Transcript Q net

Groundwater, Human Health and
the Environment
Karen G. Villholth
Groundwater Specialist
IWMI, International Water
Management Institute
Colombo, Sri Lanka
Groundwater Governance in Theory and Practice
Groundwater Governance in Asia
CSWCRTI, Dehradun, India, Oct. 16, 2006
Outline
1. Impacts of intensive
groundwater use on
the environment
2. Impacts of
groundwater use on
human health
Fundamental groundwater balance
R
R
D
S
Natural conditions
Averaged over long
term R=D and S is
constant
R
Qnet
Qnet
D
S
D=0
S
Stable groundwater
pumping
Unsustainable
condition
Qnet is equivalent to
reduction in D and S
Qnet is greater than
R, D reduces to 0
and S decreases
continuosly
Adverse impacts of GW depletion
• Water well problems
– Increasing energy costs
– Well failure
• Reduced surface water flows and storages
• Reduced drought resistance
• Land subsidence
• Deterioration of water quality
Artesian flowing wells
Example from Dhaka, Bangladesh
Δ=22%
Δ=30m over 30 years
GWL decline, example from China
Δ=20m over 35 years
Hydrograph
depicting
water-table
elevations
beneath
Luancheng
Agro-Ecological
Research
Station
(Chinese
Academy
of Sciences),
Luancheng
County, Hebei
Province,
1974-2002.
GWL decline, example from Chicago
GWL decline, example from Western USA
Effect on riparian vegetation
Subsidence
Subsidence, impacts
• Gives problems for infrastructure,
buildings, pipelines
• Flooding and drainage patterns changed
• Causes secondary GW contamination from
breakage of underground pipes and tanks
Aquifer compressibility
What is subsidence?
Before
After
Example of subsidence,
San Joaquin Valley
GWL
Landsurface level
Relation btw. subsidence and
GWL decline
Example of subsidence, Texas
Subsidence, cont.
• Especially pronounced in unconsolidated alluvial
•
•
•
inter-bedded deposits
Can be reversible or irreversible, depending on
the elasticity of the sediments
Correlation between GWL decline and
subsidence found
However, GWL decline may not be the only
reason for subsidence
Deterioration of water quality
• Salinization due to seawater ingress or
due to interception of underlying saline
formations
• Release of natural or human-induced
contaminants, e.g.
– Arsenic, upon oxidative dissolution of arsenicsulfides
– Induced/increased influx/recharge from
contaminated SW or upper aquifers
Saltwater intrusion
• Inflow of saltwater into freshwater aquifer
• Origin of saltwater:
– Seawater in coastal areas
– Geological saline deposits in deeper formations
– Influx or accumulation from irrigated agriculture
In coastal areas there is a natural
balance between salt and freshwater
ground surface
phreatic water table
sea
fresh groundwater
zone
on
i
s
u
f
f
of di
saline groundwater
impervious layer
An island is surrounded by salt water
Rainwater sustains the freshwater lens
Source: IGRAC
Ghyben-Herzberg relation
h=H: if =0.025
H = ρf/(ρs- ρf ) h = h/α ~ 40 h
h=1 m, H=40 m
H ρs = h f ρf
h
fresh
natural recharge
h
freshwater lens
sea
hf
H
s
saline
saline
f
H
hf
=(s-f)/f
Seawater intrusion
• Natural balance disturbed by:
–
–
–
–
Excessive pumping in coastal areas
Seawater rise
Land subsidence
Decreased recharge, e.g. due to urbanization
• Partially irreversible
• Causing major problems around the world
• Difficult to counteract
GW withdrawal in coastal auifers
Upconing of saline groundwater
under an extraction well
Rise of saltwater:
z = Q/(2πdKα)
Critical rise:
zcr = 0.3 d to 0.5 d
Risk of upconing of saltwater is
higher when
•
•
•
•
Pumping is intensive and continuous
Pumping is from wells close to the coast
Wells are deep
Pumping in the dry season when the
freshwater depth is smaller
How to estimate sustainable yield?
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•
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•
•
Not a straight-forward task
The extent to which water levels fall does not identify
whether an aquifer is over-exploited, the important
factor is whether the decline is acceptable or not in
terms of the impact on water users and uses
Needs assessment/knowledge of hydrogeological
setting, potential impacts, and balancing desired
advantages from use of GW with negative impacts at
the societal scale
Methods range from simple water balances combined
with GWL trend analysis, to integrated hydrological
and decision support modelling
Ultimately a political decision
Water table threshold requirements
From Brooks et al. (2004)
Other linkages between GW and
environment
• Urbanization
• Climate change
• Eutrophication
What does urbanization do to GW?
• Generally, urbanization decreases the recharge
to aquifers (pavements, storm water drainage)
• However, depending on the primary source of
water supply and the means of discharging
wastewaters, GWLs may be affected differently
Climate change
• There is accumulating evidence that the climate is
changing on a global scale
• Exact effects not known as well as the speed, extent and
local distribution of them
• Some effects seem inevitable:
– Water level rises due to global warming
– More variability and unpredictability in climate and more
‘extreme’ events
– Dry regions becoming drier
• Effects compounded by other human influences (e.g.
intensive water exploitation)
Climate change, some reasons
• Increasing levels of greenhouse gasses in the
•
atmosphere, primarily from burning fossil fuels,
but also release of chemicals from animals and
synthetic chemicals
Land use changes, e.g. increasing areas with
irrigation
Climate change,
cont…
• Predictions of effects on GW difficult
and not conclusive
• One example:
2*CO2 => T => PET => ET =>
Irrigation demand and Recharge
Eutrophication
• Due to the excessive
use of fertilizers:
• Nutrients in fertilizers released to the environment
•
end up in lakes, reservoirs, estuaries, etc., where
algal growth and oxygen depletion result
GW may be a significant source of nutrientcontaminated water, though eutrophication as
such is not a problem in the GW
Major nuisances and human health
impacts of poor GW quality
•
•
•
•
•
•
–
–
–
–
–
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Poor taste or smell:
Salts
Iron, manganese
Scaling and staining problems:
Hardness
Iron, manganese
Stomach infections:
Microbiological contamination
Dental and sceletal problems:
Fluor, arsenic
Chronic diseases (not all known):
Arsenic and other heavy metals
Pesticides
Blue baby syndrome (Methemoglobinemia):
Nitrate
Risk asessment
References
1.
2.
3.
4.
5.
Boulding, J.R., J.S. Ginn, 2004. Soil, Vadose Zone and
Groundwater Contamination. Assessment, Prevention and
Remediation. Lewis Publishers
Domenico, P.A., F.W. Schwartz, 1998. Physical and
Chemical Hydrogeology. 2nd Ed. John Wiley & Sons, Inc.
Fitts, C.R., 2002. Groundwater Science. Academic Press,
Elsevier Science.
Morris, B.L., A.R.L. Lawrence, P.J.C. Chilton, B. Adams,
R.C. Calow, B.A. Klinck, 2003. Groundwater and its
Susceptibility to Degradation: A Global Assessment of the
Problem and Options for Management. Early Warning and
Assessment Report Series, RS. 03-3. UNEP (United Nations
Environment Programme), Nairobi, Kenya
Todd, D.K., L.W. Mays, 2005. Groundwater Hydrology. 3rd
Ed. John Wiley and Sons, Inc.
Thank you!