Harrow and Hillingdon Geological Society

Hydrology of Hertfordshire

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The hydrogeology of Hertfordshire

Rob Sage
Assets Manager – Water Resources
Veolia Water Central

Veolia Water comprises 3 water companies, Central (formerly Three Rivers Water Co.), East (formerly Ipswich Water Co.) and South-east (formerly Folkestone Water Co.), which have been French-owned since privatisation in 1989. They provide water only, with sewerage services provided by other water companies (Thames Water, Anglian Water and Southern Water, respectively). Serving a population of 3.5 million, Veolia Water provides 900 megalitres per day (MLD) on average and 1,210MLD at peak times. It has 16,172 km of mains, from which leakages are about 150MLD.

6 major river and reservoir sources provide 40% of the water with 148 groundwater sources providing the remaining 60%. Groundwater is extracted from 3 aquifers, predominantly the Chalk but with minor amounts from the Thames gravels at Chertsey and from the Greensand in the north of the area. Veolia Water also imports from and exports to other water companies.

There is a large concentration of groundwater sources in the River Colne valley and on the Chalk escarpment but to the north and east there are fewer and the yields are lower. Groundwater flow is roughly downdip into the Colne and Lee catchments.

How do boreholes work?

Pumping creates low pressure in the borehole and water flows from high head to low. This causes drawdown of the water table and creates a cone or radius of influence. After some time, flow stabilises and the borehole continues to remove water from the aquifer. Boreholes are subject to natural variations in groundwater level due to changes in rainfall. Pumping is from the depth of the water table plus the drawdown after flow stabilises. Some boreholes are pumping water from over 100m below surface, which requires more energy (the biggest single cost to Veolia Water).

Source capacity

The yield (known as the deployable output) of sources depends on the reliability of abstraction and water level data. The 2005-2006 drought provided good data, which was different from that in the past. It will be affected by the impacts of climate change. Significant improvement in capability has been achieved during AMP4 (Asset Management Planning period 4 – 2005-2010 – the system used by OFWAT – the Office of Water Services regulates the investment requirements and pricing arrangements for the water and sewerage industry for the next 5-year period). We are now in AMP5, from May 2010. The company are looking for ways to improve the yield/drawdown performance. This can involve re-acidising or other development techniques to improve the flow of water into boreholes or re-drilling (at a cost of about £100,000. In summary the company are looking to optimise sources so as to achieve minimum drawdown for the maximum production of water.


Aquifers are permeable rocks that transmit water. The Chalk is a dual-porosity aquifer with slow movement through pores and rapid movement through fissures. It relies on recharge (essentially from rainfall) to replenish water in the aquifer. Discharge of water is to springs, rivers and abstractions. The water is in a constant state of flux with flows from high heads to low. The aquifer is unconfined where the Chalk crops out at the surface and confined where it is covered by later Tertiary deposits. Flow rates vary from hours to thousands of years. Flow in the upper part of the aquifer is fast but deeper down it is slower, resulting in long residence times in the confined aquifer. This enables dissolution of minerals from the rocks and reduces the quality of water.

Water-level fluctuations

Rainfall and evaporation are the main drivers for fluctuations in water level. They operate on an annual cycle, with rain falling in about equal amounts each month but evaporation increasing greatly during the summer and being very low in winter. Water levels depend on the balance between recharge and discharge and there is a constant state of flux. Recharge also requires the right sort of rain – a steady fall spread over a period of time is much better for recharge than a storm with large amounts falling in a short period, since much of the storm rainfall runs off over the ground rather than soaking into the ground. Varying flow in rivers depends on the balance between discharge from the aquifer and run-off over the ground.

The hydrograph from one borehole showed that the 1980s expe4ienced average water levels. The prolonged drought of 1996-1997, which had very little winter recharge for two successive winters, resulted in record low levels and the 1 in 1,000-year events of the winter of 2000-2001 resulted in record high levels. In April 2006, a hosepipe ban was imposed because long-range forecasts indicated that there would be a second successive winter of low recharge. This did not, in the event, happen and the ban was lifted in February 2007. This winter has had 80-85% of the average rainfall and water levels are relatively low – water levels only started to rise in January, as opposed to November in the average year.

Chalk streams

In winter springs discharge and there is an increase in flow downstream as more water comes from the aquifer. In summer, water levels are lower and springs discharge further downstream, producing the winterbourne effect of an ephemeral portion of the stream. Chalk groundwater is clear and generally at a constant temperature of 11oC (the average annual temperature) and many of the organisms in the streams are favoured by this. Borehole abstraction will not generally lower the groundwater level below the river but it may impede flow of water towards the river.

