OR/18/011 UK aquifers

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Stuart, M E. 2018. Review of denitrification potential in groundwater of England. British Geological Survey Internal Report, OR/18/011.

Literature[edit]

Foster et al. (1985)[1] set out some of the first collated evidence for biological denitrification in British aquifers. This was related to the Lincolnshire Limestone near Lincoln and the Chalk of Norfolk and was based on penetration of modern tritium-containing groundwater to depths where NO3 is negligible and the presence of bacterial consortia with denitrifying capability to depth. Denitrification has been demonstrated in other major aquifers in England, such as the Sherwood Sandstone where these become confined or very deep and there is a clear redox boundary (Edmunds et al., 1982[2]; Smedley and Edmunds, 2002[3]). There are also other more complex settings, where denitrification has been invoked to explain observations, such as zones of aquifers constrained by important marl bands. Other settings include floodplains and the hyporheic zone.

Approaches to the estimation of denitrification in the aquifers of the UK have been reviewed by Rivett et al. (2007)[4]. The text below has been predominantly summarised from this review. These studies indicate that the potential for denitrification in the unsaturated and unconfined saturated zones of major British aquifers is low.

Unsaturated zone[edit]

Chalk[edit]

Mass balance investigations to track the progress of a packet of water through the unsaturated zone was used by Kinniburgh et al. (1999)[5] on an extensive set of repeated drillings undertaken at five sites in East Anglia (Parker et al., 1991a[6]). Although the approach was of limited value in quantifying small amounts of denitrification, it could be concluded that, at the scale of metres to tens of metres, denitrification was probably insignificant in the unsaturated zone of the Chalk.

Gale et al. (1994)[7] examined evidence for denitrification in core profiles at Ogbourne St. George on the Wiltshire Chalk. The site exhibited a NO3 front moving downward at about 0.8 m/year with input NO3 peaks smoothed by the large seasonal water table fluctuations of about 20 m. Nitrate, DO and OC were all thought to be replenished annually by rapid fissure flow to the water table. Nitrate, N2O and excess N2 were estimated to represent, at most, a few per cent decrease in the NO3 load. Supporting microbiological studies showed that denitrifiers were present at all depths. Laboratory microcosms using core indicated a 2% NO3 to NO2 conversion over 21 days.

More recent studies of gases in the Chalk at Bridget’s Farm, Hampshire, used carbon isotope evidence to indicate that most of the unsaturated zone CO2 was from bacterial breakdown of organic matter in overlying soils (Kinniburgh et al., 1999[5]). Although some N2O was detected the source could have been the soil zone or nitrification of NH4 (Buss et al., 2004[8]). Marginally elevated N2/Ar ratios indicating an excess N2 of about 0.5% were indicative of denitrification, but accounted for a decrease in NO3 of 0.4 mg N/L, compared with the mean unsaturated zone concentration of 26 mg N/L. Slightly negative values of δ15N-N2 supported the above, implying minor denitrification at rates that were not quantifiable. Examination of the δ15N composition of infiltrating NO3 relative to the underlying groundwater NO3 similarly confirmed that denitrification was not a significant process in the unsaturated zone.

Sherwood sandstone[edit]

The most significant NO3 attenuation study on the Sherwood Sandstone unsaturated zone was undertaken at Gleadthorpe, Nottinghamshire (Kinniburgh et al., 1999[5]). The unsaturated zone thickness was 8–12 m and influenced by nearby public water supply abstractions. Similar research methods were employed as at the Bridget’s Farm site. Denitrification was not found to be a significant process overall, although there was some evidence of its occurrence, including the following:

  • Nitrate depletion just beneath the water table in one borehole, where NO3 concentrations declined with depth whereas nitrite increased;
  • In one near-surface sample, δ15N-N2 and N2/Ar data were consistent with minor denitrification;
  • Denitrifying bacteria were found at all depths, indicating the potential for denitrification;
  • N2O was above atmospheric concentrations, though it was unclear whether this was due to soil-zone denitrification or oxidation of ammonium: sufficient (small) quantities of ammonium were present throughout.

The N2/Ar data suggested that denitrification accounted for a maximum decrease of 0.8 mg N/L compared with an average unsaturated zone concentration of 37 mg N/L. Kinniburgh et al. (1999)[5] concluded that denitrification is limited in the sandstone unsaturated zone as a result of low supplies of labile organic carbon, leading to low rates of microbial activity and little chance of the development of anaerobic hotspots.

Saturated zone[edit]

Chalk[edit]

Circumstantial evidence for denitrification within the drift and Chalk aquifer system of Norfolk, UK was reported by Parker and James (1985). Results from core analyses showed that tritium had penetrated to the base of the overlying 30 m of glacial sand (19–58 TU) and into the upper levels of the Chalk (9–39 TU). This confirmed that there was at least a component of modern water in the Chalk, which in the absence of NO3 (<0.2 mg N/L) in the Chalk groundwater, was evidence for denitrification. A bacteriological survey in the area revealed potential denitrifiers in cores from the top of the Chalk. Regular analysis of pumped water from the Chalk showed that the mobile fissure water contained less than 2 mg/L DOC. Preliminary investigations suggested that the pore water contained 10–20 mg/L DOC) and that the rock matrix perhaps greater than 1000 mg of organic carbon per kg dry weight. If correct, to obtain a porewater DOC of 30 mg/L for Chalk with a porosity of 40% would have required the dissolution of about 1% of the matrix. However, Bishop and Lloyd (1990) used isotope and hydrochemical modelling to suggest that substantial amounts of NO3 reduction had not occurred and that the low NO3 concentrations in the confined part of the aquifer were predominantly the result of lower NO3 inputs in the past.

