Topic trcers in quifers hs been suggested Edmunds 2001- i
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1.2 Hydrogeochemical controls
1.2.1 Generation of the natural baseline
The generation of groundwater quality can
be viewed as a gigantic open system, but
unevenly equilibrated chemical reactor
involving rain water, soils and rock strata.
The acquisition of solutes may be regarded
as a series of processes and reactions taking
place within the hydrological cycle which
may occur over natural timescales of days to
millennia. The baseline geochemical system
is a response to variable recharge rates, rainfall
compositions, as well as the processes that
have taken place along flow lines in aquifers
in response to climate and environmental
change including adjustments in water levels
brought about by sea-level change as water
fl owed toward discharge areas (Fig. 1.1).
Residence times of groundwater can typically
be measured in hundreds to tens of
thousands of years (Fig. 1.1). A sequence of
relative timescales based largely on isotopic
tracers in aquifers has been suggested
(Edmunds 2001):
i. Palaeowater, that is, water recharged
during or before the last glacial era,
ii. Pre-industrial Holocene water (free of
any anthropogenic components),
iii. Industrial era but pre-thermonuclear era
water (water free of tritium), and
iv. Modern water, that is, water younger
than ca. 50 years, identified by presence of
tritium from thermonuclear tests in atmosphere,
or by presence of recent man-made
contaminants such as CFCs.
Disturbances in flow and quality commenced
with the onset of the Industrial
Revolution and mechanised recovery of
groundwater (Fig. 1.2). Before this time, natural
flow regimes became established, adjusting
most recently to the restoration of the
present-day coastline in the early Holocene.
Stratification of age and quality of groundwater
would have taken place. Human impacts
may, however, have been important in terms
of water quality before the modern era, with
forest clearances and land use changes involving
land drainage, for example. The modern
era, coinciding with roughly the last 200
years caused a number of adverse impacts
and significant changes to the hydrochemical
system, notably, deterioration in air quality,
diffuse and point source pollution from
agricultural and industrial sources, as well
as mixing of water of different qualities as
development of groundwater proceeded. The
legacy of these impacts was generally slow to
be recognised due to the hysteresis caused by
the nature of the recharge process.
The natural stratified groundwater quality
has been progressively disturbed by well
and borehole drilling and pumping, especially
during the twentieth century with
the concomitant advance of contaminants
from diffuse and point sources as well as by
a changing atmospheric background from
industrial sources (Fig. 1.2). Thus, it is necessary
to recognise the extent and chemical
character of the naturally evolved water and,
as far as possible, recognise and differentiate
this from the human impacts on the natural
baseline. This represents the main target of
this chapter and forms an important starting
point for water quality management.
1.2.2 Inert and reactive tracers
In order to understand the baseline characteristics
of groundwater, it is first necessary
to consider which constituents of the
groundwater (chemical elements/species,
isotopes or gases) can be used as tracers. It is
convenient to distinguish between inert and
reactive tracers. Inert tracers include chemically
unreactive species such as Cl and several
isotopic signatures such as δ18O, which
remain unchanged in composition over short
timescales, or for which a major shift in
composition indicates a new input source of
groundwater. Reactive tracers include H+ and
common cations contributed by weathering
and reactions taking place along flow lines.
Examples of tracer types are
Inert tracers:
Cl, Br (Br/Cl), NO3, 36Cl, δ18O, δ2H, 3H,
noble gases (isotopes and ratios, e.g.
81Kr).
Reactive tracers:
Major ions and ionic ratios, H+, Si, trace
elements (e.g. B, Li, F, Sr), isotope
ratios (87Sr/86Sr, δ34S, δ13C, 14C).
Inert tracers may be used to track the
inputs to groundwater from the atmosphere,
land surface and soils, which in turn can help
track changes in past climates, environment
and water origins, and also provide estimates
of age. It is noted that nitrate may be used
as an inert tracer in aerobic systems since it
is stable in the presence of oxygen. Reactive
tracers, on the other hand, chart the water–
rock interaction and help to characterise the
rock type, mineralogy, flow pathways and
geochemical controls leading to the unique
mineral composition of the natural water.
In order to fully understand the evolution
of a groundwater body and to characterise its
baseline properties it is usually necessary to
adopt a multi-tracer approach. For example,
a combination of inert and reactive tracers
was used to deduce the evolution of groundwaters
from present day to pre-Holocene in
the Permo-Triassic East Midlands aquifer of
England (Edmunds and Smedley 2000). This
example, described later, allowed changes in
baseline with time, palaeoclimate influences
and the impacts of modern pollution to be
assessed with some degree of confidence. A
similar multi-tracer approach is adopted for
the reference aquifers in this book since this
provides one of the clearest illustrations of
the separation of natural trends from human
influences.
1.2.3 Controls on baseline groundwater
systems in the hydrochemical cycle
Rainfall modified by evapotranspiration may
be a significant contributor to defining groundwater
compositions. Hydration of carbon
dioxide acquired during passage through the
soil provides the key reactant (H2CO3) controlling
weathering of the rocks encountered
at shallow depths. The soil and unsaturated
zone provide a highly reactive environment
for mineral dissolution with strong changes in
moisture content (water–rock ratio), recycling
of solutes, pH changes neutralising acidic
inputs and also high microbial populations
which may catalyse geochemical reactions
at shallow depths. In this way, the distinctive
mineral content of many groundwaters
is determined in the top 5–10 m. Lithology
and mineralogy (especially the presence or
absence of rapidly reacting carbonate minerals),
hydrophysical properties and residence
time determine the detail of the eventual
groundwater quality. Saturation controls by
minerals, such as calcite, may inhibit further
reaction and significant modification of water
quality in the deeper saturated water bodies,
although redox controls are of considerable
importance. The main processes whereby
baseline groundwater quality is determined
are shown conceptually in Fig. 1.3.
1.2.3.1 Rainfall inputs
Rainfall originates mainly in the oceans
and carries with it a characteristic aerosol
composition dominated by Na and Cl
accompanied by high Mg/Ca ratio. As the rain
passes over land, rain-out of aerosols
will take place so that the residual water
vapour becomes depleted in Cl. Similarly,
the isotopic composition of water vapour
evolves as it becomes more continental
(Clark and Fritz 1997). Thus, the baseline
chemistry may differ significantly between
coastal and inland areas in the same aquifer
(as described, for example, for the Chalk of
Dorset in Chapter 9). Pristine rainfall is naturally
acidic with a pH of around 5.6, and,
although it contains low CO2, it can be a significant
agent for water–rock interaction.
1.2.3.2 Soils
The chemistry of groundwater depends to a
large extent on the output from the base of
the soil zone. There is scope for considerable
modification of the rainfall composition
in the soil, notably by evapotranspiration
causing concentration, typically by a factor
of 3 in temperate regions but up to 10 or
more in semi-arid parts of Europe. The diurnal
and seasonal fluctuations in temperature
and the variable rainfall cause wide ranges
in water–rock ratio and ionic strength of the
soil solutions, leading to mineral saturation,
with some precipitation in dry spells and subsequent
dissolution. The main impact of the
soil is owing to the biological production of
CO2, which under natural baseline conditions
may raise the concentration by up to 10–100
times that of the atmosphere. The resulting
carbonic acid is the main control on weathering,
both of carbonate and silicate minerals.
Organic soils may be naturally acidic
(pH 2.9–3.3). Acidity will be neutralised to
near neutral pH in alkaline soils above carbonate
rocks, but in non-carbonate rocks the
pH of moisture leaving the soil may remain
in the range 4.0–5.0 (Dahmke et al. 1986;
Edmunds et al. 1992).
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