My client, a brewery in North West England, operates a groundwater abstraction to provide water for the brewing process. A reverse osmosis plant was installed in 2004. Since then water quality from the borehole has deteriorated; this has been monitored directly and has been experienced as increasingly frequent replacement of the plant membranes. I was asked to assess the available data, take and analyse water samples, and provide advice to recommend a remedial strategy.
On 30 December 2013 I visited the site and monitored water quality throughout the period of pumping (which was continual). Change in water quality through the day was significant; changes in electrical conductivity and TDS are shown below.
Five water samples were taken for more detailed water quality analysis and it was clear that the relative concentrations of dissolved species changed with time. As pumping progressed, the water became enriched in magnesium and calcium sulphates, as shown below.
The explanation is down to borehole construction. The principal local aquifer is the Chester Pebble Beds Formation (a coarse, pebbly, well-fractured sandstone). At the borehole this is 80 m thick; beneath is c. 50 m of low permeability Manchester Marl Formation, then the borehole penetrates 35 m of Collyhurst Sandstone Formation, a poorly-cemented, fine-grained sandstone. Water quality in the Collyhurst Sandstone aquifer is relatively poor.
Rest water level in the borehole is about 20 m below ground, leaving a saturated thickness of 60 m in the Chester Pebble Beds. Pumped water level is around 40 m below ground. Throughout the day pumping therefore reduces the saturated thickness (and transmissivity) of the Chester Pebble Beds by 66%. Hence the decreasing water quality from the borehole is related to an increased reliance on water sourced from the Collyhurst Sandstone.
Geochemical modelling of sampled borehole water quality shows that the waters are over-saturated with iron oxide and oxy-hydroxide phases. Iron oxides are expected to have been precipitated onto the borehole wall, which appears to have led to the observed progressive deterioration of pumped water quality.
Borehole rehabilitation was the recommended solution to restore the previous water quality. The recommended solution was to backfill the bottom 75 m of the borehole to exclude water from the Collyhurst Sandstone, coupled with cleaning of the hole that is open to the Chester Pebble Beds. Hydraulic performance would need to be assessed before and after works to ensure that they have had the desired effect.