Nitrate Removal Rates in a 15-Year-Old PermeableReactive Barrier Treating Septic System Nitrate

by W.D. Robertson, J.L. Vogan, and P.S. Lombardo

AbstractPermeable reactive barriers (PRBs) have gained popularity in recent years as a low-cost method for ground water reme-

diation. However, their cost advantage usually requires that these barriers remain maintenance free for a number of yearsafter installation. In this study, sediment cores were retrieved from a pilot-scale PRB consisting of a sand and wood particle(sawdust) mixture that has been in continuous operation for 15 years treating nitrate from a septic system plume in southernOntario (Long Point site). Reaction rates for the 15-year-old media were measured in dynamic flow column tests and werecompared to rates measured in year 1 using the same reactive mixture. Nitrate removal rates in the 15-year-old media varied,as expected, with temperature in the range of 0.22 to 1.1 mg N/L/d at 6 �C to 10 �C to 3.5 to 6.0 mg N/L/d at 20 �C to 22�C. The latter rates remained within about 50% of the year 1 rates (10.2 6 2.7 mg N/L/d at 22 �C). Near the end of the year15 column test, media particles >0.5 mm in diameter, containing most of the wood particles, were removed from thereactive media by sieving. Nitrate removal subsequently declined by about 80%, indicating that the wood particles were theprincipal energy source for denitrification. This example shows that some denitrifying PRBs can remain maintenance freeand be adequately reactive for decades.

IntroductionPermeable reactive barriers (PRBs) provide passive

remediation of ground water contaminants by promot-ing a variety of mineral precipitation, redox, and sorptionreactions. Reactive mixtures containing zero-valent iron,organic mulches, and zeolites have been used to treat chlo-rinated organics (Gillham and O’Hannesin 1994; Tratnyeket al. 2003), trace metals (Benner et al. 1999; Wilkin andPuls 2003), and nitrate (Schipper and Vojvodic-Vukovic1998), among others. PRBs are normally installed by trench-ing, which may involve substantial initial capital cost, par-ticularly as depths increase. However, PRB technology canbe cost-effective compared to less-passive treatment tech-nologies such as pump and treat because of lower operatingand maintenance costs.

Nitrate, largely derived from agricultural activity, isconsidered the most ubiquitous ground water contaminantworldwide (Spalding and Exner 1993). In the UnitedStates, more than 20% of rural wells in some states haveNO3-N concentrations above the drinking water limit of10 mg/L (Spalding and Exner 1993), while in Canada, 14%of 1292 rural farm wells surveyed in southern Ontarioexceeded the drinking water limit for nitrate (Goss et al.1998). Previous studies have examined a variety of

carbonaceous solids and immiscible liquids for their abilityto stimulate denitrification in PRBs treating nitrate. Thesehave included cracked corn, molasses, cotton burr, newspa-per, vegetable oil, leaf mulch, wheat straw, sawdust, and soforth (Erickson et al. 1974; Volokita et al. 1996a, 1996b;Soares and Abeliovich 1998; Hunter 2001; Kim et al. 2003;Bedessem et al. 2005; Greenan et al. 2006). However, mostof these studies were laboratory tests that were less thana year in duration. For example, in the last study, Greenanet al. (2006) tested nitrate removal in laboratory micro-cosms that used corn stalks, cardboard, woodchips, andvegetable oil–soaked woodchips as carbon sources fordenitrification. All the mixtures, particularly the cornstalks, had higher initial removal rates than the woodchipsalone, but these rates declined over the 180-d test period,such that by the end of the test, the woodchips approachedor exceeded the removal rates of the other substrates. Inseveral of the other studies noted previously, decliningreaction rates were also observed after several months ofcolumn operation (Volokita et al. 1996b; Soares and Abe-liovich 1998). Thus, although a number of these carbona-ceous mixtures have achieved relatively high nitrateremoval rates at early times in the laboratory, it is uncertainif these rates could be sustained over multiyear timeframesunder field conditions.

PRBs may contain sufficient reactive mass to poten-tially provide treatment for many years, but longevity can

Copyright ª 2008 The Author(s)Journal compilationª 2008National GroundWater Association.

