Geografia d'Europa

Summary

Groundwater quality in the London Basin area is monitored using a network of public supply and private abstraction boreholes. The majority of the area is covered by London Clay with only small inliers of Lower London Tertiary deposits and Chalk breaking through this cover. The major aquifer in the area is the Chalk which lies in hydraulic continuity with the Basal Sands, a minor aquifer that lies above. London Clay is a non-aquifer that confines the Chalk and prevents infiltration from above. The main geological structure in the area is a syncline with an axis running broadly northeast-southwest from Chertsey to Walthamstow.

The main recharge areas for the Chalk are the Chilterns to the north and the Downs to the south. Groundwater flows down dip towards the axis of the syncline. Consequently the age of the water increases towards the centre of the basin. The confining clay layer leads to anaerobic reducing conditions occurring within the Chalk. In the past the groundwater reached the surface via a number of artesian springs close to the Thames. However large scale abstraction from boreholes and wells in London has caused the de-watering of the upper part of the aquifer. This has caused reverse groundwater flow in some areas close to the tidal sections of the Thames leading to recharge of the aquifer by saline waters.

The combined effect of high residence times and saline intrusion has produced relatively poor groundwater quality under Central London with waters concentrated in magnesium, sodium, chloride and sulphate. Magnesium increases at the expense of calcium through the incongruent dissolution of carbonates in the Chalk combined with calcite recrystallization, while sulphate is supplied by the breakdown of pyrite and gypsum particularly in the Basal Sands. The clay cover protects the groundwater from surface pollutants, so concentrations of nitrates, phosphates and total organic carbon are low. Anoxic conditions would also quickly reduce any nitrate present, while maintaining ammoniacal nitrogen. Other minor ions such as boron, bromide and fluoride are found at elevated concentrations possibly due to ion exchange reactions with minor minerals in the Chalk.

Recently groundwater levels have been rising, particularly in Central London, as industries have declined or moved away. The Chalk has also been recharged by drinking water when surplus is available, in the North London Artificial Recharge Scheme. Changes in groundwater chemistry may be seen to occur, often caused by intrusion of water into the previously unsaturated zones of the Chalk and the Basal Sands, leading to dissolution reactions.

1. Geographical Area

The area covers the majority of the London Basin (Figure 1). Its northern edge lies along the southern catchment boundary of the Upper River Colne and the northern catchment boundary of the River Roding, with the sections in between cutting across the Rivers Thames, Colne and Lee sub-parallel to the Chalk outcrop pattern. The eastern edge is defined by the Thames Region boundary of the Environment Agency, which runs along the eastern catchment boundaries of the Rivers Roding and Ingrebourne. The south of the area is mainly defined by the outcrop pattern of the Chalk of the North Downs, while the western border runs along the catchment boundaries of the River Wey and The Cut near Maidenhead.

The River Thames flows west to east across the middle of the area. Its principal tributaries within the area are the rivers Colne, Brent, Lee, Roding, Beam and Ingrebourne which drain southwards and the rivers Wey, Mole, Ravensbourne, Wandle, Beverley Brook, Chertsey Bourne and The Cut which drain northwards. Most of the area is covered by urban development, principally London and its suburbs, with other urban centres being Slough, Staines and Epping. The only agricultural areas are found in the extreme south west and north east.

2. Geology/Hydrogeology

The majority of the area is overlain by London Clay which lies at the centre of the London Basin syncline. The London Clay is a non-aquifer and forms a confining layer over a thin sequence of Lower London Tertiary deposits which in turn overlie the Chalk. The Lower London Tertiaries consist of a mixed sequence of clays, sands and pebble beds which vary both laterally and vertically. The principal units are the Blackheath Beds, Woolwich and Reading Beds and the Thanet Beds. The proportion of sand in the Lower London Tertiaries increases significantly eastward so that in the east of the area the unit is almost completely sand or pebble beds. The Basal Sands, the unit extending from the Chalk up to the first major clay horizon, form a minor aquifer which is in hydraulic continuity with the Chalk major aquifer. The Lower London Tertiaries crop out within the margins of the area and as a few isolated windows in the London Clay.

