OR/15/042 Localities

MAURICE, L, FARRANT, A R, BUTCHER, A AND ATKINSON, T C. 2015. Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015. British Geological Survey Internal Report, OR/15/042.

Mirams copse stream sink

Mirams Copse stream sink (Figure 7) is a fairly typical chalk stream sink. It is situated about 80 m above sea level and is fed by small springs in the Palaeogene and some very local surface runoff. It has a maximum flow of about 5–10 l/s, when water sometimes backs up to form a small pool. It has no flow following dry periods. The stream sink is located on thin Palaeogene deposits overlying the Chalk, and the unsaturated zone is about 17–24 m thick.

Initial tracer testing using the optical brightener Photine CU produced negative results in 6 different tracer tests that involved injecting progressively more tracer (125 to 5000 mls of a 20% solution). Monitoring was carried out using passive cotton detectors at 13 spring, borehole and river sites (Figure 8). During a later test using bacteriophage, tracer was conclusively detected 1.7 km away at the Blue Pool spring. Groundwater velocities were very rapid — 4.3 km/d based on first arrival of the tracer. However tracer attenuation was high: <0.00005 % of the tracer was recovered. This was a big contrast to tracer tests from Smithcroft Copse stream sink (Figure 8) which showed similar rapid flow rates of 5 km/day but combined with much lower attenuation (about 25 % of the tracer was recovered).

Figure 7 Mirams Copse stream sink.

Figure 8 Tracer testing in the Pang catchment.

Rushall Farm Quarry

Streams sinking into the Chalk commonly do so at or near the boundary between the Chalk and the overlying Palaeogene deposits. The unconformable contact between these strata is often characterised by dissolution pipes which are visible in quarries, and present a geotechnical hazard during engineering projects.

The Quarry at Rushall Farm gives a clear section through the Palaeogene- Chalk contact (Figure 9). The unconformity surface is marked by small scale dissolution pipes infilled with sand and clay derived from the overlying strata. Some small conduits are visible in the Chalk (Figure 10).

Figure 9 The quarry at Rushall Farm in 2004.

Figure 10 Small scale conduits exposed in quarry face, 2004.

Blue pool spring

The Blue Pool spring comprises a series of powerful upwellings that form several pools (Figure 11). The spring is at an elevation of about 55–60 m above sea level. The upwellings combine to form a single channel which discharges into the River Pang (Figure 12). The Blue Pool has a fairly consistent flow of around 200 l/s and continued to have a good flow throughout the 1976 drought. Water level monitoring over the past few years in the outlet channel has revealed a seasonal increase in stage, and a rapid response to rainfall that only occurs after the seasonal increase in stage.

Figure 11 Layout of the Blue Pool Spring. Contains Ordnance Survey data © Crown copyright and database rights 2015.

Figure 12 The Blue Pool outlet channel.

The first tracer tests demonstrating rapid groundwater flow to the Blue Pool were carried out from Holly Grove and Tylers Lane stream sinks (Banks et al., 1994). Subsequent quantitative tests revealed rapid groundwater flow from Smithcroft Copse and Mirams Copse stream sinks, combined with variable attenuation (Maurice et al., 2010). Attenuation can be low (Figure 13). 610 grams of a 70% solution of Sodium Fluorescein injected at Smithcroft Copse stream sink was visible at the Blue Pool, 5.1 km away, after a travel time of only 24 hours. The dye concentration at the spring was about 60 ug/l, and the tracer recovery was about 20%. This clearly demonstrates the vulnerability of the Chalk aquifer to pollution.

Figure 13 Low attenuation over 5.1 km.

Calversley Farm Borehole

Borehole investigations in the Chalk have revealed that most groundwater flow occurs through dissolutional fissures and conduits (Schurch and Buckley 2002; Maurice et al., 2012). Calversley Farm borehole is an Environment Agency groundwater level monitoring borehole. Imaging of the borehole walls has shown that it intercepts a number of dissolutional fissures and conduits, and that these may be sediment filled (Figure 14). We will use a borehole CCTV system which can be moved around to look at the borehole walls from different angles.

Figure 14 Dissolutional conduit and fissure in Calversley Farm borehole.

Groundwater ecology sampling has found a diverse community of invertebrates in Calversely Farm borehole including Niphargus kochianus, copepoda, springtails, nematoda, oligocheata and mites. The most common invertebrate found in the borehole is the amphipod Niphargus kochianus (Figure 15). This is a stygobitic species, usually between 0.5 and 5 mm in length, which is only found in groundwater, and is specially adapted to live in an environment with no light and limited resources. Genetic work has demonstrated that the British population of Niphargus kochianus separated from European populations about 2.9 million years, so it is an ancient endemic lineage that provides an important contribution to biodiversity (McInerary et al., 2014). Stygobitic invertebrates are widespread in the Chalk and a recent study found them in more than 70% of boreholes sampled. Whilst we have not observed the stygobitic invertebrates in Calversley Farm borehole with CCTV before, we have observed them using CCTV at another borehole in the catchment (Figure 16).

Figure 15 Niphargus kochianus.

Figure 16 Stygobite observed in Cow Down borehole using CCTV.

Yattendon stream sink

The Yattendon stream sink is an unusual example of a Chalk stream sink where the stream can be observed to sink into a cavity within the Chalk. At this site, a small stream has eroded through the Palaeogene material to reveal the Chalk underneath (Figure 17). The water sinks into a narrow vadose canyon, disappearing into a chalk cave about 30 cm wide and several metres deep (Figure 18). Most streams sinking into the Chalk sink into clay and sand deposits overlying the Chalk and therefore the karst features within the Chalk cannot be observed. It is possible that many other Chalk stream sinks are underlain by features like this which are concealed under the Palaeogene sediments.

Figure 17 Yattendon Chalk cave.

Figure 18 Schematic cross section of Yattendon Chalk Cave.