A RQUITECTURA Y PATRONES

Water quality standards, including specific electrical conductance (SEC) or total dissolved solids (TDS), are used to regulate supply and use of groundwater in England and elsewhere. The Council of the European Union (1998) specified that the maximum SEC at 20°C should be 2500 µS/cm (about 1625 mg/l TDS), and the water should not be aggressive. This limit has been embodied in the water supply regulations for England that specify the maximum admissible concentrations and values for parameters in drinking water for both public supply (The Water Supply (Water Quality) Regulations (2016)) and private water supplies for human consumption (Private Water Supplies

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(England) Regulations (2016)). The World Health Organisation (WHO) describes water with a TDS of < 600 mg/l as good quality and that with a TDS of >1000 mg/l as increasingly unpalatable (WHO, 2011). For comparison, the US EPA (2017), states a guideline maximum TDS value of 500 mg/l in the Secondary Drinking Water Standards but considers an underground drinking water source to have a TDS of < 10,000 mg/l.

Understanding of groundwater quality at depth is integral to 3D groundwater vulnerability and risk assessments and, arguably, the related policy development and management decisions. Water quality tends to deteriorate with increasing depth as lower hydraulic gradients and slower groundwater movement result in longer residence times during which the water can interact with the host rock and result in increased mineralisation. There are some exceptions to this, for example, in the East Midlands better quality Sherwood Sandstone groundwater occurs below more mineralised shallower waters associated with the overlying gypsiferous Mercia Mudstone and groundwater subjected to recent near surface pollution from agriculture or coal mining.

3.3.1 Deep groundwater quality in England

There are 13 public water supply sources with depths over 400 m in the BGS Wellmaster database (a comprehensive database of water borehole logs in the UK) in England, though none are over 500 m deep.

Table 3.3 indicates that these boreholes all terminate in sandstone aquifers (either the Lower Greensand or the Sherwood Sandstone). Water in silicate aquifers is generally less mineralised than that in carbonate ones. However, it is not clear whether the lack of boreholes > 400 m deep in carbonate aquifers is due to a decrease in dissolution and secondary fractures affecting yields or poor groundwater quality at depth within these aquifers.

Table 3.3 Public water supplies from > 400 m depth in BGS’ Wellmaster database

Aquifer Number of supplies Depth (m)

Palaeogene, Chalk and Folkestone

Sherwood Sandstone Group 5 400, 414, 430, 431, 500

The Geothermal Data Catalogues provide the most complete data on groundwater quality at depth, including information on locations, depths, sample types and aquifers. The first comprehensive catalogue of underground temperature, heat flow and hydrogeochemical data was published in 1978 by the Department of Energy (Burley and Edmunds, 1978). This was updated by the British Geological Survey’s ‘Investigation of the geothermal potential of the UK’ project in the 1980s and published in three revisions (Burley and Gale, 1982; Burley et al, 1984; Rollin, 1987). The majority of the data were derived from drill stem tests (Table 3.4).

Data from the Geothermal Data Catalogues have been digitised and anomalous values removed.

Site locations given to the nearest 1 km (in some cases 10 km) were cross-referenced by location names and depth and identified to at least the nearest 100 m. Mine drainage data were also removed, since these analyses may not be representative of natural groundwater conditions and the depth from which the water drains is ambiguous. The remaining 500 analyses range from springs (surface) to a maximum borehole depth of 2385 m, although the number of observations decrease significantly with depth (Table 3.5). Where the source rock was not recorded but a depth provided, borehole logs were used to identify the formation from which the water sample was most

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likely derived. Where only a borehole depth (not sample depth) was available, the sample was assumed to be from the formation at the final borehole depth.

Analyses were from a range of formations, but primarily the Chalk, Sherwood Sandstone, Zechstein Group, Coal Measures, Millstone Grit and Carboniferous Limestone (Table 3.6). Where no TDS was recorded in the original data, it was assumed to be the sum of all the ions quoted (major ions plus silica), although in many cases some ionic concentrations (mainly potassium, bicarbonate, sulphate and silica) were not recorded and hence the calculated TDS content is a minimum estimate. The dataset is a collection of all data available at the time of compilation, rather than being a comprehensive review of water quality from different formations at specific depths. For example, many more data exist for aquifers at shallow depths which are not included, and these data indicate that the lowest TDS range for any unit in Table 3.6 relates to the London Clay at depths of > 150 m, not a formation generally considered to form a significant aquifer, is anomalous and an artefact of the way data was originally selected for inclusion in the catalogues.