The Chalk is the major aquifer across the area. It varies from about 180-245 metres thick and is divided into the Upper, Middle and Lower Chalk. The Chalk dips gently south-south-east from the Chilterns under the Lower London Tertiaries to the synclinal axis. This runs west-south-west to east-north-east, approximately on a line through Chertsey to Walthamstow and beyond. To the south of the axis the Chalk dips to the north. The Chalk crops out as the North Downs and dips under the London Tertiaries. The majority of the Chalk outcrop lies outside the area, although super-imposed on the main syncline are a number of minor folds and faults which produce local modification of the structure that result in the breaching of the clay cover in places. This results in the Lower London Tertiaries (e.g the Leyton Dome) and even the Chalk (e.g. the Deptford and Windsor Domes or the Greenwich-Woolwich section) cropping out as inliers. In some valley floors erosion has cut into the London Clay allowing Lower London Tertiaries or Chalk to crop out or come into contact with Quaternary cover such as river gravels or alluvium (e.g. the Lee Valley at Hoddesdon). The other significant exposure of Chalk is found in the south-east of the area around Dartford, where it crops out on both sides of the River Thames.

Infiltration into the Chalk aquifer is principally from precipitation on the main outcrop areas, the Chilterns and the North Downs, which lie outside the area. From here the groundwater flows in the Chalk primarily through a system of fissures which are best developed in the upper sections of the Chalk. Slower groundwater flow occurs through pores within the Chalk. The groundwater flows down dip under the London Clay towards the synclinal axis.

Groundwater has been dated in the London Basin and measurements show increasing age towards the centre, where ages in excess of 25,000 years have been recorded. In the past, when the piezometric level in the Chalk was much higher, groundwater reached the surface via artesian springs close to the River Thames and the lower reaches of the principal tributaries (Roding, Lee, Colne, Wey and Mole). With the de-watering of the upper layer of the Chalk by abstraction from boreholes and wells, piezometric lows formed as two troughs close to the synclinal axis. One trough occurs under central London with the other near Dagenham, separated from each other by a ridge parallel to the Lee valley. There is evidence from tritium studies that this ridge is composed of a tongue of more recent water.

Another effect of the de-watering of the Chalk has been saline intrusion from the tidal section of the Thames especially along sections where the river is in hydraulic continuity with the Chalk, particularly in the East London area. The effects of saline intrusion are usually more marked to the north of the Thames because of the groundwater flow regime.

A small amount of infiltration can take place via leakage through the London Clay cover.

3. The Quality of Infiltration

As the major proportion of the infiltration into the Chalk occurs outside of the area, a more detailed understanding of the processes involved can be gained by consulting other reports in this series on the Chilterns and the North Downs (Leatherhead-Orpington). Groundwater vulnerability is not really an issue for this area except in the isolated places mentioned above where the Chalk or Basal Sands breach the clay cover. Here they are classified as having soils with high leaching potential (Groundwater Vulnerability sheets 39 and 40).

As the unconfined-confined boundary in the Chalk is passed, aerobic conditions give way to anoxic conditions under the clay, allowing reducing reactions to occur. With increasing residence time, as groundwater moves towards the centre of the basin, further dissolution and cation exchange reactions between minerals in the Chalk and the groundwater are able to proceed.

4. Groundwater Quality

The groundwater quality network in this area (as of October 1997) comprises of 4 public supplies and 33 private supplies (shown in red on Figure 1). Each sample point reference number relates to an individual borehole. Where several reference numbers are given for one site, this indicates a multiple borehole source, and data are given for each borehole. All sites abstract from the Chalk alone, except site 1397 which abstracts also from the Thanet Sands and is included for comparison. A number of sites have been added recently to the network; caution must be used when considering data from sites with just one or two analyses.

In addition to the groundwater monitoring network outlined above, the Environment Agency also monitors water quality from 6 sites (shown in orange on Figure 1) around the North London Artificial Recharge Scheme (NLARS). Artificial recharge of both the Chalk and Basal Sands aquifers was carried out in the Lee Valley intermittently during the 1950s, 60s and 70s. Extensive tests were carried out to determine changes to groundwater quality (Flavin and Joseph, 1983). Since 1995 recharge using fully treated drinking water has taken place occasionally within the Enfield-Haringey section.

