OR/15/048 Regional geology and hydrogeology

Bearcock, J M, Smedley, P L and Milne, C J. 2015. Baseline groundwater chemistry: the Corallian of the Vale of Pickering, Yorkshire. British Geological Survey Internal Report, OR/15/048.

Regional geology

Geological setting

During the Jurassic Period Britain was situated between 30 and 40º north of the equator. The warm, humid climate represented a change in climatic regime following the arid Permian and Triassic (Cope, 2006; McLeish, 1992). Throughout much of the Jurassic, Britain was covered by shallow seas; the sea rises being considered related to the creation of oceanic ridges as the supercontinent Pangaea started to break up (McLeish, 1992). Jurassic sedimentary deposition was hence affected by a series of transgressions and regressions (Allen et al., 1997). The Jurassic Period is subdivided into three series, Lower, Middle and Upper. The Upper Jurassic series is further subdivided into Oxfordian, Kimmeridgian and Tithonian. The Jurassic rocks in the Vale of Pickering are of Oxfordian age and can be conveniently divided into three divisions based on lithological character. These divisions are the Oxford Clay Formation, the Corallian Group, and the West Walton, Ampthill and Kimmeridge Clay formations (Reeves et al., 1978). The Upper Jurassic rocks are entirely of marine origin. Marine regression had interrupted eustatic sea-level rise, causing the deposition of the Corallian Group (Powell, 2010). The subsequent re-advance of the sea resulted in the deposition of bituminous clay over the Corallian strata (McLeish, 1992). The Corallian strata form the major Jurassic aquifer. The stratigraphy of the Jurassic units is presented in Table 1 and Figure 6.

The Corallian Group is well-developed in north-east England. It mainly comprises ooidal and micritic limestone and calcareous fine-grained sandstone (Powell, 2010), but also includes a variety of facies from muds, to micritic limestone and oolites, to bioclastic limestones with interbeds of silts and sands (Cope, 2006; Reeves et al., 1978). These facies represent a shallow shelf depositional environment (Allen et al., 1997) and include a low influx of terrestrial material from the surrounding areas. The absence of debris flows and turbidites suggests that depositional slopes were very low (Hallam, 1992 cited in Allen et al., 1997). The aquifer crops out in areas including the Hambleton Hills, the Howardian Hills, the Tabular Hills and the North York Moors, forming the northern and south-western boundaries of the Vale of Pickering (Allen et al., 1997). The Corallian consists of three formations: the Lower Calcareous Grit (formally known as Passage Beds), the Coralline Oolite (formerly known as Coral Rag (Cope, 2006)) and the Upper Calcareous Grit Formation (Allen et al., 1997). The 'grit' subdivisions of the Corallian are neither true sandstones, nor true limestones, but are generally fine-grained calcareous sandstones. Both the grits and the oolites vary in lithology but the most marked variations occur in the oolites where reefs are present (Wilson, 1948). The Cleveland Basin is an inversion structure and was a major basin of deposition during the Jurassic followed by uplift in the Cretaceous. There are a number of large faults which traverse the area including the Vale of Pickering Fault, the Weaverthorpe Fault and the Coxwold-Gilling- Linton Fault. The throw on the Pickering and Weaverthorpe faults is of the order of 200 m (Allen et al., 1997).

Table 1 Correlation of the formations and members within the Jurassic Corallian Series (adapted from Allen et al., 1997; Cope, 2006; Powell, 2010; Reeves et al., 1978; Wilson, 1948)

Period	Formation		Members	Thickness (metres)
Recent	Post Glacial Superficial Deposits			0 to 10
Pleistocene	Glacial Deposits
Upper Jurassic	Lower Kimmeridge Clay Formation			340
	Ampthill Clay Formation			45 to 50
	Corallian Group	Upper Calcareous Grit Formation		10 to15
		Coralline Oolite Formation	Coral Rag	Up to 9
			Malton Oolite	Up to 40
			Middle Calcareous Grit	5 to 25
			Birdsall Calcareous Grit	Up to 30
			Hambleton Oolite	Up to 34
		Lower Calcareous Grit Formation		Approx 15
	Oxford Clay Formation			20 to 50

Oxford Clay

The base of the Oxford Clay marks the start of the Upper Jurassic period in the Vale of Pickering (Powell, 2010) (Table 1). This formation is found below the Corallian facies and consists of grey-green calcareous mudstone and silty mudstone (Powell, 2010), there is very little variation within these rocks (Reeves et al., 1978).

