OR/15/048 Regional hydrogeochemistry - Revision history

Ajhil at 12:05, 3 December 2019

2019-12-03T12:05:11Z

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	Summary statistical data (Table 6, Figure 26 and Figure 27) and spatial variations (Figure 28 to Figure 50) for minor and trace constituents are presented in this section. Where all the determinations are below the detection limit, the element is not discussed here (see Table 6).		Summary statistical data (Table 6, Figure 26 and Figure 27) and spatial variations (Figure 28 to Figure 50) for minor and trace constituents are presented in this section. Where all the determinations are below the detection limit, the element is not discussed here (see Table 6).

	===Phosphorus ===		===Phosphorus===
	Phosphorus has a concentration range of <0.02 to 0.043 mg L<sup>-1</sup>, and 17 of the 24 analyses are below the detection limit (Table 6). The concentrations of P are generally lower than those found in the groundwaters of the Cotswold Oolite and the Corallian of Oxfordshire and Wiltshire, which ranged from <0.02 to 0.12 mg L<sup>-1</sup> (Cobbing et al., 2004<ref name="Cobbing 2004"> COBBING, J, MOREAU, M, SHAND, P, and LANCASTER, A. 2004. The Corallian of Oxfordshire and Wiltshire. ''British Geological Survey and Environment Agency'', BGS Report CR/04/262N; Environment Agency Report NC/99/74/14 (Keyworth and Solihull). </ref>; Neumann et al., 2003<ref name="Neumann 2003"> NEUMANN, I, BROWN, S, SMEDLEY, P L, and BESIEN, T. 2003. The Great and Inferior Oolite of the Cotswolds District. British Geological Survey and Environment Agency, BGS Report CR/03/202N; Environment Agency Report NC/99/74/7 (Keyworth and Solihull). </ref>). There are no spatial trends evident in the Vale of Pickering (Figure 28), which is probably a function of the large number of censored data.		Phosphorus has a concentration range of <0.02 to 0.043 mg L<sup>-1</sup>, and 17 of the 24 analyses are below the detection limit (Table 6). The concentrations of P are generally lower than those found in the groundwaters of the Cotswold Oolite and the Corallian of Oxfordshire and Wiltshire, which ranged from <0.02 to 0.12 mg L<sup>-1</sup> (Cobbing et al., 2004<ref name="Cobbing 2004">COBBING, J, MOREAU, M, SHAND, P, and LANCASTER, A. 2004. The Corallian of Oxfordshire and Wiltshire. ''British Geological Survey and Environment Agency'', BGS Report CR/04/262N; Environment Agency Report NC/99/74/14 (Keyworth and Solihull). </ref>; Neumann et al., 2003<ref name="Neumann 2003"> NEUMANN, I, BROWN, S, SMEDLEY, P L, and BESIEN, T. 2003. The Great and Inferior Oolite of the Cotswolds District. British Geological Survey and Environment Agency, BGS Report CR/03/202N; Environment Agency Report NC/99/74/7 (Keyworth and Solihull). </ref>). There are no spatial trends evident in the Vale of Pickering (Figure 28), which is probably a function of the large number of censored data.

	===Halogen elements ===		===Halogen elements ===
	Fluoride has a concentration range of <0.25 to 0.311 mg L<sup>-1</sup>, a 5th to 95th percentile range of 0.024 to 0.252 mg L<sup>-1</sup>, and a median of 0.06 mg L<sup>-1</sup> (Table 6). All these values are well within the drinking water limit of 1.5 mg L<sup>-1</sup> (The Water Supply Regulations, 2010<ref name= WSR 2010"> THE WATER SUPPLY REGULATIONS. 2010. Statutory Instrument 2010 No. 991. </ref>). One of the samples contained F below the detection limit of 0.25 mg L<sup>-1</sup>. However this detection limit is higher than some of the measured values, of which the minimum is 0.033 mg L<sup>-1</sup>. The F range in the Corallian of the Vale of Pickering is much smaller than those of the comparable groundwaters found in the Cotswold Oolite and the Corallian of Oxfordshire and Wiltshire, which are between <0.05 to 4.8 mg L<sup>-1</sup>, and 0.05 to 1.98 mg L<sup>-1</sup> respectively (Cobbing et al., 2004<ref name="Cobbing 2004"> COBBING, J, MOREAU, M, SHAND, P, and LANCASTER, A. 2004. The Corallian of Oxfordshire and Wiltshire. British Geological Survey and Environment Agency, BGS Report CR/04/262N; Environment Agency Report NC/99/74/14 (Keyworth and Solihull). </ref>; Neumann et al., 2003<ref name="Neumann 2003"></ref>). In these the higher concentrations were the result of ion exchange.		Fluoride has a concentration range of <0.25 to 0.311 mg L<sup>-1</sup>, a 5th to 95th percentile range of 0.024 to 0.252 mg L<sup>-1</sup>, and a median of 0.06 mg L<sup>-1</sup> (Table 6). All these values are well within the drinking water limit of 1.5 mg L<sup>-1</sup> (The Water Supply Regulations, 2010<ref name= WSR 2010"> THE WATER SUPPLY REGULATIONS. 2010. Statutory Instrument 2010 No. 991. </ref>). One of the samples contained F below the detection limit of 0.25 mg L<sup>-1</sup>. However this detection limit is higher than some of the measured values, of which the minimum is 0.033 mg L<sup>-1</sup>. The F range in the Corallian of the Vale of Pickering is much smaller than those of the comparable groundwaters found in the Cotswold Oolite and the Corallian of Oxfordshire and Wiltshire, which are between <0.05 to 4.8 mg L<sup>-1</sup>, and 0.05 to 1.98 mg L<sup>-1</sup> respectively (Cobbing et al., 2004<ref name="Cobbing 2004"></ref>; Neumann et al., 2003<ref name="Neumann 2003"></ref>). In these the higher concentrations were the result of ion exchange.

