Groundwater and surface water geochemical evolution : Liverpool area, UK
The PhD thesis is focused on the hydrogeology and geochemistry of the surface and
groundwater in Liverpool area. It provides a detailed understanding of the effect of
the structural geology on the groundwater flow and the geographical variation in the
groundwater geochemistry. Moreover, the studies have extended the research towards
the geochemical evolution of the fresh and saline groundwaters and surface water.
The main conclusions are that the major structural elements, especially the NNW-SSE
major faults and a gentle NE-SW fold, have subdivided the aquifer into discreet six
hydrogeological sub-basins. As a result of this, a single groundwater flow direction in
the aquifer is not likely existed; multiple local flow directions are expected instead.
The recharge of the aquifer sub-basins is mainly by vertical percolation while the
lateral mixing between the different water types and the inland invasion of seawater
are limited by the major faults.
The aquifer has two major types of groundwaters. Fresh groundwater occupies one
part, generally a few kilometres from the coast and saline groundwater in another part
that has undergone seawater intrusion from the Mersey Estuary. The recharge of the
fresh groundwater is mainly localised from surface waters (originally rainfall). The
recent recharged groundwaters are expected in spatially restricted areas with low
salinity and they broadly resemble surface waters except they are more acidic possibly
due to C02 dissolution and dissociation, nitrification or sulphide oxidation. This
immature groundwater evolved into the regionally dominant groundwaters through a
combination of congruent dissolution of dolomite, cation exchange and sulphate
mineral dissolution happening in the Sherwood Sandstone aquifer. Due to locally
advanced stage of water rock interaction, the regionally dominant groundwater has
evolved into higher salinity fresh groundwater at the southern end of a southward
flowing compartment. Close to the urban heart of Liverpool the groundwater has
undergone local pollution as reflected by the elevated salinity, Cl, S04 and N03
concentrations, The origin of the saline groundwater is mainly due to seawater
intrusion based on the similarity in chemical composition between the saline
groundwater and River Mersey water.
This study has shown that highly saline groundwater has been expected in the
Sherwood sandstone aquifer underneath Liverpool and close to the River Mersey.
From the previous and present works the saline groundwater in this part of the aquifer
mainly due to saline water intrusion from Mersey Estuary. This has been based on the
geographic distribution and chemical affinity between the saline groundwater and
Mersey Estuary water. The invaded Estuary water experienced a wide range of
geochemical processes that deviates the composition of the water away from being a
simple physical mixture between low salinity groundwater and seawater. During
progressive invasion by seawater, it seems that cation exchange (Na-capture and Ca release) occurs first with a small amount of carbonate and even anhydrite cement
dissolution. Next, cation exchange becomes relatively less important but bacterial
sulphate reduction starts to occur. The final process during the later stages of saline
invasion seems to be dolomitization of indigenous calcite accompanying more
advanced bacterial sulphate reduction and with relatively minor cation exchange.
The chemistry of the surface water has been studied in small river systems in the area
(River Alt, Downholland Brook and River Alt). The main recharge of these surface
waters is local rainfall. Dissolution of calcite and weathering of silicate minerals are
the most common processes operating in a higher relief river basin floored by
Sherwood Sandstone (Calder River regime), while the abundance of gypsum and
calcite with silicate in the Downholland Brook and River Alt bed rocks explain the
increase of the total dissolved salts and ionic composition of the former two streams
waters. The continuous influx of atmospheric CO2 and H+ ions from the dissociation
of H2C03 increases the ability of these waters dissolving minerals in contact
especially carbonates and silicates.
The concentration and lateral variation of the nitrate concentration in the surface and
groundwaters have been studied trying to assess its possible source and fate. The
results reveal that a significant part of nitrate in surface and groundwater is coming
from the application of fertilizers in addition to urban waste water in the highly
populated areas. Nitrification process in the soil zone transforms the N-compounds
(eg. NH4) into nitrate. The direct drainage of soil water to the river course carries high
nitrate to the river waters. The low nitrate concentration in the locally-recharged
groundwater is mainly due to natural denitrification processes probably in the
unsaturated and saturated zone however the high abstraction rate of the groundwater
could be responsible for yielding water with high nitrate concentration.