Lower Calcareous Grit Formation

The beginning of the Lower Calcareous Grit Formation is mostly defined by the gradual replacement of the argillaceous conditions of the Oxford Clay deposition with an influx of fine arenaceous material. The lower Calcareous Grit Formation is the same age as the Oxford Clay. The only exception to this is in the south-west part of the Hambleton Hills, where the Lower Calcaerous Grit Formation lies on Middle Jurassic beds, as the Oxford Clay is missing (Cope, 2006; Wilson, 1948; Wilson, 1949). The formation generally comprises fine- to medium-grained calcareous sandstones which attains maximum thickness in the Tabular and Hambleton Hills, thinning southwards and eastwards towards the coast (Allen et al., 1997; Wilson, 1948). Coastal sections have shown exposed Lower Calcareous Grit Formation divided into three units. The upper unit is known as the 'Ball Beds' and is 3–5 m thick. The 'Ball Beds' comprise very ferruginous sandstone containing large, gritty, fossiliferous calcareous concretions. A hard grey siliceous sandstone 1–2 m thick underlies the 'Ball Beds'. The lower unit is 12–14 m thick and comprises thickly bedded sandstone with siliceous cement. The cement is frequently concentrated into small masses which weather out in irregular nodular bands (Wilson, 1948).

Figure 6 Geology in and around the Vale of Pickering.

In the escarpments north of Kirkbymoorside and Helmsley the Grit becomes siliceous, because of the presence of siliceous spicules, and frequent chert bands. The grit varies from a hard siliceous spicule-bearing rock in the west to gritty limestone beds with soft sandstones in the southeast (Wilson, 1948).

Coralline Oolite Formation

The Coralline Oolite comprises the following members, in age order: Hambleton Oolite, Birdsall Calcareous Grit, Middle Calcareous Grit, Malton Oolite and Coral Rag.

The Hambleton Oolite Member is developed from the Vale of Pickering northwards. It caps the escarpment on the Hambleton Hills, and forms large dip slopes on the North York Moors. It consists mainly of oolitic limestones of considerable variation. There are also some sandstones, detrital limestones, and towards the west the member becomes siliceous. The member is lenticular, being at its thickest around Kirkbymoorside. In the southern portion of the Corallian outcrop the Hambleton Oolite Member is divided into the Upper and Lower Leaf, which are separated by the Birdsall Calcareous Grit Member. The Birdsall Calcareous Grit is a fine sandstone with ooids and chert lenses. (Allen et al., 1997; Cope, 2006; Powell, 2010; Wilson, 1948).

The Middle Calcareous Grit is absent in the Howardian Hills, but is present at thicknesses of five to 25 m in the Hambleton and Tabular Hills. The member is similar to the Birdsall Calcareous Grit, comprising sandstones with localised development of shelly layers and sandy and oolitic limestone beds. At outcrop this rock can be decalcified (Allen et al., 1997; Cope, 2006; Powell, 2010; Wilson, 1948).

The Malton Oolite Member and the Coral Rag Member were formerly known together as the Osmington Oolite. The Malton Oolite in particular is laterally persistent, extending round the fringe of the Vale of Pickering to the foot of the Chalk Wolds to the south. On average the member is approximately 18 m thick, reaching a maximum thickness of ca. 40 m inland. The Malton Oolite consists of dominantly pure, shelly oolites, but becomes more sandy, as it thins towards the west. The Malton Oolite is overlain by a variety of beds, which form the Coral Rag Member. Five kilometres south of Malton, and southwards from this point the Coral Rag and a significant portion of the Malton Oolite have been removed by erosion. The Coral Rag attains a maximum thickness of 9 m and consists of a variety of fossiliferous reef limestones, and includes impure oolite, shelly oolite, argillaceous limestone, and pure white limestone (Allen et al., 1997; Cope, 2006; Powell, 2010; Wilson, 1948).

