Effects of Glacial Sediment Type and Land Use on Nitrate ...

68 Groundwater Monitoring & Remediation 35, no. 1/ Winter 2015/pages 68–81 NGWA.org

© 2015, National Ground Water Association.doi: 10.1111/gwmr.12100

Effects of Glacial Sediment Type and Land Use on Nitrate Patterns in Groundwaterby Anna Best, Emmanuelle Arnaud, Beth Parker, Ramon Aravena, and Kari Dunfield

IntroductionNitrate contamination in groundwater has become a

widespread water quality issue (Goss et al. 1998; OECD 2008). This is especially true in agricultural areas, despite the reductions in nitrate loading associated with implemen-tation of best management practices (BMPs) and nutrient management planning (OECD 2008). In this context, it is most advantageous and cost-effective to focus regulation or promotion of BMPs in vulnerable areas, rather than where nitrate will be attenuated naturally in an aquifer (Refsgaard et al. 2014).

As a nonpoint-source pollutant, nitrate associated with agricultural inputs can be considered almost homogeneous at the surface at a broad scale. The rate of transport and attenuation of nitrate at depth, however, will depend on the physical and chemical properties of subsurface materials (Rivett et al. 2008; Rodvang and Simpkins 2001). Within a hydrogeologic system, there may be locations that are conducive to denitrification or vulnerable to persistence of nitrate.

A key challenge lies in the complexity and heterogene-ity of some geologic settings, which make it difficult to pre-dict nitrate transport and fate at depth. This is particularly true within glacial settings, where materials with complex or discontinuous geometries variably affect the physical

and geochemical conditions of the subsurface and thus lead to highly variable susceptibility to contamination. In previously glaciated regions such as large parts of Canada, northern US, and northern Europe and Asia, predicting the transport and fate of nitrate requires an understanding of local glacial geology and how it impacts the spatial vari-ability of subsurface characteristics. This understanding would in turn enable the implementation of more targeted BMPs.

Although nitrate research in glacial deposits of Ontario has been done in silt till aquitards (Robertson et al. 1996), sand aquifers (Aravena and Robertson 1998), and the Waterloo Moraine (Stotler et al. 2010), little work has been done to assess the impact of geologic complexity over depth in specific glacial landscapes or to gather data, using con-sistent methodology, from a more diverse array of glacial sediments. It is hypothesized that specific glacial landscapes will have characteristics that are more or less conducive to nitrate attenuation. The nature of the materials and the geom-etry of units will affect connectivity of aquifers, integrity of aquitards, and groundwater flow paths and rates, which will in turn affect geochemical conditions. For example, out-wash plains often overlie older fine-grained till sheets but erosional windows and fractures in those tills may facili-tate preferential flow to underlying aquifers. Hummocky moraines tend to have highly variable subsurface materials due to the dynamic and variable processes at the ice margin (debris flows, meltwater flows, or ponding). They are also commonly associated with depression-focused recharge and wetlands, which could lead to higher dissolved organic

AbstractGrowing population centers such as those in southern Ontario rely on fractured bedrock aquifers for drinking water. A threat to these aqui-

fers is posed by surficial nonpoint-source pollution infiltrating with rainwater and moving through the overlying Quaternary glacial deposits. Investigation of local unconsolidated sediments, and the factors affecting contaminant transport through these, is needed to assess risks to the quality of underlying groundwater resources. In this study, sites with a variety of agricultural land management practices and glacial geologic settings were investigated by employing high-resolution data collection methods. Geologic data from continuous sediment cores were com-bined with depth-discrete hydrogeologic and geochemical parameter measurements using high-resolution multilevel monitoring wells. Within a relatively small geographic area with three distinct glacially derived sediment types, the three sites exhibited greatly disparate vulnerability to nitrate contamination. The geologic setting, including surface topography and architecture and heterogeneity of sediment types at depth, influenced groundwater flow paths and water geochemistry, and subsequently, nitrate distribution. Although management practices influence the quantity of pollutants leaching to groundwater resources, the physical and chemical properties of the subsurface related to the geologic setting ultimately determine the persistence or attenuation of nitrate, and therefore become important to characterize when evaluating best nutrient and waste management practices.

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the north and the east sides. The surrounding area includes farms, golf courses, the University of Guelph Arboretum, and industrial and residential sections. The monitoring well for this study (TGI-1A) is located within the intercropped plots consisting of tile-drained crop rows (15 m wide) alter-nating with tree rows (1 m wide). Crops have been on a corn-soybeans-wheat rotation since 1986, and annual spring fertilizer applications are applied according to soil nutrient tests, typically 200 to 250 kg/ha for corn, 0 to 20 kg/ha for soybeans, and 100 to 150 kg/ha for wheat. Corn and soy-beans were grown in 2011 and 2012, respectively, with 103 kg/ha N applied as urea in 2011 and none in 2012.

The Arkell Research Station site (ARS) is used for con-ventional crop agriculture and is located on a glaciofluvial outwash plain with a gently rolling topography and broad swales next to the frontslope of the Paris Moraine. Previous studies at the site show predominantly sand and gravel at depth with variably preserved tills (Figure S3). Fields are well drained and do not have tile drains. Fields follow a corn-corn-soybeans-wheat crop rotation and routinely receive chemical fertilizer (63 to 99 kg N/ha) and liquid swine manure appli-cations (46 000–61 000 L/ha; Table S1) depending on soil nutrient testing and nutrient management plans.

A total of 18 piezometers previously installed in the lower unconsolidated sediments and the upper bedrock reveal nitrate concentrations above the recommended drink-ing water limit (10 mg/L nitrate-N) dating back to 1980 (H.R. Whiteley, personal communication, 2013). A north-west direction of groundwater flow has been established based on these piezometers (Figure 1; Opazo Gonzalez 2012). δ15N and δ18O values in nitrate from nearby downgradient piezometers suggest manure or sewage as a main nitrate source (Opazo Gonzalez 2012). Mapping of nitrate concen-trations in the station and the surrounding area has shown that elevated concentrations appear to originate within the Arkell Research Station boundaries (Opazo Gonzalez 2012). The monitoring well for this study (ARS-1A) is installed on the downgradient edge of the field where manure was applied and is located upgradient of the station’s livestock barns and septic beds.

