1. INTRODUCTION

The groundwater flow system and nitrate attenuation processes control the nitrate distribution in aquifers. The regional flow system, shaped by the geology, regulates the transport of nitrate. Nitrate is a very soluble, mobile contaminant that is readily leached into the groundwater. The aquifer in the Woodstock area is susceptible to surface contamination due to the geology. Discontinuous aquitards and highly permeable sediment allow shallow nitrate contaminated water to travel to the deeper aquifer. Identifying processes that affect nitrate can be problematic in regional cases due to the variable physical and geochemical settings that are present in the system. Relationships between the distribution of agricultural contaminants in superrficial aquifers and corresponding stream responses to contamination are somewhat complicated where regional changes in agricultural practices and groundwater residence times are on the same time scale (Bohlke and Denver, 1995).

Nitrate attenuation may occur by dilution or denitrification. Dilution occurs through mixing of groundwaters in the aquifer. Denitrification is the only geochemical process that removes nitrate permanently from aquifers. The groundwater geochemistry is controlled by redox conditions in the aquifer. Groundwater environments with sufficient amounts of oxygen will not affect nitrate geochemistry. Only reducing environments have the potential for denitrification.

Nitrate reduction can be mediated by oxidation of, organic matter (Equation 1), pyrite (Equation 2) and iron (Equation 3) under anaerobic condition.

5CH₂O + 4NO₃^- → 2N₂ + 4HCO₃ + CO₂ + 3H₂O (1)

5FeS₂ + 14NO₃^- + 4H⁺ → 7N₂ + 5Fe²⁺ + 10 SO₄^2- + 2H₂0 (2)

10Fe²⁺ + 2NO₃^- + 14H₂O → 10FeOOH + N₂ + 18H⁺ (3)

Research into regional recharge locations, geochemistry and regional flow systems is necessary in order to effectively plan and protect well fields. The goals of this project are to use geochemical and hydrogeological tools to ascertain the source of the nitrate contamination and the processes than control the distribution of nitrate from the recharge areas to the production wells. This study is a part of a major research project involving hydrogeology and the use of dating techniques to evaluate the long-term behaviour of the Thornton Well Field. Although each site will be unique, the approach, goals and techniques used in this study may be applied to other cases.

2. STUDY SITE

A two aquifer, three aquitard system has been delineated with a complex layering system. Discontinuous layers of sand, till and gravel comprise the geology that allows local recharge water to penetrate deep into the aquifer system. Water table elevations indicate that groundwater flow in the shallow aquifer is from west to east and discharges to Cedar Creek and Hodges Pond.

Go to Figure 1

The production wells are screened in the lower aquifer. Hydraulic head data suggests that groundwater from the surface aquifer is recharging the lower aquifer. A recovery test performed on well 1 showed the presence of a flow barrier between the production wells and the east region (Padusenko, 2001). Test holes and research drilling confirmed the presence of a till unit.

3. STUDY APPROACH

Analyses for dissolved oxygen (DO), Eh and pH were performed on site to determine the redox conditions at the site. Samples for chemistry were obtained and analyzed for nitrate, sulphate, chloride, iron and dissolved organic carbon (DOC) to aid in understanding the geochemical environment and in identifying processes that affect nitrate geochemistry. Isotope analysis (δ¹⁵N and δ³⁴S ) was carried out on selected water samples, sediment samples, and manure sources. The purpose of performing ¹⁵N analysis on nitrate is to identify processes that may affect the nitrate along the flow path. In addition, comparison of δ¹⁵N signatures of groundwater samples with potential sources may permit identification of the source of the nitrate reaching the production wells. The analysis of the δ³⁴S in sulphate is performed to detect if nitrate reduction by pyrite oxidation could be occurring and to provide additional information about redox conditions.

4. NITRATE DISTRIBUTION

Go to Figure 2

High nitrate concentrations (>8 mg/L N) are found only in the west region while in the north and east regions significantly lower nitrate levels are found (1-3 mg/L as N, North region; 0-1 mg/L as N, East region). Figure 3 illustrates the distribution of nitrate with depth along the cross section A-A'. High nitrate concentrations are found in the shallow and deep aquifer units in the western area.

4.1 Nitrate Geochemistry

Go to Figure 3

DO concentrations remain high in the lower aquifer (Figure 4), inhibiting denitrification.

Go to Figure 4

Piezometers in the shallow and deep aquifers and the aquitard show the hydraulic connection of production well 1 to the west region (Padusenko, 2001). Point sources of focused recharge (drainage tile discharges and depression-focused recharge) account for large nitrate loads reaching the groundwater.