	[[Image:OR15048 fig26.jpg\|thumb\|center\| 400px\| '''Figure 26''' Box plot for the minor and trace elements in the Vale of Pickering Corallian.]]		[[Image:OR15048 fig26.jpg\|thumb\|center\| 400px\| '''Figure 26''' Box plot for the minor and trace elements in the Vale of Pickering Corallian.]]

Ajhil at 12:04, 3 December 2019

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	The temperatures of the groundwaters range from 8.7 to 15.6ºC, with a 5th to 95th percentile range of 8.9 to 15.2 ºC and a median of 10.7 ºC (Table 5). This is fairly typical of modern recharge in UK aquifers. Temperature can be a good indicator of depth, and therefore to some extent indicates residence time. There are no depth data to verify this, however. The temperature tends to be higher around the margins of the confined aquifer (Figure 11), which suggests that the groundwaters in these areas are deeper, and therefore older.		The temperatures of the groundwaters range from 8.7 to 15.6ºC, with a 5th to 95th percentile range of 8.9 to 15.2 ºC and a median of 10.7 ºC (Table 5). This is fairly typical of modern recharge in UK aquifers. Temperature can be a good indicator of depth, and therefore to some extent indicates residence time. There are no depth data to verify this, however. The temperature tends to be higher around the margins of the confined aquifer (Figure 11), which suggests that the groundwaters in these areas are deeper, and therefore older.

	The pH of the groundwaters has a range of 6.96 to 7.95, a 5th to 95th percentile range of 7.02 to 7.87 and a median of 7.3 (Table 5). This range is similar to other limestone aquifers in the UK, including the Chalk, which typically has a narrow range and median values between 7.1 and 7.3 (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 (baseline) quality of groundwater in England and Wales. British Geological Survey & Environment Agency, RR/07/06 & NC/99/74/24 (Keyworth and Solihull). </ref>). Such a small range is typical of unconfined groundwaters that are dominated by carbonate equilibrium reactions, again suggesting that CaCO<sub>3</sub> is the mineral with the most influence over the aqueous geochemistry of the Corallian aquifer. The lowest pH values are found in the areas around Pickering and Malton, while the higher values are found near Scarborough and to the west of Pickering (Figure 12). There is no clear reason for this distribution; however there is little variation between the maximum and minimum values. It is very likely therefore that the entire range of pH values represents the range of baseline compositions.		The pH of the groundwaters has a range of 6.96 to 7.95, a 5th to 95th percentile range of 7.02 to 7.87 and a median of 7.3 (Table 5). This range is similar to other limestone aquifers in the UK, including the Chalk, which typically has a narrow range and median values between 7.1 and 7.3 (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 (baseline) quality of groundwater in England and Wales. British Geological Survey & Environment Agency, RR/07/06 & NC/99/74/24 (Keyworth and Solihull). </ref>). Such a small range is typical of unconfined groundwaters that are dominated by carbonate equilibrium reactions, again suggesting that CaCO<sub>3</sub> is the mineral with the most influence over the aqueous geochemistry of the Corallian aquifer. The lowest pH values are found in the areas around Pickering and Malton, while the higher values are found near Scarborough and to the west of Pickering (Figure 12). There is no clear reason for this distribution; however there is little variation between the maximum and minimum values. It is very likely therefore that the entire range of pH values represents the range of baseline compositions.