Ground Water Monitoring & Remediation 28, no. 3/ Summer 2008/pages 65–72 65

be difficult to predict because of the complicating effectsof secondary reactions. For example, reaction by-productsproduced in zero-valent iron PRBs may generate mineralcoatings, which passivate the reactive media over time(Yabusaki et al. 2001; Agrawal et al. 2002; Zhang andGillham 2005). In carbon mulch barriers designed for deni-trification (Equation 1), ancillary reactions such as reductivedissolution of ferric iron (Equation 2), sulfate reduction(Equation 3), methanogenesis (Equation 4), and excess dis-solved organic carbon (DOC) leaching (Robertson andCherry 1995; Robertson et al. 2007) may consume addi-tional carbon from the reactive media:

4NO�3 þ 5CH2O ! 5HCO3�3 þ 2N2 þ Hþ þ 2H2O (1)

4FeðOHÞ3 þ CH2Oþ 7Hþ ! 4Fe2þ þ HCO�3 þ 10H2O

(2)

SO2�4 þ 2CH2O ! 2HCO�

3 þ H2S (3)

2CH2Oþ H2O ! CH4 þ HCO�3 þ Hþ (4)

Uncertain longevity or a requirement for frequentmedia replacement can make PRB technology less attrac-tive. Thus, long-term performance monitoring is crucial.Several studies have reported successful PRB operationover timeframes of 3 to 7 years (O’Hannesin and Gillham1998; Robertson et al. 2000; Schipper and Vojvodic-Vukovic 2001; Benner et al. 2002; Wilkin and Puls 2003;Schipper et al. 2005); however, few if any decadal studiesare yet available that ultimately will demonstrate the truecost-effectiveness of this technology.

This article presents 15 years of monitoring results froma pilot-scale PRB installed in 1992, treating nitrate from aseptic system plume in southern Ontario (Long Point site,Figure 1). The PRB contains a mixture of organic carbon (OC)(sawdust) and sand. Slow biodegradation of the cellulose

and lignin compounds contained in the wood particles pro-vides a long-term source of labile DOC, capable of sup-porting denitrification. The PRB has operated continuouslysince 1992 without maintenance, treating plume NO3-Nconcentrations of up to 100 mg/L. As with most PRBs, pre-cise estimation of field reaction rates is problematic at thissite because of the inherent difficulty in measuring groundwater flow velocities migrating through the barrier. In a pre-vious study examining earlier operation of the barrier(years 1 to 7, Robertson et al. 2000), a mean denitrificationrate of 2.4 mg N/L/d was determined based on an assump-tion that the velocity through the PRB was the same as theestimated bulk horizontal ground water velocity of 6 cm/din the surrounding aquifer. However, the velocity estimatewas uncertain because it was determined from the Darcyequation using values of aquifer hydraulic conductivity (K)that were approximates only. Additionally, the hydraulicgradient varied seasonally, leading to further uncertainty inthe velocity estimate. Also, the excavation procedure usedto install the wall caused a disturbance of the native sand inand around the wall, which made it unlikely that the groundwater velocity within the PRB would remain identical tothat of the surrounding aquifer (e.g., Benner et al. 2001;Schipper et al. 2004). Thus, previous estimates of fieldreaction rates were somewhat speculative.

In the current study, nitrate removal rates for the 15-year-old reactive media were determined by undertakingdynamic flow laboratory column tests using core samplestaken from the PRB. Additionally, prior to construction ofthe field installation in 1992, similar column tests wereundertaken using fresh samples of the same reactive mix-ture (20% sawdust and 80% sand by volume) used in thefield PRB. Thus, the acquired data set presents a uniqueopportunity to rigorously compare the reaction rates ofa fresh sample of reactive media to that of an equivalentfield sample that has been in continuous operation in thesubsurface for 15 years.

Figure 1. Long Point septic system tile bed 2 showing the plume of elevated nitrate concentrations and the location of the PRBinstalled in 1992 (adapted from Robertson and Cherry 1995).

