GWD2_HO_Notes

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Characteristics of Groundwater Systems
Session GWD 1_2: Characteristics of
Groundwater Systems
How groundwater moves
Interconnection and permeability
The interconnection of the pores in rock or soil allows water to:





enter the groundwater,
be stored in an aquifer below ground,
flow through the subsurface,
discharge naturally to a river, swamp, spring or the sea,
be recovered in a well or bore
How does all of this occur? - Again it goes back to the geology of the area and the different types of
soil and rock.
EXERCISE 2: Bore Data Analysis Exercise
Hand out of Bore location map and Table of data
Q: What features are shown on the map
A: Ground elevation contours, a river, scale, locations of bores, direction indicator
Things to note about the data –
Different names of the wells – can occur because of different sources, different bore owners (eg
Govt well, NGO, local community owned well etc)
This is a good data set with depths and elevations of all bores /wells known from surveyed levels. If
the elevations weren’t known, how could they be approximated (Answer: estimated from the top
contour)
Depth to water is the most easily measured piece of data (we’ll talk about that in Monitoring)
Tasks:
Using the data from the table, work out the water table elevation for June
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Characteristics of Groundwater Systems
Plot the water table elevation at each bore / well on the map
Contour the water table elevation - work out flow direction based on the groundwater elevations –
Q: Which direction is the hydraulic gradient? A: Generally southeast
What topographic feature is the likely discharge point for groundwater in this area –
Work out the groundwater elevations for January
Q: Are the levels the same? What are the differences?
A: There is clearly a seasonal variation of around 1-2m lower seasonal variation. Do all bores show
the same variation
What are the differences and what could they mean?



Bore 4: is 5.8m higher – this is possibly a bad measurement or the bore could be damaged
Bore B has had no change – suggests an incorrect reading or missed when the
measurements were taken so the June reading was copied
HDW(2) has much greater decline than nearby bores/ wells (HDW(1) and bore 5) – could be
due to much greater use, lower rate of recovery after use suggesting lower yield. in this
particular case it would indicate limited potential for this well as a large volume source.
Prepare a cross section of the water table and the ground surface – work out the depth of water in
each bore at both June and January
Q: How is the water table related to the topography What inferences can be drawn?
A: The water table tends to mimic the ground surface and tends to flow towards drainage lines such
as the river in this exercise
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Characteristics of Groundwater Systems
Participants are to appreciate that it is important to collect good quality monitoring data (see later
section) as erroneous measurements may be misinterpreted. This is needed for proper planning
such as predicting if a resource is likely to run dry or change quality (by reversal of hydraulic
gradient).
Q&A: Different terrains in which groundwater might be possible source – how would you tell?
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Characteristics of Groundwater Systems
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Characteristics of Groundwater Systems
Groundwater Movement through Aquifers
The ease at which the groundwater flows is a function of the hydraulic gradient (mentioned above)
and the properties of the aquifer. Movement of groundwater through the soil and within aquifers
occurs by different pathways through different types of rocks as described in the section above.
Depth of Groundwater
Noted earlier that aquifers may occur at different depths below the ground in different aquifer
layers
The best (ie highest yielding, lowest salinity) aquifers are often hundreds or even thousands of
metres below the ground surface. We mentioned this for Egypt and it is also visible on the cross
sections that we discussed earlier
Relevance to first phase emergency:
This may not be helpful in a first phase emergency, unless the groundwater from these deep
aquifers has already been drilled and developed for other purposes.
As we will discuss later, deep aquifers require a substantial amount of effort to investigate and
develop.
So where these resources are known in some circumstances they may be able to be utilised.
Permeability
The permeability of a rock formation has an important impact on how groundwater flows to shallow
and deep aquifers. Essentially, the more permeable layers allow more rapid and greater volume of
groundwater flow.
Some materials have little permeability and there is limited movement of groundwater - these are
known as Aquitards. In the case where there is no ability for seepage to pass through a rock or soil
this material is considered to comprise an aquiclude.
The relative permeability of naturally occurring materials is shown in the following table (after Bear
and Verruijt, 1987). Note that the permeability of different consolidated rock types is also reflected
in the degree of fracturing that may occur as discussed previously which provides a secondary
porosity.
K (cm/s)
10²
101
Permeability
10−1
10−2
Pervious
Aquifer potential
Unconsolidated
sediments / Soils
100=1
10−3
Good
Clean
Gravel
10−5
10−6
Semi-Pervious
Clean Sand or Sand &
Gravel
10−7
10−8
10−9 10−10
Impervious
Poor
None
Very Fine Sand, Silt, Loess,
Loam
Peat
Consolidated Rocks*
10−4
Stratified clay
Oil Reservoir Rocks
Groundwater Drilling and Development _Session GWD 1_2
Sandstone
Unweathered Clay
Limestone,
Granite
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Characteristics of Groundwater Systems
Dolomite
modified from Bear and Verruijt, (1987)
Confined and Unconfined aquifers
The presence of aquitards cause a build up of pressure in an aquifer as the groundwater is
constrained from passing through. The water in the aquifer is therefore under pressure and is
referred to as being confined . If the aquifer is tapped in a bore, the water level will rise up in
response to the pressure. The distribution of pressure is called the potentiometric surface.
