This report describes the regional geochemistry of groundwater from the Sherwood Sandstone
aquifer of Devon and Somerset. In order to assess the likely natural baseline chemistry of the
groundwater in the area, information has been gathered from the strategic collection of 21 new
groundwater samples, and from collation of existing groundwater, rainfall, mineralogical and
geochemical data.
The Sherwood Sandstone aquifer results from continental deposition during the early Triassic
period. The deposition comprised thick clastic deposits in basins created during the late Permian.
There are two distinct units: the Otter Sandstone Formation and the underlying Budleigh
Salterton Pebble Beds Formation. These units form the most important aquifer in south-west
England.
The main chemical properties of the groundwater are determined by rainwater recharge reacting
with the aquifer minerals. Of these minerals carbonates are the dominant influence on the water
chemistry. While Ca is dominant in groundwaters from both aquifer units, the water types from
the two units can be distinguished: the Otter Sandstone Formation aquifer has mainly Ca-HCO3
type waters, with some mixing towards Mg, and the groundwaters in the Budleigh Salterton
Pebble Beds Formation aquifer are of Ca-HCO3 type to Na+K-Cl type. There is little variation in
the major ion proportion of the groundwaters from the Otter Sandstone Formation, most likely as
these represent groundwater in equilibrium with the minerals which give it this character. There
is greater variation in the Budleigh Salterton Pebble Beds Formation groundwaters. Besides the
difference in groundwater types, the groundwater from the Budleigh Salterton Pebble Beds
Formation is more acidic, more oxic, has lower SEC values, and HCO3, Cr, U and Mg
concentrations, and higher concentrations of Al, Be, Cd, Cr, Co, Tl and REE than the
groundwater from the Otter Sandstone Formation.
Human impacts on the water quality are evident. These are mainly visible in the presence of
indicator contaminants, such as nitrate. The widespread presence and changes of nitrate
concentrations over time indicate the extent to which the unconfined aquifer is influenced by
modern farming practices or urban pollution. Concentrations of NO3-N exceeded the current
drinking water limit of 11.3 mg L-1 in 25% of the sampled groundwaters, which probably reflects
the intensification of agriculture in the study area over the last few decades. Indeed, the dominant
land uses of the area are agriculture and grassland. However, recent legislation and the drive
towards intelligent farming have meant that in some groundwaters the concentrations of nitrate
are decreasing.
While many of the groundwaters investigated in this study are of good inorganic quality there are
some notable exceptions which contain elevated concentrations of Fe, SO4, Ni, Co and NO3. The
distribution of the poor-quality is generally so sporadic that water quality would be difficult to
predict prior to drilling a borehole.
For many analytes, the 95th percentile of the distributions has been taken as a first approximation
of an upper limit of baseline concentrations as this serves to eliminate the most extreme outlier
concentrations that likely represent a distinct population