Mühlherr and Hiscock (1997)[9] measured N2O and other nitrate species in samples collected from boreholes and springs in the unconfined chalk aquifer of Cambridgeshire in an area of intensive arable farming. A very good positive correlation between N2O and NO3 concentrations was obtained but no relationship was established between N2O and NO2 or NH4 concentrations. Concentrations of N2O and NO3 increased continuously in the direction of groundwater flow, within the range reported in previous studies of nitrification. Corresponding DO concentrations in groundwater samples were moderately undersaturated, further indicating that the main source of N2O in this area is probably nitrification. No consumption of N2O seemed to take place within the unconfined aquifer with degassing to the atmosphere apparently being the sole mechanism for N2O removal from groundwater.

Sherwood Sandstone[edit]

Although NO3 concentrations are commonly low where the sandstones are confined, there is little direct evidence of denitrification. The south Yorkshire Sherwood Sandstone aquifer is variably covered by drift deposits. Many of the large water abstractions are located on sandstone ‘islands’ that protrude through the drift). A key study showed that deep (50–100 m) penetration of NO3 in these unconfined portions contrasted with immediately adjacent areas confined beneath low- permeability superficial deposits that contained near undetectable concentrations of NO3 throughout the water column (Parker et al., 1985). It is significant that an abrupt change in NO3 concentration has been maintained at the confined–unconfined boundary, despite the presence of major abstractions in the confined zone. Denitrification, although not definitively proven at the site, was considered to be probable (Parker et al., 1991b[10]).

The potential for denitrification was also explored in the Nottinghamshire sandstone, mainly differentiated by less drift cover than the south Yorkshire counterpart and more oxygenated groundwater (Wilson et al., 1990[11]). Widespread denitrification was not observed in the aquifer except in two isolated locations, where it was ascribed to relatively low DO concentrations being present.

Lincolnshire Limestone[edit]

Foster et al. (1985)[1] provided NO3, tritium and bacterial activity data for cores taken from the Lincolnshire Limestone that supported denitrification, e.g., penetration of thermonuclear tritium further down-dip from outcrop than both NO3 and DO. Wilson et al. (1990)[11] also presented evidence for denitrification that included concentrations of excess nitrogen and isotopic ratios.

Bottrell et al. (2000) more recently explored the control exerted on bacterial reduction reactions by the dual-porosity characteristics of the Lincolnshire Limestone. By analogy with SO4- reducing reactions in the aquifer, they concluded that the potential for denitrification in the shallow confined zone is poor. Reaction rates were limited by lack of an electron donor in the fissures; were NO3 to diffuse into the pores where organic carbon and sulphides were present, NO3-reducing bacteria would be excluded by the narrow pore throats of the matrix (similar to the Chalk).

Roberts and McArthur (1998)[12] highlighted the importance of greater DOC inputs to the Lincolnshire Limestone aquifer in unconfined areas. Where the River Glen crossed the aquifer it recharged the aquifer year-round because of drawdown due to the proximity of large public water supplies. In winter, NO3-rich surface waters generated by rainfall-runoff recharge the aquifer, led to the development of NO3 plumes emanating from rivers and swallow holes. In summer, runoff was negligible and groundwater baseflow was low, so the rivers comprised mostly DOC-rich effluent from small sewage treatment works: this then recharged into the aquifer. Denitrification was significant in the anaerobic groundwaters thus created but the contribution to denitrification from reduction of in-situ sulphide, appeared greater than that from DOC.

Mühlherr and Hiscock (1998)[13] measured N2O and other N species in groundwater from important limestone aquifers in the UK. Nitrous oxide levels were generally very high, with concentrations exceeding the concentration of air-equilibrated water by up to 320 times. The correlations between N2O, NO3 and DO, as well as the spatial distribution of these chemical parameters, were used to identify nitrification as the main N2O production mechanism in the investigated aquifers. Most of the N2O in groundwater appeared to be produced via nitrification in the unsaturated zone, which was probably strongly supplemented by nitrogenous compounds from anthropogenic land applications. Nitrous oxide production in the saturated zone was less substantial and could have also been denitrification mediated; denitrification under very reducing aquifer conditions can result in nitrous oxide consumption. The observed high N2O concentrations in groundwater, which were most likely caused by agricultural aquifer pollution.

Redox boundaries[edit]

Evidence of denitrification comes from the confined (anoxic) parts of some British aquifers from the work of Edmunds and others. This has also been confirmed by the occurrence of modern tritium-enriched groundwater with negligible NO3 (Lawrence and Foster, 1986[14]) and from the demonstration of appropriate bacteria in core samples.

Sequential redox processes have been described in three contrasting principal UK aquifers that have confined zones (Edmunds and Walton, 1983[15]; Edmunds et al., 1982[2]; Edmunds et al., 1984[16]

  • The Berkshire Chalk on the western rim of the London Basin has an average thickness of 250 m. It dips to the east, is confined below Palaeogene sands and clays in the centre of the syncline and is underlain by the Upper Greensand and then the Gault Formation that serves as the basal aquiclude. It is a dual-porosity aquifer with moderate transmissivity in this area (270–450 m2/day) decreasing with depth.
  • The Jurassic Lincolnshire limestone is some 30 m thick at outcrop thinning down dip to the east. It is confined above and below by predominantly clay rich sequence of the Rutland and Grantham Formations and is artesian at depth. It showed a clear sequence of hydrochemical changes from a sharp redox boundary some 12 km from outcrop. The unaltered limestone at depth was grey due to fine-grained Fe sulphides but diagenesis by migrating groundwater has progressively oxidised the rock to a light brown. Groundwater movement is maintained by fissure flow and transmissivities are in excess of 1000 m2/day.
  • Hydrochemical processes have been studied along a downgradient profile of some 30 km in the Sherwood Sandstone. It is confined by the Mercia Mudstone and underlain by generally impermeable Permian sequences. It dips to the east at about 1 in 50 and varies in thickness between 120 m in the south to 300 m in the north. It has a typical redbed lithology devoid of reducing horizons. Groundwater movement is predominantly intergranular, but is enhanced by fissures. Transmissivities in the confined zone vary between 350 and 850 m2/day (Edmunds et al., 1982[2]).

