porosity calculated from soil cores. For the Oak
from soil cores. Fore the Oak Park case study, (Premrov et al., 2007). The average water-
202 KPsaarkt cvaasleu estsu dwye, rKes att avkaelune sf rwoemre ttahkee nli tfreormat uthree liatnerdaturet aanbdl ea rde einpdtihc atiinv e tohfi sI ripshe riod was 2 to 5 m bgl
are indicative of Irish fluvio-glacial sand and (Premrov et al.,2007 and 2008).
203 gflruavvieol- glaaqciuail fsearnsd. aKnd grvavaelul aeqsu iwfeersr.e K staht veanlu ecso wmer-e then combined with Premrov
sat
204 beit nael.d ( 2w00i7th) dPatrae tmo rcoalvc uelta tae lt.h e( 2li0ne0a7r )vedloactiaty t aon dc atrla-vel Htimoersi.z Ionn tthael Storalovheela tdi mcaese
culate the linear velocity and travel times. In Horizontal travel time must consider vertical
205 studies K and n values from the literature were used. Where these factors were not
the Solosahtead cease studies K and n values variations and lateral changes in the sediment
sat e
206 farvoamila bthlee, olirt ienr actirucruem wstaenrece us swehde.r We ashseersesm thenets oef f laince-ar vseeloqcuiteyn ics ev,e ryfr coomm plgerxa, vaes l to sand to clay/silt,
tors were not available, or in circumstances which can occur over quite short distances,
207 with karst or fractured bedrock aquifers (Moorepark), actual horizontal travel time
where assessment of linear velocity is very with domination of sand and gravel sediments
208 cuosimngp lteraxc,e ra sb rweaiktthh rokuagrhs ti no rw eflrlas cwtuarse dus ebde dtarkoecnk fromin B tahretl eayr e(a2 0(0P3r)e. mFroor vt heet al.,2007 and 2008). It
aquifers (Moorepark), actual horizontal travel can be seen from Table 1 that a range of hori-
209 Johnstown Castle study, K data (slug injection tests) from shallow groundwater
time using tracer breaksatthrough in wells was zontal travel times from 0.056 to 5.6 years is
210 upsieezdo mteatekresn ( Fefnrotomn etB aalr.,t l2e0y0 9() 2a0n0d 3n). dFatoa r( Ftehnteon eet xaple.,c 2te0d08 )f owre rae uflsuedv iotog lacial/mixed sand and
e
Johnstown Castle study, K data (slug injec- gravel aquifer.
sat
211 estimate horizontal travel time.
tion tests) from shallow groundwater
212 p iezometers (Fenton et al.,2009) and nedata Total travel time
(Fenton et al.,2008) were used to estimate The total travel time of approximately 1 to 6.6
213 hRoersiuzlotsn tal travel time. years is the combination of vertical travel time
214 The vertical and horizontal travel time results from the individual case studies are
215 presented below. 133
216
217 Sand and gravel aquifer, Oak Park, Co. Carlow
218 Vertical travel time
219 Mean precipitation during the 2005–2008 period was 857.5 mm. The estimated
220 vertical matrix travel time of approximately 1 to 2 years corresponded with mean total
221 effective drainage of 350.2 mm. Effective drainage was estimated to have occurred on
604 Te armann, the Irish journal of agri-environmental research, VOL 7, 2009
605 TabTlaeb 1le: 1O. Oaka kP Paarrkk ccaassee ssttuuddyy a aqquuifiefre pr apraamraemterest earnsd a enstdim eastteimd haoteridz ohnotarli ztroanvetal lt itmraevs.e l times.
606
Aquifer type K dh/dx n v Horizontal travel time
sat e

m day-1 % m day-1 years
Clayey gravel 0.1-1b 0.0122d 25g 4.9(cid:2)10-3 -0.488 f 280.7 - 2.81 f
Fluvio-glacial sand and gravel 100 -500a, b, c 0.0122d 25g 4.88-24.4 f 0.28 -0.056 f
Mixed sand and gravel h 5-100h 0.0122d 25g 0.244 -4.88 e 5.6 - 0.28 e
Homogeneous gravel h 100-1000h 0.0122d 25g 4.88-48.8 f 0.28 -0.03 f
607
608 a Based on Allen and Milenic (2003); b Based on Misstear et al. (2009); c Based on McConville et al. (2001); d From
609 Premrov et al. (2007); e From Premrov et al. (2007) - computed values; f This study; g From Fetter (2001); h From
610
Kruseman and de Ridder (1992), given for comparative purposes.