MethodologyData were collected at the three sites to characterize

their geology, hydrogeology, sediment geochemistry, and water geochemistry (Best 2013). Sediment cores from ground surface to bedrock were collected for geologic and geochemical properties using field and laboratory measure-ments, and borehole-specific multilevel well designs were installed and monitored over the study period (September 2011 to August 2012). Frequent water levels and samples were taken throughout three seasons (Table S2) to assess groundwater source, residence time, and chemical proper-ties, particularly the distribution of nitrate.

Initial Field ActivitiesIn fall 2011, rotosonic drilling was used to extract a

continuous core (91% recovery rate) at each site from sur-face to at least 3 m into bedrock. Cores were described in the field at the centimeter scale in terms of texture, sorting,

carbon and decrease in surface runoff—all important factors controlling the fate of nitrate in the subsurface.

The goal of this study is to investigate the distribution of nitrate at depth in typical glacial settings with nonpoint source applications. We present high-resolution hydrogeo-logical and geochemical data from three sites near the city of Guelph, with differing glacial settings, in order to char-acterize the scale of variability and the relative influence of geology on the transport and fate of nitrate in the subsurface.

Description of Study AreaThe city of Guelph and the adjacent rural areas depend

on groundwater for drinking water, sourced mainly from a fractured Silurian dolostone aquifer that underlies uncon-solidated glacial sediments. Guelph is located in intensively farmed southwestern Ontario and has a growing popula-tion of 122,000 people. Nitrate-N levels above the 10 mg/L maximum allowable concentration for drinking water have been found in some private wells and near this level in one municipal supply well. Understanding nitrate transport through the overlying unconsolidated sediments is crucial to recognizing and predicting threats to this bedrock aqui-fer and to adopting practices that minimize water treatment costs and environmental impact.

The surficial geology of the Guelph area can be char-acterized as a variably thick package of Pleistocene gla-cial sediments (Figure 1) including glaciofluvial outwash plains, kames, drumlinized till plains, and the Paris Moraine (Chapman and Putnam 1984). The underlying Guelph Formation Silurian dolostone forms an unconfined aquifer in contact with the interface aquifer of the basal unconsoli-dated sediments (Brunton 2008). A regional semi-confined aquifer used by the municipality is found within the deeper Amabel Formation (referred to as Gasport Formation by Brunton, 2008). Three sites were selected for this study, tar-geting both distinct surficial land uses and glacial geologic settings with available land access (Figure 1).

The Vance Tract site (VAN) is a 40-hectare forested plot located on a hummocky end moraine (Paris Moraine) sur-rounded with rural residential and agricultural areas. The site was an agricultural land before being reforested (red pine) in the 1960s and allowed to fill in with natural veg-etation. There are no water supply wells or septic facilities on the land, and there has been no other development since reforestation. Numerous kettle ponds and wetlands in closed depressions are found in the moraine (Golder Associates 2006), and one wetland is situated directly north of the mon-itoring well installed for this study (VAN-1A). Laterally- and vertically-variable coarse- and fine-grained sediments underlie this site (Figure S1), and the Paris Moraine in general (McGill 2012). The moraine contributes to bedrock groundwater recharge due to its permeable soils, hummocky topography, and depression-focused recharge through ket-tles. The VAN-1A well is located a few meters away from a bedrock monitoring well.

The Guelph Turfgrass Institute site (TGI) has conven-tional agriculture and tree-based intercropping. It is located on a drumlin overlying a sequence of tills and outwash sands and gravel (Figure S2) with the Eramosa River bounding

70 A. Best et al./ Groundwater Monitoring & Remediation 35, no. 1: 68–81 NGWA.org

(a) (b)

(c)

(d)

Figure 1. Maps of the study area. Created using ESRI ArcMap 10.1. (a) Surficial geology of the south of Guelph, Ontario (Ontario Geological Survey 2003); (b) Vance Tract site (VAN); (c) Turfgrass Institute site (TGI); (d) Arkell Research station site (ARS). Elevation data from the Provincial Digital Elevation Model (2007), satellite imagery from SWOOP (2006), and ARS water table estimate from Opazo Gonzalez (2012).

clast characteristics, contacts, and sedimentary structures. Seven-port continuous multichannel tubing (CMT, Model 403) wells (Einarson and Cherry 2002) were installed using sand (size #40) as backfill in the monitoring zone and coated bentonite tablets (Enviroplug 3/8 inch) for the seals. The monitoring zones generally targeted coarser-grained units or

immediately adjacent layers, and occasionally finer-grained units, to detect temporarily perched water table conditions and measure variability in hydraulic head with depth and season (Table 1). At ARS-1A and TGI-1A, two to three ports were installed in the bedrock; VAN-1A has no ports in bedrock as the site already has a bedrock monitoring well.


estimated based on grain size distributions using the SPAW computer model (Saxton 2007; Saxton and Rawls 2006).

Chemical analyses (Table 2) were carried out on sedi-ment samples of approximately 200 g, targeting key litholog-ical units and CMT-monitoring zone intervals and collected within 2 days of core extraction. This wait time was nec-essary to carry out detailed core logging and CMT design; cores in their plastic sleeves were enclosed in a core box

Laboratory Analyses of SedimentsParticle size analysis was carried out on samples, taken

from texturally-distinct units every 1 to 1.5 m, using dry-sieving and hydrometer methods (Kroetsch and Wang 2008). In the context of texture classification, diamict refers to sedi-ment with 1 to 49% gravel in a mixed mud and sand matrix as per standard description of glacial sediments (Hambrey and Glasser 2003). Saturated hydraulic conductivity (K

sat) was

Table 1Monitoring Well Construction Details

VAN-1A TGI-1A ARS-1A

ConstituentTop of inter-val (m depth)

Bottom of inter-val (m depth)

Top of interval (m depth)

Bottom of interval (m depth)

Top of interval (m depth)

Bottom of interval (m depth)

Concrete grout 0 0.91 0 0.70 0 1.52

Bentonite holeplug 0.91 11.89 0.70 4.27 1.52 3.96

Port #1 13.13 14.73 5.79 7.01 5.43 7.33

Port #2 16.82 18.49 10.61 11.31 7.95 8.53

Port #3 20.63 22.23 13.41 14.05 9.50 10.01

Port #4 23.48 24.35 16.40 17.07 10.52 10.92

Port #5 27.43 29.14 17.98 18.50 11.29 12.54

Port #6 30.48 31.55 19.08 20.10 12.96 13.95

Port #7 34.39 35.97 21.35 22.51 14.87 15.27

Note: Intervals above each port are sealed with bentonite.