	The measured dissolved oxygen (DO) has a range of <1 to 12.7 mg L<sup>-1</sup>, a 5th to 95th percentile range of 0.56 to 11.1 mg L<sup>-1</sup> and a median of 6.33 mg L<sup>-1</sup> (Table 5). This represents a large range up to DO saturated conditions. The Eh values range from 203 to 469 mV, with a 5th to 95th percentile range of 243–451 mV, and a median of 395 mV (Table 5). This represents a relatively narrow range, dominated by oxidised water. This is typical of unconfined groundwaters. The DO content of the waters is generally lowest in the east of the area (Figure 13), and around the edge of the zone confined by Jurassic Clays. The higher values are found in the west. The spatial distribution of Eh is similar with the lowest concentrations generally found in the east and around the periphery of the confining Jurassic Clay (Figure 14). The highest values are found in the north-west portion of the Corallian aquifer. The most north-westerly samples also have the highest pH values. The interdependence of Eh and pH are the important factors which determine the solubility of minerals and speciation of chemical constituents (Levinson, 1974<ref name="Levinson 1974">LEVINSON, A A. 1974. ''Introduction to Exploration Geochemistry''. (Calgary: Applied Publishing Ltd.) </ref>).		The measured dissolved oxygen (DO) has a range of <1 to 12.7 mg L<sup>-1</sup>, a 5th to 95th percentile range of 0.56 to 11.1 mg L<sup>-1</sup> and a median of 6.33 mg L<sup>-1</sup> (Table 5). This represents a large range up to DO saturated conditions. The Eh values range from 203 to 469 mV, with a 5th to 95th percentile range of 243–451 mV, and a median of 395 mV (Table 5). This represents a relatively narrow range, dominated by oxidised water. This is typical of unconfined groundwaters. The DO content of the waters is generally lowest in the east of the area (Figure 13), and around the edge of the zone confined by Jurassic Clays. The higher values are found in the west. The spatial distribution of Eh is similar with the lowest concentrations generally found in the east and around the periphery of the confining Jurassic Clay (Figure 14). The highest values are found in the north-west portion of the Corallian aquifer. The most north-westerly samples also have the highest pH values. The interdependence of Eh and pH are the important factors which determine the solubility of minerals and speciation of chemical constituents (Levinson, 1974<ref name="Levinson 1974">LEVINSON, A A. 1974. ''Introduction to Exploration Geochemistry''. (Calgary: Applied Publishing Ltd.) </ref>).
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	[[Image:OR15048 fig30.jpg\|thumb\|center\| 400px\| '''Figure 30''' Regional variation of Br in the Corallian aquifer.]]		[[Image:OR15048 fig30.jpg\|thumb\|center\| 400px\| '''Figure 30''' Regional variation of Br in the Corallian aquifer.]]

	Strontium has a concentration range of 78.9 to 447 mg L<sup>-1</sup>, a 5th to 95th percentile range of 106 to 403 mg L<sup>-1</sup>, and a median of 155 mg L<sup>-1</sup> (Table 6). This range is about half that of the geologically similar Corallian of Oxfordshire and Wiltshire, which ranges from 88.6 to 1860 mg L<sup>-1</sup> (Cobbing et al., 2004<ref name="Cobbing 2004"></ref>). Strontium is geochemically similar to Ca, and can be present at high concentrations in Ca-bearing minerals owing to the similarities in the ionic radius of Ca and Sr. In fresh groundwater there are generally no solubility controls on Sr, and it typically increases with residence time. Increases in the Sr/Ca ratio likewise reflect increasing residence time (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 (baseline) quality of groundwater in England and Wales. ''British Geological Survey & Environment Agency'', RR/07/06 & NC/99/74/24 (Keyworth and Solihull). </ref>). The Sr/Ca ratio of the data presented here has a moderate-strong relationship with the Sr concentrations (r<sup>2</sup> = 0.64). The spatial distribution of Sr in the Vale of Pickering is similar to that of Ca (Figure 32), and there is a moderate relationship between the two elements (r<sup>2</sup> = 0.38). These values indicate that, while there is no information available on the aquifer mineralogy and chemistry, is it most likely that the Sr concentrations presented here, represent baseline concentrations.		Strontium has a concentration range of 78.9 to 447 mg L<sup>-1</sup>, a 5th to 95th percentile range of 106 to 403 mg L<sup>-1</sup>, and a median of 155 mg L<sup>-1</sup> (Table 6). This range is about half that of the geologically similar Corallian of Oxfordshire and Wiltshire, which ranges from 88.6 to 1860 mg L<sup>-1</sup> (Cobbing et al., 2004<ref name="Cobbing 2004"></ref>). Strontium is geochemically similar to Ca, and can be present at high concentrations in Ca-bearing minerals owing to the similarities in the ionic radius of Ca and Sr. In fresh groundwater there are generally no solubility controls on Sr, and it typically increases with residence time. Increases in the Sr/Ca ratio likewise reflect increasing residence time (Shand et al., 2007<ref name="Shand 2007"></ref>). The Sr/Ca ratio of the data presented here has a moderate-strong relationship with the Sr concentrations (r<sup>2</sup> = 0.64). The spatial distribution of Sr in the Vale of Pickering is similar to that of Ca (Figure 32), and there is a moderate relationship between the two elements (r<sup>2</sup> = 0.38). These values indicate that, while there is no information available on the aquifer mineralogy and chemistry, is it most likely that the Sr concentrations presented here, represent baseline concentrations.

	===Alkali metals ===		===Alkali metals ===
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	[[Image:OR15048 fig40.jpg\|thumb\|center\| 400px\| '''Figure 4'''0 Regional Variation of Co in the Corallian aquifer.]]		[[Image:OR15048 fig40.jpg\|thumb\|center\| 400px\| '''Figure 4'''0 Regional Variation of Co in the Corallian aquifer.]]

