Controls on saline intrusion into the Crag aquifer of north-east Norfolk.
The River Thurne catchment on the north-cast coast of Norfolk contains large areas of
marshland, including several Ramsar-designated sites. Land drainage of much of the
marshland over the previous centuries has lowered groundwater levels to below sea-level.
Consequently, saline groundwater has intruded into a large proportion of the underlying
Norwich Crag aquifer, thereby raising the salinity of the surface water networks. Changes
in the economics of arable farming and the perception that changes in the land drainage
regimes will produce beneficial effects on the water quality of the River Thume and its
associated Broads have necessitated the need for a better understanding of the hydrogeology
of this complex, highly managed aquifer.
To provide the necessary understanding, this study has investigated the hydrogeology,
hydrology and land management of the catchment using a variety of geophysical techniques,
including electrical resistivity soundings EM surveys and reflection seismology to supplement
information collected using standard hydrogeological measurements The distribution of dyke
water levels as maintained by the drainage pumps are shown to exert important controls on
the extent and depth of saline intrusion. In the north of the catchment a further control is the
internal structure of the Crag aquifer. A clay layer of probable Baventian age divides the
Crag aquifer into two units and appear to prevent the salinization of the aquifer above this
layer. A catchment water balance has shown that land drainage pumps discharge about 95%
of the catchment recharge, so that raising dyke water levels will also result in raised
Futurel and use change to grazing marsh, while improving the water quality of drainage water
entering the River Thurne may lead to increased salinization of the aquifer, as a result of
changes in the distribution of dyke water levels.