Lets talk response zones

Response zones are important

A properly designed response zone is essential to good quality groundwater data.  Unfortunately, response zones are still poorly thought out and because of this the data collected is really very limited or in some cases completely wrong and misleading, driving inappropriate recommendations; costing clients time and money and increasing risks to the environment and health.

Here are just a few reasons everyone working in land quality assessment, construction or managing land assets needs to care about groundwater response zones:

  • Get it wrong and you could miss a shallow water table, leading to a recommendation of soakaways as SuDS, these will not work in the saturated ground, resulting in flooding after every heavy rainfall.
  • Get it wrong and you could miss a shallow water table, leading to excessive and unexpected dewatering during excavation work, not forgetting that in some cases dewatering in construction could require a groundwater activity permit and a discharge consent
  • Get it wrong and you could create migration pathways for contamination, leading the landowner to be responsible for causing, or knowingly permitting discharge of hazardous substances to a potable water supply (groundwater aquifer)
  • Get it wrong and you could mistake confined groundwater for shallow groundwater, adopting expensive drainage options when soakaways actually would have worked.

This is not an exhaustive list but are four very real examples Hart Environmental have seen in the last 12months and in all cases the cost and delay to rectifying could have been avoided by properly designed response zones.

Suitably designed response zones are also important for gas monitoring and for collection of geotechnical data.

How many response zones should there be?

It is not the case that one borehole or one response zone can deliver all monitoring needs, how many are needed and where these are targeted must always be a site-specific assessment considering exactly what data is required[1].  Does the geology match what was expected; is there multiple water tables; is free phase contamination anticipated; is hydraulic conductivity testing required; where is the ground gas source; how might gas (or vapour) migrate; will gas response zones be saturated by rising groundwater?  Just some questions that should be asked every time a monitoring well is designed and certainly before each installation.

A deep borehole might be advanced to establish the depth to rockhead for foundation design, installing this borehole with a monitoring standpipe setting the response zone to span 1m to 30m might seem ‘economically’ sensible capturing all likely groundwater strikes.  Then using this same standpipe to monitor ground gas ticks another box – saving the Client money, wining the job by the lowest quote…  These installations are still far too common, and in reality, all that is being achieved is increasing uncertainty that will usually result in poor and inappropriate site conceptualisation.  We add ‘usually’ in the above statement because, although we have not come across a single example that disproves the statement, we do have complex geology and groundwater across the UK and design should always be site specific, so it can be unwise to completely discount something in the world of land quality assessments!

So what is a response zone?

The response zone is the slotted section of pipework in a well that intersects the ground, groundwater or gas source being monitored.  Environment Agency guidance on groundwater response zones[2] (a must read), recommends that these are generally no longer than 3m, although in some cases, based on the conceptual site model there may be cause for longer ones.

Environment Agency guidance also states that a response zone should never link aquifers (continue from shallow groundwater through low permeable layer(s) into a deeper aquifer).  Other than the huge uncertainty this creates with data collection and migration pathways, it also presents a risk to water resources, providing pathways for aquifers to drain, limiting abstractable water supplies and threatening surface water basal flows.  One poor installation might result in negligible effects, multiple poor installations start to cause serious issues.

And lastly but, by no means least, after the monitoring has been completed all monitoring wells should be decommissioned properly following appropriate guidance2.  Decommissioning wells means filling the standpipe with low permeable strata such as bentonite or grout; or removing pipework and the gravel anulus and sealing the entire void.  The design of decommissioning works depends on the design of the well and surrounding environmental setting.  The purpose is to ensure monitoring wells do not present long term migration pathways for contamination, groundwater or gas.

[1] https://www.claire.co.uk/information-centre/cl-aire-publications – the CL:AIRE technical bulletins include lots of advice for designing response zones and multilayer investigations for a range of media

[2] Environment Agency Guidance on the design and installation of groundwater quality monitoring points, Science Report SC020093, 2006