	Copper has a concentration range of 0.8 to 9 µg L<sup>-1</sup>, a 5th to 95th percentile range of 0.8 to 5 µg L<sup>-1</sup>, and a median of 1.9 µg L<sup>-1</sup> (Table 6). This range is smaller than those observed in the Corallian of Oxfordshire and Wiltshire (Cobbing et al., 2004<ref name="Cobbing 2004"></ref>) and Cotswold Oolite (Neumann et al., 2003<ref name="Neumann 2003"></ref>), which range from 0.5 to 304 mg L<sup>-1</sup> and 0.1 to 115 mg L<sup>-1</sup> respectively. Copper occurs naturally as native metal, or in sulphide ore deposits, but is also a very commonly used metal in industrialised countries, and in pipework in water supply infrastructure. Anthropogenic inputs are to be expected in young groundwaters (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 (baseline) quality of groundwater in England and Wales. ''British Geological Survey & Environment Agency'', RR/07/06 & NC/99/74/24 (Keyworth and Solihull). </ref>). Where the groundwater is oxidising, Cu is most soluble under acidic conditions. As the pH increases, Cu can adsorb to organic matter or Fe and Mn oxyhydroxides (Shand et al., 2007<ref name="Shand 2007"></ref>). There is a poor correlation between pH and Cu (r<sup>2</sup>=0.02), and there is no clear spatial distribution (Figure 41). The distribution of Cu could be influenced by anthropogenic activities.		Copper has a concentration range of 0.8 to 9 µg L<sup>-1</sup>, a 5th to 95th percentile range of 0.8 to 5 µg L<sup>-1</sup>, and a median of 1.9 µg L<sup>-1</sup> (Table 6). This range is smaller than those observed in the Corallian of Oxfordshire and Wiltshire (Cobbing et al., 2004<ref name="Cobbing 2004"></ref>) and Cotswold Oolite (Neumann et al., 2003<ref name="Neumann 2003"></ref>), which range from 0.5 to 304 mg L<sup>-1</sup> and 0.1 to 115 mg L<sup>-1</sup> respectively. Copper occurs naturally as native metal, or in sulphide ore deposits, but is also a very commonly used metal in industrialised countries, and in pipework in water supply infrastructure. Anthropogenic inputs are to be expected in young groundwaters (Shand et al., 2007<ref name="Shand 2007"></ref>). Where the groundwater is oxidising, Cu is most soluble under acidic conditions. As the pH increases, Cu can adsorb to organic matter or Fe and Mn oxyhydroxides (Shand et al., 2007<ref name="Shand 2007"></ref>). There is a poor correlation between pH and Cu (r<sup>2</sup>=0.02), and there is no clear spatial distribution (Figure 41). The distribution of Cu could be influenced by anthropogenic activities.

	[[Image:OR15048 fig41.jpg\|thumb\|center\| 400px\| '''Figure 41''' Regional Variation of Cu in the Corallian aquifer.]]		[[Image:OR15048 fig41.jpg\|thumb\|center\| 400px\| '''Figure 41''' Regional Variation of Cu in the Corallian aquifer.]]

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	DO: dissolved oxygen; SEC: specific electrical conductance; DOC: dissolved organic carbon; <br>P: percentile; n(c): number censored; min and max are observed values		DO: dissolved oxygen; SEC: specific electrical conductance; DOC: dissolved organic carbon; <br>P: percentile; n(c): number censored; min and max are observed values
	</center>		</center>
	[[Image:OR/~~15/048~~ fig8.jpg\|thumb\|center\| 400px\| '''Figure 8''' Piper digram of Corallian groundwaters in the Vale of Pickering.]]		[[Image:OR/15048 fig8.jpg\|thumb\|center\| 400px\| '''Figure 8''' Piper digram of Corallian groundwaters in the Vale of Pickering.]]

	The temperatures of the groundwaters range from 8.7 to 15.6ºC, with a 5th to 95th percentile range of 8.9 to 15.2 ºC and a median of 10.7 ºC (Table 5). This is fairly typical of modern recharge in UK aquifers. Temperature can be a good indicator of depth, and therefore to some extent indicates residence time. There are no depth data to verify this, however. The temperature tends to be higher around the margins of the confined aquifer (Figure 11), which suggests that the groundwaters in these areas are deeper, and therefore older.		The temperatures of the groundwaters range from 8.7 to 15.6ºC, with a 5th to 95th percentile range of 8.9 to 15.2 ºC and a median of 10.7 ºC (Table 5). This is fairly typical of modern recharge in UK aquifers. Temperature can be a good indicator of depth, and therefore to some extent indicates residence time. There are no depth data to verify this, however. The temperature tends to be higher around the margins of the confined aquifer (Figure 11), which suggests that the groundwaters in these areas are deeper, and therefore older.

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Major constituents

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	Most samples were at equilibrium, or supersaturated, with respect to calcite, which is why the distribution of these Ca data is similar to that of the Chalk aquifers. In addition, there are no clear trends in the spatial distribution of Ca (Figure 16).		Most samples were at equilibrium, or supersaturated, with respect to calcite, which is why the distribution of these Ca data is similar to that of the Chalk aquifers. In addition, there are no clear trends in the spatial distribution of Ca (Figure 16).

