Rinko Summary May 1, 2007
Chapter 2: From Rainwater to Groundwater
Micha Sigrist (Takizawa lab,
COE research assistant)
Groundwater originates mostly from rainwater that can be seen as highly diluted seawater.
The rainwater undergoes a number of processes and influences that change its composition
already in the air. But also on the way from the top soil to the aquifer processes that deplete
or enrich the water with solutes are taking place.
An approach of using the mass balance calculations is used to decipher the hydrological
cycle, sources of constituents, the isotopic evolution of rainwater, cycling of elements in the
biosphere and to derive groundwater quality below farmland.
2.1 The Hydrological cycle
Reservoirs:






Mass transfer between the reservoirs is
estimated by measurements and calculation
from mass and energy balances.
Rather well known are the continental
precipitation and the continental surface runoff.
Evapotranspiration is estimated at 68% of the
precipitation
Ocean evaporated water contributes 32% of the
continental precipitation
Contribution of Ocean evaporated water in
rainfall depends on the distance to the sea
(at around 2500km it makes up halve of P)
Residence time:
 V
Q
with =residence time, V volume of reservoir
and Q=into or out of reservoir
Oceans
:
Atmosphere :
Groundwater :
Groundwater (river):
(from Press and Sievers: understanding earth)
39000 years
10 days
9500 years
271 years

residence times as indicator for susceptibility
towards changes

global water cycle ignores differences among
continents
recharge varies greatly as a function of latitude

- 1 -
Rinko Summary May 1, 2007
2.2 The Composition of Rainwater
The composition of rainwater is determined by the source of the water vapor and by the
ions which are acquired (or lost) by the water on its journey through the atmosphere.
- over the ocean rainwater resembles strongly diluted seawater with 10-15 mg/l Cl- sea salt content originates from marine aerosols that form from liquid droplets that are
produced when gas bubbles burst at the sea surface
- sea salt content is rapidly decreased inland as the largest particles are washed out
rapidly
-
the relative composition of rain water and seawater can be compared by using a
fractionation factor
if application of fractionation factors are close to 1, it leads to the conclusion the the
rainwater is basically strongly diluted seawater
on the other hand, if enrichment can be observed, several sources can be responsible:
o Ca2+ : soil dust, dust from cement industry, fuel and waste burning
o Na+: soil dust in arid climates
-
care has to be taken with rainwater samplers:
 if a rain sampler is continuously open, it also collects dry deposition, resulting in
higher concentrations
-
for facilitating interpretation of analysis, sea salt contribution can be subtracted from
the rainwater analysis
-
in the US and Europe, a conspicuous enrichment of the anions NO3- and SO42- was
found with sources from industrial and traffic fumes containing gaseous NOx and SO2
that form strong acids when they oxidize
o resulting in acid rain with pH  4
o decreasing SO2 emissions can higher the pH again
neutralizing bases are also emitted by
o manure spread over agricultural land (evaporation of NH4OH)
o cement industry
o and waste burning that emit particles which contribute with base
-
2.2.1 Sources and transport of atmospheric pollutants
-
Atmospheric pollution operates on an international scale due to the rapid dispersion
and world-wide transport of local emissions through the atmosphere
Particles or gases travel the distance between the US and Europe in about 4 days!
- 2 -
Rinko Summary May 1, 2007
2.3 Stable Isotopes in Rain
Stable isotopes are those that do not engage in nuclear transformations. Therefore they are
termed as stable isotopes.
18
Stable isotopes:
O, 17O, 16O, 1H, 2H (deuterium)
3
Radioactive isotopes:
H (tritium), 14C
Stable isotopes behave slightly differently in physical, chemical and biological processes
due to the difference in mass.
This leads to a fractionation between 18O and 16O as well as between 1H and 2H during
evaporation and rainout, leading to small variations in isotopic concentrations that may yeld
information on climate at the point of infiltration.
2.3.1 Isotopic ratios and the  notion
The concentration of a stable isotopes is normally given as the ratio of the least abundant
isotope over the most abundant isotope and expressed relative to a standard.
- internationally agreed standard for water is called VSMOW (Vienna Standard Mean
Ocean Water)
o 18O/16O = 2.005x10-3
o 1H/2H = 1.56x10-4
R
R
sample
s tan dard