The degree of change in groundwater quality will vary with the distance from artificial recharge sources. A site one kilometre from a point of recharge is considered far enough away for the groundwater chemistry to be unaffected. Consequently site 1215 which lies between the two arms of the NLARS, but over one kilometre from any of artificial recharge boreholes, appears unaffected enough to warrant inclusion as an ordinary groundwater monitoring network site.

4.1 Major ion chemistry

Most of the sites, provided dissolved metal analysis is available, are plotted on a Piper plot (Figure 2). The data points spread across the Piper plot indicating a mixture of water types. There appear to be three distinct end points for this data set. The first is a calcium bicarbonate type of water found at the left hand edge of the plot, that is similar to groundwater from unconfined Chalk (e.g. site 1406). The second end point represents long residence water found in the confined Chalk shown in the central section of the plot, that is a sodium bicarbonate-sodium sulphate type water (e.g. site 0693). Between these two end points there is a range of water types. This trend shows up clearly as a curved spread across the Na+K-Mg-Ca triangle. Finally the third end point is caused by saline intrusion from brackish water along the tidal section of the River Thames, which results in waters dominated by sodium chloride (e.g. site 1419). Hence the points from a few sites (1414, 1415 & 1416) close to the Thames tend to plot away from the main trend towards more sodium chloride dominated compositions. These sites also have the largest total ionic concentrations indicating addition of water rich in salts.

A number of baseline surveys on the Chalk have identified three major processes that alter groundwater quality in the confined zone. Quite close to the confining boundary, reducing reactions occur followed by dissolution reactions and cation exchange reactions with increasing residence times.

Water that infiltrates into the Chalk is soon saturated in calcite (calcium carbonate) so calcium concentrations reach around 100-150 mg/l. However, as groundwater reaches the centre of the basin, calcium concentrations drop significantly. This decline is mirrored by a corresponding increase in sodium and magnesium concentrations. Clark and Morgan (1987) suggest the following as possible explanations. Cation exchange between calcium and sodium may occur between groundwater and minerals in the Chalk, although the number of exchange sites in the Chalk is small. More likely is that the cation exchange occurs within the Basal Sands rather than the Chalk. Although these sands are largely de-watered at present, it is possible that groundwater compositions were modified by leakage or diffusion in the past when the Basal Sands were in the saturated zone. Another reason for the reduction of calcium concentrations is recrystallization of calcium-rich calcite while incongruent dissolution of carbonates releases magnesium, strontium and other elements into groundwater. Increases in the sodium component over ~30 mg/l are probably due to ancient marine water trapped within the pores of the aquifer. Such an increase will obviously be accompanied by a corresponding increase in chloride concentrations. Potassium does not have such a large variation as sodium. It is high at sites 1414, 1415, 1416 and 1419, where saline intrusion from the Thames occurs, but high values are also found at 0696 and 1053, in the east of the area, where the Lower London Tertiaries have a higher clay component. It is possible that the high potassium comes from reaction with clay minerals.

As for the major anions, bicarbonate, as indicated by alkalinity, remains fairly constant across the area, with the highest concentrations in the oldest water at the centre of the basin. Chloride concentrations also seem to be greatest at the centre of the basin or at sites where saline intrusion is expected.

Sulphate concentrations tend to increase towards the centre of the basin with some of the highest concentrations found under central London (north of the Thames) at sites 1416, 1415 and 1414. If saline intrusion were the main source for the sulphate the highest value would be expected at site 1419, but as it is lower than at some other sites most of the sulphate must have come from an alternative source. The most likely sources are from the dissolution of gypsum (calcium sulphate) or the breakdown of iron pyrites (FeS₂) which are both found in the London Clay and the Lower London Tertiaries.

High chloride concentrations are mainly dependent on saline intrusion from the Thames or connate waters, which is indicated by a weak correlation between sodium and chloride. The pH is more alkaline and more variable than from unconfined Chalk groundwaters. The highest conductivities are found where saline intrusion occurs including site 1591 which lies close to the Thames but up hydraulic gradient.