W.D. Robertson et al./ Ground Water Monitoring & Remediation 28, no. 3: 65–7266

Year 1 Column TestsPrior to initiation of the field trial, laboratory column

tests were undertaken to assess the nitrate removal poten-tial of four candidate OC substrates for use in the proposedPRB. These included delignified wood pulp (cellulose),wheat straw, alfalfa, and wood particles (sawdust). Allwere mixed with medium sand at a volumetric ratio of20% carbonaceous solids to 80% sand. For each mixture,duplicate columns 90 to 100 cm in length were loadedwith an influent solution containing approximately 70 mg/LNO3-N, similar to the concentrations observed in the sourcearea of the Long Point septic system plume. Tests were runfor a duration of 81 to 95 d at a hydraulic retention time(HRT) of approximately 1 d in the reactive media, except forthe last 20 d of the sawdust and wheat straw tests, whichwere run at a longer HRT of 1.6 to 2.1 d (Vogan 1993).Zero-order nitrate removal rates were determined by observ-ing the difference between influent and effluent nitrate con-centrations and then dividing by the HRT in the column. Thecellulose, alfalfa, and wheat straw columns provided thehighest initial reaction rates (86 to 127 mg N/L/d), but theserates declined rapidly by more than 50% by the end of thetests (Figure 2). The sawdust columns provided lower reac-tion rates (9 to 16 mg N/L/d), but rates appeared more stable(Figure 2). Figure 2 omits some early data from the cellu-lose, alfalfa, and wheat straw columns when nitrate in theeffluent was below detection and therefore zero-order re-action rates could not be calculated and for the sawdustcolumn when the difference between influent and efflu-ent nitrate concentrations was small enough during the ini-tial higher flow period that measurement error becamesignificant.

Although the cellulose, alfalfa, and wheat straw col-umns achieved higher reaction rates, sawdust was selectedas the preferred media for the field trial because of severalconsiderations. As described previously, calculations usingthe Darcy equation indicated that the ground water veloc-ity in the aquifer at the site was about 6 cm/d (Robertsonand Cherry 1995). Thus, reactive mixtures using celluloseor alfalfa, with reaction rates on the order of 50 to 100 mgN/L/d, would achieve complete plume nitrate removal inabout 1 d, which equates to a travel distance of only 6 cmin the aquifer. The installation technique used for the field

trial (manual excavation using a trench box, as described inRobertson and Cherry 1995) resulted in a PRB width of60 cm, however. This implied an HRT of about 10 d in thePRB, which was much better suited to the lower reactionrate of approximately 10 mg N/L/d provided by the saw-dust media (Figure 2). Furthermore, geochemical profilingalong the length of the highly reactive alfalfa columnshowed that subsequent to complete nitrate depletion,Eh declined abruptly, sulfate reduction commenced, andDOC concentrations escalated rapidly to up to 162 mg/L(Figure 3). A high ratio of OC to nitrate can favor dissimi-latory nitrate reduction to ammonium (DNRA) over deni-trification (Tiedje 1988), and indeed up to 15 mg/L ofNH4-N was detected in the alfalfa column effluent duringtesting (Vogan 1993). Figure 3 shows that the increase inNH4-N to approximately 5 mg/L within the alfalfa columnon day 46 coincided with the zone where nitrate was beingdepleted (20 to 70 cm), whereas little further increase inNH4 occurred after this, even though DOC continued toincrease. This supports the likelihood that NH4 was gener-ated by DNRA rather than from mineralization of organicN contained in the alfalfa. In contrast, the sawdust columnsmaintained NH4-N of less than 0.1 mg/L and DOC of lessthan 6 mg/L after day 20 of the tests (Vogan 1993). In thecontext of the Long Point PRB, sulfate reduction, excessDOC leaching, and ammonium production would be con-sidered undesirable because they could lead to water qual-ity degradation in the downgradient aquifer. NH4 couldoxidize back to NO3 if aerobic conditions are subsequentlyencountered farther along the flowpath (Korom 1992).Other PRB studies have also observed the onset of these re-actions once nitrate removal is complete (Robertson et al.2007, 2008). Thus, the use of highly reactive media that isnot well matched to the HRT in the barrier can be unwisebecause the excess carbon may stimulate ancillary reactions

Days after Start

Rat

e (m

g N

/L/d

)

0 20 40 60 80 1000

50

100

150 CelluloseAlfalfaWheat strawSawdust

Figure 2. Nitrate removal rates of four candidate reactivemixtures measured in column tests prior to initiation of thefield trial (data from Vogan 1993).

Con

cent

ratio

n (m

g/L)

Eh (m

V)

0 -450

30 -300

60 -150

90 0

120 150

150 300

180 450

DOCSO4Eh

Distance along Column (cm)

Con

cent

ratio

n (m

g/L)

0 20 40 60 80 1000

15

30

45

60

75

NO3-NNH4-N

Figure 3. Profiles of redox-sensitive parameters along alfalfacolumn 3, year 1 column test, day 46 of test (data from Vogan1993).

