Karst hydrogeology of the Bedhampton and Havant springs

The Chalk of the South Downs, like other limestone areas of the UK, hosts features indicative of a karst landscape, where drainage is underground through dissolutionally enlarged cavities, via features including:

• caves
• dolines
• dry valleys
• large springs
• solution pipes
• stream sinks

The Bedhampton and Havant spring complex in Hampshire is one of the best examples of Chalk karst springs in the UK. The springs are large, with a combined flow of approximately 104 000 m³/day (Atkinson and Smith, 1974) — enough to fill 40 Olympic-sized swimming pools every day. Such a high discharge is typical of karst springs fed by discrete open conduits.

Their karstic nature has also been demonstrated by tracer testing from stream sinks in the Horndean area to the springs (Atkinson and Smith, 1974; Barton et al., 1996). The springs are used as a drinking water source by Portsmouth Water Ltd.

The geology, surface karst, and the total catchment area of the springs, based on the modelled drinking-water source protection zones, are shown in Figure 1. Figure 1 also divides the catchments into three ‘karst zones’ using the method developed by Maurice et al. (2006) in the Berkshire Chalk.

The karst zones are defined on the basis of the geology as well as the density and type of surface karst features present. This is an overview of the karst hydrogeology in the catchment of the springs, based on available data for the catchment and current knowledge of the nature of Chalk karst in England.

Geology

The Bedhampton–Havant region is underlain by Upper Cretaceous-aged chalk and sands, silts and clays of Palaeogene age.

The Chalk Group comprises a soft, white, porous limestone, deposited between about 100 and 65 million years ago. It can be divided up into nine different formations, each characterised by differences in their physical characteristics (lithology) as well as the presence or absence of flint bands, marl seams, sponge beds and hardgrounds.

The Chalk is overlain by a sequence of Palaeogene sediments, 65 to 23 million years old. The oldest is the Lambeth Group, dominated by the clay-rich Reading Formation. This is overlain by the London Clay Formation. The youngest rocks in the region are the sands, silts and clays of the Bracklesham Group. These clay-rich units are impermeable and so support surface drainage.

Parts of the Chalk outcrop are covered in younger, superficial deposits including alluvium and the Clay-With-Flints Formation. The latter is a residual deposit derived from the weathering of the Palaeogene strata. As its name implies, it comprises a flinty clay, which can be several metres thick.

In this area, the rocks have been folded into a large, east–west-trending synclinal (trough) structure known as the Chichester Syncline. The Chalk of the South Downs dips southwards, disappearing beneath the overlying Palaeogene sediments, which are at the surface in the core of the syncline, before rising up to the surface along the Portsdown anticline (ridge) (Figure 2). The Bedhampton and Havant springs are found at the boundary between the Chalk and the overlying Lambeth Group, on the southern side of the Chichester Syncline.

Karst geomorphology

Several types of karst feature occur in the Bedhampton–Havant area.

Stream sinks

Stream sinks (also known as swallow holes) are surface karst features where water from surface streams sinks rapidly into the ground. In the Bedhampton–Havant area, they generally occur where drainage from the overlying Palaeogene clay-rich sediments sinks into the Chalk (Figure 3). Stream sinks are therefore concentrated along the boundary between these rocks at the surface, notably around Soberton Heath, Lovedean, Horndean and Rowlands Castle, where about 50 discrete sinks were reported (Barton et al., 1996). However, some of these may have been dolines and many of the hydrologically active stream sinks have been built over or culverted, so it is difficult to assess how many there were.

Dolines

Dolines are depressions on the ground surface caused by the dissolution of the rocks beneath and can be common in the Chalk, especially in association with the Chalk–Palaeogene boundary. For example, in Dorset, up to 157 dolines per square kilometre are reported by Sperling et al. (1977). In the Bedhampton–Havant area, they appear to be locally common along the Chalk–Palaeogene boundary, and some are associated with stream sinks. Away from the boundary, dolines become progressively less common.

Dolines may be ancient stream sinks that are no longer active, or they may have been formed by other karst processes. For example, many dolines in the Chalk are likely to be formed by a process called ‘suffosion’, in which loose material such as soil, sand or silt is present above the Chalk and is gradually washed down into fissures within the Chalk itself. This causes the loose material to slump into these fissures, creating a depression at the surface.