611

612 (ap proximately 1 year) through the unsaturat- m on site, which means travel time at different
ed zone (according to Hooker, 2005)and the locations will reflect this depth. Bartley (2003)
613 fas test horizontal travel times from Table 1 used a surface Br- tracer recharge experiment
through the saturated zone to a virtual surface to quantify the vertical unsaturated travel time
614 rec eptor 500 m away from the chosen site to ceramic cups at 1 m bgl and to the screened
(0.056 to 5.6 years - for fluvioglacial/mixed section of a well at 29 to 32 m bgl in ground-
615
sand and gravel). This reflects the best case water. Maximum vertical travel velocity due to
scenario in terms of lag time between changed preferential flow paths through the unsaturat-
616
practices and improved water quality. ed zone was estimated as 1.66 m day-1, giving
617 a travel time of approximately 17 days. On a
KKaarrsstt lliimmeessttoonnee aaqquuiiffeerr,, CCuurrttiinnss ffaarrmm,, neighbouring farm, also located on thin soils
618 FFeerr mmooyy,, CCoo.. CCoorrkk (depth to bedrock 1 m) underlying a karstified
Vertical travel time limestone, first occurrence of tracer in a
619
Mean precipitation during the 2005–2008 screened interval at approximately 22 m bgl
period was 953 mm. The vertical matrix trav- using the same methodology was 34 days
620
el time of approximately 1 to 2 years corre- (Richards et al.,2005). Maximum soil solu-
621 spo nded with mean total effective rainfall of tion concentrations were observed 34-65 days
437.2 mm. Mean effective rainfall was esti- after application, giving a vertical travel time
622 ma ted to have occurred on 158.5 days, which of 34 to 65 days (Richards et al.,2005). ). It
equates to an average vertical pore velocity of was concluded that matrix flow through the
623 10 .03 mm day-1through the unsaturated zone overburden and preferential flow through the
and depth of infiltration in this period of 1.2 remainder of the unsaturated zone (karst lime-
624
m. The depth to bedrock is approximately 1–4 stone) is quantitatively more important for
631
625
Table 2: Moorepark, horizontal travel time from K range and actual horizontal travel time
632 Table 2: Moorepark, horizontal travel time from K rasnagte and actual horizontal travel time from a
662363 from tra ac etrr aexcepre reimxpeenrt iimn tehnet kianr stth aeq ukifaerrs tt oa rqeuciefpetro rt o5 0rs0aet cme patwoary 5 f0ro0m m th ea wsoauyr cfer.o m the source.
634
627
Tracer Horizontal Travel time
Tracer breakthrough Horizontal Travel
K breakthrough at based on Tracer
sat at point 4 time based on K
628 point 10 sat
m day-1 days days time months

629 0.004 to 27a 44 51 Days to Years 2 to 3

635
663306 a Based on Bartley (2003), slug injection tests
637
638
134
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
FENTON, COXON, HARIA, HORAN, HUMPHREYS, JOHNSON, MURPHY, NECPALOVA, PREMROW, RICHARDS:
Variations in travel time for N loading to groundwaters in four case studies in Ireland:
Implications for policy makers and regulators
transport of conservative tracers togroundwa- responded with mean total effective rainfallof
ter. Over two monitoring years, the mean 481 mm. Mean effective rainfall was estimat-
annual farm average NO -N concentrations ed to have occurred on 91 days, which equates
3
were 11.9 and 15.2 NO -N L-1 with consid- to an average vertical pore velocity of 52 mm
3
erable temporal variation observed within day-1through the unsaturated zone and depth
years (Bartley, 2003). of infiltration in this period of 4.8 m. The
mean (mean watertable depth ±standard devi-
Horizontal travel time ation) perched shallow watertable on site dur-
The horizontal travel time in the karst aquifer ing this time varied from 1.5±0.3 m bgl (sum-
was difficult to estimate as considerable data mer) to 0.6±0.2m bgl (winter). Vertical trav-
was required (ne, fracture density, fracture el time to the perched groundwater is achiev-
truncation, fracture orientation, fracture trace able within one drainage season. Infiltration
length, fracture spacing, mechanical aperture, may take several years to reach the boundary
effective hydraulic aperture, and aperture of the sand and gravel confined layer. During
opening size) to estimate K values. A K this time period effective rainfallranged from
sat sat
range was available from slug injection tests 312 to 605 mm, which means that in some
(Bartley, 2003) but was not indicative or com- years vertical travel time in 1 year was not
parable with actual horizontal flow travel achievable.