1000
sample
R
s tan dard
R = isotopic ratio
The least abundant isotope is specified with the  symbol, for example the ratio 18O/16O in
rain is described as 18Orain.
The  value of a mixture of n waters can be calculated by adding the  values of the
contributing waters multiplied by the mass fraction of the reference isotope, and dividing by
the concentration of the reference isotope in the mixture:
n

 i ( xi ref m i )
 mixture  i  1
ref
mmixture
where xi is the fraction of water i, which has concentration refmi of the reference isotope
- 3 -
Rinko Summary May 1, 2007
2.3.2 The Rayleigh process
-
-
-
Rayleigh originally derived a formula to calculate the products of distillation for a
mixture of liquids with different boiling points. The principles, however, are
applicable to any separation process with a constant fractionation factor, as is the case
for stable isotopes:
o Heavier H218O condenses more readily than H216O, producing a water that is
isotopically heavier than the vapor.
 the remaining water vapor becomes enriched in the light isotope 16O
It is difficult to predict the value of a fractionation factor, but in general it is related
to the movement of individual molecules and it decreases with increasing
temperature.
 temperature dependent
Qualitative reasoning suggests that the denser phase (liquid compared to gas) or the
denser component (CaCO3 vs CO2) will contain more of the heavy isotopes.
Heavier isotopes are discriminated in kinetic and diffusion processes, and therefore in
biological activities as well.
2.3.3 The isotopic composition of rain
The rainout of atmospheric water vapor is driven by the cooling of air during transport to
higher latitude ore altitude.
The isotopic composition of rainwater from various locations on earth, show that the
stable isotopes of hydrogen and oxygen in rainwater obey an empirical relation:
2H = 8   18O + 10
known as the meteoric water line
-
-
as isotopic fractionation is temperature dependent, seasonal fluctuations can be
observed
o the rain turns isotopically lighter in the colder season
o near coastal areas have a smaller range in isotopic composition than
continental areas
the further inland, the more vapor is recycled vapor (precipitated and re-evaporated)
which lightens the atmospheric vapor
2.4 Dry Deposition and Evapotranspiration
-
-
Dry deposition comprises both the deposition of particulate aerosols and of
atmospheric gases by adsorption.
Cl- concentration is used as a conservative parameter in groundwater and therefore, it
is important to estimate the contribution of dry deposition to the total atmospheric
input of Cl-.
The importance of dry deposition shows up in the difference between wet-only rain
gauges and rainwater collected by bulk sampler
- 4 -
Rinko Summary May 1, 2007
-
Wet-only and bulk sampler differences are fundamental when estimating the
atmospheric input of different elements to the aquifer
Dry deposition is strongly influenced by wind, location and vegetation
Dry deposition is difficult to measure as it may be affected by canopy-exchange or
ions may have been lost by adsorption to the canopy, flushing salts from the last dry
period and leachates from the vegetation
 estimates over chemical balances of watersheds or lysimeter measurements
Evapotranspiration (E) may be estimated using Cl- concentrations if there is no extra
source such as manure or waste disposal or if the chloride mass transfer has changed over
time (ex: land use)
E  Pt 
C Cl ,rain  Pt  D
C Cl , soil
where E = Evapotranspiration (mm/y), Pt = total precipitation (mm/y), CCl,rain = Clconcentration (mg/l) in rain and CCl,soil = Cl- concentration (mg/l) in the soil- or groundwater
D = 1575 for a deciduous or mixed fores and 0 for bare or grass-covered land
2.5 Mass Balances and Ecosystem Dynamics
-
-
-
-
Mass balances can be used to estimate the water quality
o For agricultural areas, terms related to harvesting, application of manure and
fertilizers are most important.
o NO3- may be overestimated because of evaporation (ammonia during field
application) and denitrification processes
comparison of calculated and observed concentrations reveal whether processes have
been neglected in the mass balance
concentrations may greatly differ along a depth profile
mass balances calculations are an overall summary in which concentration fluctuations
in time and space are averaged and internal biochemical cycles are neglected
be aware that cycling and uptake of elements by vegetation, as well as decay of
organic matter can profoundly influence the concentrations in water leached from the
soil
Rain input of elements may not cover the needs of the vegetation
Some elements may be recycled in the soil-biomass system
Natural vegetation masterfully stores essential elements with limited availability
 disturbing this balance by logging trees or bush fires may greatly disturb the fluxes
by leaving the biosphere
Important in the chemical balance is also the below-ground production, that makes up
0.2 to 0.3 (for forests) and up to 0.6 (grasses) of the net primary production
- 5 -
Rinko Summary May 1, 2007
2.5.1 Water quality profiles in the unsaturated soil
-
-
-
-
-
The effects of variations in rainwater composition and dry deposition, and of seasonal
fluctuations in infiltration and evapotranspiration on the groundwater chemistry are
seen best in the unsaturated zone where no highly reactive minerals like carbonates are
contained
In the example, large variations in Na+ and Cl- are observed which are of marine origin
o Accumulated salts will be washed down with infiltrating water
