Options for water supply in Cambridge.

CambridgeshireLive news recently reported that plans for new homes in Cambridge could be stymied by lack of water supply. I have been reading the report behind these findings [1]. It is good to know that the water supply constraints are being taken seriously at last  – and that solutions are on the table too. Demand side measures include leakage reduction, rainwater harvesting in new developments, and prioritising public water supply over agricultural use. Measures to increase supply include land management schemes to improve water retention and possibly hooking up to a new reservoir in Lincolnshire.

The minimum growth scenario is higher than Cambridge Water have allowed for.

Cambridge City Council has now commenced drafting the new local plan and are considering three growth scenarios with between 36,700 and 57,000 new dwellings over 20 years. (This includes those already in the current Local Plan.) The lowest scenario is based on the minimum under national policy guidelines. The council is not allowed to consider lower rates that this. However even this exceeds the growth that Cambridge Water Company planned for in its Water Resources Management Plan (WRMP), though they used the rates that were expected at the time (see chart)

Chart showing growth scenarios, Cambridge Water's plan in red from [1]

We already take too much water and climate change is making this worse.

All stakeholders are agreed that current water usage is too high. The chalk aquifer that supplies our water also supplies chalk streams that in turn feed into the River Ouse and the Cam. They are in poor condition due to chemicals and low flow rates. The chemicals come from agriculture and road runoff and in some cases sewage outflows. Higher flow rates would help to dilute the chemicals but low flow is becoming the norm and climate change is making this worse due to more frequent dry years. Cambridge Water company operates well within its licence conditions, but the Environment Agency is expected to reduce them. 

We could prioritise water for people, not agriculture.

Cambridge Water is not the only company abstracting water from the aquifer. Farmers also have licences to take water for irrigation. One possibility is for these users to trade in their licences and build reservoirs on their land instead. This should leave more water for the public supply. (However, agriculture may need more irrigation rather than less due to climate change.)

We could retain more rain water to recharge the aquifer.

Historically, flood management has meant ensuring that heavy rain is channelled as quickly as possible out to the sea. This is a waste - retaining the water on a flood plain makes it available for use later. It would help maintain flows in streams and rivers and even recharge the aquifer. There will be funding available for this sort of project from Environmental Land Management Schemes (ELMS) that is part of the new national agricultural policy. There is going to be at least one pilot scheme in Cambridgeshire. With climate change bringing more extreme weather events this approach could be very important.

New dwellings will use less water but it is not clear how much less. 

The less water the new homes use, the more homes we can have. The national average water consumption (for domestic use) is 141 litres/person/day. Cambridge Water customers are more careful than this: last year we used 131 l/p/d [2]. The national standard for new homes is 125 l/p/day but local authorities are allowed to set a tighter standard of 110 l/person/day in planning controls. Obviously, Cambridge will do this. 

Eddington uses 40% less.

However, it is possible to do much better than that. At Eddington, in North West Cambridge, rainwater and surface water run-off are captured for re-use, and the open water storage ponds are incorporated into a pleasant green landscape for leisure and wildlife. This water receives some basic treatment on site and is used for flushing toilets and washing clothes; it is still not suitable for drinking. The net result is that Eddington customers take less than 80 l/p/day from the main supply: 40% less than the Cambridge average. 

Unfortunately it is not practical to retrofit this sort of infrastructure into existing developments and it is only economic when done on a large scale. This is one reason why the best options for housing growth are for large new developments rather than infill in existing villages or in the city.

Leakage can be reduced.

As of 2017, the national average leakage rate was about 23%. This is average for Europe, though higher than France (20%), Germany (7%) and the Netherlands (5%) [3]. The current rate in Cambridge is about 15% but their WRMP plans reduce this to less than 13% by 2024. With other efficiency savings, they expect to reduce demand by 4 Ml/day, out of current demand just over 80 Ml/day. However the expected growth will use up this margin quickly.

A new reservoir in Lincolnshire.

All Cambridge’s water currently comes from aquifers. Some parts of these are more heavily used than others but there is very limited scope to increase the overall supply. In fact the whole region is struggling to find enough water supplies and Water Resources East, which covers all of the east of England has plans for a new large reservoir in Lincolnshire. There are a number of water companies partnering on this project and Cambridge was not one of them but could be. In any case, it will be 2035 before water from the reservoir would be available.

Imported water has to be kept separate.

Apparently it is not practical to allow imported water (such as from the proposed reservoir) to mix with the water from the chalk aquifer. The report says: Water quality and chemistry concerns mean that it is not practical to transfer mix water sources within the existing network.

This is another reason for growth to be in large new developments as they can be built with a separate water supply that will be fed by imported water. It is also possible to separate existing bits of the network if they are reasonably isolated already, such as Cambourne. However growth in Cambourne is currently constrained by capacity of water recycling facilities (i.e. sewage treatment).

A variety of options but they will take time.

As is often the case there is no magic bullet solution for our water supply. It is going to take a whole combination of measures to maximise water availability and make sure we use the water we have sensibly with less waste. Some of the solutions, in particular hooking up to the proposed reservoir in Lincolnshire are going to take time and it is likely that growth will be constrained at least in the short term. But even with no growth we would still need to make changes because our streams and rivers are already in poor condition and climate change is making this worse.

[1] Greater Cambridge Local Plan Strategic Spatial Options Assessment: Integrated Water Management Study, November 2020

[2] Discover Water - the amount we use

[3] Reality Check: Have water companies cut leaks by a third? (BBC) August 2018