W.D. Robertson et al./ Ground Water Monitoring & Remediation 28, no. 3: 65–72 67

that may be undesirable rather than productively treating thecontaminant of concern. Additionally, the wood particlemedia that was selected for the field trial was locally avail-able at low cost and mixed easily with sand.

Wall InstallationThe field trial was installed in September 1992 at

a location 7 m downgradient from the edge of the LongPoint septic system tile bed (Figure 1). The septic systemis of conventional design and services a seasonal use camp-ground that has 120 overnight campsites. The tile bed dis-charges to an unconfined sand aquifer that extends to about6 m depth (Figure 1) and generates a ground water plumewith NO3-N concentrations in the range of 30 to 100 mg/Lin the shallow zone, although nitrate is depleted naturally,by denitrification, at depth (Robertson and Cherry 1992;Aravena and Robertson 1998). The campground comfortstation also has a second tile bed located 300 m to the east,but during this study period (1992 to 2007), most of thesewage flow was directed to the tile bed where the reactivebarrier was installed (tile bed 2). However, flow diversionto tile bed 1 did occur periodically, which affected theground water velocities and nitrate concentrations reachingthe barrier.

The reactive barrier was 1.2 m long, 0.6 m wide in thedirection of ground water flow, and installed to a depth of1.5 m (0.8 m below the water table) with the aid of a ply-wood trench box (Robertson and Cherry 1995). The instal-lation procedure involved manually excavating the aquifersand through the bottom of the trench box, mixing theexcavated sand with 20% by volume of sawdust, returningthe reactive mixture into the excavation, and then finally,removing the trench box. Although attempts were made tothoroughly mix the media, the excavation remained water-filled during installation, and thus some hydraulic sortingwas unavoidable. After installation, two multiple-depthpiezometer bundles were installed at the upgradient anddowngradient edges of the barrier (Figure 1) to allow mon-itoring of nitrate removal within the barrier.

Methods

Year 15 CoringIn 2007, to obtain a sample of the 15-year-old reactive

media for column testing, a continuous, undisturbed sedi-ment core was retrieved through the center of the PRB to adepth of 1.2 m. The core was obtained in a 5-cm-diameteraluminum core tube fitted with a core catcher, which wasadvanced into the subsurface by percussion. A second core,which served as a control, was retrieved to a similar depth atlocation 1.5 m west of the end of the barrier. After retrieval,the core tubes were split lengthwise with a saw blade, andsediment characteristics were visually logged.

Year 15 Column TestsTwo column tests were conducted simultaneously, one

using the core sample from the PRB and the second usingthe sample from the control location outside the barrier.

Sediment from the same depth interval (0.7 to 1.2 m) coin-ciding with the approximate depth position of the reactivebarrier below the water table was used in both tests. Thesame column apparatus used in the year 1 tests (6.5-cm-diameter Plexiglas columns, 90 to 100 cm in length;Vogan 1993) was used in the year 15 tests. The core wasremoved from the core tubes, placed into the columns witha minimum of disturbance, and then lightly tamped into thebottom of the columns. The available sediment filled thecolumns to a length of 18 to 19 cm, which was less thanthat of the year 1 tests (90 to 100 cm, Vogan 1993). Prior toloading the columns, 30-g subsamples of the cored sedi-ment were retained at 10-cm depth increments for OC con-tent analysis. The column tests were run for a period of125 d, using an influent solution that was retrieved froma farm field drainage tile in southern Ontario and that hada known elevated nitrate content. The columns were ini-tially loaded at a flow rate of 0.43 L/d, which led to anHRT in the columns of 0.55 d. Flow was then reduced to0.18 L/d on day 28 of the test, resulting in longer HRTs of1.4 to 1.5 d during the latter part of the test. The columnswere placed in a refrigerator during some stages of the testto assess reaction rates over a range of temperatures (6 �Cto 22 �C). Near the end of the test (day 105), the influentsolution was changed temporarily to an NaCl tracer solu-tion with elevated electrical conductivity (EC). Columnpore volumes were then determined by measuring thebreakthrough of the influent-normalized (C/C0) EC valueof 0.5. Immediately after the tracer test, the media in thePRB column was extracted and sieved to remove the meanparticles >0.5 mm in diameter. This size fraction comprisedabout 70% sawdust particles and represented 9% dryweight of the bulk media. Sawdust particles were muchless prevalent in the remaining fraction of less than 0.5 mmsize. The column was then repacked, and the test continuedfor an additional 20 d with the depleted media, in order toestablish comparative reaction rates with most of the woodparticles removed.