Upper Calcareous Grit Formation

The uppermost formation in the Corallian Group is the Upper Calcareous Grit Formation. This comprises around 15 m of well bedded fine grained calcareous sandstone and siltstone. Abundant beds of clayey limestone occur in the middle of the unit. The Upper Calcareous Grit Formation provides an intermittent cap to some of the southern ridges of the Tabular Hills, and is continuous from Pickering into the south-east part of the Hambleton Hills. This formation is commonly leached and re-cemented with silica (Allen et al., 1997; Tattersall and Wilkinson, 1974; Wilson, 1948).

Kimmeridge Clay and Ampthill Clay

A thick deposit of Kimmeridge Clay and Ampthill Clay overlies the Corallian Group, covering the floor of the Vale of Pickering (Tattersall and Wilkinson, 1974 ; Wilson, 1948).

Superficial deposits

Over much of the outcrop area the only superficial deposits present are alluvium and river terrace deposits. However widespread glacial deposits exist near Scarborough. Along the southern margin of the Corallian, extensive sand and gravel deposits mask the boundary with the overlying Kimmeridge Clay. These deposits are locally underlain by 10–20 m of till (Wilson, 1948).

Regional hydrogeology

The Corallian Group forms the main aquifer in the Jurassic sequence of the Cleveland Basin (Allen et al., 1997). It was developed in response to increasing demands for water supplies in South Yorkshire after the rejection of proposals to build a regulating reservoir in the North York Moors in 1970 (Reeves et al., 1978). This aquifer is underlain by the Oxford Clay and overlain by the Kimmeridge Clay, which are both poorly permeable (Allen et al., 1997). Table 2 shows the hydrogeological significance of the Jurassic formations in the Vale of Pickering.

Aquifer properties

The aquifer is highly fractured and groundwater storage and movement takes place predominantly within the fractures. Large yields can be obtained close to such fractures or faults and springs (Allen et al., 1997).

In the unconfined part of the aquifer groundwater is likely to be in hydraulic continuity with surface water courses. Groundwater levels and stream flow both respond rapidly to rainfall due to large areas of impermeable catchment (Reeves et al., 1978). Groundwater is particularly important during summer months for maintaining stream flows with a significant baseflow component.

Groundwater discharge from the Corallian occurs mainly via a series of springs controlled by faulting. The springs usually discharge at the boundary between the Corallian and the overlying clays but may also occur along fault lines. In summer the total discharge from all of the springs exceeds the total discharge from the aquifer as all of the flow from some of the rivers disappears into swallow holes and re-emerges as springs. In these areas, solution-enhanced fractures have developed and the aquifer becomes karstic in nature. At East Ness flow rates through solution enhanced fractures range from 2 to 3500 m³/d (Allen et al., 1997).

Core data

Allen et al. (1997) presented a summary of sediment core data. There were limited data available for porosity and permeability, and only for the Corallian Oolite. Based on a total of 25 samples from 5 boreholes the mean porosity of the Corallian Oolite is 17.4 % with an interquartile range of 10.9 to 27.2%. The interquartile range for hydraulic conductivity (K) (based on plug samples) is 1.4 x 10^-5 to 1.6 x 10^-3 m d^-1, with a mean of 1.8 × 10^-4 m d^-1. Neither the vertical K nor the horizontal K seem to be consistently greater than each other and in general, high K values correlate with higher porosities (Allen et al., 1997).

Pumping test results

Allen et al. (1997) also summarised the results of pumping tests of the Corallian aquifer. Only 6 values of storage coefficient are available, which range from 4 x 10^-7 to 0.023 (Allen et al., 1997). Transmissivity values have been recorded. The interquartile range of these measurements is 38 to 2249 m² d^-1, and the mean is 318 m²/d. Transmissivities in the main outcrop area of the aquifer are generally low (<10 m² d^-1), which is a reflection of the small saturated thickness. The exception to this is the area around Malton. Transmissivity values are generally higher towards the confined aquifer and where boreholes are sited near to major springs where values can be up to 3800 m² d^-1.

Aquifer resources

A detailed investigation into the water resources of the aquifer was conducted between 1970 and 1974 by Reeves et al. (1978). Groundwater movement and chemistry were found to be closely related to geological structure. It was concluded that the greatest hydrogeological potential is to the north of the Coxwold-Gilling trough, which divides the relatively simple synclinal outcrop area in the north from the complex eastern extension of the Howardian Hills Fault Belt beneath the clay-covered Vale of Pickering. There is limited hydraulic connectivity between the two areas. The southernmost area is compartmentalised by a series of tensional easterly trending faults (Allen et al., 1997; Reeves et al., 1978).

