Aquifer storage and recovery (ASR) and artificial recharge
Artificial recharge has been an established tool for water resource management in the UK for more than 30 years now. ASR*is a more recent technology and has, in places, had a problematic track record, often due to the marginal nature of the aquifers under investigation. Issues that have been encountered include:
- high rates of mixing combined with poor native groundwater quality leading to poor recovery efficiencies;
- borehole clogging (both from injected fines and from clay mineral breakdown in-situ;
- hydrochemical issues (water-water and water-rock);
- water resource issues (down”‘gradient drift or excessive interaction with unconfined zone); and
- geotechnical issues associated with fluctuating pressures.
However, as available water resources become increasingly constrained and the demand for storage at key pinch points in water supply networks increases, ASR is being actively considered by many UK water companies.
ESI has one of the largest and most experienced teams of groundwater specialists in the UK and is well placed to advise on many of the key issues that can be associated with ASR development.
Scoping Studies
ESI’s team (meet our team here) includes senior staff with many years experience of artificial recharge, ASR, hydrochemistryand water resource assessment. This is combined with an excellent appreciation of water companies’ water resource development agenda and anunrivaledbreadth of experience in the hydrogeology/hydrochemistryof the most likely target aquifers.
Water Resource Issues and Mixing
Most of the ASR investigations to date have been in marginal aquifers, or parts of major aquifers that have not been previously investigated. A clear and well designed strategy for drilling and testing the aquifer is an essential starting point for these investigations. ESI’s senior staff have years or experience of designing, testing and interpreting results from borehole investigations and are thus able to provide practical and well thought through advice on this aspect.
Groundwater modelling has a vital role to play in assessing the likely effectiveness of ASR/artificial recharge schemes and any uncertainty associated with the potential outcome. There are particular benefits in using groundwater modelling in an iterative manner throughout the scoping, investigation and development phases.
ESI has the strongest team of groundwater modellers in the UK andhave modelled a wide range of aquifers and borehole configurations.Particular strengths in the use of FEFLOW have been developedwhich allows both the flow and transport elements of the problem to be modelled simultaneously.
Geochemical Modelling of Aquifer StorageandRecovery
ESI has developed a tool for modelling groundwater chemistry changes in ASR schemes, using the geochemical code PHREEQC. PHREEQC is a software tool that enables simulation of geochemical processes including ion exchange, dissolution/precipitation, dual porosity, kinetic reactions plus many more. ESI hasimplemented a radial flow model within PHREEQC so that geochemical processes withinandaround the ‘bubble’ of fresh water can be simulated. (This is believed to bethe first use of geochemical modelling in PHREEQC, with radial flow, in theUK.)
In addition, ESI hasdeveloped the ability to use probabilistic analysis in PHREEQC to model the range of potential outcomes of hydrochemical reactions (once agin this is believed to befirst time this has been done in theUK). This can be used to assess reasonable worse-case scenarios in terms of solute concentrations, duration of the settling-in phase, etc.
Together, these provide a valuableandstate-of-the-art set of tools to assess the impact of natural variability of groundwater chemistryandphysico-chemical properties of the aquifer on the viabilityandpotential constraints on the operational effectiveness of the proposed ASR schemes.
This model has been used to investigate hydro-geo-chemical issues in a proposed ASR scheme in which the injection of oxygenated water into a confined aquifer was giving rise to mineralogical changes and the release of minerals into the water with potential impacts on the effectiveness of the scheme. The model proved capable of simulating the observed field results and provided critical insights into the processes occurring. This provided the developers with the information they needed to take the scheme forward into production.
*Distinguished from artificial recharge by the use of the same point for injection and abstraction and by general targeting of aquifers that would not otherwise be considered as water resources in their own right.