OR/18/011 Processes, measurements and indicators - Revision history

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Modelling approach

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	===Modelling approach===		===Modelling approach===
	Keuskamp et al. (2012)<ref name="Keuskamp 2012">KEUSKAMP, J A, VAN DRECHT, G, and BOUWMAN, A F. 2012. European-scale modelling of groundwater denitrification and associated N<sub>2</sub>O production. ''Environmental Pollution'', Vol. 165, 67–76.</ref> describe a spatial model for simulating the fate of N in both soil and groundwater with the aim of predicting both N leaching and N<sub>2</sub>O emissions. This used an improved version of the global IMAGE model (Bouwman et al., 2006<ref name="Bouwman 2016">BOUWMAN, A F, KRAM, T, and KLEIN GOLDEWIJK, K. 2006. Integrated modelling of global environmental change. An overview of IMAGE 2.4. ''Netherlands Environmental Assessment Agency Publication 500110002/2006'' (Bilthoven).</ref>) which takes spatial heterogeneity at the European scale into account.		Keuskamp et al. (2012)<ref name="Keuskamp 2012"></ref> describe a spatial model for simulating the fate of N in both soil and groundwater with the aim of predicting both N leaching and N<sub>2</sub>O emissions. This used an improved version of the global IMAGE model (Bouwman et al., 2006<ref name="Bouwman 2016">BOUWMAN, A F, KRAM, T, and KLEIN GOLDEWIJK, K. 2006. Integrated modelling of global environmental change. An overview of IMAGE 2.4. ''Netherlands Environmental Assessment Agency Publication 500110002/2006'' (Bilthoven).</ref>) which takes spatial heterogeneity at the European scale into account.

	This model uses a 1×1 km grid and computes a steady state annual water balance at the surface.		This model uses a 1×1 km grid and computes a steady state annual water balance at the surface.

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	\| Use δ<sup>15</sup>N and δ<sup>18</sup>O (plus S and C isotopes)		\| Use δ<sup>15</sup>N and δ<sup>18</sup>O (plus S and C isotopes)
	\| Mass spectrometry		\| Mass spectrometry
	\| Aravena and Robertson (1998)<ref name="Aravena 1998">ARAVENA, R, and ROBERTSON, W D. 1998. Use of multiple isotope tracers to evaluate denitrification in ground water: study of nitrate from a large-flux septic system plume. ''Ground Water'', Vol. 36, 975–982.</ref>; Böhlke and Denver (1995)<ref name="Böhlke 1995">BÖHLKE, J, and DENVER, J. 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, 2319–2339. </ref>; Böhlke et al. (2002)<ref name="Böhlke 2002">BÖHLKE, J, WANTY, R, TUTTLE, M, DELIN, G, and LANDON, M. 2002. Denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota. ''Water Resources Research'', Vol. 38, 1105. </ref>; Böttcher et al. (1990)<ref name="Böttcher 1990">BÖTTCHER, J, STREBEL, O, VOERKELIUS, S, and SCHMIDT, H-L. 1990. Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer. ''Journal of Hydrology'', Vol. 114, 413–424. </ref>; Chen and MacQuarrie (2005)<ref name="Chen 2005">CHEN, D J Z, and MACQUARRIE, K T B. 2005. Correlation of δ15N and δ18O in NO3- during denitrification in groundwater. ''Journal of Environmental Engineering and Science'', Vol. 4, 221–226. </ref>; Fukada et al. (2003)<ref name="Fukada 2003">FUKADA, T, HISCOCK, K M, DENNIS, P F, and GRISCHEK, T. 2003. A dual isotope approach to identify denitrification in groundwater at a river-bank infiltration site. ''Water Research'', Vol. 37, 3070–3078. </ref>; Kellman and Hillaire-Marcel (1998		\| Aravena and Robertson (1998)<ref name="Aravena 1998">ARAVENA, R, and ROBERTSON, W D. 1998. Use of multiple isotope tracers to evaluate denitrification in ground water: study of nitrate from a large-flux septic system plume. ''Ground Water'', Vol. 36, 975–982.</ref>; Böhlke and Denver (1995)<ref name="Böhlke 1995">BÖHLKE, J, and DENVER, J. 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, 2319–2339. </ref>; Böhlke et al. (2002)<ref name="Böhlke 2002"></ref>; Böttcher et al. (1990)<ref name="Böttcher 1990">BÖTTCHER, J, STREBEL, O, VOERKELIUS, S, and SCHMIDT, H-L. 1990. Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer. ''Journal of Hydrology'', Vol. 114, 413–424. </ref>; Chen and MacQuarrie (2005)<ref name="Chen 2005">CHEN, D J Z, and MACQUARRIE, K T B. 2005. Correlation of δ15N and δ18O in NO3- during denitrification in groundwater. ''Journal of Environmental Engineering and Science'', Vol. 4, 221–226. </ref>; Fukada et al. (2003)<ref name="Fukada 2003">FUKADA, T, HISCOCK, K M, DENNIS, P F, and GRISCHEK, T. 2003. A dual isotope approach to identify denitrification in groundwater at a river-bank infiltration site. ''Water Research'', Vol. 37, 3070–3078. </ref>; Kellman and Hillaire-Marcel (1998
	<ref name="Kellman 1998">KELLMAN, L M, and HILLAIRE-MARCEL, C. 1998. Nitrate cycling in streams: using natural abundances of NO<sub>3</sub><sup>-</sup>-δ<sup>15</sup>N to measure ''in-situ'' denitrification. Biogeochemistry, Vol. 43, 273–292. </ref>, 2003<ref name="KELLMAN 2003">KELLMAN, L M, and HILLAIRE-MARCEL, C. 2003. Evaluation of nitrogen isotopes as indicators of nitrate contamination sources in an agricultural watershed. ''Agriculture, Ecosystems & Environment'', Vol. 95, 87–102. </ref>); Komor and Anderson (1993)<ref name="Komor 1993">KOMOR, S C, and ANDERSON, H W. 1993. Nitrogen isotopes as indicators of nitrate sources in Minnesota sand-plain aquifers. ''Ground Water'', Vol. 31, 260–270.</ref>; Mariotti et al. (1988)<ref name="Mariotti 1988">MARIOTTI, A, LANDREAU, A, and SIMON, B. 1988. 15N isotope biogeochemistry and natural denitrification process in groundwater: Application to the chalk aquifer of northern France. ''Geochimica et Cosmochimica Acta'', Vol. 52, 1869–1878.</ref>; Wilson et al. (1994)<ref name="Wilson 1994">WILSON, G B, ANDREWS, J N, and BATH, A H. 1994. The nitrogen isotope composition of groundwater nitrates from the East Midlands Triassic Sandstone aquifer, England. ''Journal of Hydrology'', Vol. 157, 35–46. </ref>		<ref name="Kellman 1998">KELLMAN, L M, and HILLAIRE-MARCEL, C. 1998. Nitrate cycling in streams: using natural abundances of NO<sub>3</sub><sup>-</sup>-δ<sup>15</sup>N to measure ''in-situ'' denitrification. Biogeochemistry, Vol. 43, 273–292. </ref>, 2003<ref name="KELLMAN 2003">KELLMAN, L M, and HILLAIRE-MARCEL, C. 2003. Evaluation of nitrogen isotopes as indicators of nitrate contamination sources in an agricultural watershed. ''Agriculture, Ecosystems & Environment'', Vol. 95, 87–102. </ref>); Komor and Anderson (1993)<ref name="Komor 1993">KOMOR, S C, and ANDERSON, H W. 1993. Nitrogen isotopes as indicators of nitrate sources in Minnesota sand-plain aquifers. ''Ground Water'', Vol. 31, 260–270.</ref>; Mariotti et al. (1988)<ref name="Mariotti 1988">MARIOTTI, A, LANDREAU, A, and SIMON, B. 1988. 15N isotope biogeochemistry and natural denitrification process in groundwater: Application to the chalk aquifer of northern France. ''Geochimica et Cosmochimica Acta'', Vol. 52, 1869–1878.</ref>; Wilson et al. (1994)<ref name="Wilson 1994">WILSON, G B, ANDREWS, J N, and BATH, A H. 1994. The nitrogen isotope composition of groundwater nitrates from the East Midlands Triassic Sandstone aquifer, England. ''Journal of Hydrology'', Vol. 157, 35–46. </ref>