At the time of reporting all three aquifers had high input levels of anthropogenic NO3. The occurrence of NO3-free groundwater was coincident with the redox boundary and with the complete reaction of DO in all three. Edmunds and Walton (1983)[15] followed several indicators of agricultural pollution (Ca, K, SO4 and NO3) across the redox boundary in the Lincolnshire Limestone. The migration eastwards of Ca, K and SO4 but not NO3 provided circumstantial evidence of denitrification. Ammonia was found only in the NO3-free zones in all three aquifers, produced by DNRA or to the accumulation of NH4+ by incongruent reaction of clays.

Water quality in the Lincolnshire Limestone has been interpreted in a number of ways. Peach (1984) found that in the northern Lincolnshire Limestone, modern recharge appeared not to flow down dip to a large extent, as there was little evidence for high NO3, SO4, Cl or Ca groundwater downgradient. In the north, Peach described three ages of water with different origins:

  1. Relatively modern recharge water with leakage of high-Na water from the overlying Rutland Formation (Thorncroft Sand);
  2. Old recharge water, probably post-last glaciation to present;
  3. Very old saline water, probably emplaced during Rutland Formation times.

Edmunds (1973, 1976) studied hydrochemical variations along a 28-km flow line in the southern Lincolnshire Limestone in 1969 and found solution, redox and ion-exchange reactions, SO4 reduction and mixing with saline formation water to all be important processes within the aquifer. Edmunds and Walton (1983) re-sampled the same aquifer profile after 10 years to look at hydrogeochemical evolution with time and found that the overall sequence was remarkably similar in both distance from outcrop and absolute concentration but that, close to the redox boundary, down-gradient increases in Ca, SO4 Cl and NO3 had occurred, together with a less abrupt redox boundary. Edmunds and Walton (1983)[15] postulated that these changes were initiated by the migration of agrichemical pollutants and that the results indicate that the aquifer has a considerable capacity for in-situ NO3 reduction.

Howard (1985)[17] defines water type in his study of the Lincolnshire Chalk by origin and his confined water represents old low-salinity water that replenished the aquifer when groundwater levels began to recover after the Devensian glacial episode. This has low N but does not necessarily represent denitrification as initial N would have depended on vegetation at the time of recharge. Some of this reducing water contained NH4.

Table 4.2 summarises the range of values measures for key parameters in redox zones of the three main UK aquifers. These demonstrate a consistent pattern with the anaerobic zone characterised by DO <0.4 mg/L, NO3 generally <1, Fe >100 µg/L Mn>4 µg/L. Edmunds and Elliot also suggest the use the decline of DO to indicate reducing conditions. Smedley and Edmunds (2002)[18] however use temperature (in this case 13°C) as an indicator of confinement.

Table 4.1    Ranges of literature values of chemical characteristics of
redox zones identified in UK aquifers (all in mg/L except Fe and Mn (µg/L) and Eh (mV)).
Lithology Area Zone 1 (Aerobic) Zone 2 (Intermediate) Zone 3 (Anaerobic) Reference
Chalk Berkshire DO = 4–10
NO3 = 20–30
Eh = +350–00
NH4 = <0.01
Fe = <0.3
Mn = <0.03
DO = <LOD
NO3 = <LOD
Eh = 0–+50
NH4 = 0.05–0.85
Fe = 7–170
Mn = 4–57
Edmunds et al. (1984)[16]
Lincolnshire NO3 = 26–77
Eh = >+300
NO3 = <4
Eh = --50–+100
Howard (1985)[17]
London Basin NO3 = 2.5–51 NO3 = 2.5 NO3= 0.04 Elliot et al. (1999)[19]
Sherwood Sandstone East Midlands DO = 2.5–10
NO3 = 6.8–54
Eh = +127–336
Fe = <4–236
Mn = <0.5–21
DO = <0.5–8.7
NO3 = 0.02–6.7
Eh = -7–+406
Fe = 6–372
Mn = 1.5–6
DO = <0.1
NO3 = 0.04–1.15
Eh = -5-0–+78
Fe = 115–1970
Mn=5.5–37
Edmunds et al. (1982)[2]
DO = 2–10
NO3 = 3–54
Eh = +250–400
Fe = <15 µg/L
DO = <LOD
NO3 = <LOD
Eh = 0–+100
Fe=130–2000
Edmunds et al. (1984)[16]
DO = 0.5–10.1
NO3 = <0.04–67
Eh = +116–485
DO = <0.4
NO3 = <0.4–2.1
Eh =- 99–+183
Smedley and Edmunds (2002)[18]
Lincs. limestone Lincolnshire 1969 DO = 1.5–7
NO3 = 6–40
Eh = +400
Fe = <LOD
DO = <LOD
NO3 = <LOD
Eh = 0–+100
Fe = 100–1800
Edmunds et al. (1984)[16]
Lincolnshire 1983 DO = 0.5–7
NO3 = 10–62
Eh = +400
Fe = <LOD
DO = <LOD
NO3 = <LOD
Eh = 0-+100
Fe = 30–2800
Edmunds et al. (1984)[16];
Edmunds and Walton (1983)[15]
Lincolnshire NO3 = 25 NO3 = <5 Lawrence and Foster (1986)[14]

Deeper systems[edit]

In deeper aquifer systems, input of organic carbon from the soil reservoir is not important, and so the likely source of degradable organic carbon is the geologic material comprising the aquifer matrix (Hiscock et al., 1991[20]).