Ground Water SamplingIn the current study, ground water samples were also

retrieved from the original piezometer bundles installedat the upgradient and downgradient edges of the PRB(Figure 1). Samples were collected using a peristalticpump, were filtered (0.45 lm) in-line prior to atmosphericexposure, and were accompanied by field measurements ofEC and temperature. Nitrate was analyzed at the Soil andNutrient Laboratory, University of Guelph, Ontario, usinga colorimetric technique (Cd reduction) with a Techni-conTM TRAACS-800 autoanalyzer (Technicon Instruments,Tarrytown, New York). Several samples were also analyzedfor a more complete suite of anions (NO3, SO4, and Cl) atthe Earth and Environmental Sciences Department, Univer-sity of Waterloo, by ion chromatography using a Dionexmodel ICS-90 (Dionex, Sunnyvale, California). A secondsubset of samples was also analyzed for Fe content. Thesewere collected in duplicate sample bottles, which wereacidified (pH < 2) with HCl immediately after collection.Fe analyses were completed at the University of Guelph

W.D. Robertson et al./ Ground Water Monitoring & Remediation 28, no. 3: 65–7268

laboratory by atomic adsorption spectrometry. DOC wasmeasured in the Earth and Environmental Sciences depart-ment using a Dohrman DC-190 total carbon analyzer(Dohrman, Santa Clara, California). Sediment OC contentwas analyzed at the University of Guelph by measuringmass loss upon ignition (Heiri et al. 2001).

Results

Long-Term Barrier PerformanceFigure 4 shows that over 15 years of operation, the

field PRB provided relatively complete removal of plumeNO3-N concentrations of up to 100 mg/L. However,Figure 4 also shows that NO3-N concentrations enteringthe barrier were highly variable and occasionally were atvery low levels (<2 mg/L). These low values represent non-plume, low-EC background ground water that enters thebarrier seasonally, associated with the period when noloading occurs to the tile bed (November to May). Addi-tionally, the tile bed receives high sewage loading of about10 cm/d (Robertson and Cherry 1992) during peak parkusage in July and August. This likely increases the horizon-tal ground water velocity near the tile bed at this time.Thus, seasonal variability of both the nitrate concentrationsand the ground water velocity made the determination oflong-term treatment rates difficult using the field dataalone.

Table 1 gives the concentrations of several redox-sensitiveconstituents (NO3, SO4, and Fe) and DOC, upgradient anddowngradient of the barrier in year 15 (November 2007).At the time of sampling, low EC background ground waterwas migrating through the barrier due to a preceding periodof reduced tile bed loading. Although only a small amountof NO3-N was being removed (0.02 mg/L), the distinctincrease in Fe concentration (17 mg/L) indicated thatreductive dissolution of ferric iron minerals (Equation 2)was occurring in the barrier at that time.

Core DescriptionThe PRB core was enriched in OC by a factor of 18

compared to the control core (mean of 2.47 vs. 0.14 wt %

OC, Table 2). All depths in the PRB core were enriched inOC, particularly the bottom sample, which had an OC con-tent of 9.5 wt %. This was from a localized zone severalcentimeters in thickness that was substantially enriched insawdust and was presumably the result of imperfect mixingor hydraulic sorting that occurred during installation.Assuming that the PRB has a dry bulk density of 1.5 g/cm3,the barrier volume of 0.58 m3 would weigh 870 kg. After 15years, the mean OC content of 2.47 wt% represents 21 kg C,which is 19% less than the estimate of the initial C masscontained in the barrier (26 kg, Robertson et al. 2000). Thisamount of carbon loss (19% over 15 years) is consistentwith a first-order degradation rate of �4E-5/d. Althoughthis value is uncertain because of the highly variableOC content in the barrier (Table 2), it is consistent withsome previous estimates of wood particle degradationrates. For example, Schuman and Beldon (1991) measureda degradation rate of �2E-4/d for wood particle residuesburied in reclaimed mine spoils.