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	\| Use environmental tracers as age indicators		\| Use environmental tracers as age indicators
	\| CFCs, SFsub>6</sub>, <sup>3</sup>H, <sup>14</sup>C		\| CFCs, SFsub>6</sub>, <sup>3</sup>H, <sup>14</sup>C
	\| Böhlke et al. (2002)<ref name="Böhlke 2002">BÖHLKE, J, WANTY, R, TUTTLE, M, DELIN, G, and LANDON, M. 2002. Denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota. ''Water Resources Research'', Vol. 38, 1105. </ref>; Cook and Böhlke (2000)<ref name="Cook 2000">COOK, P G, and BÖHLKE, J-K. 2000. Determining timescales for groundwater flow and solute transport. 1–30 in ''Environmental tracers in subsurface hydrology''. (Springer.) </ref>		\| Böhlke et al. (2002)<ref name="Böhlke 2002"></ref>; Cook and Böhlke (2000)<ref name="Cook 2000">COOK, P G, and BÖHLKE, J-K. 2000. Determining timescales for groundwater flow and solute transport. 1–30 in ''Environmental tracers in subsurface hydrology''. (Springer.) </ref>
	\|- style="vertical-align:top;"		\|- style="vertical-align:top;"
	\| Molecular approaches		\| Molecular approaches

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	Zhang et al. (2009)<ref name="Zhang 2009"></ref> estimated the rate of denitrification in the Oostrum study to be 0.6 mM NO<sub>3</sub>/year for a 5-m section of depleted aquifer at the top of a well with NO<sub>3</sub> concentration of 3 mM in the lower sections.		Zhang et al. (2009)<ref name="Zhang 2009"></ref> estimated the rate of denitrification in the Oostrum study to be 0.6 mM NO<sub>3</sub>/year for a 5-m section of depleted aquifer at the top of a well with NO<sub>3</sub> concentration of 3 mM in the lower sections.

	Jahangir et al. (2013)<ref name="Jahangir 2013">JAHANGIR, M M R, JOHNSTON, P, ADDY, K, KHALIL, M I, GROFFMAN, P M, and RICHARDS, K G. 2013. Quantification of in situ denitrification rates in groundwater below an arable and a grassland system. ''Water, Air, & Soil Pollution'', Vol. 224, 1–14. </ref> measured in-situ groundwater denitrification rates in subsoil, at the bedrock interface and in bedrock at two sites in Ireland, grassland and arable, using an in-situ ‘push–pull’ method with <sup>15</sup>N-labelled NO<sub>3</sub><sup>-</sup>. Measured groundwater denitrification rates ranged from 1.3 to 469.5 μg N/kg/day Exceptionally high denitrification rates observed at the bedrock interface at the grassland site (470 ± 152 μg N/kg/day) suggest that deep groundwater can serve as substantial hotspots for NO<sub>3</sub><sup>-</sup>N removal. However, denitrification rates at the other locations were low. Denitrification rates were negatively correlated with ambient DO, redox potential, permeability and NO<sub>3</sub><sup>-</sup> (all p values, p<0.01) and positively correlated with SO<sub>4</sub><sup>2-</sup> (p<0.05). A higher mean N<sub>2</sub>O/(N<sub>2</sub>O+N<sub>2</sub>) ratios at an arable site (0.28) compared to a grassland site (0.10) revealed that the arable site had higher potential to indirect N<sub>2</sub>O emissions.		Jahangir et al. (2013)<ref name="Jahangir 2013"></ref> measured in-situ groundwater denitrification rates in subsoil, at the bedrock interface and in bedrock at two sites in Ireland, grassland and arable, using an in-situ ‘push–pull’ method with <sup>15</sup>N-labelled NO<sub>3</sub><sup>-</sup>. Measured groundwater denitrification rates ranged from 1.3 to 469.5 μg N/kg/day Exceptionally high denitrification rates observed at the bedrock interface at the grassland site (470 ± 152 μg N/kg/day) suggest that deep groundwater can serve as substantial hotspots for NO<sub>3</sub><sup>-</sup>N removal. However, denitrification rates at the other locations were low. Denitrification rates were negatively correlated with ambient DO, redox potential, permeability and NO<sub>3</sub><sup>-</sup> (all p values, p<0.01) and positively correlated with SO<sub>4</sub><sup>2-</sup> (p<0.05). A higher mean N<sub>2</sub>O/(N<sub>2</sub>O+N<sub>2</sub>) ratios at an arable site (0.28) compared to a grassland site (0.10) revealed that the arable site had higher potential to indirect N<sub>2</sub>O emissions.

	<center>		<center>

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	There appear to be very few estimates of denitrification rates in groundwater. Korom (1992)<ref name="Korom 1992"></ref> tabulated a range of laboratory denitrification rates for aquifer samples in the range of 0.004 to 1.16 mgN/kg dry sediment/day and for aquifers of <LOD to 3.1 mgN/L/day. They also include half-lives of 1.2 to 2.1 years in sand and gravelly sand from Kölle et al. (1985)<ref name="Kölle 1985">KÖLLE, W, STREBEL, O, and BÖTTCHER, J. 1985. Formation of sulfate by microbial denitrification in a reducing aquifer. ''Water Supply'', Vol. 3, 35–40. </ref> and Böttcher et al. (1989)<ref name="Böttcher 1989">BÖTTCHER, J, STREBEL, O, and DUYNISVELD, W H. 1989. Kinetik und Modellierung gekoppelter Stoffumsetzungen im-Grundwasser eines Lockergesteins-Aquifers. ''Geologisches Jahrbuch Reihe C'', Vol. 51, 3–40.</ref>. Tesoriero et al. (2000)<ref name="Tesoriero 2000">TESORIERO, A J, LIEBSCHER, H, and COX, S E. 2000. Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths. ''Water Resources Research'', Vol. 36, 1545–1559.</ref> investigated the rate and mechanisms of NO<sub>3</sub> removal in an unconfined sand and gravel aquifer using a series of well nests in the Abbotsford-Sumas aquifer on the west USA-Canadian border. Little or no denitrification was observed in the upland portions of the aquifer but a gradual redox gradient as water moved deeper into the aquifer was observed. A complete loss of NO<sub>3</sub> was observed and pyrite oxidation was considered to be the electron source. Denitrification rate estimates were based on mass balance calculations using NO<sub>3</sub> and excess N<sub>2</sub> coupled with groundwater travel times. Denitrification rates in the deep, upland portions of the aquifer were found to range from <0.01 to 0.14 mM of N per year; rates at the redox gradient along the shallow flow path range from 1.0 to 2.7 mM of N/year.		There appear to be very few estimates of denitrification rates in groundwater. Korom (1992)<ref name="Korom 1992"></ref> tabulated a range of laboratory denitrification rates for aquifer samples in the range of 0.004 to 1.16 mgN/kg dry sediment/day and for aquifers of <LOD to 3.1 mgN/L/day. They also include half-lives of 1.2 to 2.1 years in sand and gravelly sand from Kölle et al. (1985)<ref name="Kölle 1985">KÖLLE, W, STREBEL, O, and BÖTTCHER, J. 1985. Formation of sulfate by microbial denitrification in a reducing aquifer. ''Water Supply'', Vol. 3, 35–40. </ref> and Böttcher et al. (1989)<ref name="Böttcher 1989">BÖTTCHER, J, STREBEL, O, and DUYNISVELD, W H. 1989. Kinetik und Modellierung gekoppelter Stoffumsetzungen im-Grundwasser eines Lockergesteins-Aquifers. ''Geologisches Jahrbuch Reihe C'', Vol. 51, 3–40.</ref>. Tesoriero et al. (2000)<ref name="Tesoriero 2000">TESORIERO, A J, LIEBSCHER, H, and COX, S E. 2000. Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths. ''Water Resources Research'', Vol. 36, 1545–1559.</ref> investigated the rate and mechanisms of NO<sub>3</sub> removal in an unconfined sand and gravel aquifer using a series of well nests in the Abbotsford-Sumas aquifer on the west USA-Canadian border. Little or no denitrification was observed in the upland portions of the aquifer but a gradual redox gradient as water moved deeper into the aquifer was observed. A complete loss of NO<sub>3</sub> was observed and pyrite oxidation was considered to be the electron source. Denitrification rate estimates were based on mass balance calculations using NO<sub>3</sub> and excess N<sub>2</sub> coupled with groundwater travel times. Denitrification rates in the deep, upland portions of the aquifer were found to range from <0.01 to 0.14 mM of N per year; rates at the redox gradient along the shallow flow path range from 1.0 to 2.7 mM of N/year.