times. Instead, tracer breakthrough in several
wells originating from the vertical travel time Horizontal travel time
study was used to calculate horizontal travel Horizontal travel times on the poorly produc-
time. The horizontal travel time to the recep- tive aquifer were estimated to be 3424 years in
tor 500 m away from the source following this the shallow perched groundwater to 152 years
pathway was estimated as 2 to 3 months. in the confined aquifer (Table 3). Estimates of
the horizontal travel time could be improved
Total travel time further through investigation of K on the
sat
Vertical travel time was estimated at 1 to 2 site.
years to allow for differences in soil depth.
Ceramic cup and borehole tracer data indicat- Total travel time
ed faster preferential flow paths on site. Once Combining vertical travel time of 3 years and
water and nutrients pass through the soil layer, the horizontal travel time of 3424 years for the
travel time increases rapidly. Horizontal travel shallow perched groundwater, the total travel
time was estimated as 2 to 3 months. Total time is estimated to be 3427 years. For zones
travel time from source to receptor following that are connected to the confined aquifer the
groundwater flow direction was approximate- travel time may be shorter.
ly 1.5 – 2.5 years and was dependent on soil
thickness. In such a complex environment a PPoooorrllyy pprroodduuccttiivvee aaqquuiiffeerr,, JJoohhnnssttoowwnn CCaassttllee,,
modelling exercise (e.g. global or distribution CCoo.. WWeexxffoorrdd
methods) at the correct scale should be under- Vertical travel time
taken. Mean precipitation during the 2005–2008
period was 1046 mm. The vertical matrix
PPoooorrllyy pprroodduuccttiivvee aaqquuiiffeerr,, SSoolloohheeaadd,, CCoo.. travel time of approximately 1 year corre-
TTiippppeerraarryy sponded with mean total effective rainfall of
Vertical travel time 553 mm. Mean effective rainfall was estimat-
Mean precipitation during the 2005–2008 ed to have occurred on 178 days, which
period was 1059 mm. The mean vertical equates to an average vertical pore velocity of
matrix travel time of approximately 1 year cor- 9.7 mm day-1 through the unsaturated zone
135
Tearmann, the Irish journal of agri-environmental research, VOL 7, 2009
657
658 TabTlaeb l3e: 3S: oSloolhoheeaadd,, hhoorriizzoonntatal lt rtarvaevl etli mtiem ine sihna llsohwal alonwd daenepde rd gereopuenrd wgraoteurn tdo wsuartfearc et ow astuerr fvaicretu awl ater
659 virrteucaelp rtoerc e5p0t0o mr 5a0w0a ym fr oamw athye f srooumrc et.h e source.
660
Groundwater Horizontal
layer K dh/dx n v Travel time
sat e
m day-1 % m day-1 years

A 0.01a 0.004 10c 0.0004 3424
B 0.224b 0.004 10c 0.00896 152
661
662
A Shallow perched groundwater
663
B Deeper Confined Aquifer
664 a, c Based on Fetter (2001)
665 b Based on Daly and Teillard (2001)
666
687
667
688 TTabablele 4 4:: JJoohhnnssttoowwnn C Casatsletl,e m, meane ahno rhizoorniztaoln trtaavl etlr ativmeel tinim siex imn osnixit omreodn piltootrse tdo pa lsoutrsf atcoe aw sauterrf ace water
68696 8 rerce ecpepttoorr 550000 mm a wawaya yfr ofrmo mth et hsoeu srcoeu. rce.