An example of denitrification at depth in a confined aquifer is described by Lawrence and Foster (1986)[14] for the Lincolnshire Limestone in the UK. In this fissured limestone aquifer, an eastward decline in NO3 concentration accompanies progressive removal of DO and lowering of the redox potential. Over a distance of 10 km the groundwater NO3 concentration declines from a maximum of 25 mg N/I at outcrop to less than 5 mg N/I when confined, representing a NO3 removal rate of 0.01–0.05 mg N/l/d (expressed as NO3 loss per rate of flow of groundwater through the aquifer). Four cored boreholes were drilled along a groundwater flow line and the material recovered examined for organic carbon content and the presence of denitrifying bacteria. The results showed that the DOC content of the pore water ranges between 13 and 28 mg/1 and that of the mobile groundwater between 1.6 and 3.4 mg/l. Denitrifying bacteria were cultured from samples scraped from fissure walls, but not from samples incubated with pore water. Thus, it appears that the source of organic carbon supporting denitrification is contained in the limestone matrix, and that the very small pore size of the matrix restricts denitrification to short distances from fissure walls.

Floodplains[edit]

Gooddy et al. (2014)[21] investigated a peri-urban floodplain of the River Thames adjoining the city of Oxford, UK, over a period of three years through repeated sampling of groundwaters from existing and specially constructed piezometers. An intensive study of nitrogen dynamics through the use of N-species chemistry, nitrogen isotopes and dissolved nitrous oxide reveals that there is little or no denitrification in the majority of a landfill plume, and neither is the ammonium significantly retarded by sorption to the aquifer sediments.

Baseline data[edit]

Baseline data for NO3 concentrations in water for England and Wales have been assumed to demonstrate denitrification where NO3 concentrations were below the limit of detection (Shand et al., 2007[22]). These authors also assess the presence of NO2 as a good indicator of on-going denitrification.

Figure 4.1 shows that the redox potential (Eh) and DO concentration provide the primary indicators of the redox status of natural groundwaters in the Lincolnshire Limestone (Griffiths et al., 2006[23]). As the aquifer becomes increasingly confined, DO becomes depleted due to redox reactions. Once contact with O2 ceases, Eh falls by ca. 300 mV. This marked redox boundary has considerable implications for the subsequent down-gradient hydrogeochemisty. On the basis of the samples collected as part of the Baseline project the initial Eh values appear to have been lower (+100 to +300 mV) and the reduction in Eh with distance down gradient less marked. The change in redox conditions is more obviously seen in the DO concentrations that were reduced to less than detection (<0.1 mg/L) within a couple of kilometres of the aquifer becoming confined.

The decrease in NO3 concentrations down the hydraulic gradient (Figure 4.1) was consistent with denitrification: NO3 is only stable in the presence of DO. Although old (pre-intensive agriculture) groundwaters were expected to have lower concentrations, it is likely that recharge waters contained detectable NO3. Edmunds and Walton (1983) observed that NO3 levels decreased gradually due to dispersion and in situ microbial reduction and became consistently low (<1 mg/L as NO3-N) once DO was below 0.2 mg/L as NO3-N (or even less).

Figure 4.1    Variation in redox sensitive parameters and species across the Lincolnshire Limestone aquifer. Samples plotted across down-dip sections in the north (red) centre (green) and south (blue) of the study area to illustrate geochemical evolution as groundwater moves downgradient from the outcrop area (from Griffiths et al. (2006)[23]). The corresponding edge of the outcrop area for each section across the aquifer is represented as a dashed line.

Table 4.2 sets out data from the confined and unconfined zones where these have been separately tabulated within the Baseline project. For the Chalk and the limestone, the differences in NO3, Fe and Mn are clear and are in line with suggested indicator values for low redox zones. The Lower Greensand is low in Mn and elevated concentrations are not seen. Denitrification is identified in many of the other Baselines reports and these are discussed below.

Table 4.2    Comparison of median data from unconfined and confined zones from baseline data (N species as N).
Area

Unconfined

Confined

Reference
NO3-N (mg/L) NO2-N (mg/L) Fe (µg/L) Mn (µg/L) NO3-N (mg/L) NO2-N (mg/L) Fe (µg/L) Mn (µg/L)
Yorkshire Humber Chalk 8.81 <0.01 <30 <10 <0.5 <0.01 2270 100 Smedley et al. (2004)[24]
Colne-Lee Chalk 5.9 0.002 15 5 0.1 0.002 140 6 Shand et al. (2003b)[25]
North Downs Chalk 6.4 <0.005 <30 <10 <0.3 <0.005 191 19.6 Smedley et al. (2003)[26]
Lincolnshire Limestone 11.5 0.002 <30 <50 <0.003 0.0008 340 <50 Griffiths et al. (2006)[23]
Lower Greensand–
Folkestone Formation
4.23 0.004 65 33 0.02 0.5 185 11 Shand et al. (2003a)[27]

Chalk[edit]