Table 2Selected Sediment and Water Sample Analysis Methods

Analyte Laboratory Reporting LimitSample

Campaigns Method

Sediment

Nitrate-N University of Guelph Agriculture & Food Lab, Guelph, Ontario

0.1 mg/kg dry Samples from core: Fall 2011

KCl extraction followed by colorimetry (USEPA 600/4-79-020: Method 350.1)

Ammonium-N 0.2 mg/kg dry KCl extraction followed by cadmium reduction to nitrite and colorimetry (USEPA 600/R93/100: Method 353.2)

Total sulfur 0.01% dry weight LECO combustion (Nelson and Sommers 1982)

Solid organic carbon 0.05% dry weight

Water geochemistry

Nitrate-N Maxxam Analytics, Mississauga, Ontario

0.1 to 1.0 mg/L Fall 2011, Spring 2012, Summer 2012

Cadmium reduction to nitrite followed by automated colorimetry (SM 4500 NO3 I; SM 4500 NO2 B)

Ammonium-N 0.05 mg/L Automated colorimetry (US GS I-2522-90)

DOC (Dissolved organic carbon)

0.2 mg/L High-temperature combustion (SM 5310 B)

Water isotopes

Tritium University of Waterloo Environmental Isotope Lab, Waterloo, Ontario

0.8 TU; precision 0.5 TU

Spring 2012; additional samples in spring 2013 at TGI-1A

Electrolysis followed by liquid scintillation counting (Taylor 1977)

δ Deuterium precision 0.8‰ Chromium reduction to hydrogen gas and mass spectrometry (Drimmie et al. 2001)

δ 18-oxygen precision 0.2‰ Continuous flow, isotope ratio mass spectrometry (Epstein and Maveda 1953; Drimmie and Heemskerk 2001)


of bedrock encountered at 37.50 m. Sediments are highly variable, with thick sand and gravel packages and mud beds between basal and cap diamict units (Figure 2a).

Bedrock is dolostone with many millimeter- to centimeter-scale pores, and one to two horizontal fractures per meter with black staining, suggesting active groundwater flow. The basal diamict lies above the bedrock from 37.50 to 35.95 m bgs. It is dense, with low clast content and inter-mediate texture. The overlying sand package extends from 35.95 to 23.45 m and is dominated by fine sand, with minor gravel and silt beds. A silt and clay unit lies in the middle of the succession from 23.45 to 20.75 m above a sharp con-tact with the underlying sand. At the base, the sediment is deformed, laminated silt, and clay, whereas at the top, it is moderately sorted silt and very fine sand. The overlying package from 20.75 to 17.70 m bgs comprises multiple thin beds of fine to coarse sand, silt, and gravel (small pebbles and granules). The upper gravel unit extends from 17.05 to 10.05 m bgs, with clasts ranging in size from granules to cobbles. The gravel has a sandy matrix with low mud con-tent at the base, whereas at the top, it has a muddy sand matrix and includes several thin muddy gravel beds. The uppermost package begins 10.05 m bgs with a bottom sec-tion composed of several beds of cohesive diamict, sand and gravel, and silty fine sand. From 7.00 m to the soil profile is a loose, clast-rich, intermediate diamict, which is crudely stratified in terms of clast content and matrix texture. This upper diamict is likely Wentworth Till, a sandy, stony till that typically covers the Paris Moraine (Karrow 1987).

VAN-1A HydrogeologyThe water table is present within the base of the upper-

most diamict unit in VAN-1A (Figure 2b). There were sea-sonal fluctuations of 1 to 2 m in all ports over the study period, with the highest water elevations in March 2012 (end of winter) and the lowest in late summer. Water elevations in the seven ports show an overall downward gradient in the vertical component between port 1 and port 4, and an over-all upward gradient between port 7 and port 4. The bedrock monitoring well a few meters away from VAN-1A shows a downward gradient within the bedrock beneath the base of the VAN-1A well, indicating recharge of groundwater from the unconsolidated deposits into the bedrock (Figure S4).

Vertical head gradients within the VAN-1A water col-umn show several changes over depth (Figure 2c). Port 1 and port 2, within the upper gravel and sand units, have similar water elevations that show a very small average downward gradient of 0.002. There was usually a downward gradient between ports 2 and 3 (average value 0.08), but during July and August 2012 when temperatures were high and water levels were dropping in all ports, port 3 water lev-els dropped more slowly, resulting in a temporary upward gradient between port 3 and port 2 (average value 0.05). Port 3 is completed in a fine-grained clay, silt, and very fine sand unit, which is expected to have a lower hydrau-lic conductivity than the other monitoring zones; there was also sometimes a lag of up to several days after pumping in recovering to expected water levels. A consistent upward vertical gradient (average 0.06) is present in the lower sand units, from port 7 (above the bedrock) to port 4.

to help prevent degradation. Samples were stored in labeled Ziploc bags at 4°C and were delivered to the laboratory for analysis the same or the next day. Nitrate-N, ammonium-N, and any other soluble compound were determined from pore-water extracted by KCl extraction, while sulfur and organic carbon were determined from the whole solid sample.