4.2 Other determinands

Nitrate concentrations from all sites except 1168-1172 and 1510 are low, close to detection limits. Nitrate generally enters the groundwater from anthropogenic sources such as fertilizers or sewage. The London Clay will act as a barrier to infiltration of nitrate rich waters from the surface. Nitrate that does reach confined Chalk quickly breaks down under the reducing conditions. Some of the nitrogen released may contribute to the ammoniacal nitrogen concentrations which increase significantly in the unconfined zone, with the highest concentrations found towards the centre of the basin. The higher nitrate concentrations at site 1168-1172 are due to the fact that the Chalk and Lower London Tertiaries crop out at the Windsor Dome within the catchment zone of the wells. This allows a route for nitrate pollution and also oxygenated water from the surface or the River Thames to recharge the groundwater. Site 1510, which also shows higher nitrate concentrations, lies in the Ravensbourne valley close to the Lower London Tertiaries-London Clay boundary. It is possible that the river has eroded the London Clay sufficiently to allow a passage for some recharge from the surface through fissures, scour holes or even abandoned boreholes.

Orthophosphate concentrations are generally low, being close to detection limits across the area. Similarly Total Organic Carbon (TOC) for most sites is at concentrations within the narrow range of 1-2 mg/l, which is below what might be expected from specific pollution sources. The main exception is site 1419 at Beckton where saline intrusion from the tidal Thames is known to occur.

Strontium, fluoride, bromide, iron and boron concentrations all occur at concentrations in excess of those usually found in the unconfined Chalk aquifer. Generally all of these determinands increase with longer residence times found for groundwater towards the centre of the basin. However, superimposed on this pattern are local variations. Strontium is known to correlate strongly with magnesium in Chalk groundwaters, the increase thought to result from incongruent dissolution of the carbonates at the expense of calcium that recrystallizes as calcite. A good correlation is also observed in this area, especially if the two sites with the highest magnesium concentrations are omitted. Saline intrusion has occurred at those sites and the presence of sea water, which has a high Mg:Sr ratio, upsets the correlation.

The fluorite saturation concentration of about 2000 m g/l is reached at sites 0692, 1053, 1415 and 1505 which are close to centre of the basin, residence time being an important factor leading to the dissolution of fluoride minerals within the Chalk. Sites 1168, 0721, 0833 and 1406, close to the London Clay/Lower London Tertiary boundary have lower fluoride concentrations. Other surveys (e.g. Lee and Clark, 1987) noted lower concentrations of a number of determinands along the Lee, Wandle and Ravensbourne valleys caused by more rapid recharge by immature calcium bicarbonate type waters. Site 1215 in the Lee Valley shows a lower fluoride concentration than other sites at a similar distance towards the centre of the basin (e.g. 1398, 0695). Similarly site 1510 in the Ravensbourne Valley has a low fluoride concentration. Boron appears to follow a similar pattern to fluoride, although with several sites having only a single analysis, caution should be observed in making conclusions. Bromide concentrations are highest close to the centre of the basin (sites 0692, 1415, 1416). The value for site 1215 looks anomalous, while that for 1419 reflects the saline intrusion.

Iron has higher concentrations where anaerobic conditions occur in the Chalk. The highest concentrations occur at sites 1416, 1415, 1512 and 1513 in the centre of the basin. However superimposed on this general trend are significant local variations such as the very low iron at site 1053 while site 0833 is relatively high. The breakdown of iron-rich minerals such as pyrite which are more common in the Basal Sands are thought to be the source of iron. Variations in the composition and degree of saturation of the Basal Sands will lead to a more complex distribution pattern of iron-rich groundwaters. Manganese concentrations are low across the Chalk, often close to detection limit. Only in Central London, sites 1414,1415 and 1416, site 0721 which lies next to unconfined Blackheath Beds and site 1419 at Beckton show any variation from this pattern.

5. North London Artificial Recharge Scheme (NLARS)

Artificial recharge will change the groundwater quality by mixing water types so these sites are considered separately.

Prior to recharge the saturated zone lay mainly within the Chalk across the area covered by the NLARS, with groundwater reaching the Basal Sands at only a few points. Artificial recharge has the effect of locally raising the groundwater levels by up to 4 metres, often well into the unsaturated Basal Sands. The area of the NLARS contains a two well developed 'window' zones, where the Lower London Tertiaries are not confined by London Clay. The larger window lies at the southern end of the NLARS in the Lee Valley near Hackney, while a smaller window lies at the northern end at Enfield. These windows provide a possible route for both infiltration into and aeration of the Basal Sands. Six private abstractions are used to monitor the artificial recharge area.