	Zhang et al. (2009)<ref name="Zhang 2009">ZHANG, Y-C, SLOMP, C P, BROERS, H P, PASSIER, H F, and VAN CAPPELLEN, P. 2009. Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer. ''Geochimica et Cosmochimica Acta'', Vol. 73, 6716–6726.</ref> estimated the rate of denitrification in the Oostrum study to be 0.6 mM NO<sub>3</sub>/year for a 5-m section of depleted aquifer at the top of a well with NO<sub>3</sub> concentration of 3 mM in the lower sections.		Zhang et al. (2009)<ref name="Zhang 2009"></ref> estimated the rate of denitrification in the Oostrum study to be 0.6 mM NO<sub>3</sub>/year for a 5-m section of depleted aquifer at the top of a well with NO<sub>3</sub> concentration of 3 mM in the lower sections.

	Jahangir et al. (2013)<ref name="Jahangir 2013">JAHANGIR, M M R, JOHNSTON, P, ADDY, K, KHALIL, M I, GROFFMAN, P M, and RICHARDS, K G. 2013. Quantification of in situ denitrification rates in groundwater below an arable and a grassland system. ''Water, Air, & Soil Pollution'', Vol. 224, 1–14. </ref> measured in-situ groundwater denitrification rates in subsoil, at the bedrock interface and in bedrock at two sites in Ireland, grassland and arable, using an in-situ ‘push–pull’ method with <sup>15</sup>N-labelled NO<sub>3</sub><sup>-</sup>. Measured groundwater denitrification rates ranged from 1.3 to 469.5 μg N/kg/day Exceptionally high denitrification rates observed at the bedrock interface at the grassland site (470 ± 152 μg N/kg/day) suggest that deep groundwater can serve as substantial hotspots for NO<sub>3</sub><sup>-</sup>N removal. However, denitrification rates at the other locations were low. Denitrification rates were negatively correlated with ambient DO, redox potential, permeability and NO<sub>3</sub><sup>-</sup> (all p values, p<0.01) and positively correlated with SO<sub>4</sub><sup>2-</sup> (p<0.05). A higher mean N<sub>2</sub>O/(N<sub>2</sub>O+N<sub>2</sub>) ratios at an arable site (0.28) compared to a grassland site (0.10) revealed that the arable site had higher potential to indirect N<sub>2</sub>O emissions.		Jahangir et al. (2013)<ref name="Jahangir 2013">JAHANGIR, M M R, JOHNSTON, P, ADDY, K, KHALIL, M I, GROFFMAN, P M, and RICHARDS, K G. 2013. Quantification of in situ denitrification rates in groundwater below an arable and a grassland system. ''Water, Air, & Soil Pollution'', Vol. 224, 1–14. </ref> measured in-situ groundwater denitrification rates in subsoil, at the bedrock interface and in bedrock at two sites in Ireland, grassland and arable, using an in-situ ‘push–pull’ method with <sup>15</sup>N-labelled NO<sub>3</sub><sup>-</sup>. Measured groundwater denitrification rates ranged from 1.3 to 469.5 μg N/kg/day Exceptionally high denitrification rates observed at the bedrock interface at the grassland site (470 ± 152 μg N/kg/day) suggest that deep groundwater can serve as substantial hotspots for NO<sub>3</sub><sup>-</sup>N removal. However, denitrification rates at the other locations were low. Denitrification rates were negatively correlated with ambient DO, redox potential, permeability and NO<sub>3</sub><sup>-</sup> (all p values, p<0.01) and positively correlated with SO<sub>4</sub><sup>2-</sup> (p<0.05). A higher mean N<sub>2</sub>O/(N<sub>2</sub>O+N<sub>2</sub>) ratios at an arable site (0.28) compared to a grassland site (0.10) revealed that the arable site had higher potential to indirect N<sub>2</sub>O emissions.
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	They recommended that other redox indicators, such as NH<sub>4</sub>, H<sub>2</sub>S, CH<sub>4</sub>, and H<sub>2</sub>, should be measured whenever possible. An internally consistent set of threshold criteria was developed and applied to the aquifers using data from the NWQA Program. These were then related to both natural (As) and anthropogenic (NO<sub>3</sub> and VOCs) contaminant distribution. The redox conditions explained many of the observed water quality trends at the regional scale. Identification of zones of redox heterogeneity were also important for assessing the fate and transport of contaminants. This approach is similar to that of Lyngkilde and Christensen (1992)<ref name="Lyngkilde 1992"></ref> and Bjerg et al. (1995)<ref name="Bjerg 1995"></ref> above but uses lower concentrations.		They recommended that other redox indicators, such as NH<sub>4</sub>, H<sub>2</sub>S, CH<sub>4</sub>, and H<sub>2</sub>, should be measured whenever possible. An internally consistent set of threshold criteria was developed and applied to the aquifers using data from the NWQA Program. These were then related to both natural (As) and anthropogenic (NO<sub>3</sub> and VOCs) contaminant distribution. The redox conditions explained many of the observed water quality trends at the regional scale. Identification of zones of redox heterogeneity were also important for assessing the fate and transport of contaminants. This approach is similar to that of Lyngkilde and Christensen (1992)<ref name="Lyngkilde 1992"></ref> and Bjerg et al. (1995)<ref name="Bjerg 1995"></ref> above but uses lower concentrations.

	Zhang et al. (2009)<ref name="Zhang 2009">ZHANG, Y-C, SLOMP, C P, BROERS, H P, PASSIER, H F, and VAN CAPPELLEN, P. 2009. Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer. ''Geochimica et Cosmochimica Acta'', Vol. 73, 6716–6726. </ref> studied NO<sub>3</sub> removal from a sandy aquifer below cultivated fields and forested areas at Oostrum in the Netherlands. Nitrate loss correlated with SO<sub>4</sub> production, and increase in dissolved Fe<sup>2+</sup> and pyrite-associated trace metals (e.g. As, Ni, Co and Zn) in a zone between 10 and 20 m deep. These results indicated that denitrification coupled to pyrite oxidation is a major process in the aquifer. Significant NO<sub>3</sub> loss coupled to SO<sub>4</sub> production was further confirmed by comparing historical estimates of regional SO<sub>4</sub> and NO<sub>3</sub> loadings to age-dated groundwater SO<sub>4</sub> and NO<sub>3</sub> concentrations, for the period 1950–2000. This highlights a warning against always anticipating SO<sub>4</sub> removal as redox decreases.		Zhang et al. (2009)<ref name="Zhang 2009"></ref> studied NO<sub>3</sub> removal from a sandy aquifer below cultivated fields and forested areas at Oostrum in the Netherlands. Nitrate loss correlated with SO<sub>4</sub> production, and increase in dissolved Fe<sup>2+</sup> and pyrite-associated trace metals (e.g. As, Ni, Co and Zn) in a zone between 10 and 20 m deep. These results indicated that denitrification coupled to pyrite oxidation is a major process in the aquifer. Significant NO<sub>3</sub> loss coupled to SO<sub>4</sub> production was further confirmed by comparing historical estimates of regional SO<sub>4</sub> and NO<sub>3</sub> loadings to age-dated groundwater SO<sub>4</sub> and NO<sub>3</sub> concentrations, for the period 1950–2000. This highlights a warning against always anticipating SO<sub>4</sub> removal as redox decreases.