Hydrogeologic and Water Geochemical AnalysesWater depths were measured manually with a Skinny

Dipper water meter (Heron Instruments Inc., Dundas, Ontario) and recorded to the nearest 0.03 m. Elevations at the top of each well were obtained using a Leica Diva GPS with 1 cm accuracy. Three water sampling campaigns were carried out at each site, targeting the fall 2011, spring 2012, and summer 2012 seasons. Due to time constraints and variable flow rates, sample dates varied between the sites (Table S2). Groundwater sampling was carried out using a peristaltic pump and polyethylene or TeflonTM sample tub-ing and samples were collected after purging at least three well volumes. At ARS-1A and VAN-1A, the water table was too deep to allow continuous pumping, and water was extracted by bailing and discharging the water inside the tubing. Because of large well volumes and the extended time required to extract water this way, the VAN-1A ports were purged of three well volumes over the course of 1 to 2 weeks prior to the sample date, and then at least one volume purged immediately prior to sampling, so that samples from all ports could be collected within 1 to 2 days. Two ports in TGI-1A, located in fine-grained diamict, exhibited slow recovery and were pumped dry several times in the days prior to sampling, and then sampled directly. Although standing water in a well may experience increased dissolved oxygen and related chemical changes, especially at the top of the water column, this impact was not very pronounced after a 1-week time lapse in till aquitard wells (Schilling 2011). At all sites, sample tubing was pushed to the bottom of the well tubing to target the more representative water within the fil-ter pack, which is in connection with the formation water. Field parameters (temperature, pH, conductivity, dissolved oxygen [DO], and oxidation-reduction potential [ORP]) were measured using a calibrated handheld multiparameter probe (YSI 556, YSI Incorporated, Yellow Springs, Ohio), either in a small static container or with a flow-through cell configuration whenever possible. Because water was typi-cally extracted by bailing at ARS-1A and VAN-1A, accurate DO measurements at those sites were not available. Water samples were collected in polyethylene bottles and stored in a cooler with ice until transferred to refrigerator at 4°C. Ammonium samples were preserved by acidification. Water samples were then sent to commercial (Maxxam Analytics, Mississauga, Ontario) and research laboratories for specific geochemical and isotopic analyses (Table 2).

Results

Vance Tract (VAN-1A)VAN-1A Continuous Core

The VAN-1A borehole was continuously cored to a depth of 41.15 m below ground surface (bgs), with the top


and organic carbon (approx. 0.4 to 3% dry) in the 0.6 m deep soil horizon. The sediments beneath the soil horizon have relatively low concentrations of nitrate and organic carbon (average 0.3 mg/kg nitrate-N and 0.2% organic car-bon). Ammonium-N is highly variable over depth (generally <0.02 to 8.8 mg/kg), with many of the higher concentrations found within the middle mud bed and lower sand package. Total sulfur was not detectable (<0.01%).

Nitrate was not detected in any water samples from VAN-1A (<0.1 mg/L nitrate-N). Ammonium was detectable in all ports, with peak concentrations in ports 2 and 3, to a maximum of 1.6 mg/L ammonium-N (Figure 2f). Dissolved organic carbon (DOC) ranges from 6 to 17 mg/L for ports 2 to 7, which is relatively higher than typical concentra-tions of DOC in groundwater (1 to 3 mg/L; Aravena and Wassenaar 1993). ORP measurements are typically negative in each port and fairly consistent with depth, indicating the groundwater is under reducing conditions.

Turfgrass Institute (TGI-1A)

TGI-1A Continuous CoreThe TGI-1A borehole was continuously cored to a

depth of 22.55 m bgs, with top of bedrock encountered at 19.05 m (Figure 3a). The dolostone bedrock is porous and vuggy, with cavities on the millimeter- to-centimeter scale. The lowest 1.2 m of the 3.5 m bedrock core includes three

Ksat

estimates range from 10−3 to 10−4 cm/s in the inter-mediate (coarse-grained) diamict, sand and gravel and from 10−4 to 10−5 cm/s in the muddy interval in the middle of the succession (Figure S5). K

sat estimates for diamict are rela-

tively high (10−4 cm/s). However, the diamict in the Guelph area is considered coarse-grained and the values are still in the range identified for glacial till (Freeze and Cherry 1979).

The 5-year average tritium concentration in precipitation from the Ottawa tritium record from 1974 to 2007 (IAEA/WMO 2013), decayed to 2012, varied between 8 and 15 TU. Tritium values in shallow groundwater in the Guelph region representing more recent precipitation ranged between 15 and 20 TU (Camillo 2013; AECOM 2009). Tritium concen-trations of 13.4 to 14.1 TU in ports 1, 4, and 7 indicate that modern water is present throughout the sediment profile, with notably very close values between the three depths (Figure 2d). The stable isotope data show values of −10.2 and −71.2 ‰ for δ18O and δ2H, respectively, in port 1. More enriched values, which range from −8.7 to −7.9 ‰ for δ18O and from −61.2 to −66.3 ‰ for δ2H, are observed in ports 3, 4, and 7. The isotopic composition of groundwater in the Guelph area ranged between −9.4 and −11.8 ‰ for δ18O and −68.1 and −76.9 ‰ for δ2H (Opazo Gonzalez 2012).

VAN-1A GeochemistrySediment geochemistry in the VAN-1A core (Figure 2e)

shows higher nitrate (approx. 1 to 4 mg/kg nitrate-N dry)

(a) (b) (c) (d) (e) (f)

Figure 2. Summary of key data from VAN-1A. (a) Geologic log. Top layer is soil. (b) Multilevel well design. (c) Static water eleva-tions. (d) Tritium concentrations in multilevel ports. (e) Selected sediment geochemical data. (f) Selected water geochemical data. See Table S2 for specific water sampling dates.


the study period, there were large seasonal fluctuations up to 3.5 m for most ports, with the highest elevations occurring in the mid-to-late winter and the lowest elevations occurring in the fall (Figure 3c). A sharp increase in water elevations occurred in late November 2011, following heavier precipi-tation and indicating recharge with colder and wetter fall weather conditions.