In the Chalk aquifer of the North Downs NO3-N concentrations in groundwaters from the unconfined aquifer often reach close to or in excess of the EC maximum permissible value for drinking water of 11.3 mg/L (up to 28 mg/L observed) (Smedley et al., 2003[26]). In the confined aquifer, the reducing conditions observed produce groundwaters with low NO3 concentrations, either due to denitrification or to the presence of pre-modern (unpolluted) recharge. The relatively high concentrations of NH4-N observed in the confined aquifer (up to 4.4 mg/L) are a relatively common feature of groundwaters from reducing aquifers and were considered to be naturally-derived, rather than as a result of local pollution. In the Colne-Lee Chalk, the most dramatic changes in water quality occur where the Chalk becomes confined beneath Palaeogene sediments coinciding approximately with the redox boundary in the aquifer (Shand et al., 2003b [25]). This is partly due to the development of a redox boundary, situated close the boundary with overlying Palaeogene rocks. Oxygen decreases to less than detection limit in the confined aquifer and consequently NO3 is removed from the system by denitrification or NO3 reduction and Fe and Mn increase. In the Chalk of Dorset NO3 concentrations were uniformly high across the unconfined aquifer, even at deeper sites Edmunds et al. (2002)[3]. Deeper groundwater is probably slow moving Holocene recharge separated from the main groundwater circulation of the present day. To the east of the unconfined/confined contact groundwaters were reducing.

Other areas of the Chalk have similar water quality. In the Chalk of Yorkshire the presence of NO3 at concentrations greater than the EC maximum permissible value (11.3 mg/L) in some groundwater sources is one of the most significant problems (Smedley et al., 2004[24]). Further east where the Chalk is covered by thick Superficial deposits, the aquifer becomes confined. Here, redox processes exert an important influence, with the redox boundary marking a zone of significant change in the concentrations of a number of elements. In the semi-confined dip-slope groundwaters of the Beverley–Driffield area, where superficial cover is largely arenaceous, NO3-N concentrations were generally in the range 2–10 mg/L. Concentrations in the confined aquifer were low (usually <1 mg/Land often much lower), either as a result of denitrification or recharge of the groundwaters at a time before the use of modern agricultural chemicals. Since the NO3-N concentrations in pre-pollution recharge are likely to have been of the order of 3–4 mg/L this implies that some degree of denitrification has taken place in the confined aquifer.

In the Chalk of north Norfolk NO3-N concentrations vary across the aquifer, with lowest concentrations in boreholes beneath the Palaeogene cover and highest in the unconfined coastal aquifer, and shows an inverse relationship with Fe (Smedley et al., 2004[24]). Both Fe and Mn would be expected to be ubiquitous in the aquifer, with the Mn being released from the calcite phase during re-precipitation. Ammonium (NH4+) concentrations were elevated in several of the samples associated with reducing conditions, and were probably derived from clays The low redox potential, and thus NO3-N concentrations found in the Waveney catchment may reflect the protective function of the more argillaceous nature of the till that is dominant in this area, compared to North Norfolk. The groundwaters of the Crag have varying redox status across the outcrop. Many samples in the Waveney catchment did not contain DO, whilst in the North they were generally oxidising. Concentrations of Fe generally exceeding 2 mg/Lin the Waveney catchment while concentrations of both Mn and Fe were generally low in the North Norfolk region.

In contrast, Ander et al. (2004) only found one low NO3 sample associated with low DO, and high Mn and Fe in the Chalk of central East Anglia and Stuart and Smedley (2009)[28] found the Chalk aquifer of Hampshire to have elevated NO3 concentrations and low concentrations of Fe, Mn, NO2 and NH4.

In summary, a pattern of reducing conditions in the Chalk is found where the aquifer is confined by overlying Palaeogene strata or by low permeability argillaceous superficial deposits.

Permo-triassic sandstone[edit]

For the Sherwood Sandstone Shand et al. (2002)[29] describe the presence of ‘windows’ in the superficial deposits in the Vale of York, where NO3-rich recharge occurs. Beneath the impermeable superficial deposits, redox boundaries have developed allowing NO3 reduction to occur. The redox control is also manifested by increases of Fe, Mn and NO2 along flow lines beneath the clays. Griffiths et al. (2003a)[30] reported that little denitrification was occurring in the sandstones of Cheshire at Mickledale but that low NO3 found in some samples in the survey may indicate that some denitrification or NO3 reduction has occurred. In Staffordshire Tyler-Whittle et al. (2002)[31] found that oxidising conditions extended up to depths of 380 m and concentrations of NO3 were high throughout most of the region. Reducing conditions were present where the sandstone is confined beneath the Mercia Mudstone, to the southeast and at isolated sites within the outcrop due to the presence of less permeable horizons within the aquifer. The reduced nitrogen species NO2 and NH4 were only present in the confined reduced groundwater, taken to indicate that denitrification and/or NO3 reduction has occurred. The decrease in NO3 concentrations at depth in the unconfined aquifer was thought to represent baseline conditions not reduced water. In the Manchester area, the presence of thick impermeable drift over much of the aquifer has allowed reducing conditions to exist even at shallow depths. This has given rise to high dissolved Fe, and NO3 concentrations were very low due to denitrification (Griffiths et al., 2003b[32]).

For the Liverpool area, Griffiths et al. (2005)[33] found that high NO3 concentrations were often associated with permeable drift, especially in some of the shallower boreholes, or unconfined conditions. High concentrations were also found in areas covered by impermeable drift deposits but these tend to be located along river valleys where drift thicknesses may be less due to erosion and local drift windows. NO3 concentrations were higher, as expected, in oxidising waters with denitrification probably creating low concentrations where the groundwaters were reducing. For Shropshire (Smedley et al., 2005) considered that the limited occurrence of observed reducing groundwaters suggested that either such conditions are of localised extent or that they have not been developed significantly for groundwater use. The reducing groundwaters were apparently distributed sporadically across the aquifer, related to the distribution of superficial deposits. Bearcock and Smedley (2012)[34] do not report evidence of denitrification in the Otter Sandstone of Devon and Somerset.