	Magnesium has a concentration range of 2.12 to 19.4 mg L<sup>-1</sup>, a 5th to 95th percentile range of 2.53 to 17.1 mg L<sup>-1</sup> and a median of 6.91 mg L<sup>-1</sup> (Table 5). This range is similar to that found in the Cotswold Oolite and the Corallian of Oxfordshire and Wiltshire, as well as the Chalk aquifers. The cumulative-probablity plot shows that the distribution of Mg concentrations is log normal. The lowest concentrations of Mg are all found in the north-west corner of the study area (Figure ~~5.10~~); the highest concentrations are in the northern portion. There is scant information regarding the mineralogy of the Vale of Pickering in the available memoirs, but it is most likely that the source of Mg is within calcite or clays. It is unlikely, however, that the source is dolomite, as all the waters analysed in this study are undersaturated with respect to it.		Magnesium has a concentration range of 2.12 to 19.4 mg L<sup>-1</sup>, a 5th to 95th percentile range of 2.53 to 17.1 mg L<sup>-1</sup> and a median of 6.91 mg L<sup>-1</sup> (Table 5). This range is similar to that found in the Cotswold Oolite and the Corallian of Oxfordshire and Wiltshire, as well as the Chalk aquifers. The cumulative-probablity plot shows that the distribution of Mg concentrations is log normal. The lowest concentrations of Mg are all found in the north-west corner of the study area (Figure 17); the highest concentrations are in the northern portion. There is scant information regarding the mineralogy of the Vale of Pickering in the available memoirs, but it is most likely that the source of Mg is within calcite or clays. It is unlikely, however, that the source is dolomite, as all the waters analysed in this study are undersaturated with respect to it.

	Sodium has a concentration range of 8.71 to 29.7 mg L<sup>-1</sup>, a 5th to 95th percentile range of 8.77 to 21.5 mg L<sup>-1</sup> and a median of 11.4 mg L<sup>-1</sup>. This range is relatively narrow as Na in many UK groundwaters ranges over two orders of magnitude (Shand et al., 2007<ref name="Shand 2007"></ref>). The data presented here lack the positive skew seen in data from comparable groundwaters of the Cotswold Oolite and Corallian of Oxfordshire and Wiltshire. While the highest concentration is found close to the coast, there is no trend of Na decreasing away from the sea, which suggests that the influence of marine aerosols or saline intrusion is limited (Figure 18). It is likely that many of the lower concentrations represent evapotranspired rainwater. The remaining Na is probably sourced from percolation through overlying soils, and minor proportions of feldspars or clays that may be present in the gritty layers of the Corallian aquifer.		Sodium has a concentration range of 8.71 to 29.7 mg L<sup>-1</sup>, a 5th to 95th percentile range of 8.77 to 21.5 mg L<sup>-1</sup> and a median of 11.4 mg L<sup>-1</sup>. This range is relatively narrow as Na in many UK groundwaters ranges over two orders of magnitude (Shand et al., 2007<ref name="Shand 2007"></ref>). The data presented here lack the positive skew seen in data from comparable groundwaters of the Cotswold Oolite and Corallian of Oxfordshire and Wiltshire. While the highest concentration is found close to the coast, there is no trend of Na decreasing away from the sea, which suggests that the influence of marine aerosols or saline intrusion is limited (Figure 18). It is likely that many of the lower concentrations represent evapotranspired rainwater. The remaining Na is probably sourced from percolation through overlying soils, and minor proportions of feldspars or clays that may be present in the gritty layers of the Corallian aquifer.

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Major constituents

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	The pH of the groundwaters has a range of 6.96 to 7.95, a 5th to 95th percentile range of 7.02 to 7.87 and a median of 7.3 (Table 5). This range is similar to other limestone aquifers in the UK, including the Chalk, which typically has a narrow range and median values between 7.1 and 7.3 (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 (baseline) quality of groundwater in England and Wales. British Geological Survey & Environment Agency, RR/07/06 & NC/99/74/24 (Keyworth and Solihull). </ref>). Such a small range is typical of unconfined groundwaters that are dominated by carbonate equilibrium reactions, again suggesting that CaCO<sub>3</sub> is the mineral with the most influence over the aqueous geochemistry of the Corallian aquifer. The lowest pH values are found in the areas around Pickering and Malton, while the higher values are found near Scarborough and to the west of Pickering (Figure 12). There is no clear reason for this distribution; however there is little variation between the maximum and minimum values. It is very likely therefore that the entire range of pH values represents the range of baseline compositions.		The pH of the groundwaters has a range of 6.96 to 7.95, a 5th to 95th percentile range of 7.02 to 7.87 and a median of 7.3 (Table 5). This range is similar to other limestone aquifers in the UK, including the Chalk, which typically has a narrow range and median values between 7.1 and 7.3 (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 (baseline) quality of groundwater in England and Wales. British Geological Survey & Environment Agency, RR/07/06 & NC/99/74/24 (Keyworth and Solihull). </ref>). Such a small range is typical of unconfined groundwaters that are dominated by carbonate equilibrium reactions, again suggesting that CaCO<sub>3</sub> is the mineral with the most influence over the aqueous geochemistry of the Corallian aquifer. The lowest pH values are found in the areas around Pickering and Malton, while the higher values are found near Scarborough and to the west of Pickering (Figure 12). There is no clear reason for this distribution; however there is little variation between the maximum and minimum values. It is very likely therefore that the entire range of pH values represents the range of baseline compositions.