Water elevation profiles show an overall downward gra-dient throughout ports 2 to 7. Port 1 within the upper sand bed has levels similar to those of port 2 in the driest part of the year but lower than those of the other ports through-out the wetter winter. There is a drop in head from port 2 in the middle diamict unit to port 3 in the lower sand bed (average gradient 0.23). Port 4 in the muddy basal diamict tends to be similar to port 3, with a slight upward gradient between them (0.007) in warmer months, and a slight down-ward gradient (0.007) from October to March during winter recharge conditions. Another head drop occurs throughout the basal diamict to the bedrock, from ports 4 to 6 (aver-age gradient 0.27). Bedrock ports 6 and 7 track each other throughout the year with a consistent small downward gradi-ent (0.02). During well purging and sampling, ports 4 and 5 in the muddy, dense basal diamict consistently ran dry and required almost a day to recover to regular water levels. K

sat

estimates decline steadily from 10−4 to 10−5 cm/s from the diamict at surface to the diamict at the base of the succession consistent with changes in texture; whereas K

sat estimates for

sand interbeds are in the 10−3 cm/s range (Figure S6).Tritium concentrations decrease steadily throughout

TGI-1A from 16.8 TU in port 1 to non-detectable levels

horizontal fractures with black staining and one with iron oxide staining, suggesting active groundwater flow. The overlying sediments are composed of three thick packages of diamict separated with sharp contacts by two relatively thinner sand or sand and gravel units. The diamict is gener-ally more consolidated with depth, and the texture varies, with muddy, clast-poor diamict at the base; intermediate, clast-rich diamict at the surface; and a variable package of layered diamict in the middle.

The basal, muddy, and clast-poor diamict package (19.05 to 15.55 m bgs) is dense and stiff, with a high silt and clay fraction in the matrix (82 to 85%), and relatively few pebbles and granules (Figure 3a). The lower sand bed (14.95 to 13.40 m bgs) is well-sorted, silty very fine sand. The middle diamict package (13.40 to 7.75 m bgs) has 5 to 30% clast content and its matrix texture varies from muddy at the base to intermedi-ate at the top. Multiple centimeter-scale sand beds are found within the diamict unit between 12.00 and 10.65 m bgs, and a 0.3-m bed of very fine sand is found near the top.

The upper sand and gravel unit (7.75 to 5.80 m bgs) fines upward from gravel to a gravelly, poorly-sorted sand and then to a more uniform fine-to-medium sand. The upper diamict package (7.90 m bgs to the soil) is clast-rich and intermediate and varies from loose to cohesive. Based on regional mapping (Karrow 1987), the diamict at surface is likely the Port Stanley till.

TGI-1A HydrogeologyThe water table at TGI-1A varies from the middle of the

upper diamict unit to the upper sand bed (Figure 3b). Over

(a) (f)(b) (c) (d) (e)

Figure 3. Summary of key data from TGI-1A. (a) Geologic log. Top layer is soil. (b) Multilevel well design. (c) Static water elevations on four dates. (d) Tritium concentrations in multilevel ports. (e) Selected sediment geochemical data. (f) Selected water geochemical data. See Table S2 for specific water sampling dates.


Similarly, dissolved oxygen, measured using a flow cell to limit contact with the atmosphere, is present in port 1 (2.7 to 5.2 mg/L); port 2 (0.6 to 0.7 mg/L); and ports 3, 6, and 7 (≤ 0.4 mg/L). Dissolved oxygen could not be accurately measured in ports 4 and 5 since slow recovery prevented use of a flow cell.

Arkell Research Station (ARS-1A)

ARS-1A Continuous CoreThe ARS-1A borehole was continuously cored to a

depth of 15.25 m with top of bedrock encountered at 11.60 m bgs (Figure 4a). The Silurian dolostone bedrock has mil-limeter-scale pores and is extensively fractured, including a 0.6-m subvertical fracture. Some of this fracturing may have occurred mechanically during drilling; however, there are two to three horizontal fractures per meter with iron oxide staining, suggesting active groundwater flow. The lower sediment package (11.60 to 7.00 m bgs) is composed of a cohesive, clast-rich, intermediate diamict with a 0.45-m bed of poorly-sorted gravel. The upper package in ARS-1A begins with a bed of loose, sandy, clast-rich diamict, which is visually and texturally distinct from the lower interme-diate diamict. This bed coarsens upward to loose, poorly sorted gravel, which extends from 6.50 m bgs to the soil horizon. The gravel has a silty sand matrix and is crudely stratified, with variations in matrix texture and clast size and abundance.

ARS-1A HydrogeologyA deep unsaturated zone was present at ARS-1A, with

the water table in the dolostone bedrock (Figure 4b). No perched water table was observed in the lower diamict unit

(<0.8 TU) in bedrock ports 6 and 7 (Figure 3d). The consis-tent decline suggests increasing groundwater residence time with depth, with pre-1953 recharge suggested by the non-detectable tritium levels in the bedrock ports. The stable isotope data show values between −9.8 and −10.6 ‰ for δ18O and −69.6 and −71.3 ‰ for δ2H, which is within the range of values reported for groundwater in the Guelph area (Opazo Gonzalez 2012).

TGI-1A GeochemistrySediment geochemistry (Figure 3e) shows higher nitrate

(approx. 0.6 to 3 mg/kg nitrate-N dry) and organic carbon (approx 0.4 to 2% dry) in the 0.4-m deep soil horizon. The sediment beneath the soil generally has lower con-centrations of nitrate and organic carbon (average 0.4 mg/kg nitrate-N and 0.2% organic carbon). However, organic carbon ranges up to 0.7% in the lower sand unit and basal diamict below 13.2 m, whereas it is lower and often non-detectable (<0.05%) in the upper sediment between the soil and 13.2 m depth. Ammonium is variable over depth (0.8 to 7.2 mg/kg dry) and total sulfur was not detectable (<0.01%).

Nitrate was detected in the upper two ports with con-centrations ranging from 0.4 to 1.7 mg/L nitrate-N in port 1 and from 3.9 to 5.4 mg/L nitrate-N in port 2, and was not detected in the lower five ports (Figure 3f). Ammonium was usually not detected in the upper three ports (<0.05 mg/L ammonium-N), but is present in the lower ports at low concentrations to a maximum of 0.27 mg/L ammonium-N. Dissolved organic carbon is present throughout the water column from 1 to 5 mg/L, with no clear trends over depth. The ORP values tend to be positive in the upper two ports, but decrease to consistently negative values in ports 4 to 7.

(a) (b) (c) (d) (e) (f)

Figure 4. Summary of key data from ARS-1A. (a) Geologic log. Top layer is soil. (b) Multilevel well design. (c) Static water elevations on four dates. (d) Tritium concentrations in multilevel ports. (e) Selected sediment geochemical data. (f) Selected water geochemical data. See Table S2 for specific water sampling dates.


natural land. The VAN-1A sediment core revealed high variability in sediment type over depth. This heterogene-ity is consistent with other research cores from the moraine (McGill 2012) and local water well records (Figure S2), as well as the variable processes, and consequently vari-able deposits, expected in an end moraine environment (Krzyszkowski and Zielinski 2002).