In summary, a pattern of reducing conditions in the Permo-Triassic Sandstone is found where the aquifer is confined by overlying Mercia Mudstone Group strata or by low permeability argillaceous superficial deposits.

Other limestones[edit]

The Jurassic contains a number of limestone-bearing strata that provide significant aquifers. In the Corallian of Oxfordshire and Wiltshire there is a large range of NO3 concentrations (Cobbing et al., 2004[35]). The highest were from unconfined sources with shallow groundwater the most seriously affected. However the median was low at 0.25 mg/L (NO3-N), reflecting the reducing conditions in many of the groundwaters. The highest Fe concentrations were found in the circumneutral low-Eh reducing groundwaters. Manganese is more soluble over a wider range of pH-Eh conditions than Fe. In the Corallian of the Vale of Pickering, most of the groundwaters sampled had NO3 concentrations consistent with oxidising conditions, although the lowest suggest that conditions were reducing in some (Bearcock et al., 2015[36]). Iron has a wide range of concentrations similar to the Cotswolds Oolites, but less than the Corallian of Oxfordshire and Wiltshire.

In the Great and Inferior Oolite of the Cotswolds the majority of unconfined groundwaters, and in particular, spring waters in the region were affected by high NO3 concentrations (Neumann et al., 2003[37]). In confined groundwaters, where DO is reduced or absent, NO3 concentrations were lower indicating that in-situ denitrification is likely to have occurred. However, some of these waters may even be of sufficient age to represent pre-modern water. In the Great Oolite limestone, the onset of reducing conditions at depths occurs before the aquifer becomes confined. Within the Inferior Oolite aquifer, all unconfined groundwaters appear to be oxidising. The redox boundary lies within the confined part of the Inferior Oolite aquifer. All groundwater samples obtained from the confined Inferior Oolite show reducing conditions the redox-boundary occurs some 7–8 km down gradient of nominal confinement. Iron concentrations increase in groundwaters beyond the redox boundary, reaching up to 1260 μg /L towards the end of the sampled flow line. Manganese concentrations are also high in some confined groundwater samples. Sulphate reduction is believed to have occurred in two boreholes drawing water from the confined limestone aquifers.

In the Lincolnshire Limestone NO3-N concentrations display a particularly wide range from below detection limit to 20.7 mg/L which reflects the high NO3 concentrations of the unconfined aquifer and low concentrations within the confined aquifer to the east (Griffiths et al., 2006[23]). However, low NO3 concentrations in the confined part of the aquifer were considered to be predominantly the result of lower NO3 inputs in the past.

In the Permian Magnesian Limestone of County Durham and north Yorkshire (Bearcock and Smedley, 2009[38]), the complex lithology and variable chemistry of the strata mean that sites with groundwaters of a very different chemistry occur in close proximity. The most oxidised waters were found in the east of the aquifer, and there is a general north-south trend, with groundwaters becoming more reducing towards the south. The NO3-N has the largest concentration range, which can be related to the large variation in redox conditions, the population of low NO3-N concentrations could represent NO3 lost in reducing conditions through denitrification. Concentrations of Fe, Mn and SO4 were particularly variable. Of those investigated, the element most frequently exceeding the drinking-water limit was Mn (28% of samples in the dataset had concentrations exceeding 50 μg/L and concentrations ranged up to 1300 μg/L).

In the Carboniferous Limestone of northern England concentrations of NO3 were generally low (Abesser et al., 2005b[39]). Some groundwaters clearly show enhanced levels particularly shallow groundwaters (<40 m) and springs, but even in the deeper parts of the aquifer. Because many of the waters were relatively reducing, denitrification or DNRA may have lowered the concentrations. Nitrite and in particular NH4-N concentrations above the EU MAC of 0.03 mg/Land 0.38 mg/L occur more commonly, although median values for the aquifer were low. Oxygen depletion and low redox conditions were typically associated with deeper groundwaters that frequently show increased Fe and Mn concentrations. However, concentrations in excess of the drinking water limits of 250 µg/L for Fe and 50 µg/L for Mn only occur where DO levels <2 mg/L and Eh <350 mV provide favourable conditions for reductive oxide dissolution. Similarly, NH4-N concentrations in excess of 0.5 mg/L were found in deeper, mostly reducing (Eh <350 mV) groundwaters. The NO3-N levels of these high NH4-N groundwaters were generally low, signifying that DNRA may take place. The distribution of redox status in the groundwaters of the study area is complex, due to the variations in the occurrence of impermeable superficial deposits as well as due to the layered nature of the aquifer where limestones provide the water-bearing strata whereas the interbedded mudstones act as aquicludes or aquitards.

In the Derbyshire Dome, NO3 is present in most groundwaters (<0.05–12.6 mg/Las N) (Abesser and Smedley, 2008[40]). Concentrations were mostly low (median 3.06 mg/L) but were generally higher in the groundwaters on the Carboniferous Limestone outcrop than on the Millstone Grit (except for one very young groundwater) as fractures in the limestone provide pathways of rapid transport to the water tables. Concentrations were generally highest in oxic groundwaters Nitrate is also detectable in most thermal waters (except for St Anne’s Well), indicating the presence of a component of recent recharge. Denitrification may have lowered NO3-N concentrations in some groundwaters, as indicated.