Environmental impacts

Chalk rivers are a rare and endangered habitat and low flows are a major concern. The Veolia Central region had 3 of the original 1991 Aggravated Low Flow projects, which were implemented in AMP2 (1995-2000). A major national environmental protection programme has been in operation covering 2000-2015 and sustainability changes have already been noted for post-AMP5 (2015 onwards). There will be more to come but it is difficult and expensive to replace water and there seems to be little regard paid to cost-benefit analysis. Stopping pumping in some boreholes has had little or no impact on low flows in some areas. The example was cited of the River Beane, which flows through Stevenage. Its flow is actually exceeded by the flow of sewage from the town, which is piped directly to a sewage works in the Lee Valley. A smaller works at Stevenage would enable treated sewage to be fed back into the Beane but this would be an uneconomic proposition.


Pollution is a major issue and many sources are impacted by nitrates, pesticides, legacy landfills and industrial sources contributing turbidity and microbiological pollution. In the unconfined Chalk there is little protection, though locally puttied Chalk provides an effective seal, and the rapid karst flows mean that the threat of pollution is constant. It is normally managed by installing treatment, blending or sometimes abandonment/replacement of sources. Investigation and removal of the source of pollution is very difficult and expensive, legally complicated and subject to long time delays. In consequence, polluter pays is not often the end-result. The company has drinking water safety plans approved by the Drinking Water Inspectorate.

Impacts on Chalk

Chalk is the main aquifer and comprises blocks separated by fissures. Water movement within the blocks is slow (less than 1m per year) but is does occur while the source is contributing pollution. If the pollution source is removed the blocks, which are hard to clean, may well contain most of the pollution which has migrated into them over time. Meanwhile, rapid flow in the fissures will have moved the pollution away but pollutants will leach from the blocks into the fissures, allowing rapid migration away from the source. Leaching fr4om the blocks is a very long-term process, which may take 100 or more years. Fissures are easier to clean.

In one case, 10,000L of red diesel was spilt on gravels overlying the Chalk. Action was immediately taken with a sump dug in a bid to recover the fuel by direct pumping. The water/diesel mix was passed through separators and diesel was recovered at a rate of 1,000L per day. This has been ongoing for 5 years and there is still some diesel there.


Bromate is a major pollutant of the Chalk aquifer. It derives from a chemical plant in St Albans, which closed and had the buildings demolished. It was then left as an open site, on which rain could fall and leach pollutants on site directly into the aquifer, for a number of years before housing was constructed and effectively removed the source of pollution by preventing further infiltration. The bromate pollution was first discovered in 2000 and was impacting both Veolia Water and Thames Water Utilities Ltd. A remediation notice under Part IIA of the Control of Pollution Act 1994 was issued in 2005, which the polluter appealed. Following a public inquiry in 2007, the Secretary of State for the Environment issued a remedial notice in July 2009, which enables Veolia Water to claim from the polluter the cost of all works carried out to remediate the problem since its issue. Costs incurred before July 2009 cannot be claimed. Monitoring is ongoing and Veolia Water is operating a pump-and-treat system to protect downstream sources.

Bromate occurs in high concentrations from St Albans to Hatfield. Further away, the pollution is more dilute but the zero impact is as far away as Hoddesdon and almost to Cheshunt, 20-22km away. The Chalk has many swallow holes, by which water enters the aquifer and karst flows mean travel tiems from entering the swallow hole to discharging at springs can be 2-3km per day. A new conceptual model of bromate flow shows a big karst system from Potters Bar flowing north past Hatfield into the main River Lee. However the pollution plume suggests there is possibly an additional pathway to the south and maybe others too.

At the Hatfield borehole, bromate is 3-400μg/L and at Essenden up to 50μg/L. The Hatfield water source was abandoned and at Essenden a blending solution was adopted but this can take a maximum of 30μg/L. In mid-2005, bromate levels at Essenden were seen to be rising but reabstraction from Hatfield to remove bromate reduced the impact at Essenden, allowing the blending solution to continue. Because of the high iron content (used to remove the bromate), water abstracted from Hatfield goes to the sewage works for treatment.


The Buncefield explosion was recognised early on as having potential to impact on Veolia Water Central sources. The first concern was overland flow from the surface water drainage containment site at Buncefield down a dry valley to a Veolia source on the River Ver. In the longer term, there could be escape of fluids percolating down to the water table. In particular, a nearby road has soakaway drainage to within 2-3m of the water table.


Groundwater is a critical component of the water supply available to Veolia Water Central. There is little scope for further development to improve efficiency. There are major concerns on environmental issues related to low flows in Chalk streams. Contamination remains a major threat.

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