Observations of two tiles suggest that discharge from these tile drains recharge the groundwater at high nitrate concentrations (17 and 7 mg/L) during times of low flow. The discharge from WO Tile 23 has a δ¹⁵N signature in nitrate (+7.30‰ at high flow, +5.90‰ at low flow) similar to that of the production wells (+6.5‰ to +6.60‰). There is no evidence that denitrification is occurring in the region west of the production wells. The oxidizing geochemical environment inhibits denitrification. High nitrate concentrations exist throughout the region, without evidence of a decrease in nitrate concentration. The majority of δ¹⁵N values are within a small range throughout the west region (+5.70‰ to +8.50‰) supporting the lack of denitrification in the western area.

The geochemistry and hydrogeology provide evidence that water from the shallow aquifer is recharging the Thorton Well field. It is evident in the geochemical data collected along the transect A-A', that the high DO and nitrate water is being drawn down into the production wells. A correlation of the geochemical parameters and the hydraulic connection of the aquifer to the production wells provide strong evidence that the region west of the well field is responsible for the high nitrate observed in the production wells.

4.1.2 East Region
The hydraulic barrier separating this region from the well field, excludes the east region from the regional flow system. The origin of groundwater recharge for the east region is not specifically known. Recharge through fertilizer and manure produces chloride values in the range found in the region (Altman and Parizek, 1995). The presence of tritium in groundwater samples from piezometers and a private well in this region suggests that the groundwater was recharged after 1953, and is young enough to be impacted by current land use practices (Sebol, 2000). Low DO (0-1 mg/L), high DOC (3.2-10.9 mg/L), high sulphate (46-120 mg/L), and low nitrate concentrations (0-1 mg/L) characterize the groundwater conditions in the region east of the well field. The east region is a reducing environment with noticeable hydrogen sulphide odours from piezometers.

High DOC concentrations are due to groundwater recharge through organic rich materials either wetland sediments or cultivated farm fields. The low carbon content of the sand suggests a surface source and a shallow water table (0.5 to 2.5 m b.s.) which limits the diffusion of oxygen into the recharge water (Starr and Gillham, 1993).

The reducing environment created by the physical system provides the necessary environment for nitrate attenuation. The presence of tritium suggests that the absence of nitrate is not due to presence of old groundwater that has not been impacted by current land uses. Only one shallow piezometer has a significant nitrate concentration (4 mg/L), while the remaining piezometers in the region have nitrate concentrations below 1 mg/L. Dilution is not likely the cause, since the concentrations of other dissolved species such as chloride and sulphate remain at similar values with depth. The depleted δ³⁴S values in the east region ranging between -8.50‰ and -15.80‰, indicate that the high sulphate concentration observed in the east region is associated with pyrite oxidation (Krouse and Grenenko, 1991).

The geochemical environment in the east region is inconsistent with the observed geochemistry in the production wells. The depleted δ³⁴S signature of the sulphate in this region does not match the signature found in the production wells and the west region that range between -1.40‰ and 3.50‰ (Heagle, 2000). The lack of nitrate and DO also confirms that the shallow aquifer in the east region appears to be hydraulically isolated from the production wells making this region an unlikely source of groundwater for the production wells.

4.1.3 North Region
Regional flow through this area begins from west to east but curves north-east, away from the well field (Figure 1). Private well logs indicate a till unit borders the north section of the well field, although lenses of sand do exist. A potential flow barrier, the till unit, aids in the explanation of the differences in geochemistry between the north region and the production wells. Tritium was found in private wells indicating that the groundwater is young (post 1953) and recharge water would be affected by recent land uses (Heagle, 2000).

Low nitrate (non-detect to 0.5 mg/L) and high sulphate values (11-94 mg/L) characterize the region to the north. Varying DO concentrations (below detection limit to 12 mg/L) and low DOC concentrations (<1 mg/l) also exist in this region. manure application provides a source of nitrate to leach to the groundwater. the surficial till in the area (tavistock till) provides two mechanisms for maintaining low nitrate concentrations in groundwater. visual observation indicated that during large recharge events overland flow dominates the drainage due to the low permeability of the till. instead of infiltrating into the aquifer, nitrate will be transported from the north region via intermittent stream or drainage swales. nitrate concentrations of 4.3 mg/l found in the unsaturated zone, were not present in the saturated zone immediately below. recharge water that infiltrates into the till will be attenuated through denitrification in the unsaturated layer. sulphur bearing aquitards have been observed to reduce nitrate concentrations in infiltrating water (robertson et al., 1996). the increasing sulphate concentration and depleted δ³⁴S values suggest that denitrification is related to oxidation of pyrite, equation 2, which is occurring under reducing conditions.