A drop in head over the fine-grained mud bed in the middle of the core, combined with increasing head with depth over the lower sand package, suggests a two-layered groundwater flow system (Figure 5a). The mud bed appears to act as an aquitard with a K

sat estimate at its base of 4.6 x

10–5 cm/s (Figure S5), but its absence in nearby boreholes (Figure S2) indicates that it is relatively localized. Modern tritium ages at the bottom of the water column indicate that recent recharge makes its way to the base of VAN-1A with relative ease despite the mud bed. Water likely recharges through permeable units that are laterally connected with the lower sand package, resulting in the upward vertical gradient observed here. Localized (m’s to 10’s m scale) lat-eral flow is thus inferred (Figure 5a) based on the down-core changes in hydraulic head together with tritium and hydrogeological data, though this requires confirmation with additional points.

No nitrate was detected in VAN-1A groundwater. Although no recent nutrient additions have occurred directly on-site, this indicates that any nitrate from agricultural prac-tices in the 1960s and earlier has been attenuated or flushed out, and that any nitrate from the surrounding agricultural areas and septic tanks either is not transported to this site or is attenuated along with any naturally occurring nitrate from the forest. Conditions conducive to denitrification are con-sistently found in VAN-1A, with reducing conditions and elevated DOC in the groundwater. The presence of ammo-nium with concentrations ranging up to 1.6 mg/L is also an indication of reducing conditions.

Water slightly more enriched in 18-oxygen and deu-terium in ports 3, 4, and 7 coincides with higher dis-solved organic carbon in these ports compared with port 1. In the context of the site topography, nearby wetland, and hydrogeologic data, these data are thought to record the influence of wetland recharge on the geochemistry of the lower flow path (Krabbenhoft et al. 1994; Kehew et al. 1996). Collection of water in depressions including the adjacent wetland would allow some evaporation to occur prior to recharge, enriching the water in 18-oxy-gen and deuterium. Wetland recharge may also promote reducing conditions due to the supply of organic carbon (Kehew et al. 1996; Denver et al. 2014). Overall, the pre-dominance of reducing waters associated with recharge in closed depressions within the moraine, and the abun-dant electron donor supply (dissolved organic carbon), is a combination that could effectively mitigate potential nitrate contamination from farms or septic beds in the area.

TGI-1A Integration of DatasetsThe Turfgrass Institute site is situated on a drumlin

used for field crops intercropped with tree rows to mitigate nutrient leaching (Figure 1). The TGI-1A sediment core is

during the study period. Water elevations fluctuated by about 0.8 m seasonally, with the highest elevations present in late winter and the lowest in August 2012 (later sum-mer; Figure 4c). The bedrock at ARS-1A is highly frac-tured, and during drilling, large volumes of drilling water were quickly lost in the top meter of bedrock while trying to cool the barrel, suggesting high permeability of this upper bedrock. FLUTE profiling at a site approximately 500 m from ARS-1A has shown high transmissivity in the Guelph Formation dolostone attributed to fracture flow (Opazo Gonzalez 2012). A small downward gradient (average 0.06) is present between ports 6 and 7.

Ksat

estimates range from 10–3 to 10–4 cm/s in the gravel and intermediate diamict, with very similar values for these two lithologies consistent with the coarse-grained matrix and clast-rich nature of this diamict (Figure S7).

Tritium concentrations of 19.1 and 19.7 TU (Figure 4d) indicate that ARS-1A groundwater is within the range of modern precipitation samples (AECOM 2009; Camillo 2013). The stable isotope data show values of approxi-mately −11.5 ‰ for δ18O and −77‰ for δ2H, which are in the range of values for groundwater in the Guelph region (Opazo Gonzalez 2012).

ARS-1A GeochemistrySediment geochemistry results (Figure 4e) show higher

nitrate (approx. 2 to 6 mg/kg nitrate-N dry) and organic car-bon content (approx. 1 to 2% dry) in the 0.9-m deep soil horizon. Below the soil, there is a relatively thick zone with elevated nitrate and organic carbon compared with the val-ues found deeper in the core. Nitrate values range up to 1.4 mg/kg to a depth of 4.0 m and organic carbon ranges up to 0.5% to a depth of 2.6 m. This layer contrasts with lower average values of 0.6 mg/kg and 0.1%, respectively, in the deeper sediment. Ammonium-N concentrations are variable (0.2 to 4.9 mg/kg dry).

Nitrate concentrations in ARS-1A are above the allow-able drinking water maximum at 13 to 14 mg/L nitrate-N in port 6 and 14 to 18 mg/L nitrate-N in port 7 (Figure 4f). Ammonium was not detectable in any samples (<0.05 mg/L ammonium-N). Dissolved organic carbon is present from 1.2 to 4.4 mg/L, and ORP values are positive, between 40 and 300 mV.

DiscussionThe goal of this study was to investigate the distribution

of nonpoint source nitrate at depth in different glacial depos-its. Concentrations of nitrate found at each site were con-sistent with land management practices in terms of relative levels found in the subsurface. However, downcore changes in nitrate concentrations, hydrogeology, and geochemistry also clearly reflected the geologic setting and geologic het-erogeneity. Once nitrate infiltrated into these different gla-cial settings, localized conditions determined the transport, persistence, or attenuation of the nitrate (Figure 5).

VAN-1A Integration of DatasetsThe Vance Tract is a forested site situated on the hum-

mocky Paris Moraine, surrounded by rural agricultural and


There are several geochemical trends over depth observed within TGI-1A. Nitrate and relatively oxidiz-ing conditions are present in the upper two ports, whereas the lower ports show non-detectable nitrate and relatively reducing conditions. The evolution of reducing conditions over depth or over flow paths with a concurrent attenuation of nitrate is frequently seen in groundwater (Postma et al. 1991; Aravena and Robertson 1998). With a loss of nitrate between ports 2 and 3, denitrification appears to occur in the groundwater. The intermediate diamict surrounding port 2 (estimated K

sat of 2.6 x 10–4) changes to a muddy diamict

(estimated Ksat

of 1.3 x 10–4) above the sand surrounding port 3 (Figure S6). This relatively finer-grained layer may slow the downward movement of water and facilitate the consumption of oxygen and the subsequent reduction of nitrate. Denitrification may be mediated by dissolved or solid organic carbon present at the site.