	The measured dissolved oxygen (DO) has a range of <1 to 12.7 mg L<sup>-1</sup>, a 5th to 95th percentile range of 0.56 to 11.1 mg L<sup>-1</sup> and a median of 6.33 mg L<sup>-1</sup> (Table 5). This represents a large range up to DO saturated conditions . The Eh values range from 203 to 469 mV, with a 5th to 95th percentile range of 243–451 mV, and a median of 395 mV (Table 5). This represents a relatively narrow range, dominated by oxidised water. This is typical of unconfined groundwaters. The DO content of the waters is generally lowest in the east of the area (Figure 13), and around the edge of the zone confined by Jurassic Clays. The higher values are found in the west. The spatial distribution of Eh is similar with the lowest concentrations generally found in the east and around the periphery of the confining Jurassic Clay (Figure 14). The highest values are found in the north-west portion of the Corallian aquifer. The most north-westerly samples also have the highest pH values. The interdependence of Eh and pH are the important factors which determine the solubility of minerals and speciation of chemical constituents (Levinson, 1974<ref name="Levinson 1974">LEVINSON, A A. 1974. ''Introduction to Exploration Geochemistry''. (Calgary: Applied Publishing Ltd.) </ref>).		The measured dissolved oxygen (DO) has a range of <1 to 12.7 mg L<sup>-1</sup>, a 5th to 95th percentile range of 0.56 to 11.1 mg L<sup>-1</sup> and a median of 6.33 mg L<sup>-1</sup> (Table 5). This represents a large range up to DO saturated conditions. The Eh values range from 203 to 469 mV, with a 5th to 95th percentile range of 243–451 mV, and a median of 395 mV (Table 5). This represents a relatively narrow range, dominated by oxidised water. This is typical of unconfined groundwaters. The DO content of the waters is generally lowest in the east of the area (Figure 13), and around the edge of the zone confined by Jurassic Clays. The higher values are found in the west. The spatial distribution of Eh is similar with the lowest concentrations generally found in the east and around the periphery of the confining Jurassic Clay (Figure 14). The highest values are found in the north-west portion of the Corallian aquifer. The most north-westerly samples also have the highest pH values. The interdependence of Eh and pH are the important factors which determine the solubility of minerals and speciation of chemical constituents (Levinson, 1974<ref name="Levinson 1974">LEVINSON, A A. 1974. ''Introduction to Exploration Geochemistry''. (Calgary: Applied Publishing Ltd.) </ref>).

	[[Image:OR/15048 fig9.jpg\|thumb\|center\| 400px\| '''Figure 9''' Box plot of major ions in the Corallian aquifer.]]		[[Image:OR/15048 fig9.jpg\|thumb\|center\| 400px\| '''Figure 9''' Box plot of major ions in the Corallian aquifer.]]