Table 2 Hydrological significance of Jurassic Formations in the Vale of Pickering (adapted from Allen et al., 1997; Cope, 2006; Reeves et al., 1978; Wilson, 1948)

Formation	Hydrological Significance
Lower Kimmeridge Clay	Aquitard
Ampthill Clay
Upper Calcareous Grit	Major Aquifer—the Corallian
Coralline Oolite
Lower Calcareous Grit
Oxford Clay	Aquitard

Reeves et al. (1978) divided the aquifer into five hydrogeological zones, determined largely by structure and characterised by water quality (Table 3, mapped in Reeves et al., (1978)) They concluded that groundwater chemistry of groundwater in the Corallian aquifer was primarily determined by rate of groundwater movement (which is largely determined by structure). All the sample sites in this study were located within zones D or E.

Aquifer mineralogy and chemistry

There is sparse information available on the mineralogy of the Corallian aquifer in the Vale of Pickering owing to a scarcity of BGS memoirs in the area. The following section is paraphrased from Fox-Strangways (1880) and Powell (2010). All formations in the Corallian aquifer are dominated by calcite, either as the main mineral or as a cement. The Lower Calcareous Grit Formation is a calcareous sandstone. It is known to contain siliceous bands and nodules. The Coralline Oolite Formation is a varied sequence of ooidal limestone, with fine grained sandstone, and various reef facies. The Upper Calcareous Grit is a fine grained calcareous sandstone. The Corallian aquifer is therefore dominated by quartz and calcite.

Rainfall chemistry

Average annual rainfall over the Vale of Pickering and surrounding areas varies between 717–987 mm year^-1 (based on the 1961–1990 averages), with the greatest rainfall tending to fall in the north-west of the region in the North York Moors (NRFA, 2008). The average annual rainfall over the outcrop is approximately 840 mm year^-1; evapotranspiration accounts for ~380 mm year^-1. The potential for infiltration therefore amounts to a maximum of 450 M L day^-1 (Tattersall and Wilkinson, 1974). It is estimated that only 10 to 15% of this amount occurs to the Gilling-Malton aquifer block which occurs to the south of the Gilling-Ampleforth faults (Reeves et al., 1978).

Table 3 Description of hydrogeological zones in the Vale of Pickering, described by Reeves et al. (1978)

Zone	Area	Groundwater Chemistry
A	Central area of the vale. Corallian aquifer isolated from outcrop by thick clay cover. Only 1 borehole penetrates the Corallian.	Limited recharge, gives Na-HCO3 type groundwater. Thought to be product of ion exchange.
B	Westermost area of the vale. North and South of Caulkleys Bank. Corallian aquifer in restricted hydraulic continuity with outcrop area.	Flow from recharge area is restricted, Na-HCO3 type groundwater.
C	North and north-western part of vale, east of Caulkleys Bank. Aquifer is beneath clay, but in hydraulic continuity with the outcrop.	Aquifer is in full or almost full continuity with outcrop groundwater, gives Ca-HCO3 type groundwater.
D	Aquifer is exposed or beneath thin clay cover around the margins of the vale. Major springs occur in this zone and transmissivity is greater than average.	Analyses from springs indicated groundwater is Ca-HO3 type.
E	Upland outcrop area	Recharge zone where groundwater is Ca- HCO3 type.

Table 4 shows the chemical composition of rainfall from the Moor House monitoring site [NY 757 328] located in the north Pennines, near to the source of the River Tees, 115 km north-west of Pickering.