690
669 Horizontal
Plot K dh/dx n a v Travel time
sat e
m day-1 % m day-1 Years
670

1 0.009a 0.39 32 0.011 125
671 2 0.008a 0.24 32 0.006 228
3 0.011a 0.35 32 0.012 114
4 0.011a 0.38 32 0.013 105
672 5 0.012a 0.32 32 0.012 114
6 0.008a 0.32 32 0.008 171
691 a From Fenton et al. (2008)
69627 3
and depth of infiltration in this period of 1.7 and therefore high residence times of water in
69637 4 m (Fenton et al.,2008). certain aquifers means that reversing this
The depth to the median watertable during increasing trend may not be achievable within
69647 5
this period was 1.01 m (Fenton et al.,2009). the 2015 timeframe. A comparison of total
69657 6 travel times from all the contrasting case stud-
Horizontal travel time ies is presented in Table 5.
69667 7 H o rizontal travel time ranged from 105 to For the purposes of illustration, a simple
228 years (Table 4.) model of transport has been used to show dif-
69677 8 ferences in first breakthrough travel time from
Total travel time source to receptor in four contrasting sites.
69687 9 V e rtical travel time was estimated at less than This simple methodology shows the wide vari-
1 year and horizontal travel time was estimat- ability in travel times that can be expected
69698 0
ed from 105 to 228 years. This gives a total within catchments in Ireland (several months
70608 1 tr a vel time of approximately 106 to 229 years to years to reach a receptor 500 m away). In
to the receptor 500 m away from the source. reality considerable heterogeneities and
70618 2 anisotropies in soil, sediment and bedrock
DDiissccuussssiioonn hydrogeological properties make transport
70628 3 Ir is h case studies pathways and times highly variable. Examples
This is the first attempt at calculating nitrate of this include discrepancies between travel
70638 4
lag times in Ireland and there are no direct time based on matrix flow (depth of infiltra-
70648 5 m e thodologies to calculate lag time across tion) or preferential flow (tracer breakthrough
Europe. Groundwaters in Ireland are at risk of data). In some instances N transfer to shallow
70658 6 fa i ling to meet the water qualitytargets of the groundwater is rapid with little chance for
WFD by 2015. Slow groundwater travel times attenuation. Zones of high conductivity asso-
706
707 1 36
708
709
710
711
FENTON, COXON, HARIA, HORAN, HUMPHREYS, JOHNSON, MURPHY, NECPALOVA, PREMROW, RICHARDS:
712 VaTraiabtlieo n5s.V inu ltnrearvaebli ltitiym ec aftoerg oNr yl,o aatdtienngu atoti ognr ocuapnadcwiatyt,e rtost ainl efsotuimr actaesde tsratuvdeli etsim ine sI raenlda nd:
Implications for policy makers and regulators
713 Talbagle t5im:Ve uislnseureasb filoitry t hcaet efogourry c, oanttternaustaitniogn c acsaep asctiutdy,i etos.t a l estimated travel times and
lag time issues for the four contrasting case studies.
Case study Vulnerability Attenuation Total Lag time issues
Case study Vulcnaeteragboirlyit y Atctaepnuacaittiyo n Ttroatvaell Lag time issues
category capacity travel
time
time
Indicative of initial breakthrough
Contaminated shallow groundwater
Months interaction with groundwater and
Oak Park, Co
High Moderate to surface water. Flushing utilising
Carlow
Decades specific yield of the aquifer should be
incorporated into lag time.
N mineralisation in soils, travel time
will change depending on soil N
Moorepark, status, thickness of soil and
Extreme None Years
Co. Cork preferential flow paths. Flushing
utilising specific yield of the aquifer
should be incorporated into lag time.
Connectivity to semi confined aquifer.
Solohead, Co. Phosphorus lag time to surface water
Moderate High Decades
Tipperary and groundwater. NO unlikely to be
3
problematic.