In summary, limestones commonly exhibit a complex distribution of zones of low redox. These do occur in confined zones although the distance of the redox boundary from the confined/unconfined boundary is variable. Distribution may also be related to the distribution of superficial deposits and to layering in the aquifer. It has also been demonstrated that low redox does not necessarily imply that denitrification does occur.

Other sandstones[edit]

In the confined sand aquifers of the Palaeogene in southeast England there is a large range of redox conditions with some samples exhibiting low NO3 (Bearcock and Smedley, 2010[41]). There appear to be ‘pockets’ of reduced groundwaters throughout the London Palaeogene beds. There is a weak negative correlation between the Eh and the NH4-N concentration, suggesting that NO3 reduction occurs under the most reducing conditions. However, several fully oxidised sites have a relatively high NH4-N concentration. A site near Ascot shows strongly reducing conditions sufficient to support SO4 reduction. This is evidenced by a low SO4 concentration (0.215 mg/L), a low NO3-N concentration (0.23 mg/L), and a corresponding high NH4-N concentration (6.7 mg/L). In Wessex the majority of unconfined groundwaters in the Palaeogene sediments have elevated NO3 concentrations (Neumann et al., 2004[42]). However, 70% of sampled groundwaters were strongly reducing, with some waters having a strong H2S smell. In these waters, Fe and Mn concentrations were elevated while NO3 concentrations were low indicating that denitrification is likely to have occurred.

In the Cretaceous Lower Greensand aquifer of southern England, NO3 varies in concentration from below detection limit in the more reducing groundwaters to 16.3 mg/L NO3-N in the more oxidising groundwaters (Shand et al., 2003a[27]). The unconfined Folkestone Formation has predominantly high NO3. The Hythe Formation show a wide range in NO3-N concentrations (<0.03–15.8 mg/L) the lower concentrations being from parts of the aquifer confined beneath the Folkestone, Bargate and Sandgate strata. Many reducing groundwaters also have concentrations of NO3 below the detection limit. In the Slough boreholes, the presence of H2S gas was noted whilst sampling. The confined parts of the aquifer therefore, contain elevated concentrations of Fe. In contrast, Mn is often low most likely due to the low Mn concentrations in the aquifer.

Nitrate concentrations were variable in the Jurassic Bridport Sand Formation aquifer of southwest England (Shand et al., 2004[43]). The highest concentrations occur in the more oxidising groundwaters. Fe and Mn provide a good indicator of the redox conditions in these circum-neutral pH groundwaters where DO and Eh are poorly constrained. Water quality is consistent with the removal of NO3 by denitrification under reducing conditions. Water quality is influenced by both limitations on vertical hydraulic conductivity and by compartmentalisation of the aquifer by fracturing.

Nitrate concentrations were generally low throughout the Carboniferous Millstone Grit aquifer of northern England (mean <1 mg/L) partly due to denitrification or DNRA in these relatively reducing waters and partly due to the protection of the aquifer provided by its multi-layered nature as well as by the overlying superficial deposits (Abesser et al., 2005a[44]). Groundwaters with Eh values below 350 mV and low concentrations of DO also contain high concentrations of dissolved Fe.

In the Devonian Old Red Sandstone aquifers in Wales and Herefordshire, groundwater is moderately pristine but with local inputs of NO3 (Moreau et al., 2004[45]). Most of the groundwaters studied contained DO and had moderately high Eh. The samples where NO3 was below the limit of detection were moderately reducing and have most likely undergone denitrification. Ammonium concentrations were generally highest in the reducing groundwaters (up to 0.07 mg/L), but the sample with the highest NO3 (15.6 mg/L NO3-N) also contained NH4 with a concentration of 0.11 mg/L, probably as a result of pollution. The abundance of oxidising groundwaters is reflected in low median concentrations of Fe and Mn (0.02 and 0.003 mg/L respectively), even though Fe oxides are abundant as cements in the aquifer. However, under reducing conditions, Fe and Mn are mobile and the more reducing groundwaters contain relatively high concentrations, up to 3.2 and 0.95 mg/L respectively.

In summary, the Lower Greensand demonstrates low redox zones related to confinement by the overlying Gault, but the pattern in the Palaeogene is more complex. Distribution may also be related to the distribution of superficial deposits and to layering in the aquifer. In older indurated sandstones, NO3 concentrations were generally lower.

Other[edit]

In the Silurian and Ordovician strata of the Plynlimon catchment in Wales (Shand et al., 2005[46]) NO3 concentrations were low in both the upper catchments of the Severn and Wye. Nitrate concentrations varied from <LOD up to 1–2 mg l–1 and NH4 is present at up to 0.7 mg/L in the more reducing groundwaters. Manganese and Fe were high in many of the shallow groundwaters, particularly in the Wye catchment. This may be due to reducing conditions caused by more extensive superficial deposits in this part of the Wye catchment. In the Teifi catchment NO3 concentrations were higher ranging up to 9.3 mg/L in the bedrock and 4.3 mg/L in the superficial deposits and NH4 is present up to 1.7 mg/L in the bedrock. TOC concentrations may also reach moderately high concentrations, up to 7 mg/L. A strong smell of H2S was noted in several boreholes, indicating that SO4-reducing conditions occur at depth, and in shallow ground waters beneath organic-rich soils and clay.

In the granites of southwest England, a large range of NO3</su b>-N concentrations is observed at the shallowest depths (Smedley and Allen, 2004[24]). Only three samples in the database used in this study were from boreholes with recorded depths greater than 80 m. These had comparatively low, though still detectable, NO3-N concentrations around 5 mg/L.

In summary, whilst there is evidence of low redox conditions, related to confinement by organic-rich superficial deposits, there is no direct evidence for denitrification.