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	Values of δ<sup>13</sup>C are an index of the evolution of the dissolved inorganic carbon system (DIC) (Clark and Fritz, 1997<ref name="Clark 1997">CLARK, I, and FRITZ, P. 1997. ''Environmental Isotopes in Hydrogeology''. (Boca Raton, USA: Lewise Publishers.)</ref>; Darling et al., 2005<ref name="Darling 2003"></ref>). Groundwater in calcareous sedimentary terrain acquires DIC through reaction with soil CO<sub>2</sub> and reaction with carbonate minerals in the soil and aquifer. The δ<sup>13</sup>C composition of DIC is fundamentally governed by interaction between soil CO<sub>2</sub> (δ<sup>13</sup>C ~ -26‰) and carbonate minerals (δ<sup>13</sup>C ~ 0‰), resulting in a δ<sup>13</sup>C-DIC value of around ~ -13‰, but further modified depending on the nature of the carbonate system. If open (typical of unconfined aquifer conditions), further exchange with soil-derived CO<sub>2</sub> will result in δ<sup>13</sup>C-DIC values <-13‰. If closed (typical of confined conditions), δ<sup>13</sup>C-DIC will start to evolve towards the rock composition, resulting in δ<sup>13</sup>C-DIC values rising progressively above -13‰. The compositions in the Corallian groundwaters range between -18.0 and -12.0‰ with an average of -15‰. Most of the sites are therefore relatively depleted in δ<sup>13</sup>C, which suggests a relatively immature DIC system of generally limited residence time. This conclusion supports the δ<sup>18</sup>O and δ<sup>2</sup>H results insofar as they suggest no significant modification (i.e. mixing with old groundwater) since recharge. However the most δ<sup>13</sup>C enriched sites tend to be close to the valley floor and probably signify slightly older waters. (Figure 54)		Values of δ<sup>13</sup>C are an index of the evolution of the dissolved inorganic carbon system (DIC) (Clark and Fritz, 1997<ref name="Clark 1997">CLARK, I, and FRITZ, P. 1997. ''Environmental Isotopes in Hydrogeology''. (Boca Raton, USA: Lewise Publishers.)</ref>; Darling et al., 2005<ref name="Darling 2003"></ref>). Groundwater in calcareous sedimentary terrain acquires DIC through reaction with soil CO<sub>2</sub> and reaction with carbonate minerals in the soil and aquifer. The δ<sup>13</sup>C composition of DIC is fundamentally governed by interaction between soil CO<sub>2</sub> (δ<sup>13</sup>C ~ -26‰) and carbonate minerals (δ<sup>13</sup>C ~ 0‰), resulting in a δ<sup>13</sup>C-DIC value of around ~ -13‰, but further modified depending on the nature of the carbonate system. If open (typical of unconfined aquifer conditions), further exchange with soil-derived CO<sub>2</sub> will result in δ<sup>13</sup>C-DIC values <-13‰. If closed (typical of confined conditions), δ<sup>13</sup>C-DIC will start to evolve towards the rock composition, resulting in δ<sup>13</sup>C-DIC values rising progressively above -13‰. The compositions in the Corallian groundwaters range between -18.0 and -12.0‰ with an average of -15‰. Most of the sites are therefore relatively depleted in δ<sup>13</sup>C, which suggests a relatively immature DIC system of generally limited residence time. This conclusion supports the δ<sup>18</sup>O and δ<sup>2</sup>H results insofar as they suggest no significant modification (i.e. mixing with old groundwater) since recharge. However the most δ<sup>13</sup>C enriched sites tend to be close to the valley floor and probably signify slightly older waters. (Figure 54)

	[[Image:~~OR15048 fig51~~.jpg\|thumb\|center\| 400px\| '''Figure 51''' O and H stable isotopic composition of the Corallian aquifer. WMWL = world meteroric water line (Craig, 1961<ref name="Craig 1961"> CRAIG, H. 1961. Isotopic variations in natural waters. ''Science'', Vol. 133, 1702–1703 </ref>).]]		[[Image:OR15048fig51.jpg\|thumb\|center\| 400px\| '''Figure 51''' O and H stable isotopic composition of the Corallian aquifer. WMWL = world meteroric water line (Craig, 1961<ref name="Craig 1961"> CRAIG, H. 1961. Isotopic variations in natural waters. ''Science'', Vol. 133, 1702–1703 </ref>).]]

	==Chemical variations with depth==		==Chemical variations with depth==
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	In many cases the major ions follow similar trends over the period they were measured. For example at one site about 2km south-east of Helmsley [SE 63 82], Ca, Cl, and Mg all increase gradually, but steadily, over a period of 10 years (Figure 55). At the same site this is mirrored by a general decrease in the HCO<sub>3</sub> concentration (Figure 56) over the same time period. There is no obvious reason for these changes.		In many cases the major ions follow similar trends over the period they were measured. For example at one site about 2km south-east of Helmsley [SE 63 82], Ca, Cl, and Mg all increase gradually, but steadily, over a period of 10 years (Figure 55). At the same site this is mirrored by a general decrease in the HCO<sub>3</sub> concentration (Figure 56) over the same time period. There is no obvious reason for these changes.

	[[Image:~~OR15048 fig52~~.jpg\|thumb\|center\| 400px\| '''Figure 52''' Regional variation of δ<sup>18</sup>O in the Corallian aquifer.]]		[[Image:OR15048fig52.jpg\|thumb\|center\| 400px\| '''Figure 52''' Regional variation of δ<sup>18</sup>O in the Corallian aquifer.]]
	[[Image:~~OR15048 fig53~~.jpg\|thumb\|center\| 400px\| '''Figure 53''' Regional variation of δ<sup>2</sup>H in the Corallian aquifer.]]		[[Image:OR15048fig53.jpg\|thumb\|center\| 400px\| '''Figure 53''' Regional variation of δ<sup>2</sup>H in the Corallian aquifer.]]

	At the same site near Helmsley there is an increase in NO<sub>3</sub>-N over a period of six years. This is not an unusual trend in areas dominated by agriculture, as NO<sub>3</sub>-N is a common diffuse pollutant derived from agricultural practices. At this site the concentration increases from values around the drinking water limit (11.3 mg L<sup>-1</sup> NO<sub>3</sub>-N)at an average of 1.7 mg L<sup>-1</sup> each year, to twice the drinking water limit over a period of seven years (Figure 57).		At the same site near Helmsley there is an increase in NO<sub>3</sub>-N over a period of six years. This is not an unusual trend in areas dominated by agriculture, as NO<sub>3</sub>-N is a common diffuse pollutant derived from agricultural practices. At this site the concentration increases from values around the drinking water limit (11.3 mg L<sup>-1</sup> NO<sub>3</sub>-N)at an average of 1.7 mg L<sup>-1</sup> each year, to twice the drinking water limit over a period of seven years (Figure 57).