Contaminated shallow groundwater
Johnstown
interaction with surface water in close
Castle, Co. Moderate High Decades
proximity to excess nutrients. N
Wexford
mineralisation in soils.
ciated with gravel channel deposits or fracture groundwater (with the exception of sand and
systems, may connect a source area with a gravel aquifers) under the WFD definition.
receptor so that travel times may be signifi- Groundwater contribution to surface waters in
cantly less than estimated. In this instance a poorly productive aquifers may be as low as
small area of land may have a disproportionate 5% but can be as high as 30% when flow
effect on water quality at the receptor. along the subsoil/bedrock interface is consid-
Saturated soil and subsoil is not deemed ered; by contrast, in productive aquifers such
137
Tearmann, the Irish journal of agri-environmental research, VOL 7, 2009
as sand and gravel and karstified limestone, the coarser gravel lenses present at this site
the contribution may be from 80% to 90% Irish bedrock aquifers are deemed to have
(Misstear et al.,2009). Components of deep low attenuation potential due to their frac-
groundwater, intermediate (interflow and tured and karstified nature and protection is
shallow groundwater) and overland flow that mainly provided by the overlying glacial tills
contribute to stream flow were assessed in (Daly and Warren, 1998). In karst environ-
seven pilot catchments and used to constrain ments K measurements are only indicative
sat
and inform the numerical model NAM of how fast groundwater flows at a specific
(Nedbør-Afstrømnings-Model) in Ireland point, which leads to vast ranges in K e.g. In
sat
(Jennings et al., 2007). This model was then Waulsortian limestone (a lithological sub-
extended to regional catchments. The model group of Carboniferous Limestone), K val-
sat
estimated large amounts of intermediate flow ues range from 4 to 2500 m day-1
contributed to stream flow,e.g. 318 mm y-1in (Goldscheider and Drew, 2009). In the vicin-
the Owenduff poorly productive aquifer. In ity of the case study site, K values such as a
sat
such a catchment deep groundwater con- public water supply, 873 m day-1 and nearby
tributed 128 mm y-1 and overland flow con- water abstraction wells range from 10 to 200
tributed 1322 mm y-1. In the Suck pilot karst m day-1. From slug injection tests, bedrock
catchment intermediate flows were 362, 171 K values in the case study area ranged from
sat
and 124 mm y-1 respectively. Such informa- 0.004 to 27 m day-1(Bartley, 2003). The soils
tion, when combined with nitrate concentra- here have high total nitrogen levels and thus
tions leaving the rooting zone can be used to elevated mineralisation rates. Even if fertiliser
calculate loads of nutrients in intermediate inputs are reduced it will take a long time to
and groundwater flow. This is important for prevent losses (Grimvall et al.,2000) from the
calculating aquifer flushing times to certain unsaturated zone. In this respect groundwater
EU threshold values. trend monitoring will not reflect mitigation
For groundwater classification chemical measures in place at farm level but instead will
and quantification status under the WFD sev- represent a mixture of bio-geochemical
eral tests must be complied with. For example processes.
“no significant diminution of surface water In the Oak Park and Moorepark case stud-
chemistry and ecology” determines if ground- ies,shorter travel times and less denitrification
water impacts on a surface water body achiev- potential in the soils and aquifer put receptors
ing the goals of the WFD. The groundwater at risk. In terms of maximising the effective-
threshold values used are surface water quality ness of resources applied to improve water
standards with a dilution factor. This dilution quality, it is likely to be most successful if
factor is determined by the shallow and deep problem receptors are identified and mitiga-
groundwater contributions, but not the inter- tion measures targeted in areas that contribute
flow components. Further differentiation of to that receptor. Implementing measures on
intermediate flow is needed. sites for which there is no receptor water qual-
Drilling records for the Oak Park site indi- ity problem or for which the travel time and N
cate that the clay/silt lenses are not continuous remediation potential is such that changes in
and may not represent a barrier for transport management are not likely to impact water
of water soluble pollutants in this fluvio- quality (a blanket approach) is not an efficient
glacial deposit aquifer (Premrov et al.,2007). use of resources. The time it takes to flush an
Therefore, the estimated travel time of 1 to entire plume of nitrate through interflow,
6.6 years was shown as an indication of the shallow groundwater and deep groundwater
fastest possible total travel time through the pathways should be considered as an impor-
unsaturated and shallow groundwater through tant component of lag time. This of course
138