The increase in nitrate from port 1 to port 2 might be a result of changes in land management. The treed intercrops were introduced in 1980 and were demonstrated to lower nutrient leaching to tile drains compared with an adjacent

composed of diamict packages and thinner sand beds, con-sistent with a drumlinized till plain setting where the drum-lin form and several diamict packages record repeated ice advances in the area.

Hydraulic head profiles indicate a flow system with an overall downward vertical gradient locally, with down-ward infiltration through diamict and toward more conduc-tive units (sand layers and bedrock). This downward flow appears inhibited at the contact between the lower sand bed (K

sat estimate of 2.7 x 10–3 cm/s) and the dense, muddy,

basal diamict (Ksat

estimate of 9.9 x 10–5 and 4.1 x 10–5 cm/s), with similar water elevations in ports 3 and 4 and a slight upward gradient present from April to September 2012. This suggests that groundwater tends not to infiltrate through the basal silty dense diamict except during winter recharge conditions. Slow flow through the basal diamict is indicated by the ports running dry during purging, and the sediment exhibiting the lowest estimated hydraulic con-ductivity (10–5 cm/s) in the profile (Figure S6), low tritium concentrations in port 4, and non-detectable tritium in the bedrock (ports 6 and 7).

Figure 5. Conceptual diagrams integrating geology, hydrogeology, and geochemistry for each study site. (a) Vance Tract, forested end moraine setting; (b) Turfgrass Institute, agroforestry plots with fertilizer application, drumlin setting; (c) Arkell Research Station, conventional crop agriculture with fertilizer and manure application, glacial outwash plain setting. DOC = dissolved organic carbon, SOC = sediment organic carbon.

(a)

(b) (c)


field without trees (Dougherty et al. 2009). The tile drains themselves were installed in 1992 and could also remove some leached nutrients from the subsurface. The lack of nitrate within the older water at depth could be attributable to lower nutrient application in the past plus attenuation by denitrification (Puckett and Cowdery 2002).

ARS-1A Integration of DatasetsThe Arkell Research Station is an agricultural site with

a history of elevated groundwater nitrate. The ARS-1A core exhibits coarse surficial gravel and intermediate-textured basal diamict, consistent with findings from a nearby ground-penetrating radar survey (Sadura et al. 2006) and similar to the heterogeneous units of gravel, sand, and diamict found throughout the station (Figure S4). These spatially-variable coarse-grained sediment packages were likely deposited by meltwater and debris flows in an end moraine outwash fan environment (Krzyszkowski and Zielinski 2002; Sadura et al. 2006).

A deep unsaturated zone was present at this site with the water table found in the highly transmissive, fractured dolostone bedrock. There is apparently fast infiltration of precipitation through the subsurface, evidenced by the lack of runoff ditches or artificial drainage of the fields, dry sedi-ment in the core, and modern tritium concentrations in the bedrock groundwater as well as supported by the relatively high estimated hydraulic conductivity (10–3 to 10–4 cm/s; Figure S7). The dynamic meltwater flows that deposited outwash gravels at this site likely eroded into the basal diamict in some areas (Figure S4). As a result, there are likely preferential flow paths created by units with higher permeability in connection with each other (Anderson et al. 1999). Other possible reasons for the fast recharge to bed-rock could be depression-focused infiltration in the swales of the gently rolling topography (Derby and Knighton 2001; Gerke et al. 2010) or the presence of fractures in the basal diamict (Harrar et al. 2007).

Long-term, repeated applications of animal manure often results in higher soil organic matter and higher nitrate in the subsoil layer compared with long-term use of chemi-cal fertilizer alone (Edmeades 2003). Such nitrate stores in the unsaturated zone can serve as a source of nitrate in groundwater (Geyer et al. 1992). Considering that both fertilizer and liquid swine manure have been repeatedly applied immediately upgradient of ARS-1A, this may explain the high nitrate concentrations at this site. These high concentrations are not attenuated at depth, despite the thick overburden, due to rapid infiltration through inter-connected coarse-grained sediment units with relatively high estimated hydraulic conductivity, and oxidizing con-ditions at the water table. Additionally, ammonium stores adsorbed to clay particles in the basal diamict may be oxi-dized to nitrate upon release to groundwater considering the observed readily available oxygen. Compounds such as organic carbon that are readily oxidized and thus would deplete the oxygen appear to be limited in the lower vadose zone and shallow groundwater. Although some dissolved organic carbon is present in the groundwater, this may be composed of less reactive forms of carbon (Starr and Gillham 1993).

Importance of Geological Context When Evaluating Land Management Practices

Although land management regulations are often applied to large politically-defined regions, it can be more advanta-geous to target BMPs on vulnerable areas (Refsgaard et al. 2014). Nitrate concentrations in this study were consistent with land management practices, with elevated nitrate at a conventional agricultural site, lower nitrate with tree-based intercropping, and no nitrate detected at a reclaimed for-ested site. However, this study shows that the geological characteristics of each glacial setting exert a major control on nitrate transport from the surface, its distribution in the subsurface, and the rate of attenuation in the groundwater.

Denitrification depends on oxygen depletion and the presence of electron donors in dissolved or solid phases, each of which will be influenced by the sediment heteroge-neity and architecture as well as sediment sources (for solid electron donors such as organic matter and sulfide minerals). Moraine settings such as VAN-1A, with variable subsurface stratigraphy, moraine wetland-influenced organic carbon, and reducing conditions conducive to denitrification, may be able to serve as natural nitrogen sinks in the landscape. Nitrate attenuation was also observed in the drumlinized till plain setting of TGI-1A, but promoted by the presence of diamict aquitards that slow water flow and allow reducing conditions to develop. These two glacial settings may thus provide considerable protection from nitrate contamination to underlying aquifers, depending on site-specific condi-tions. In contrast, outwash plain settings such as ARS-1A can have relatively simple sediment architecture, coarse-grained and dry sediments, preferential pathways through erosional windows and/or fractures in underlying till as well as high dissolved oxygen and low DOC related to the thick, permeable unsaturated zone; these characteristics make them particularly vulnerable to nitrate leaching and persis-tence. At ARS-1A, this was compounded by the high per-meability and fractured nature of the underlying bedrock. It is, therefore, critical that any evaluation of BMPs be made in the context of site geology. While BMPs may influence the amount of nitrate reaching the subsurface environment, site-specific conditions will determine the transport and fate of the nitrate in the groundwater and its potential for con-taminating underlying aquifers.