Table 3.4 Sources of water quality samples, data from the Geothermal Data Catalogues (Burley et al., 1984; Rollin, 1987)

Data source Number of sites

Spring (including thermal springs)

10

Depth sample 9

Interstitial 23

Pumped sample 71

Artesian discharge 10

Drill stem test 309

Unknown 68

Table 3.5 Depths of water quality samples, data from the Geothermal Data Catalogues (Burley et al., 1984; Rollin, 1987)

Depth (m) Number of sites

0-500 176

500-1000 137

1000-1500 115

1500-2000 65

2000-2500 6

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Table 3.6 Water quality analyses by formation, data from the Geothermal Data Catalogues (Burley et al., 1984; Rollin, 1987)

Period Formation Number of

sites

Depth range (m)

TDS range (mg/l)

Palaeogene London Clay 3 167-179 129-298

Cretaceous Chalk 28 90-532 124-35287

Upper Greensand 7 120-626 181-5350

Lower Greensand 13 0-687 110-7999

Wealden 2 665-759 2314-6965

Jurassic Portland 3 804-865

14186-116890

Corallian 3 580-1258 19993-93725

Kellaways and Oxford Clay formations

4 105-833 10812-47625

Great Oolite 5 224-1246 11259-67304

Inferior Oolite 6 158-1369 375-131736

Bridport Sand

Mercia Mudstone 2 321-683 1474-52418

Dolomitic

Zechstein 57 151-1918 296-331597

Rotliegendes 4 1316-1814

103015-315711

Carboniferous Coal Measures 83 90-2375 365-275911

Millstone Grit 94 282-2266 950-317298

Bowland Shale 2 0 (springs) 637-1195

Figure 3.1 shows TDS as a function of depth for all of the Geothermal Data Catalogue data. Figures A3.2 to A3.7 in Appendix 3 show TDS as a function of depth highlighting data for the Chalk, Sherwood Sandstone, Zechstein Group, Coal Measures, Millstone Grit and Carboniferous Limestone. Figure 3.1 shows that there is significant variation in TDS at any given depth. For

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example, at ~ 400 m bgl, measured TDS may vary by over three orders of magnitude from ~100 to >100,000 mg/l. The corollary of this is that a given TDS may be found over a wide range of depth intervals. For example, TDS values of 10,000 mg/l have been reported from the near surface down to depths of >1km.

Figure 3.1 TDS as a function of depth for England based on data from the Geothermal Data Catalogues (Burley et al., 1984; Rollin, 1987).

However, groundwater at greater depths will generally be older, allowing more time for water-rock interaction and hence more mineralised. Hence there is a broadly linear lower bound to the distribution of TDS (Figure 3.1). This means that for a given depth interval an equivalent minimum TDS can be approximately identified. The lower TDS bound for a given depth indicates a maximum depth of ~ 900 m for potable groundwater in England (maximum TDS ~ 1625 mg/l based on the current statutory SEC limit for potable groundwater of 2500 µS/cm) (Figure 3.2).

Groundwater below ~1,750m is likely to be more saline than seawater (35,000 mg/l TDS) (Figure 3.2). However, estimation of depth intervals associated with specific TDS thresholds will depend on the precise location and shape of the lower bound to the TDS-depth trend and these figures should only be taken as approximate values. A similar lower bound to groundwater quality-depth data has also been described for data from California (Kang and Jackson, 2016). In California, however, the lower bound is lower than for England, reflecting lower TDS at greater depths. This difference could result from a range of factors, including the length of time that groundwater has been in contact with the host rocks which in turn is a function of the hydrogeological setting, rock hydraulic conductivity, and rock solubility.

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Figure 3.2 TDS as a function of depth for England with interpolated depths associated with limit of potable water (<1625 mg/l) and groundwater more saline than seawater (>35,000 mg/l).