	Lee et al. (2008)<ref name="Lee 2008">LEE, J-J, JANG, C-S, WANG, S-W, LIANG, C-P, and LIU, C-W. 2008. Delineation of spatial redox zones using discriminant analysis and geochemical modelling in arsenic-affected alluvial aquifers. ''Hydrological Processes'', Vol. 22, 3029–3041. </ref> characterised the redox conditions in an arsenic-affected aquifer in the Lanyang Plain, Taiwan. Discriminant analysis was adopted to delineate three redox zones (oxidative, transitional and reductive zones) in different aquifers and yielded over 90% agreement with groundwater quality data. According to the DA results, the groundwater of the Lanyang plain was classified as Zone 1 (oxidizing zone) if DO ≥ 1 mg/L or NO<sub>3</sub> ≥0.35 mg/L, Zone 2 (transitional zone), if DO was between 0.3 and 1 mg/L or NO<sub>3</sub> < 0.35 mg/and Zone 3 (reducing zone) if DO <0.3 mg/L or HS-≥ 0.07 mg/L.		Lee et al. (2008)<ref name="Lee 2008">LEE, J-J, JANG, C-S, WANG, S-W, LIANG, C-P, and LIU, C-W. 2008. Delineation of spatial redox zones using discriminant analysis and geochemical modelling in arsenic-affected alluvial aquifers. ''Hydrological Processes'', Vol. 22, 3029–3041. </ref> characterised the redox conditions in an arsenic-affected aquifer in the Lanyang Plain, Taiwan. Discriminant analysis was adopted to delineate three redox zones (oxidative, transitional and reductive zones) in different aquifers and yielded over 90% agreement with groundwater quality data. According to the DA results, the groundwater of the Lanyang plain was classified as Zone 1 (oxidizing zone) if DO ≥ 1 mg/L or NO<sub>3</sub> ≥0.35 mg/L, Zone 2 (transitional zone), if DO was between 0.3 and 1 mg/L or NO<sub>3</sub> < 0.35 mg/and Zone 3 (reducing zone) if DO <0.3 mg/L or HS-≥ 0.07 mg/L.

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Redox sequence ions

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	\| Seitzinger et al. (2006)<ref name="Seitzinger 2006">SEITZINGER, S, HARRISON, J A, BÖHLKE, J K, BOUWMAN, A F, LOWRANCE, R, PETERSON, B, TOBIAS, C, and DRECHT, G V. 2006. Denitrification across landscapes and waterscapes: a synthesis. ''Ecological Applications'', Vol. 16, 2064–2090. </ref>; Tiedje (1988)<ref name="Tiedje 1988">TIEDJE, J M. 1988. Ecology of denitrification and dissimilatory nitrate reduction to ammonium. 179–244 in ''Biology of anaerobic microorganisms''. ZEHNDER, A J B (editor). (New York: John Wiley & Sons.) </ref>		\| Seitzinger et al. (2006)<ref name="Seitzinger 2006"></ref>; Tiedje (1988)<ref name="Tiedje 1988">TIEDJE, J M. 1988. Ecology of denitrification and dissimilatory nitrate reduction to ammonium. 179–244 in ''Biology of anaerobic microorganisms''. ZEHNDER, A J B (editor). (New York: John Wiley & Sons.) </ref>
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	</center>		</center>

	After consumption of NO<sub>3</sub><sup>-</sup>, a further sequence of ions can be used as electron acceptors with decreasing energetic yields in a step-wise process. These include reduction of Mn<sup>4+</sup> then Fe<sup>3+</sup> to soluble oxidation states (Mn<sup>2+</sup> and Fe<sup>2+</sup> species) with increase in observed concentrations, reduction of sulphate (SO<sub>4</sub><sup>2-</sup>) to S species, and finally the reduction of carbon dioxide to methane (Figure 2.1 and Table 2.1). The presence or absence of these parameters can be used as indicators of low redox and therefore of denitrification potential (McMahon and Chapelle, 2008<ref name="McMahon 2008">MCMAHON, P, and CHAPELLE, F. 2008. Redox processes and water quality of selected principal aquifer systems. ''Ground Water'', Vol. 46, 259–271.</ref>). This reaction sequence is commonly seen along groundwater flow lines (Edmunds et al., 1982<ref name="Edmunds 1982">EDMUNDS, W M, BATH, A H, and MILES, D L. 1982. Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England. ''Geochimica et Cosmochimica Acta'', Vol. 46, 2069–2081. </ref>; Edmunds et al., 1984<ref name="Edmunds 1984">EDMUNDS, W M, MILES, D L, and COOK, J M. 1984. A comparative study of sequential redox processes in three British aquifers. 55–70 in ''Hydrochemical Balances in Freshwater''. ERIKSSON, E (editor). Vol. 150. (Wallingford: IAHS-AISH.)</ref>) typically as aquifers become confined.		After consumption of NO<sub>3</sub><sup>-</sup>, a further sequence of ions can be used as electron acceptors with decreasing energetic yields in a step-wise process. These include reduction of Mn<sup>4+</sup> then Fe<sup>3+</sup> to soluble oxidation states (Mn<sup>2+</sup> and Fe<sup>2+</sup> species) with increase in observed concentrations, reduction of sulphate (SO<sub>4</sub><sup>2-</sup>) to S species, and finally the reduction of carbon dioxide to methane (Figure 2.1 and Table 2.1). The presence or absence of these parameters can be used as indicators of low redox and therefore of denitrification potential (McMahon and Chapelle, 2008<ref name="McMahon 2008">MCMAHON, P, and CHAPELLE, F. 2008. Redox processes and water quality of selected principal aquifer systems. ''Ground Water'', Vol. 46, 259–271.</ref>). This reaction sequence is commonly seen along groundwater flow lines (Edmunds et al., 1982<ref name="Edmunds 1982">EDMUNDS, W M, BATH, A H, and MILES, D L. 1982. Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England. ''Geochimica et Cosmochimica Acta'', Vol. 46, 2069–2081. </ref>; Edmunds et al., 1984<ref name="Edmunds 1984"></ref>) typically as aquifers become confined.