The sawdust particles remained easily visible and ex-hibited a form and texture similar to fresh sawdust,although they were more friable. In the freshly openedcore, the sawdust particles were black in color but thenchanged to a pale brown color after several hours of air

Years after Startup

NO

3-N (m

g/L)

0 5 10 150

25

50

75

100Wall InWall Out

Figure 4. Nitrate removal in the Long Point PRB over 15years of operation. Monitoring locations are shown onFigure 1. Years 1 to 7 data have been reported previously(Robertson et al. 2000).

Table 1Ground Water Composition Upgradient and

Downgradient of the Long Point Barrier in Year 15

Parameter Up1 Down2

EC (uS/cm) 378 593NO3-N (mg/L) 0.02 <0.01SO4 (mg/L) 14.1 14.4Cl (mg/L) 1.7 2.5Fe (mg/L) 0.42 17.5DOC (mg/L) 5.1 5.6

Note: Sampling date (November 24, 2007) corresponds to a period of lownitrate loading to the barrier.1Piezometer LD2, 1.1 m depth.2Piezometer LD3, 1.1 m depth.

Table 2OC Content of Sediment Cores Retrieved fromthe Center of the Long Point PRB in Year 15(April 2007) and from a Control Location

1.5 m West of the End of the Barrier

Depth (m)

OC (wt %)

PRB Core Control Core

0.6 0.200.7 1.14 0.060.8 0.54 0.030.9 0.60 0.251.0 0.54 0.181.1 9.53Mean 2.47 0.14

W.D. Robertson et al./ Ground Water Monitoring & Remediation 28, no. 3: 65–72 69

exposure. There were no visual indications of biomassdevelopment or secondary mineral precipitates on thewood particles or in the surrounding media. Occasionalcarbonaceous solids were also observed throughout thecontrol core. These were in the form of fine (<0.5 mmdiameter) rootlets, but these were much less prevalent thanthe sawdust particles in the PRB core.

Year 15 Column TestsNO3-N concentrations in the control column effluent

remained within a narrow range throughout the test (9.5 6

0.3 mg/L, n ¼ 28) and were only slightly lower than theinfluent values (9.7 6 0.2 mg/L, n ¼ 24, Figure 5), indicat-ing that nitrate removal was relatively inactive or absent inthe control column. In contrast, NO3-N concentrationswere substantially lower in the PRB column effluent(5.9 6 2.2 mg/L, n ¼ 31), indicating that nitrate removalwas active in this column. Removal rates varied exponen-tially with temperature as expected (R2 ¼ 0.96, Figure 6),ranging from 0.22 to 1.1 mg N/L/d at 6 �C to 10 �C, com-pared to 3.5 to 6.0 mg N/L/d at 20 �C to 22 �C. When themedia particles >0.5 mm in diameter containing most ofthe sawdust particles were removed from the PRB columnmedia, nitrate removal declined abruptly by about 80%(Figure 5). This indicated that the sawdust particles werelikely the principal carbon (energy) source supportingdenitrification.

DiscussionDenitrification rates measured at 20 �C to 22 �C in

the year 15 column test (4.6 6 0.7 mg N/L/d, n ¼ 17,Figure 6) were only about 50% lower than that of the ratesmeasured in the year 1 column tests (10.2 6 2.7 mg N/L/d,n ¼ 10). Likewise, the previous estimate of the mean reac-tion rate in the field PRB during the first 7 years of opera-tion (2.4 mg N/L/d, Robertson et al. 2000), although

speculative, was similar to the rates measured in the year15 column test at 14 �C to 15 �C (Figure 6). Temperaturesin the shallow water table zone at the field site varied sea-sonally in the range of 8 �C to 15 �C, but during the sum-mer and fall period, when most of the monitoring data werecollected, were consistently in the range of 14 �C to 15 �C.

The long-term persistence of the carbon source in theLong Point PRB is consistent with previous mass balanceestimates of longevity. These indicated that denitrificationconsumed less than 1% of the initial carbon mass annually(Robertson et al. 2000). Furthermore, although recent mon-itoring confirmed that secondary reactions such as reduc-tive iron dissolution remained active, the rate of carbonconsumption from these reactions has apparently not beenhigh enough to substantially deplete the reactive media.From Equation 2, the observed Fe increase of 0.3 mM/L(17 mg/L, Table 1) would consume 0.076 mM C/L (0.91mg C/L). Ground water flow through the barrier was esti-mated previously at 15 L/d (Robertson et al. 2000). Usingthis flow value, carbon consumption from reductive disso-lution of iron would be 14 mg/d or 5 g/year, which is aninsignificant fraction of the initial C mass present of thebarrier (26 kg, Robertson et al. 2000).