Concentrations of Na and Cl in the Moor House analyses suggest that there are maritime influences. This site is >60 km from the coast. Concentrations are therefore likely to be higher in the Vale of Pickering rainfall than the Moor House rainfall. The Vale of Pickering is proximal to the coast, and likely to be influenced by airborne marine salts (Shand et al., 2007). Rainwater is naturally acidic owing to the dissolution of atmospheric CO₂, which forms a weak solution of carbonic acid with an equilibrium pH of around 5.7. The Moor House rain water is pH 5.6 indicating there are few airborne pollutants present, such as oxides of sulphur (SOx) and nitrogen (NOx) which are capable of further acidification (Shand et al., 2007). Sites within the study area that are closer to industrial atmospheric inputs are likely to contain higher concentrations of SOx and NOx and have a lower pH. If it is assumed that all the NH₄ in rainfall oxidises to NO₃ on infiltration to groundwater, baseline concentrations of dissolved NO₃-N derived from rainfall alone are likely to be in the order of 0.73 mg L^-1 or less. The pH of recharge may be even lower than that measured in the rainfall owing to the oxidation of NH₄⁺ ions to NO₃ which results in the release of H+ ions and concentration by evapotranspiration. This process is reflected in the pH of recharge, which is typically pH 3–5 (Shand et al., 2007; Smedley and Allen, 2004). Mineral dissolution reactions within the calcareous beds will be responsible for buffering these pH values, indeed the pH of waters in calcareous soils and aquifers rarely occurs below 6.5 (Abesser et al., 2005  ; Kinniburgh and Edmunds, 1986).

Current issues in groundwater quality

The main aim of the Environment Agency’s (EA) Water Framework Directive (WFD) is for water bodies to achieve good chemical status. The EA, tasked with planning and delivering better water environments, defined the 11 river basin districts for which management plans were created to identify the current status of groundwater. The Vale of Pickering Corallian aquifer is within the Humber River Basin District. This aquifer is divided into two groundwater bodies, separated by the Gilling-Ampleforth faults. In both groundwater bodies the chemical status is assessed as poor; and it is predicted that the chemical status will remain poor owing to the disproportionate expense associated with achieving this target. It is anticipated, however, that good status will be achieved by 2027 (EA, 2009).

The groundwaters are monitored to assess the risks of not meeting water quality standards. The risks to the inorganic water quality are the presence of nutrients (namely nitrate and phosphate), the presence of hazardous substances or other pollutants, and saline intrusion as a result of abstraction and other artificial flow pressures. The groundwater bodies are classified as drinking- water protection areas, which mean they need to be managed for this use, and are protected by WFD legislation. In particular, action needs to be taken to prevent or limit the inputs of NO₃ to the groundwater. This will ensure that there is no future deterioration of groundwater quality, and the increasing NO3 trends can be reversed. Actions to achieve this are by a combination of regulatory and voluntary measures. Nitrate vulnerable zones (NVZs) were first designated by the EA in 1996, and then updated in 2002 and 2008. Currently almost 70% of England is designated as an NVZ. Farms within NVZs have to comply with rules affecting their careful planning, storage and usage of NO₃ rich substances (DEFRA, 2011). Almost all of the Corallian aquifer at outcrop in this region is located within a NVZ (DEFRA, 2011  ; Natural England, 2015). Voluntary measures, such as catchment sensitive farming (CSF), are also encouraged in order to protect the groundwater from NO3 contamination (DEFRA, 2011). There are significant problems of nitrate and phosphate throughout the Vale of Pickering, which is exacerbated by the high connectivity of the drainage system within the vale (Natural England, 2015).

Table 4 Rainfall chemistry from the Moor House monitoring site [NY 757 328].
The average rainfall composition was calculated from data provided by the Environmental Change Network (ECN) (www.ecn.ac.uk). The mean annual rainfall at this site is 2012 mm y^–1 (1991–2007).

Parameter	Units	Average rainfall composition (2007)	Concentrated rainfall composition (enriched by evapotranspiration)
pH		5.6
SEC	µS cm-1	17	51
Ca	mg L–1	0.19	0.57
Mg	mg L–1	0.33	0.99
Na	mg L–1	2.85	8.54
K	mg L–1	0.10	0.31
Cl	mg L–1	4.95	14.85
SO4	mg L–1	0.41	1.24
NO3–N	mg L–1	0.09	0.73*
NH4–N	mg L–1	0.15

*Assuming all NH₄-N is oxidised to NO₃ on infiltration.

Figure 7 Source Protection Zones in the study area.

Source protection zones (SPZs) around boreholes have been identified by the EA (Figure 7). These show the risk of contamination from activities which may cause pollution. They are zoned according to the risk in terms of time for the pollution to travel to the borehole and extent of the contamination risk. Within the Corallian large SPZs are located around Pickering, Scarborough and East Ness. Local SPZs (~500m in diameter) are sparsely distributed around the periphery of valley bottom, within the Hambleton and Howardian Hills, and in the area south of Malton (EA, 2015).