Groundwater monitoring in Lancashire

Groundwater-bearing rocks

Figure 1: baseline monitoring network (with Superficial geology).

Figure 2: baseline monitoring network (with bedrock geology).

There are two significant aquifers across Lancashire: a shallow aquifer formed of superficial glacial sand and gravel interbedded with clay (Figure 1), and a deeper aquifer below formed by the Sherwood Sandstone (Figure 2).

The shallow aquifer is up to 40 m thick and is designated by the Environment Agency as a Secondary B aquifer. It is used for private drinking water supply, farms and golf course irrigation. In the area of the proposed shale–gas sites, this aquifer is underlain by a thick layer (up to 350 m) of a low–permeability mudstone, the Mercia Mudstone. Water moves slowly through this mudstone and it is not classed as an aquifer. Below this is the Sherwood Sandstone, which reaches a thickness of up to 750 m. The Sherwood Sandstone is classed by the Environment Agency as a Principal aquifer.

The Sherwood Sandstone is too deep to be practically accessible in the area below the Fylde. However, to the east of the proposed shale–gas exploration (east of the Woodsfold Fault – see Figure 1), the aquifer is much closer to the surface. In this area it is used for both public and private water supply.

The aquifer units overlie the deeper shale that is the proposed shale–gas target. In the area of the proposed gas–exploration sites, the shale units occur at some 2000 m below surface.

Water sampling

BGS has established a network of water sampling sites consisting of existing boreholes, wells and streams within a radius of some 10 km of the proposed shale–gas exploration sites (Figures 1, 2). The network comprises 15 groundwater sites from the Superficial and Sherwood Sandstone aquifers and 11 streams. We have been monitoring the water quality at these sites at quarterly frequency since February 2015.

In addition, we have drilled new boreholes in the vicinity of the proposed gas–exploration sites (Figures 1, 2) for more detailed groundwater investigation. These have been monitored quarterly as for the groundwater sites in the monitoring network since February 2016. A new deep borehole into the Sherwood Sandstone aquifer (500 m deep) is providing new samples for characterisation of deep groundwater below the proposed exploration sites.

Groundwater quality monitoring

Monitoring of existing boreholes and wells, together with newly drilled boreholes, involves sampling and analysis of a wide range of physic–chemical parameters, including:

  • water level
  • temperature, pH, conductivity, redox potential
  • major ions and trace elements
  • dissolved gases (O2, CH4, CO2, Rn)
  • organic chemicals (e.g. total petroleum hydrocarbons, volatile organic compounds)
  • stable isotopes (18O,2H of water,13C of inorganic carbon and methane)
  • groundwater 'age' indicators (CFCs)

Monitoring equipment installed in the new boreholes provides for near real–time measurements of additional water–quality parameters.

Results

Groundwater

Results from the baseline monitoring of pumped groundwater samples (Figure 3) show that the groundwater in both Superficial and Sherwood Sandstone aquifers has near–neutral pH and is largely anoxic (low or no dissolved oxygen). As a result, concentrations of nitrate (NO3) are low, those of iron (Fe) and manganese (Mn) are high and concentrations of arsenic (As) and ammonium (NH4) are high in some. Methane (CH4) is also often detected, though rarely at high concentrations. The composition of CH4, where present, suggests that it has been produced in the superficial sediments by microbial reaction of organic matter.

Analysis of organic chemicals suggests that the groundwater contains detectable quantities of some pesticides, perfluorinated compounds and other synthetic chemicals, but the concentrations are low and the numbers of chemicals detected are small. The presence of these substances does, however, indicate effects on the shallow groundwater as a result of human activity (Figure 3).

Monitoring of groundwater in the two aquifers (Figures 4, 5) has shown that the chemical characteristics have been broadly consistent over time, although more detailed investigation of the temporal variability is ongoing. Concentrations of naturally–occurring methane up to 3 mg/L are observed in groundwater from the Superficial aquifer; concentrations in the Sherwood Sandstone groundwater are generally low.

Figure 4: Time series of selected inorganic constituents in groundwater samples from the monitoring network (Superficial aquifer).
Figure 5: Time series of selected inorganic constituents in groundwater samples from the monitoring network (Sherwood Sandstone aquifer).
Figure 6: Time series of selected inorganic constituents in groundwater samples from the new shallow BGS boreholes (Superficial aquifer).
Figure 7: Time series of selected inorganic constituents in streamwater samples from the monitoring network.

Deep Sherwood Sandstone groundwater

Investigation of groundwater quality from the new deep (500 m) borehole into the Sherwood Sandstone at Roseacre Wood has revealed that the groundwater is saline. Samples taken from discrete depths within the open-hole section (356–500 m below ground level) have compositions dominated by sodium (Na) and chloride (Cl):


Depth (m) Ca Mg Na K HCO3 Cl SO4 NO3
mg/L
360 448 73.0 15,200 42.0 48.0 24,000 1530 <60
500 1120 396 56,100 74.0 90.0 92,400 3100 <150

For comparison, the Na and Cl concentrations of open seawater are 10,500 mg/L and 19,000 mg/L respectively. This makes the deep Sherwood Sandstone aquifer in this part of the Fylde unviable as a water resource.

Surface water

Analysis of water from first–order streams shows generally lower concentrations of dissolved solids than are found in the local shallow groundwater, though with slightly higher concentrations of nitrate, nitrite (NO2) and dissolved organic carbon. Many trace elements also have lower concentrations than local groundwater, due to a combination of differing redox conditions and more limited interaction with rocks and soils.

Current activities

Figure 8: Groundwater sampling.

Quarterly monitoring of sites in the monitoring network as well as the newly–drilled boreholes is ongoing. Water sampling is next due to be carried out during November 2017. We are also evaluating the data being produced to provide a detailed characterisation of the water–quality baseline in Fylde.

Contact

Contact BGS enquiries for further information.