	Most likely denitrifying organisms possess truncated pathways which require synergistic relationships among different denitrifying species to complete reduction to N<sub>2</sub> (Jones et al., 2013<ref name="Jones 2013">JONES, C M, GRAF, D R, BRU, D, PHILIPPOT, L, and HALLIN, S. 2013. The unaccounted yet abundant nitrous oxide-reducing microbial community: a potential nitrous oxide sink. ''The ISME Journal'', Vol. 7, 417. </ref>) and this can be the source of the accumulation of N<sub>2</sub>O as a greenhouse gas (Müller et al., 2014<ref name="Müller 2014">MÜLLER, C, LAUGHLIN, R J, SPOTT, O, and RÜTTING, T. 2014. Quantification of N<sub>2</sub>O emission pathways via a <sup>15</sup>N tracing model. ''Soil Biology and Biochemistry'', Vol. 72, 44–54. </ref>). Seitzinger et al. (2006)<ref name="Seitzinger 2006">SEITZINGER, S, HARRISON, J A, BÖHLKE, J K, BOUWMAN, A F, LOWRANCE, R, PETERSON, B, TOBIAS, C, and DRECHT, G V. 2006. Denitrification across landscapes and waterscapes: a synthesis. ''Ecological Applications'', Vol. 16, 2064–2090. </ref> argue that groundwater is an important location for denitrification due to long groundwater residence times, but the uncertainty is large.		Most likely denitrifying organisms possess truncated pathways which require synergistic relationships among different denitrifying species to complete reduction to N<sub>2</sub> (Jones et al., 2013<ref name="Jones 2013">JONES, C M, GRAF, D R, BRU, D, PHILIPPOT, L, and HALLIN, S. 2013. The unaccounted yet abundant nitrous oxide-reducing microbial community: a potential nitrous oxide sink. ''The ISME Journal'', Vol. 7, 417. </ref>) and this can be the source of the accumulation of N<sub>2</sub>O as a greenhouse gas (Müller et al., 2014<ref name="Müller 2014">MÜLLER, C, LAUGHLIN, R J, SPOTT, O, and RÜTTING, T. 2014. Quantification of N<sub>2</sub>O emission pathways via a <sup>15</sup>N tracing model. ''Soil Biology and Biochemistry'', Vol. 72, 44–54. </ref>). Seitzinger et al. (2006)<ref name="Seitzinger 2006">SEITZINGER, S, HARRISON, J A, BÖHLKE, J K, BOUWMAN, A F, LOWRANCE, R, PETERSON, B, TOBIAS, C, and DRECHT, G V. 2006. Denitrification across landscapes and waterscapes: a synthesis. ''Ecological Applications'', Vol. 16, 2064–2090. </ref> argue that groundwater is an important location for denitrification due to long groundwater residence times, but the uncertainty is large.
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	It is anticipated that there will be a sequence of redox changes as water migrates from upland recharge areas to lowland discharge areas under confined conditions. Champ et al. (1979)<ref name="Champ 1979">CHAMP, D R, GULENS, J, and JACKSON, R E. 1979. Oxidation–reduction sequences in ground water flow systems. ''Canadian Journal of Earth Sciences'', Vol. 16, 12–23. </ref> identified 3 zones O<sub>2</sub>–NO<sub>3</sub>, Fe-Mn, sulphide. Hiscock et al. (1991)<ref name="Hiscock 1991"></ref> showed change in redox potential is often accompanied by a sequential reduction in dissolved groundwater species which is sited as proof of denitrification.		It is anticipated that there will be a sequence of redox changes as water migrates from upland recharge areas to lowland discharge areas under confined conditions. Champ et al. (1979)<ref name="Champ 1979">CHAMP, D R, GULENS, J, and JACKSON, R E. 1979. Oxidation–reduction sequences in ground water flow systems. ''Canadian Journal of Earth Sciences'', Vol. 16, 12–23. </ref> identified 3 zones O<sub>2</sub>–NO<sub>3</sub>, Fe-Mn, sulphide. Hiscock et al. (1991)<ref name="Hiscock 1991"></ref> showed change in redox potential is often accompanied by a sequential reduction in dissolved groundwater species which is sited as proof of denitrification.

	This reaction sequence is commonly seen along groundwater flow lines (Edmunds et al., 1982<ref name="Edmunds 1982">EDMUNDS, W M, BATH, A H, and MILES, D L. 1982. Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England. ''Geochimica et Cosmochimica Acta'', Vol. 46, 2069–2081. </ref>; Edmunds et al., 1984<ref name="Edmunds 1984">EDMUNDS, W M, MILES, D L, and COOK, J M. 1984. A comparative study of sequential redox processes in three British aquifers. 55–70 in ''Hydrochemical Balances in Freshwater'' ERIKSSON, E (editor). Vol. 150. (Wallingford: IAHS-AISH.) </ref>) typically as aquifers become confined. Water at recharge is generally saturated with DO at the partial pressure of the atmosphere (10–12 mg/L depending upon barometric conditions). Passing through the soil and the unsaturated zone some of this O<sub>2</sub> will react as a result of microbiological processes and oxidation-reduction reactions. However, almost all water reaching the water table still contains several mg/L O<sub>2</sub>. Geochemical and microbial reactions progressively remove the O<sub>2</sub> along flow lines. Once all the O<sub>2</sub> has reacted an abrupt change of water chemistry takes place (redox boundary). Down-gradient of the redox boundary, denitrification occurs and it is likely that Fe<sup>2+</sup> concentrations will increase. Sulphate reduction and the production of sulphide (H<sub>2</sub>S as S<sub>2</sub><sup>-</sup> in solution) may also occur at greater depths (Figure 2.2).		This reaction sequence is commonly seen along groundwater flow lines (Edmunds et al., 1982<ref name="Edmunds 1982">EDMUNDS, W M, BATH, A H, and MILES, D L. 1982. Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England. ''Geochimica et Cosmochimica Acta'', Vol. 46, 2069–2081. </ref>; Edmunds et al., 1984<ref name="Edmunds 1984"></ref>) typically as aquifers become confined. Water at recharge is generally saturated with DO at the partial pressure of the atmosphere (10–12 mg/L depending upon barometric conditions). Passing through the soil and the unsaturated zone some of this O<sub>2</sub> will react as a result of microbiological processes and oxidation-reduction reactions. However, almost all water reaching the water table still contains several mg/L O<sub>2</sub>. Geochemical and microbial reactions progressively remove the O<sub>2</sub> along flow lines. Once all the O<sub>2</sub> has reacted an abrupt change of water chemistry takes place (redox boundary). Down-gradient of the redox boundary, denitrification occurs and it is likely that Fe<sup>2+</sup> concentrations will increase. Sulphate reduction and the production of sulphide (H<sub>2</sub>S as S<sub>2</sub><sup>-</sup> in solution) may also occur at greater depths (Figure 2.2).

	Denitrification can also be important in shallow groundwater where recharge contains an elevated loading of organic carbon.(Smith et al., 1991<ref name="Smith 1991">SMITH, R L, HOWES, B L, and DUFF, J H. 1991. Denitrification in nitrate-contaminated groundwater: Occurrence in steep vertical geochemical gradients. ''Geochimica et Cosmochimica Acta'', Vol. 55, 1815–1825. </ref>; Spalding and Parrott, 1994<ref name="Spalding 1994">SPALDING, R F, and PARROTT, J D. 1994. Shallow groundwater denitrification. ''Science of the Total Environment'', Vol. 141, 17–25. </ref>; Zarnetske et al., 2011b<ref name="Zarnetske 2011b">ZARNETSKE, J P, HAGGERTY, R, WONDZELL, S M, and BAKER, M A. 2011b. Labile dissolved organic carbon supply limits hyporheic denitrification. ''Journal of Geophysical Research: Biogeosciences'', Vol. 116, G04036. </ref>). In their review of floodplain processes, Stuart and Lapworth (2011)<ref name="Stuart 2011">STUART, M E, and LAPWORTH, D J. 2011. A review of processes important in the floodplain setting. ''British Geological Survey Open Report OR/11/030''. </ref> tabulated a set of criteria characterising the redox zones (Table 2.3) based on earlier work. These used a series of indicators including the electron acceptors O<sub>2</sub>, NO<sub>3</sub> and SO<sub>4</sub>, intermediates NO<sub>2</sub> and N<sub>2</sub>O and the solid phase acceptors Mn<sup>4</sup> and Fe<sup>3</sup>, indicated by the presence of dissolved Mn and Fe, and eventually methane. The values shown in Table 2.3 were applied to landfill leachate plumes and some concentrations, particularly for Fe, are high.		Denitrification can also be important in shallow groundwater where recharge contains an elevated loading of organic carbon.(Smith et al., 1991<ref name="Smith 1991">SMITH, R L, HOWES, B L, and DUFF, J H. 1991. Denitrification in nitrate-contaminated groundwater: Occurrence in steep vertical geochemical gradients. ''Geochimica et Cosmochimica Acta'', Vol. 55, 1815–1825. </ref>; Spalding and Parrott, 1994<ref name="Spalding 1994">SPALDING, R F, and PARROTT, J D. 1994. Shallow groundwater denitrification. ''Science of the Total Environment'', Vol. 141, 17–25. </ref>; Zarnetske et al., 2011b<ref name="Zarnetske 2011b">ZARNETSKE, J P, HAGGERTY, R, WONDZELL, S M, and BAKER, M A. 2011b. Labile dissolved organic carbon supply limits hyporheic denitrification. ''Journal of Geophysical Research: Biogeosciences'', Vol. 116, G04036. </ref>). In their review of floodplain processes, Stuart and Lapworth (2011)<ref name="Stuart 2011">STUART, M E, and LAPWORTH, D J. 2011. A review of processes important in the floodplain setting. ''British Geological Survey Open Report OR/11/030''. </ref> tabulated a set of criteria characterising the redox zones (Table 2.3) based on earlier work. These used a series of indicators including the electron acceptors O<sub>2</sub>, NO<sub>3</sub> and SO<sub>4</sub>, intermediates NO<sub>2</sub> and N<sub>2</sub>O and the solid phase acceptors Mn<sup>4</sup> and Fe<sup>3</sup>, indicated by the presence of dissolved Mn and Fe, and eventually methane. The values shown in Table 2.3 were applied to landfill leachate plumes and some concentrations, particularly for Fe, are high.