The water table is at the same pressure as the atmosphere and is therefore said to be unconfined.
When a well taps an unconfined aquifer, the water level in the well is the watertable. Unconfined
aquifers are easier to establish bores or wells and are usually much shallower than confined
aquifers. This will be discussed later in the session
Groundwater Discharge
Water will preferentially flow through aquifers that have higher permeability and will try to move to
areas that have least restriction on flow.
If the aquifer is confined by overlying aquitards this may require long flow times and the water may
be contained subsurface until it eventually reaches a point where the constraint is released – this
means the groundwater will discharge.
Q & A : GROUNDWATER DISHARGE
Q: if groundwater flows from areas of higher head to lower head, where do participants think it will
ultimately end up?
A: Ultimately the sea but also rivers or lakes
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Characteristics of Groundwater Systems
A: Could also be at springs where the water table actually intersects the ground surface to allow the
groundwater to seep or flow out at springs
Follow up with the figure below to discuss concepts
Based on Bear and Verruijt (1987)
Discharge of Groundwater and Relationship of Groundwater to Topography
Local and regional scale flow paths are affected by recharge location, discharge sites and aquifer
types, with groundwater generally discharging at lowest points in the landscape.
We saw this in the mapping of the water table – the water table followed the topography and
graded towards the stream.
Springs
EXERCISE 3: Springs
Go back to the cross section we drew – look at the water levels for January
Now Sketch the watertable in bores 1 and A to be two metres below the surface
Q: Would the water table intersect the ground surface? What would be the result of this?
A: Springs would develop
What are the implications as a water source for an emergency?
Groundwater flows underground until the water table intersects the land surface and the
flowing water becomes surface water in the form of springs, .In order to discharge as springs,
the watertable in the recharge area must be higher than that in the area where it discharges. The
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Characteristics of Groundwater Systems
more relief an area has, the more potential there is for groundwater to flow and thus to emerge as
springs.
Springs are a common source of groundwater arising from natural discharge.
We saw in the section above that springs can seep out at the surface if there is a pathway for
groundwater even before it reaches the water table – ie we have a perched water table
Commonly the springs emanate from consolidated rocks such as limestone and crystalline rocks.
Some springs are fed by shallow groundwater seepage out of the rock, others are fed by
deep aquifer water discharged under artesian pressure that finds it way through the
aquitard to the ground surface.
Spring types under different groundwater conditions, After Davis and Lambert
Springs can occur as individual identifiable “spring eyes” or as seepage across a less well
defined area. .
Springs can be perennial or seasonal depending on the particular circumstances. As water table (or
water pressure in confined aquifers) falls and there is a decreasing rate of groundwater flow through
the rocks forming the springs, the spring flow decreases.
Interaction with Rivers
Groundwater discharge to rivers occurs when the water table is at a higher elevation than the water
in the river and there is a hydraulic gradient towards the river. The groundwater discharge
contributes to baseflow in the river particularly in dry seasons
From Reid et al 2009
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Characteristics of Groundwater Systems
At periods of higher flow in a river, the hydraulic gradient can reverse and there is recharge from the
river into the groundwater. This is often a seasonal feature and in the case of some arid zones,
provides the recharge that maintains the groundwater source for the future (see Case study 1)
From Reid et al 2009
In some instances the water table may be naturally well below the river level and there is no direct
connection. In this case, leakage through the bed of the river to teh aquifer can occur.
From Reid et al 2009
Similar situations occur where there are lakes or wetlands.
Relevance to an Emergency
The conditions around streams can be important in an emergency where groundwater and stream
water are both part of the water source. A reduction in the amount of groundwater migrating to the
stream can limit the available supply particularly in dry periods.
In wet seasons, flow of water in rivers (eg wadis in Africa) can be direct source of water, fill the
layers of sand in the river bed to become a groundwater supply once the floods recede, and also
allow seepage from the river into surrounding rocks to replenish groundwater resources taken from
bedrock.
Coastal Zones
Under natural undisturbed conditions, there is a state of equilibrium in which sea water beneath the
freshwater in coastal zones. This saline water is often referred to as the “salt water wedge”.
Freshwater is less dense than salty water. To maintain the equilibrium and keep the salt water
wedge from moving inland, the fresh water has to be at a higher elevation than the adjacent
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Characteristics of Groundwater Systems
waterbody in order to stop the salty water displacing the fresh water. It is this principle that
provides fresh water to many low-lying island systems.