Wood particle barriers have already been used toprovide nitrate removal in a number of agricultural andwaste water settings, but none has the 15-year monitoringrecord of the Long Point PRB. Schipper and Vojvodic-Vukovic (1998) used a reactive wall consisting of a similarsand-sawdust mixture to treat nitrate generated from a fieldwhere manure was applied and observed consistent treat-ment at a potential denitrification rate of about 0.5 mg N/L(media)/d over a 5-year period (Schipper and Vojvodic-Vukovic 2001). van Driel et al. (2006) used a lined trenchcontaining coarse wood particle media to provide ‘‘end-of-pipe’’ nitrate treatment for a farm field drainage tile and

Temperature (ºC)

Rat

e (m

g N

/L/d

)

0 5 10 15 20 250

4

8

12

Year 1 columns (sawdust)Year 15 column (sawdust)Year 1-7 field PRBy = 0.17 exp(0.16x),R2 = 0.96

Figure 6. Comparison of nitrate removal rates in year 1 andyear 15 sawdust columns and in the field PRB during years 1to 7 of operation. PRB rate is from Robertson et al. (2000).Year 1 rate is mean value measured in duplicate columns dur-ing days 71 to 94 of test (10.2 6 2.7 mg N/L/d, n ¼ 10). Datafrom Vogan (1993). Solid line is exponential regression fit ofyear 15 column data.

Tem

pera

ture

(ºC

)

0

10

20

30

Pore Volumes

NO

3-N (m

g/L)

0 25 50 75 100 1250

4

8

12

Flow x 0.4 Sawdust Removed

InfluentEffluent, control columnEffluent, PRB column

Figure 5. Nitrate removal in the year 15 column tests.

W.D. Robertson et al./ Ground Water Monitoring & Remediation 28, no. 3: 65–7270

observed a mean reaction rate of 8 mg N/L/d over the first2 years of operation. Jaynes et al. (2008) used denitrifica-tion walls containing woodchips that were installed alongthe length of a drainage tile and measured a reduction in Ndischarge from the tile of 29 kg N/ha/year over a 5-yearperiod. This represented a decrease of 55% compared toa control tile and indicated a reaction rate within the wallof about 1.0 mg N/L/d. Robertson et al. (2007) used a PRBlayer containing coarse wood particles to treat nitrate froma poultry manure composting yard and observed a reactionrate of about 10 mg N/d during the first year of operation.In another study using wood particle media at four otherseptic system sites in Ontario, influent NO3-N averaging14 to 38 mg/L was attenuated by 87% to 98% over 3 to5 years of operation at an indicated reaction rate of 7 togreater than 10 mg N/L/d (Robertson et al. 2005). The cur-rent demonstration of decadal longevity at the Long Pointsite, at reaction rates that are well suited to the groundwater retention times that commonly occur in PRBs,should add to the attractiveness of wood particle media foruse in such barriers. This is particularly so when very long-term, maintenance-free operation is desired.

ConclusionsLaboratory column tests demonstrated that after 15

years of continuous operation, denitrification rates in theLong Point PRB media (4.6 6 0.7 mg N/L/d at 20 �C to22 �C) remained within about 50% of rates measured inyear 1. Although secondary reactions such as reductiveiron dissolution remained active in the PRB, carbon con-sumption from these reactions has apparently not beenhigh enough to deplete the reactive media. This demon-stration of decadal longevity should enhance the attractive-ness of wood particle media for use in PRBs when verylong-term, maintenance-free operation is desired.

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Biographical SketchesW.D. Robertson, corresponding author, is at the Department of

Earth and Environmental Sciences, University of Waterloo, Water-loo, Ontario, N2L 3G1, Canada; (519) 885-1211; fax: (519) 746-7484; wroberts@sciborg.uwaterloo.ca.J.L. Vogan is a senior hydrologist, at EnviroMetal Technolo-

gies Inc., Waterloo, Ontario, Canada.P.S. Lombardo is president of Lombardo Associates Inc.,

Newton, MA.

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