Dbk at 11:10, 2 December 2019

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	====Autotrophic denitrification====		====Autotrophic denitrification====
	Korom (1992)<ref name="Korom 1992">~~KOROM, S F. 1992. Natural denitrification in the saturated zone: a review. ''Water Resources Research'', Vol. 28, 1657–1668.~~</ref> also discuss the denitrification process in the context of addition of NO<sub>3</sub> to groundwater where Mn, Fe or SO<sub>4</sub> have already been reduced, where autotrophic denitrification may occur using the reduced inorganic compounds as electron donors. They found that groundwater containing Fe<sup>2+</sup> did not contain any observable NO<sub>3</sub><sup>-</sup>. Autotrophic denitrification coupled to sulphide or Fe oxidation has been proven for microbiological isolates ((Straub et al., 1996<ref name="Straub 1996">STRAUB, K L, BENZ, M, SCHINK, B, and WIDDEL, F. 1996. Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. ''Applied and Environmental Microbiology'', Vol. 62, 1458–1460. </ref>; Weber et al., 2006<ref name="Weber 2006">WEBER, K A, POLLOCK, J, COLE, K A, O'CONNOR, S M, ACHENBACH, L A, and COATES, J D. 2006. Anaerobic nitrate-dependent iron (II) bio-oxidation by a novel lithoautotrophic betaproteobacterium, strain 2002. ''Applied and Environmental Microbiology'', Vol. 72, 686–694. </ref>) but demonstrating this at the field-scale is more difficult. Nitrate reduction by oxidation of pyrite should lead to increased concentrations of SO<sub>4</sub>. Schwientek et al. (2008)<ref name="Schwientek 2008">SCHWIENTEK, M, EINSIEDL, F, STICHLER, W, STÖGBAUER, A, STRAUSS, H, and MALOSZEWSKI, P. 2008. Evidence for denitrification regulated by pyrite oxidation in a heterogeneous porous groundwater system. ''Chemical Geology'', Vol. 255, 60–67. </ref> looked for evidence that denitrification could be regulated by pyrite oxidation. A combination of sulphur isotopes coupled with assessment of long (c.100 years) travel times indicated that this was likely to be at a very slow rate. Zhang et al. (2009)<ref name="Zhang 2009">ZHANG, Y-C, SLOMP, C P, BROERS, H P, PASSIER, H F, and VAN CAPPELLEN, P. 2009. Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer. ''Geochimica et Cosmochimica Acta'', Vol. 73, 6716–6726. </ref> showed that NO<sub>3</sub> removal from the groundwater below cultivated fields at Oostrum, Netherlands, correlated with SO<sub>4</sub> production, and the release of dissolved Fe<sup>2+</sup> and pyrite-associated trace metals (e.g. As, Ni, Co and Zn). These results, and the presence of pyrite in the sediment matrix within the NO<sub>3</sub> removal zone, indicated that denitrification coupled to pyrite oxidation (autotrophic denitrification) was a major process in the aquifer. A number of modelling studies also indicated that pyrite oxidation was a potential pathway (e.g. Wriedt and Rode, 2006<ref name="Wriedt 2006">WRIEDT, G, and RODE, M. 2006. Modelling nitrate transport and turnover in a lowland catchment system. ''Journal of Hydrology'', Vol. 328, 157–176. </ref>). These processes via Fe or sulphur oxidation are termed chemoautotrophic denitrification (Burgin and Hamilton, 2007<ref name="Burgin 2007">BURGIN, A J, and HAMILTON, S K. 2007. Have we overemphasized the role of denitrification in aquatic ecosystems? A review of nitrate removal pathways. ''Frontiers in Ecology and the Environment'', Vol. 5, 89–96. </ref>). Jahangir et al. (2013)<ref name="Jahangir 2013">JAHANGIR, M M R, JOHNSTON, P, ADDY, K, KHALIL, M I, GROFFMAN, P M, and RICHARDS, K G. 2013. Quantification of in situ denitrification rates in groundwater below an arable and a grassland system. ''Water, Air, & Soil Pollution'', Vol. 224, 1–14.</ref> found a positive correlation between groundwater SO<sub>4</sub> concentration and denitrification rate and suggest that, due to low DOC in most groundwater environments, denitrification may well be autotrophic. A similar correlation with NH<sub>4</sub> was attributed to possible DRNA.		Korom (1992)<ref name="Korom 1992"></ref> also discuss the denitrification process in the context of addition of NO<sub>3</sub> to groundwater where Mn, Fe or SO<sub>4</sub> have already been reduced, where autotrophic denitrification may occur using the reduced inorganic compounds as electron donors. They found that groundwater containing Fe<sup>2+</sup> did not contain any observable NO<sub>3</sub><sup>-</sup>. Autotrophic denitrification coupled to sulphide or Fe oxidation has been proven for microbiological isolates ((Straub et al., 1996<ref name="Straub 1996">STRAUB, K L, BENZ, M, SCHINK, B, and WIDDEL, F. 1996. Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. ''Applied and Environmental Microbiology'', Vol. 62, 1458–1460. </ref>; Weber et al., 2006<ref name="Weber 2006">WEBER, K A, POLLOCK, J, COLE, K A, O'CONNOR, S M, ACHENBACH, L A, and COATES, J D. 2006. Anaerobic nitrate-dependent iron (II) bio-oxidation by a novel lithoautotrophic betaproteobacterium, strain 2002. ''Applied and Environmental Microbiology'', Vol. 72, 686–694. </ref>) but demonstrating this at the field-scale is more difficult. Nitrate reduction by oxidation of pyrite should lead to increased concentrations of SO<sub>4</sub>. Schwientek et al. (2008)<ref name="Schwientek 2008">SCHWIENTEK, M, EINSIEDL, F, STICHLER, W, STÖGBAUER, A, STRAUSS, H, and MALOSZEWSKI, P. 2008. Evidence for denitrification regulated by pyrite oxidation in a heterogeneous porous groundwater system. ''Chemical Geology'', Vol. 255, 60–67. </ref> looked for evidence that denitrification could be regulated by pyrite oxidation. A combination of sulphur isotopes coupled with assessment of long (c.100 years) travel times indicated that this was likely to be at a very slow rate. Zhang et al. (2009)<ref name="Zhang 2009">ZHANG, Y-C, SLOMP, C P, BROERS, H P, PASSIER, H F, and VAN CAPPELLEN, P. 2009. Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer. ''Geochimica et Cosmochimica Acta'', Vol. 73, 6716–6726. </ref> showed that NO<sub>3</sub> removal from the groundwater below cultivated fields at Oostrum, Netherlands, correlated with SO<sub>4</sub> production, and the release of dissolved Fe<sup>2+</sup> and pyrite-associated trace metals (e.g. As, Ni, Co and Zn). These results, and the presence of pyrite in the sediment matrix within the NO<sub>3</sub> removal zone, indicated that denitrification coupled to pyrite oxidation (autotrophic denitrification) was a major process in the aquifer. A number of modelling studies also indicated that pyrite oxidation was a potential pathway (e.g. Wriedt and Rode, 2006<ref name="Wriedt 2006">WRIEDT, G, and RODE, M. 2006. Modelling nitrate transport and turnover in a lowland catchment system. ''Journal of Hydrology'', Vol. 328, 157–176. </ref>). These processes via Fe or sulphur oxidation are termed chemoautotrophic denitrification (Burgin and Hamilton, 2007<ref name="Burgin 2007">BURGIN, A J, and HAMILTON, S K. 2007. Have we overemphasized the role of denitrification in aquatic ecosystems? A review of nitrate removal pathways. ''Frontiers in Ecology and the Environment'', Vol. 5, 89–96. </ref>). Jahangir et al. (2013)<ref name="Jahangir 2013">JAHANGIR, M M R, JOHNSTON, P, ADDY, K, KHALIL, M I, GROFFMAN, P M, and RICHARDS, K G. 2013. Quantification of in situ denitrification rates in groundwater below an arable and a grassland system. ''Water, Air, & Soil Pollution'', Vol. 224, 1–14.</ref> found a positive correlation between groundwater SO<sub>4</sub> concentration and denitrification rate and suggest that, due to low DOC in most groundwater environments, denitrification may well be autotrophic. A similar correlation with NH<sub>4</sub> was attributed to possible DRNA.