As the better quality groundwater is pumped, the head in the freshwater aquifer is lowered and
there is the potential for saline water to move landward. Excessive pumping can result in total
depletion of the fresh water, as the salt water mixes in with it. If a well is located above the
interface, salt water can cone upwards towards the well. Care is needed to avoid contamination of
these aquifers with salty water.
This is a problem that needed to be dealt with in response to the Indian Oceam Tsunami (See Case
Study2 – below).
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Characteristics of Groundwater Systems
Case Study 2: Impacts of the Asian
Tsunami on groundwater in Sri Lanka
Summary
The Indian Ocean Tsunami impacted coastal sandy aquifers were affected on the eastern side of
SriLanka by causing inflow of salt water into wells.
These aquifers contain a lens of fresh water sitting on top of the denser saline sea water. Recharge to
these aquifers is typically rainfall infiltration.
The Tsunami caused physical damage to many of the wells in the coastal areas, including filling with
rubbish and even removal of well lining from the groundwater.
When the tsunami struck salt water permeated into the entire aquifer, and thus all of the water supply
being highly contaminated by salt.
Immediately after the tsunami wells in the area were extensively pumped in the area by NGOs, civilians
and armed forces as it was believed that this would clean the wells.
Pumping on the wells led to increased EC. The groundwater wells often experienced an upconing effect
when being ‘pumped clean’ after the tsunami.
Many I/NGOs were attempting to pump out the salt water, however due to the increase in the size of the
salt water wedge and excessive pumping resulted in the salt water being drawn up from the salt water
wedge and increasing the salt water concentration at the borehole location
Figure 23 - Groundwater Flow Pre-Tsunami (Lytton, 2008)
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Characteristics of Groundwater Systems
Groundwater Flow Post-Tsunami (Lytton, 2008)
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Characteristics of Groundwater Systems
Emergency Situation
On the 26th of December 2004 a tsunami was triggered by a 9.0 magnitude earthquake off the coast of
Sumatra. The wave caused destruction to many countries around the Indian Ocean. The tsunami struck
the east and southern coast of Sri Lanka killing 40,000 people, and destroying numerous houses and
water supply wells. Up to 450,000 people were displaced in Sri Lanka (Tamil Information Centre, 2006).
This case study is prepared to outline some of the impacts of the Tsunami on the groundwater resource
in coastal settings
Aquifers in Sri Lanka:
Aquifers in Sri Lanka are described by Panebokke and Perera ( 2005)
The geological basement in much of Sri Lanka consists of high grade metamorphic rocks. The
metamorphic rocks are low permeability crystalline materials although weathered material (regolith) on
the upper exposed outcrop areas forms aquifers in some areas.
Deep confined aquifers are located in the northwest of the island and Miocene limestone that occurs in
the north of the country comprise karstic aquifers.
The coastal zone on the eastern side of the country has extensive shallow coastal sand and alluvial
aquifers that are highly permeable. These are the aquifers of greatest interest in this case study.
Groundwater is highly utilised in many coastal areas from the coastal sand and alluvial aquifers. The
majority of users are undocumented as they are private dug wells.
Aquifers in SriLanka (after Panabokke and Perera,
2005)
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Coastal Sand Aquifers
Along most of the coastline there are shallow aquifers located on the coastal sands. Groundwater
recovered from these aquifers support agriculture (chilli and onions), domestic use by the local
population and the tourist industry (Panabokke & Perera, 2005).
These aquifers contain a lens of fresh water sitting on top of the denser saline sea water. Recharge to
these aquifers is typically rainfall infiltration, which falls during the maha season (October – January). Due
to the varying rainfall patterns the fresh and brackish water boundary expands and contacts over time
(Panabokke & Perera, 2005).
Over-extraction of the aquifers can cause a cone of upwards intrusion into the aquifer, resulting in a
saline bore which can no longer be used (Cooray, 1967; Panabokke & Perera, 2005).
Alluvial Aquifers
Alluvial aquifers are located in or around current and old river beds, and vary in depth, extent and yield
(Cooray, 1967; Panabokke & Perera, 2005). These aquifers are generally alluvial sediments, sands, clayey
sands, and gravels which have been deposited, and then overlain with finer sediments. The beds extend
several hundred meters either side of a river and up to 35m deep (Panabokke & Perera, 2005). Thus the
water supplies to these aquifers are quickly recharged, and can support high extraction rates to varying
degrees without significant drawdown effects.
Types of Wells
There are two key types of wells in Sri Lanka: tube and dug wells. Dug wells are generally shallow, water
table aquifers which use the resources unconfined aquifers, often they are not more than 5 - 7m in depth
and approximately 1-1.5m wide and have concrete casing. These are the most common types of wells in
Sri Lanka, and are used for domestic water supply (Panabokke & Perera, 2005). The location and number
of dug wells is not recorded in Sri Lanka.