5.1 Groundwater Quality

Broadly the concentrations for most determinands fall within the ranges observed for the Chalk in the surrounding areas of the London Basin. However there are significant variations for many determinands at each site. For example the range between maximum and minimum observed concentrations for conductivity from recharge sites is at least double that from the ordinary network sites. Such variation is to be expected as recharged surfaced waters will alter the composition of the groundwater on a temporal basis by both mixing and rock-water reactions that follow when groundwater levels are raised into the former unsaturated zone, often into the Basal Sands. The Piper plot (Figure 3) of the waters from the recharge area shows they form a distinct grouping of a calcium sulphate rich water type with the exception of site 1221 that has predominantly calcium bicarbonate water type.

The major cations (particularly calcium, magnesium and potassium) tend to be enriched compared to concentrations from other network sites of a comparable distance down dip in the confined Chalk (e.g. 1232, 0693, 1412, 1215, 0695). The exception to this pattern is site 1221. As for the anions, bicarbonate (indicated by alkalinity) and chloride concentrations from the artificial recharge area are similar to other network sites, but sulphate concentrations are elevated. (Site 1221 and perhaps site 1220 are exceptions).

Of the other minor determinands measured, the majority exhibit concentrations similar to surrounding sites in the London Basin Chalk, with the main exceptions to this pattern being iron and to a lesser extent manganese and strontium, which are more concentrated in the recharge area.

5.2 Discussion

Flavin and Joseph (1983) noticed increases in calcium, magnesium, iron and sulphate from recharge boreholes during artificial recharge experiments in the 1970s. This effect was most pronounced at boreholes that lay near the 'windows' in the London Clay or where recharge water was known to have reached previously unsaturated zones within the Basal Sands aquifer. The concentrations of these ions tended to decrease with successive cycles of recharge and abstraction. Current data are not linked to any specific phases of artificial recharge. Therefore it is not possible to make more detailed comment, other than the observed variations in groundwater chemistry are broadly consistent with those noted by Flavin and Joseph (1983).

The major source of sulphate and iron is believed to be from the oxidation of pyrite in the Basal Sands, although the dissolution of gypsum may provide additional sulphate. Gale et al. (1991) believe that the main oxidant is gaseous oxygen which gains access through windows in the London Clay and through wells dug last century. They also noted that increases in calcium, magnesium, and potassium show a linear correlation with sulphate. This was interpreted as evidence that these ions are flushed from the Basal Sands at the same time as the sulphate.

Site 1221, of all the NLARS monitoring sites, appears to have groundwater chemistry least altered by the effects of artificial recharge. As this site only just lies within one kilometre of the artificial recharge area, it would appear to confirm the premise for the selection of these monitoring sites, that the chemical effects of the artificial recharge are minimal beyond one kilometre.

Environment Agency, Thames Region
Dec 1997. Data assessed for period January 1985 to October 1997

For further information, contact the Groundwater Quality section at:
Kings Meadow House, Kings Meadow Road, Reading, Berkshire, RG1 8DQ.
 

Further Information

Solid and Drift Geology Sheets 255, 256, 257 269, 270 and 271 1:50,000 Series - British Geological Survey

Groundwater Vulnerability Sheet 39 (West London) and Sheet 40 (Thames Estuary) 1:100,000 Series - Environment Agency

British Geological Survey.1996. London and the Thames Valley (4th Edition compiled by M G Sumbler). HMSO.

Water Resources Board.1972. The Hydrogeology of the London Basin.

Clark, L., and Morgan, D.J. 1987. Groundwater Quality in the London Basin. WRc.

Flavin, R.J., and Joseph, J.B. 1983. The Hydrogeology of the Lee Valley and Some Effects of Artificial Recharge. Quarterly Journal of Engineering Geology.

Gale, I.N. et al. 1991. Artificial Recharge in the London Basin: Groundwater Quality Studies. British Geological Survey Technical Report WD/90/56C.
 

Fuente:
Environment Agency
http://www.environment-agency.gov.uk/gwcl/Thames/Chalk-London-Basin/GW_QRepChalkLondon.html

MÁS INFORMES SOBRE CALIDAD DEL AGUA DE LA REGIÓN DEL TÁMESIS EN:
http://www.environment-agency.gov.uk/gwcl/GW_ThamesWQ.htm