Environment agency groundwater monitoring review reports[edit]

Table 4.3 shows summary quality data for Environment Agency Groundwater Quality Monitoring Review reports where the text identifies that parts of the aquifer are confined. The full list of report summaries are in Appendix 1. These are taken from summaries showing the percentage of concentrations over the DWL. Where these were not provided data from the mean, 95 percentile and maximum have been used.

All areas which have been assessed as partly confined, except the Magnesian Limestone of the East Midlands show elevated concentrations of Fe and Mn and many of NO2 as well. Measurements for DO do not appear to be reliable and many indicate minimum concentrations of >1 mg/L. Comments suggest that elevated concentrations of NH4 were likely to be derived from local pollution.

These areas include the Carboniferous limestones and sandstones of northern England, parts of the Magnesian Limestone, Permo-Triassic sandstones of the northwest, Jurassic limestones of the Cotswolds and the Corallian, the Lower Greensand, areas of the Chalk confined by the Palaeogene or by low permeability superficial deposits and the Crag.

This dataset will be evaluated in greater detail as part of the construction of the map.

Table 4.3    Summary data from Environment Agency Groundwater
Quality Monitoring Reports where confined conditions are identified within the area.
Report Area Monitoring unit DO
(mg/L)
NO3-N
(%>QL-
11.3 mg/L)
NO2-N
(%>QL-
0.03 mg/L)
NH4-N
(%>QL-
0.39 mg/L)
Fe
(%>QL-
200 µg/L)
Mn
(%>QL-
50 µg/L)
SO4
(%>QL-
250 mg/L)
Evidence for confined conditions Reference
Ang 2* Norfolk and Suffolk Crag 0-sat Mean>QL 95%ile>QL Yes Mean>QL Mean>QL Max>QL Yes – by drift Environment Agency (2009c)[47]
S 1* East Hampshire Chalk >1.7 Max>QL 0 0 Max>QL 0 0 Yes along tertiary edge semi-confined Environment Agency (2004d)[48]
S 13* Chichester Chalk >1.9 Max>QL Max>QL Max>QL 95%ile>QL 95%ile >QL 0 Yes in Littlehampton inlier ENTEC (2008b)[49]
Tham 3 Confined Chalk of the London Basin 0-sat 1.7 1.1 14 25.4 2.4 4.6 Yes ESI (2006a)[50]
Tham 5 Kennet Valley Chalk 0.16–12 8.9 2.7 13 8.7 0.8 0 Yes ESI (2005a)[51]
Ang 1* Norfolk and Suffolk Chalk 0-sat Mean>QL 95%ile>QL 95%ile>QL 95%ile>QL 95%ile>QL 95%ile>QL Yes Environment Agency (2009b)[52]
Ang 3* North Essex Chalk 1.1–9.8 0 - 95%ile>QL Mean>QL 95%ile>QL 0 Yes Environment Agency (2009d)[53]
Ang 4* South Essex Chalk 0–7.8 0 Max>QL Mean>QL Mean>QL 95%ile>QL Max>QL Yes mainly confined Environment Agency (2009e)[54]
S 8* Kent Lower Greensand >2.5 95%ile >QL 95%ile >QL 0 Max>QL 95%ile >QL 0 Yes. In northern boundary ENTEC (2008e)[55]
S 16* Isle of Wight Lower Greensand >1.32 Max>QL Max>QL Max>QL 95%ile>QL 95%ile >QL 0 Yes ENTEC (2008d)[56]
S 17* Western Lower Greensand >1.1 Max>QL Max>QL Max>QL 80 95%ile >QL in Folkestone 0 Yes ENTEC (2008j)[57]
Tham 6 Lower Greensand 0–8.7 6.3 2.9 5.9 27 9.5 0 Yes ESI (2006e)[58]
MID1 Jurassic Limestone of Cotswolds Edge 18.3 0 3 1.4 0 Yes Besien et al. (2006)[59]
Tham 1 Corallian 0.1–10.4 10 1 0 63 21 5 Yes Duncan et al. (2004)[60]
NW3 Permo-Triassic sandstones of the Wirral and West Cheshire 8.3 6.5 3.1 12 15 Low 2.2 Yes ESI (2006i)[61]
NW4 Permo-Triassic sandstones of Eden Valley and Carlisle Basin 5 2.3 0.95 13 21 8.1 from evaporites Yes ESI (2006b)[62]
NW8 Sherwood Sandstone of Furness and West Cumbria >0.02 All 3.67 24.7 23.7 1 Yes ESI (2006c)[63]
MID3 Magnesian Limestone of the East Midlands 1.02–9.61 22 11 0 0 0 33 Yes Besien and Pearson (2007b)[64]
NE4 Skerne Magnesian limestone 0 3.1 2.1 22 30 14 Yes ESI (2004a)[65]
NE1 Till and Northumberland rivers Fell sandstone Low except arable area 10 2.2 High in N centre. Up to 7.7 mg/L 33 9 Yes ESI (2004b)[66]
NE2 Tyne Fell Sandstone 0 NQ 0 74 All 0 Yes ESI (2004c)[67]
NW2 Carboniferous Limestone, Millstone Grit and Coal Measures of the Lune and Kent


Low, 1.2%
Low 0.79
13%
0
3.2 both 28
67
16
54
Low Yes ESI (2006f)[68]
NW6 Carboniferous Limestone, Millstone Grit and Coal Measures of the Ribble


0
0
2.1
2.1
0.03
0
8.4
0.39
6
30
37
60
33
34
52
0 Yes ESI (2006h)[69]
NW9 Millstone Grit and Coal Measures of Northern Manchester 0.2 5 - 14 72 73 71 Yes ESI (2006g)[70]

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