	==Other N cycle processes==		==Other N cycle processes==

Dbk: /* Rates */

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Rates

← Older revision		Revision as of 12:08, 2 December 2019
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	\| Inhibit the reduction of N<sub>2</sub>O to N<sup>2</sup>		\| Inhibit the reduction of N<sub>2</sub>O to N<sup>2</sup>
	\| N<sub>2</sub>O		\| N<sub>2</sub>O
	\| Bragan et al. (1997)<ref name="Bragan 1997">~~BRAGAN, R J, STARR, J L, and PARKIN, T B. 1997. Shallow groundwater denitrification rate measurement by acetylene block. ''Journal of Environmental Quality'', Vol. 26, 1531–1538.~~ </ref>; Groffman et al. (1999)<ref name="Groffman 1999">GROFFMAN, P M, HOLLAND, E, MYROLD, D D, ROBERTSON, G P, and ZOU, X. 1999. Denitrification. 272–288 in ''Standard soil methods for long term ecological research''. ROBERTSON, G P, BLEDSOE, C S, COLEMAN, D C, and SOLLINS, P (editors). (New York, USA: Oxford University Press.)</ref>; Mühlherr and Hiscock (1997<ref name="Mühlherr 1997"></ref>, 1998<ref name="Mühlherr 1998"></ref>); Weymann et al. (2008)<ref name="Weymann 2008">WEYMANN, D, WELL, R, FLESSA, H, VON DER HEIDE, C, DEURER, M, MEYER, K, KONRAD, C, and WALTHER, W. 2008. Groundwater N<sub>2</sub>O emission factors of nitrate-contaminated aquifers as derived from denitrification progress and N<sub>2</sub>O accumulation. ''Biogeosciences'', 1215–1226. </ref>		\| Bragan et al. (1997)<ref name="Bragan 1997"></ref>; Groffman et al. (1999)<ref name="Groffman 1999">GROFFMAN, P M, HOLLAND, E, MYROLD, D D, ROBERTSON, G P, and ZOU, X. 1999. Denitrification. 272–288 in ''Standard soil methods for long term ecological research''. ROBERTSON, G P, BLEDSOE, C S, COLEMAN, D C, and SOLLINS, P (editors). (New York, USA: Oxford University Press.)</ref>; Mühlherr and Hiscock (1997<ref name="Mühlherr 1997"></ref>, 1998<ref name="Mühlherr 1998"></ref>); Weymann et al. (2008)<ref name="Weymann 2008">WEYMANN, D, WELL, R, FLESSA, H, VON DER HEIDE, C, DEURER, M, MEYER, K, KONRAD, C, and WALTHER, W. 2008. Groundwater N<sub>2</sub>O emission factors of nitrate-contaminated aquifers as derived from denitrification progress and N<sub>2</sub>O accumulation. ''Biogeosciences'', 1215–1226. </ref>
	\|- style="vertical-align:top;"		\|- style="vertical-align:top;"

Dbk at 11:08, 2 December 2019

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	====Heterotrophic denitrification====		====Heterotrophic denitrification====
	There have been a number of reviews of groundwater denitrification in the UK context. Hiscock et al. (1991)<ref name="Hiscock 1991">~~HISCOCK, K M, LLOYD, J W, and LERNER, D N. 1991. Review of natural and artificial denitrification of groundwater. ''Water Research'', Vol. 25, 1099–1111.~~ </ref> reviewed the necessary environmental conditions for denitrification in groundwater and extended these to artificial denitrification. They stated that most denitrifying bacteria are heterotrophic and are able to utilize a wide range of carbon compounds (sugars, organic acids, amino acids) as electron sources. Nutrient requirements are further discussed by Champ et al. (1979)<ref name="Champ 1979">CHAMP, D R, GULENS, J, and JACKSON, R E. 1979. Oxidation–reduction sequences in ground water flow systems. ''Canadian Journal of Earth Sciences'', Vol. 16, 12–23. </ref> and Bitton and Gerba (1984)<ref name="Bitton 1984">BITTON, G, and GERBA, C P. 1984. ''Groundwater pollution microbiology'' (John Wiley and Sons, Inc., New York.) </ref>. Historical evidence appeared to show that NO<sub>3</sub> reduction was not observed at concentrations above 0.2 mg/L (Skerman and MacRae, 1957<ref name="Skerman 1957">SKERMAN, V, and MACRAE, I. 1957. The influence of oxygen availability on the degree of nitrate reduction by pseudomonas denitrificans. ''Canadian Journal of Microbiology'', Vol. 3, 505–530.</ref>). This did not take account of more modern concepts of small, protected niches (hotspots) which enable organisms to live in conditions different from the bulk conditions. A biofilm even just a few cells thick can provide enough cover to have an anaerobic layer in an ostensibly aerobic environment. Dependence on pH range and temperature are covered by Gauntlett and Craft (1979)<ref name="Gauntlett 1979">GAUNTLETT, R B, and CRAFT, D G. 1979. Biological removal of nitrate from river water. ''Water Research Centre Technical Report TR98'' (Medmenham). </ref>. However Rivett et al. (2008)<ref name="Rivett 2008" >RIVETT, M O, BUSS, S R, MORGAN, P, SMITH, J W N, and BEMMENT, C D. 2008. Nitrate attenuation in groundwater: a review of biogeochemical controlling processes. ''Water Research'', Vol. 42, 4215–4232. </ref> concluded that the critical limiting factors are oxygen tension and electron donor concentration and availability. Variability in other environmental conditions such as NO<sub>3</sub> concentration, nutrient availability, pH, temperature, presence of toxins and microbial acclimation appears to be less important, exerting only secondary influences on denitrification rates. Korom (1992)<ref name="Korom 1992">KOROM, S F. 1992. Natural denitrification in the saturated zone: a review. ''Water Resources Research'', Vol. 28, 1657–1668. </ref> included both denitrification and dissimilatory reduction to ammonia (DNRA) in their review of saturated zone processes. They concluded that natural denitrification can decrease NO<sub>3</sub> contamination in modern waters but that it was difficult to predict the rate.		There have been a number of reviews of groundwater denitrification in the UK context. Hiscock et al. (1991)<ref name="Hiscock 1991"></ref> reviewed the necessary environmental conditions for denitrification in groundwater and extended these to artificial denitrification. They stated that most denitrifying bacteria are heterotrophic and are able to utilize a wide range of carbon compounds (sugars, organic acids, amino acids) as electron sources. Nutrient requirements are further discussed by Champ et al. (1979)<ref name="Champ 1979">CHAMP, D R, GULENS, J, and JACKSON, R E. 1979. Oxidation–reduction sequences in ground water flow systems. ''Canadian Journal of Earth Sciences'', Vol. 16, 12–23. </ref> and Bitton and Gerba (1984)<ref name="Bitton 1984">BITTON, G, and GERBA, C P. 1984. ''Groundwater pollution microbiology'' (John Wiley and Sons, Inc., New York.) </ref>. Historical evidence appeared to show that NO<sub>3</sub> reduction was not observed at concentrations above 0.2 mg/L (Skerman and MacRae, 1957<ref name="Skerman 1957">SKERMAN, V, and MACRAE, I. 1957. The influence of oxygen availability on the degree of nitrate reduction by pseudomonas denitrificans. ''Canadian Journal of Microbiology'', Vol. 3, 505–530.</ref>). This did not take account of more modern concepts of small, protected niches (hotspots) which enable organisms to live in conditions different from the bulk conditions. A biofilm even just a few cells thick can provide enough cover to have an anaerobic layer in an ostensibly aerobic environment. Dependence on pH range and temperature are covered by Gauntlett and Craft (1979)<ref name="Gauntlett 1979">GAUNTLETT, R B, and CRAFT, D G. 1979. Biological removal of nitrate from river water. ''Water Research Centre Technical Report TR98'' (Medmenham). </ref>. However Rivett et al. (2008)<ref name="Rivett 2008" >RIVETT, M O, BUSS, S R, MORGAN, P, SMITH, J W N, and BEMMENT, C D. 2008. Nitrate attenuation in groundwater: a review of biogeochemical controlling processes. ''Water Research'', Vol. 42, 4215–4232. </ref> concluded that the critical limiting factors are oxygen tension and electron donor concentration and availability. Variability in other environmental conditions such as NO<sub>3</sub> concentration, nutrient availability, pH, temperature, presence of toxins and microbial acclimation appears to be less important, exerting only secondary influences on denitrification rates. Korom (1992)<ref name="Korom 1992">KOROM, S F. 1992. Natural denitrification in the saturated zone: a review. ''Water Resources Research'', Vol. 28, 1657–1668. </ref> included both denitrification and dissimilatory reduction to ammonia (DNRA) in their review of saturated zone processes. They concluded that natural denitrification can decrease NO<sub>3</sub> contamination in modern waters but that it was difficult to predict the rate.