Low nitrate concentrations (non-detect to 0.5 mg/L), and high sulphate concentrations (11-94 mg/L) provide evidence that this region is not contributing significant quantities of groundwater to the production wells. Hydrogeological evidence of the regional flow direction and the presence of a till barrier prevent hydraulic connection between these regions.

4.2 Sources of Nitrate
The nitrate geochemistry, including high nitrate concentrations, of the west region and the hydraulic connection between this region and the production wells suggests the west region is the likely source of nitrate for the contamination of the Thorton Well Field. Potential sources of nitrate in the region west of the Thorton Well field include manure spreading, inorganic fertilizer, manure piles and soil organic nitrogen. Nitrate concentrations from private wells and piezometers in the west region and well 1, well 3 and well 5, are within the same range (9 to 13 mg/L as N). Production well δ¹⁵N values range between 6.5‰ and 6.60‰. Similar δ¹⁵N values were found throughout the west region (+5.70‰ to +8.50‰).

The δ¹⁵N values in nitrate are in the range of nitrate associated with soil organic nitrogen and manure (Kendall and Aravena, 2000). Based on the agricultural practices in the area, manure is probably the main source of nitrate. The contamination of groundwater is not solely due to diffuse recharge of nitrate. Due to the draining of large areas of land, tile drains and runoff may be responsible for nitrate entering the groundwater as well. Tile discharge water with high nitrate concentrations (up to 17 mg/L) recharge the groundwater at discrete locations. Runoff appears to be focused into low-lying areas where permeable aquifer sediments are at the surface.

5. CONCLUSIONS

Nitrate input into the groundwater flow system occurs by two methods, broad-scale diffuse percolation from agricultural fields and through point sources of focused recharge (drainage tile discharges; and depression focused recharge). Nitrate contaminated groundwater from the west region is drawn from the upper aquifer to the lower aquifer resulting in nitrate concentrations above the drinking water limit in the production wells. The redox environment of the aquifer is the primary control on nitrate geochemistry. Oxidizing conditions are present in the west region meanwhile the east and north region show the effect of reducing conditions on nitrate concentrations. The approach of this research may be applied to other cases where agricultural practices encounter urban water needs. Identifying sensitive areas for recharge of production aquifers is site specific due to hydrogeological considerations and land use practices. This study outlines some of the methods of identification of nitrate sources and the processes that affect nitrate concentration in groundwater.

6. REFERENCES

• Altman, S.J., and R.R. Parizek, 1995. Dilution of Nonpoint-Source Nitrate in Groundwater. Journal of Environmental Quality, 24:707-718.
• Bohlke, J.K., and J.M. Denver, 1995. Combined use of groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds, Atlantic coastal plain, Maryland. Water Resources Research, Vol. 31, No. 9, pp. 2319- 2339.
• Goss, M.J., D.A.J. Barry, and D.L. Rudolph, 1998. Contamination in Ontario Farmstead Domestic Wells and Its Association With Agriculture: 1. Results From Drinking Water Wells. Journal of Contaminant Hydrology, Vol.32, pp.267-293.
• Heagle, 2000. Nitrate Geochemistry of a Regional Aquifer in a n Agricultural Setting, Woodstock, Ontario. M.Sc. Thesis, Department of Earth Sciences, Waterloo, Ontario, Canada.
• Kendall C. and R.O. Aravena. 2000. Nitrate Isotopes in Groundwater Systems. In: Cook, G. and A.L. Herczeg (Eds), Environmental Tracers in Subsurface Hydrology. Kluwer Academic Publishers, Boston, p. 261-297.
• Krouse, H.R., and V.A. Grinenko, (Eds.). Stable isotopes: Natural and Athropogenic Sulphur in the Environment. John Wiley, New York, 1991, p. 440.
• Padusenko, G.R. 2001. Regional Hydrogeologic Evaluation of a Complex Glacial Aquifer System in An Agricultural Landscape: Implications for Nitrate Distribution. M.Sc. Thesis, Department of Earth Sciences, Waterloo, Ontario, Canada.
• Robertson, W.D., B.M. Russell and J.A. Cherry, 1996. Attenuation of Nitrate in Aquitard Sediments of Southern Ontario. Journal of Hydrology, Vol.180, Nos.1-4, pp.267- 281.
• Sebol, L.A., 2000. Determination of groundwater age using CFCs in three shallow aquifers in Southern Ontario. M.Sc. Thesis, Department of Earth Sciences, Waterloo, Ontario, Canada.
• Starr, R.C., and R.W. Gillham, 1993. Denitrification and Organic Carbon Availability in Two Aquifers. Ground Water, Vol.31, No.6, pp.934-947.