o Weather may bring additional sea salts and
 it is difficult to separate these processes
o Also transpiration may influence the maximal concentrations
Cl- concentration may be high below forests where dry deposits and
evapotranspiration are explaining a big part of the concentration (evaporation may
amount to 40% of the precipitation)
Mostly, unsaturated profiles show higher concentrations of Cl- in the upper area
 indicating short-circuiting the rain to greater depth via root channels and water
repellent, humic surfaces of soil cutans
o This implies that Cl- mass balances must be based on water quality from below
the root zone
o It also explains that seasonal isotope and Cl- signals are quickly lost in the soil
solution
in areas with very high evapotranspiration, very high concentrations of salts may
accumulate in the soil solution
o selective removal of water molecules by roots
2.6 Overall controls on water quality
-
-
a frequency plot for ions for the USA suggests that the concentration of some species
depends on their availability in rocks, very slowly dissolving minerals or control by
biological processes which have a random character on the continental scale
steepening of the gradient could indicate that solubility of minerals places upper limits
on maximal concentrations of species in natural waters
On a watershed scale, the complexity increases as many processes affect groundwater
chemistry:
Evaporation
Transpiration
Selective uptake by vegetation
Osidation/Reduction
Cation exchange
Dissolution of minerals
Precipitation of secondary minerals
Mixing of waters
Leaching of fertilizers, manure
Pollution
Lake/sea biological processes
- 6 -
Rinko Summary May 1, 2007
-
-
Oxidation/reduction processes are of importance and often relate to the decay of
organic matter
Oxidation may occur in the soil but also within aquifers where fossil organic matter is
present as peat, lignite, etc
 once oxygen becomes exhausted, other electron acceptors like nitrate, iron oxides
or sulfates may mediate the oxidation of organic matter resulting in
o High iron concentration
o Bad smelling water (hydrogen sulfide)
o Methane bearing
Production of CO2 as result of organic matter oxidation may enhance weathering
-
Weathering and dissolution of minerals releases elements to the water.
The geology of an area influences the water chemistry:
-
Element
Na+
K+
Mg2+
Concentrations (mmol/l)
0.1 – 2
0.01 – 0.2
0.05 – 2
Ca2+
0.05 – 5
ClHCO3SO42NO3Si
Fe2+
PO4
0.05 – 2
0–5
0.01 – 5
0.001 – 0.2
0.02 – 1
0 – 0.5
0 – 0.02
Source
Feldspar, Rock-salt, Atmosphere, Cation exchange
Feldspar, Mica
Dolomite, Serpentine, Pyroxene, Amphibole,
Olivine, Mica
Carbonate, Gypsum, Feldspar, Pyroxene,
Amphibole
Rock-salt, Atmosphere
Carbonates, Organic matter
Atmosphere, Gypsum, Sulfides
Atmosphere, Organic matter
Silicates
Silicates, Siderite, Hydroxides, Sulfides
Organic matter, Phosphates
If total dissolved solid concentration is plotted against the mean annual surface runoff as a
function of rock type are, 2 trends can be identified:
o The total amount of dissolved solids in the water is almost independent from
the amount of runoff for limestone, volcanics, sand and gravel deposits
o The amount of dissolved solids varies for a specific value of annual runoff for
different rocks
-
The differences are related to the solubilities of the minerals present in the parent rock
and the rate of dissolution of these minerals
o Granite and sandstones show concentrations depending on the runoff and
indicate that the minerals contained show very slow dissolution kinetics
o Volcanics and limestones contain minerals that dissolve fast compared to the
residence time of the water
o For limestones, an equilibrium approach is appropriate
- 7 -
Rinko Summary May 1, 2007
Minerals may precipitate in soils or aquifers
- weathering of silicate minerals leads generally to the formation of clay minerals
- these may be altered by “stripping off” cations until very low soluble Al-hydroxide
remains; if Fe is present in the parent rock, Fe-Oxides can be expected
- in arid climates, evaporation may lead to the precipitation of minerals like calcite,
gypsum and chloride salts
Ion exchange reactions between dissolved cations and those sorbed to mineral surfaces
may profoundly change the water chemistry
- if infiltrating water differs profoundly from the preexistent water, ion exchange is to
be expected resulting in a smoothening of concentration gradients for:
o fresh and saltwater displacement in coastal aquifers
o spreading out pollution plumes from waste sites
o acid rain moving downward the soil
Mixing of different water qualities occurs in coastal zones but also in groundwater
seepage zones and near springs.
Anthropogenic activities can thoroughly change the water quality. The contributions may
be of many kinds:
- NaCl from households
- Heavy metal pollution
- Poisonous organic constituents
- Air pollution resulting in acid rain
- NO3- and pesticides leaching from agricultural fields
Comments:
Kumar:
Doing any kind of estimation for the infiltration, there are always big uncertainties to it.
- especially the evaporation is attributed with big questions. Several methods are used.
They may be consisting of evaporation pan, lysimeter or mathematical calculations
based on empirical experiences.
- The Cl- concentration method might be very difficult to apply in developed areas as
there are many Cl- input sources that affect the natural Cl- concentrations
Takeda:
-
Cl concentration in urban areas might mostly originate from the sewerage, urine may
contain a high Cl concentration
- 8 -