Some dolines may have been infilled and others may have been dug deeper or wider by humans in the past to enable extraction of chalk for brickworks or lime. The presence of dolines is indicative of karst; however, it can be difficult to distinguish natural karst dolines from artificial chalk pits dug hundreds of years ago. Some surface depressions may be wrongly classified as dolines (Prince, 1964).

Solution pipes

Sediment-filled solution pipes can also be extremely common in the Chalk, especially beneath the Clay-With-Flints Formation (Waltham et al., 2005), so it is likely that these will be present in the Bedhampton–Havant area. These solution pipes can be up to 10 to 15 m deep, but have no surface expression (Figure 4).

Karst zones

Using a method developed in the Pang–Lambourn catchments in Berkshire (Maurice et al., 2006), the catchment for the Bedhampton and Havant springs can be divided into three karst zones based on the distance from the Chalk–Palaeogene boundary (Figure 1).

• Karst zone 1 is characterised by frequent stream sinks and dolines associated with the boundary
• Karst zone 2 is an intermediate area where the Clay-With-Flints Formation is present (not shown on Figure 2). Dolines and solution pipes are likely to occur in this zone, but few hydrologically active stream sinks are present
• Karst zone 3 is the area furthest from the geological boundary, where stream sinks are absent and dolines are rare

Karst hydrogeology

Groundwater contours indicate the elevation and gradient of the water table. Contours reported by Day (1964) suggest that the general flow direction within the catchment of the springs is from north to south, with focused concentrations of flow down valley features (Figure 5).

Tracer tests

Tracer tests can be used to determine connections between swallow holes and springs or boreholes, and to demonstrate how fast the water is moving in the subsurface. They involve tracing water as it moves through the subsurface by introducing a harmless substance, called a tracer, into the ground and sampling the water at springs or boreholes to detect the tracer.

Tracer tests have been carried out from five swallow holes located on the Chalk–Palaeogene boundary (Atkinson and Smith, 1974; Barton et al., 1996). Tracer from four of these swallow holes was detected at both the Bedhampton and Havant springs, demonstrating very rapid groundwater flow of several kilometres per day, based on time to first arrival of the tracer at the springs (Figure 1 and Table 1).

These velocities are extremely high and suggest flow through a well-connected system of karstic conduits and fissures extending over distances of many kilometres. The highest velocities of more than 10 km/day from the Rowlands Castle No. 39 swallow hole are substantially higher than the mean velocity of 1.94 km/day reported for 3015 tracer tests in karst aquifers by Worthington and Ford (2009).

The amount of tracer detected can also be used to calculate the proportion of injected tracer that has been recovered, which gives an indication of how much dilution or filtering of the tracer there has been. Atkinson and Smith (1974) reports tracer recoveries of 69.7 per cent for the test from Hazleton Wood swallow hole, which indicates relatively little loss of tracer along the flowpath, or relatively low dilution at the springs. Tracer recoveries were not reported for the tests by Barton et al. (1996).

A tracer test was also carried out from a swallow hole at Lovedean (#13) that demonstrated a connection with the Lovedean Well about 0.9 km away. The recorded velocity of 0.6 km/day was slower than those in the tracer tests from the swallow holes, but was still rapid.

The tracer tests demonstrate the presence of karstic groundwater flow linking the stream sinks along the Chalk–Palaeogene boundary to the springs in the southerly parts of the catchments. The nature of karst in the Chalk for the more northern parts of the catchments is more difficult to define as there have been no studies of it there.

Transmissivity

Other measurements of the rate at which water flows through the rock (‘transmissivity’) can be used to indicate where solutional fissures and conduits may be occurring, as high transmissivity is likely to indicate their presence. However, there are no transmissivity records within the catchment. The nearest transmissivity measurements (Allen et al., 1997) are 10 to 20 km to the east and are highly variable. They include fairly low (57 m²/day) and very high (9100 m²/day) values within karst zone 2 (Figure 6).

Karst zones 2 and 3 in other Chalk areas

Several studies in other areas of the Chalk in England provide insight into the nature of karst in areas away from the Palaeogene outcrop. Some have demonstrated the presence of connected karstic conduits and fissures in karst zones 2 and 3.

Conclusions

The Bedhampton and Havant karst springs have been proven to be connected to karstic stream sinks located along the Chalk–Palaeogene boundary, in karst zone 1. It is also likely that connected networks of dissolutionally enlarged fissures and conduits are present within karst zones 2 and 3 and contribute to the spring discharge. Whilst it may be more likely that they occur beneath the dry valleys, the location of such networks is not known and they may occur less frequently than in karst zone 1.

Further reading