Moraines are often considered important areas of ground-water recharge, and protection of these areas is a priority for local communities (Golder Associates 2006). Their complex and heterogeneous geology requires detailed investigation to characterize the groundwater system within them and devise appropriate source water protection plans and best management practices (Stotler et al. 2010). Drumlinized till plains are a common, and spatially extensive, environment among glaciated terrains. Although internally, drumlins are less heterogeneous than moraines, their internal composi-tion can vary significantly depending on the subglacial con-ditions at the time of their formation and prior glacial history in that area. As such, the variable subsurface of drumlinized till plains and moraines makes site-specific analysis critical to confirm the presence of aquitard materials and reducing conditions. Data from this study show that high-resolution vertical profiles (geologic, hydrogeologic, and geochemical


datasets) from extracted core and many short, depth-discrete monitoring zones allow the complexity of groundwater flow paths and geochemical conditions important to contaminant mobility and fate to be inferred. From this single vertical profile, an evidence-based conceptual model can help guide further hydrogeological characterization studies.

Nitrate contamination in unconfined aquifers is more probable with permeable soils and coarse unconsolidated sediments (Goss et al. 1998). It is clear that care must be taken at agricultural sites situated on glacial outwash sedi-ments, which are common in Ontario. This appears to be the case even when the water table is deep, in this case, approximately 13 m at ARS-1A, despite the fact that con-tamination risks are often considered greater with a shallow water table (Nolan et al. 2002). Contrary to the common assumption that tills will provide some protection to the underlying groundwater, the common occurrence of a basal diamict across the Arkell Research Station does not appear to mitigate nitrate contamination to the unconfined bed-rock aquifer. This could be due to the limited thickness and the lateral discontinuity of the diamict, its coarse-grained nature, or to the preferential flow through fractures in the diamict. It is clear from the findings at ARS-1A that more intensive conservation practices may be needed in this par-ticularly vulnerable geologic setting.

ConclusionsThis study investigated three sites with differing glacial

geology, with a focus on its influence on nitrate transport and attenuation, based on detailed measurement of physi-cal, hydrogeologic, and geochemical properties in vertical profiles. Nitrate concentrations were consistent with land use, but physical and chemical properties and style or archi-tecture of the sediment variability clearly played a major role in determining the distribution of nitrate in the subsur-face. Coarse diamict and outwash sediments overlying frac-tured bedrock allowed fast drainage at the Arkell Research Station resulting in a deep water table with elevated nitrate in the bedrock aquifer. The predominance of finer-grained diamict at the Turfgrass Institute slowed water movement, allowing reducing conditions conducive to denitrification to develop. A dense basal diamict slowed water movement to the degree that bedrock groundwater at this site appears relatively old compared with the other two sites, lowering the risk of impact by high nutrient application rates in recent decades. At the forested Vance Tract, consistent reducing conditions were likely promoted by more complex sedi-ment layering and elevated dissolved organic carbon in the groundwater, which could originate from recharge through the numerous kettle ponds and wetlands found within the moraine. Subsurface sediment heterogeneity in the imme-diate area around the well allowed modern groundwater to reach the bottom of the sediments, which could pose a threat to the bedrock aquifer if nitrate-attenuating condi-tions were not present. These findings clearly suggest that sediment heterogeneity in glacial geologic settings affects the hydrogeological and geochemical conditions at a site, which then influences contaminant fate at depth. The glacial geological setting must be considered when evaluating and

implementing best management practices, as some sites will be better suited to mitigate the impact of nitrate in agricul-tural regions.

AcknowledgmentsThis study was primarily funded through the Ontario

Ministry of Agriculture and Rural Affairs-University of Guelph Partnership (Environmental Sustainability theme). Additional financial support was provided by the Natural Sciences and Engineering Research Council, the Ontario Research Fund-Research Excellence, and the Geological Survey of Canada, as well as in-kind contributions from Solinst Canada Ltd. We are grateful for site access provided by the University of Guelph (TGI and ARS) and the Grand River Conservation Authority (VAN). We also acknowledge the field and the laboratory assistance from staff and stu-dents of G360 Centre for Applied Groundwater Research and the School of Environmental Sciences.

The research presented herein was funded by OMAFRA, with contributions from NSERC, ORF-RE, and the Geological Survey of Canada. There is no conflict of interest associated with this manuscript.

Supporting InformationThe following supporting information is available for

this article:Figure S1. Geological cross section—Vance Tract.Figure S2. Geological cross section—Trufgrass Institute.Figure S3. Geological cross section—Arkell Research

Station.Figure S4. Static water elevations in the sediment and

bedrock ports at Vance tract.Figure S5. Particle size analysis and K

sat estimates—

Vance tract.Figure S6. Particle size analysis and K

sat estimates—

Turfgrass Institute.Figure S7. Particle size analysis and K

sat estimates—

Arkell Research Station.Table S1. Nutrient application rates for Arkell Research

Station (2001–2012). Table S2. Schedule of geochemical water sampling at

all three sites.

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Biographical SketchesAnna Best, M.Sc., was at School of Environmental Sciences,

University of Guelph and G360 Centre for Applied Groundwater Research.

Emmanuelle Arnaud, Ph.D., corresponding author, is at School of Environmental Sciences, University of Guelph and G360 Centre for Applied Groundwater Research. [email protected]

Beth Parker, Ph.D., is at School of Engineering, University of Guelph and G360 Centre for Applied Groundwater Research.

Ramon Aravena, Ph.D., is at Department of Earth and Environmental Sciences, University of Waterloo.

Kari Dunfield, Ph.D., is at School of Environmental Sciences, University of Guelph.

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