	[[Image:~~OR15048 fig54~~.jpg\|thumb\|center\| 400px\| '''Figure 54''' Regional variation of δ<sup>13</sup>C in the Corallian aquifer.]]		[[Image:OR15048fig54.jpg\|thumb\|center\| 400px\| '''Figure 54''' Regional variation of δ<sup>13</sup>C in the Corallian aquifer.]]
	[[Image:~~OR15048 fig55~~.jpg\|thumb\|center\| 400px\| '''Figure 55''' Temporal changes in Ca, Mg, and Cl at a site near Helmsley.]]		[[Image:OR15048fig55.jpg\|thumb\|center\| 400px\| '''Figure 55''' Temporal changes in Ca, Mg, and Cl at a site near Helmsley.]]

	Another four of the EA’s monitoring sites show similar NO<sub>3</sub>-N trends, and are distributed throughout the Vale of Pickering. This finding is consistent with previous reports which document the high concentrations of NO<sub>3</sub>-N and PO<sub>4</sub><sup>3-</sup> in the region (EA, 2009<ref name="EA 2009">EA. 2009. River Basin Management Plan Humber River Basin District, Environment Agency.</ref>; Natural England, 2015<ref name="Natural 2015"> NATURAL ENGLAND. 2015. National Character Area Profile: 26. Vale of Pickering www.gov.uk/natural-england. </ref>).		Another four of the EA’s monitoring sites show similar NO<sub>3</sub>-N trends, and are distributed throughout the Vale of Pickering. This finding is consistent with previous reports which document the high concentrations of NO<sub>3</sub>-N and PO<sub>4</sub><sup>3-</sup> in the region (EA, 2009<ref name="EA 2009">EA. 2009. River Basin Management Plan Humber River Basin District, Environment Agency.</ref>; Natural England, 2015<ref name="Natural 2015"> NATURAL ENGLAND. 2015. National Character Area Profile: 26. Vale of Pickering www.gov.uk/natural-england. </ref>).
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	A more unusual temporal variation was observed in groundwaters at a site near Kirbymoorside [SE 70 87]. Concentrations of the major elements were relatively constant between 1995 and 1998, most elements increased until 1999, where they stayed relatively constant, with the exception of three lower values recorded on 9/11/99, 17/10/03 and 20/10/05. The exceptions to this are Na and K, which mirror this trend (Figure 58 and Figure 59). There is no clear reason for this trend.		A more unusual temporal variation was observed in groundwaters at a site near Kirbymoorside [SE 70 87]. Concentrations of the major elements were relatively constant between 1995 and 1998, most elements increased until 1999, where they stayed relatively constant, with the exception of three lower values recorded on 9/11/99, 17/10/03 and 20/10/05. The exceptions to this are Na and K, which mirror this trend (Figure 58 and Figure 59). There is no clear reason for this trend.

	[[Image:~~OR15048 fig56~~.jpg\|thumb\|center\| 400px\| '''Figure 56''' Temporal changes in HCO<sub>3</sub> at a site near Helmsley.]]		[[Image:OR15048fig56.jpg\|thumb\|center\| 400px\| '''Figure 56''' Temporal changes in HCO<sub>3</sub> at a site near Helmsley.]]
	[[Image:~~OR15048 fig57~~.jpg\|thumb\|center\| 400px\| '''Figure 57''' Temporal changes in NO<sub>3</sub>–N at a site near Helmsley.]]		[[Image:OR15048fig57.jpg\|thumb\|center\| 400px\| '''Figure 57''' Temporal changes in NO<sub>3</sub>–N at a site near Helmsley.]]
	[[Image:~~OR15048 fig58~~.jpg\|thumb\|center\| 400px\| '''Figure 58''' Temporal changes in Ca, Cl, SO<sub>4</sub>, HCO<sub>3</sub> at a site near Kirkbymoorside.]]		[[Image:OR15048fig58.jpg\|thumb\|center\| 400px\| '''Figure 58''' Temporal changes in Ca, Cl, SO<sub>4</sub>, HCO<sub>3</sub> at a site near Kirkbymoorside.]]
	[[Image:~~OR15048 fig59~~.jpg\|thumb\|center\| 400px\| '''Figure 59''' Temporal changes in Mg, Na, K and NO<sub>3</sub>–N at a site near Kirkbymoorside.]]		[[Image:OR15048fig59.jpg\|thumb\|center\| 400px\| '''Figure 59''' Temporal changes in Mg, Na, K and NO<sub>3</sub>–N at a site near Kirkbymoorside.]]

	==References==		==References==

	[[category:OR/15/048 Baseline groundwater chemistry: the Corallian of the Vale of Pickering, Yorkshire \| 08]]		[[category:OR/15/048 Baseline groundwater chemistry: the Corallian of the Vale of Pickering, Yorkshire \| 08]]