	[[Image:OR18011fig2.1.jpg\|thumb\|center\|400px\| '''Figure 2.1'''    The sequence of reaction zones developing as groundwater moves along flow pathways from recharge to confined conditions (after Shand et al., 2007<ref name="Shand 2007">SHAND, P, EDMUNDS, W M, LAWRENCE, A R, SMEDLEY, P L, and BURKE, S. 2007. The natural		[[Image:OR18011fig2.1.jpg\|thumb\|center\|400px\| '''Figure 2.1'''    The sequence of reaction zones developing as groundwater moves along flow pathways from recharge to confined conditions (after Shand et al., 2007<ref name="Shand 2007">SHAND, P, EDMUNDS, W M, LAWRENCE, A R, SMEDLEY, P L, and BURKE, S. 2007. The natural
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	==Other potential indicators==		==Other potential indicators==
	===Redox sequence ions===		===Redox sequence ions===
	It is anticipated that there will be a sequence of redox changes as water migrates from upland recharge areas to lowland discharge areas under confined conditions. Champ et al. (1979)<ref name="Champ 1979">CHAMP, D R, GULENS, J, and JACKSON, R E. 1979. Oxidation–reduction sequences in ground water flow systems. ''Canadian Journal of Earth Sciences'', Vol. 16, 12–23. </ref> identified 3 zones O<sub>2</sub>–NO<sub>3</sub>, Fe-Mn, sulphide. Hiscock et al. (1991)<ref name="Hiscock 1991">~~HISCOCK, K M, LLOYD, J W, and LERNER, D N. 1991. Review of natural and artificial denitrification of groundwater. ''Water Research'', Vol. 25, 1099–1111.~~</ref> showed change in redox potential is often accompanied by a sequential reduction in dissolved groundwater species which is sited as proof of denitrification.		It is anticipated that there will be a sequence of redox changes as water migrates from upland recharge areas to lowland discharge areas under confined conditions. Champ et al. (1979)<ref name="Champ 1979">CHAMP, D R, GULENS, J, and JACKSON, R E. 1979. Oxidation–reduction sequences in ground water flow systems. ''Canadian Journal of Earth Sciences'', Vol. 16, 12–23. </ref> identified 3 zones O<sub>2</sub>–NO<sub>3</sub>, Fe-Mn, sulphide. Hiscock et al. (1991)<ref name="Hiscock 1991"></ref> showed change in redox potential is often accompanied by a sequential reduction in dissolved groundwater species which is sited as proof of denitrification.

	This reaction sequence is commonly seen along groundwater flow lines (Edmunds et al., 1982<ref name="Edmunds 1982">EDMUNDS, W M, BATH, A H, and MILES, D L. 1982. Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England. ''Geochimica et Cosmochimica Acta'', Vol. 46, 2069–2081. </ref>; Edmunds et al., 1984<ref name="Edmunds 1984">EDMUNDS, W M, MILES, D L, and COOK, J M. 1984. A comparative study of sequential redox processes in three British aquifers. 55–70 in ''Hydrochemical Balances in Freshwater'' ERIKSSON, E (editor). Vol. 150. (Wallingford: IAHS-AISH.) </ref>) typically as aquifers become confined. Water at recharge is generally saturated with DO at the partial pressure of the atmosphere (10–12 mg/L depending upon barometric conditions). Passing through the soil and the unsaturated zone some of this O<sub>2</sub> will react as a result of microbiological processes and oxidation-reduction reactions. However, almost all water reaching the water table still contains several mg/L O<sub>2</sub>. Geochemical and microbial reactions progressively remove the O<sub>2</sub> along flow lines. Once all the O<sub>2</sub> has reacted an abrupt change of water chemistry takes place (redox boundary). Down-gradient of the redox boundary, denitrification occurs and it is likely that Fe<sup>2+</sup> concentrations will increase. Sulphate reduction and the production of sulphide (H<sub>2</sub>S as S<sub>2</sub><sup>-</sup> in solution) may also occur at greater depths (Figure 2.2).		This reaction sequence is commonly seen along groundwater flow lines (Edmunds et al., 1982<ref name="Edmunds 1982">EDMUNDS, W M, BATH, A H, and MILES, D L. 1982. Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England. ''Geochimica et Cosmochimica Acta'', Vol. 46, 2069–2081. </ref>; Edmunds et al., 1984<ref name="Edmunds 1984">EDMUNDS, W M, MILES, D L, and COOK, J M. 1984. A comparative study of sequential redox processes in three British aquifers. 55–70 in ''Hydrochemical Balances in Freshwater'' ERIKSSON, E (editor). Vol. 150. (Wallingford: IAHS-AISH.) </ref>) typically as aquifers become confined. Water at recharge is generally saturated with DO at the partial pressure of the atmosphere (10–12 mg/L depending upon barometric conditions). Passing through the soil and the unsaturated zone some of this O<sub>2</sub> will react as a result of microbiological processes and oxidation-reduction reactions. However, almost all water reaching the water table still contains several mg/L O<sub>2</sub>. Geochemical and microbial reactions progressively remove the O<sub>2</sub> along flow lines. Once all the O<sub>2</sub> has reacted an abrupt change of water chemistry takes place (redox boundary). Down-gradient of the redox boundary, denitrification occurs and it is likely that Fe<sup>2+</sup> concentrations will increase. Sulphate reduction and the production of sulphide (H<sub>2</sub>S as S<sub>2</sub><sup>-</sup> in solution) may also occur at greater depths (Figure 2.2).

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