Tube wells are much smaller in diameter, and draw resources from much deeper in the aquifer. They
often screen unconfined aquifers, not reachable by dug wells, and service both domestic, agricultural and
industrial uses. They are often constructed with PVC pipe and fitted with a hand or motorised pump.
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Characteristics of Groundwater Systems
Distribution of tube wells in Sri Lanka (from Panabokke and
Perera, 2005)
Impact of the Tsunami on Groundwater
Damage to Wells
The Tsunami caused physical damage to many of the wells in the coastal area, including filling with
rubbish and even removal of well lining from the groundwater.
When the tsunami struck salt water permeated into the entire aquifer, and thus all of the water supply
being highly contaminated by salt. In a study by (Saltori & Giusti, 2006) it was noted that in the few day
following the tsunami that the water within local drinking wells was up to 28,000μS/cm, however
decreased to 3,000 – 8,000μS/cm in a few weeks.
The Ampara district experienced three methods of salt water intrusion after the tsunami (Lytton, 2008):
 infiltration from sea water,
 the strength and pressure of the wave causing an underground saline wave into the aquifer
(indicated by several witness reports of geysers beyond the wave extent), and
 the lagoons situated behind the dunes of the beach filling with saline water and slowly releasing
it into the groundwater system (Lytton 2008).
Lytton found EC decreased with distance from the coast line. Most wells, even those not inundated by
the wave, still recorded values over that acceptable to the human palate of 1800uS/cm at the time of
testing. Bacterial content of the wells recorded colonies too numerous to count.
Leclerc et al.’s (2007) testing results concluded that the main cause of saline intrusion in the area was
from direct inundation. Their testing regime involved wells in one local area, and then over an extended
area, and comparing wells of similar conditions.
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Characteristics of Groundwater Systems
As a result of the tsunami, large numbers of wells were without water due to well pollution due to saline
intrusion or to pollution related to latrines and burial pits in the vicinity of wells.
Saltori and Giusti also concluded that with the elimination of fish industry immediately after the tsunami
that the agriculture industry increased and that with the electric pumps supplied by aid agencies that the
agricultural industry could be contributing to the salinity problems in the area due to up coning, as well as
contamination from nitrates and pesticides.
Rehabilitation of wells
Immediately after the tsunami wells in the area were extensively pumped in the area by NGOs, civilians
and armed forces as it was believed that this would clean the wells. In addition, many members of the
public believed that pumping need to take place in order to be able to use their well again.
Various studies showed that pumping on the wells led to increases the EC. Well cleaning was taking place
in this area by various organisations, however most did not take readings before or after their activities,
and frequently the extracted water was dumped next to the well, allowing it to permeate back into the
aquifer, causing a contamination loop (Lytton, 2008). It appears that pumping the well only destroys any
remaining fresh water lens within the aquifer by either pumping it to its extent, the pumping causing
mixing of fresh and saline water, or the recycling of the water causing in effect the same situation as
another tsunami would do locally.
The research concluded that there was no way to clean and restore the aquifers in the short term.
As a result of the problems being produced in attempts to clean the wells, by the end of 2005 UNICEF
produced guidelines on the cleaning of wells and the processes to be undertaken (UNICEF, 2005). The
rain events in this area seem to have decreased the EC in the wells, however in some cases it caused an
increase between events perhaps as salt stored in the soils leaching into the groundwater system
(Piyadasa et al. 2006).
The groundwater wells often experienced an upconing effect when being ‘pumped clean’ after the
tsunami. Many I/NGOs were attempting to pump out the salt water, however due to the increase in the
size of the salt water wedge and excessive pumping resulted in the salt water being drawn up from the
salt water wedge and increasing the salt water concentration at the borehole location (Figure 24). The
pumping was not beneficial for this reason, additionally many organisations when purging the bore let
the contaminated water flow straight onto the ground next to the bore hole, causing recontamination
(Lytton, 2008). In alluvial aquifer systems it was noted that while the salinity decreased over time, in
places the salinity spiked after the first rain period. This was due to a flushing of the salts captured in the
soils by the fresh water as it entered into the groundwater stream.
References
Bhattacharya, & al, e. (2008). Groundwater for Sustainable Development: Problems, Perspectives and
Challenges. London: Taylor and Francis Group.
Cooray, P. (1967). An Introduction To The Geology of Ceylon. Colombo: The Government Press.
Leclerc, J.-P., Berger, C., Foulon, A., Sarraute, R., & Babert, L. (2007, May 7). Tsunami impact on
shallow groundwater in the Ampara district in Eastern Sri Lanka: Conductivity measurements and
qualitative interpretations. Retrieved March 10, 2010, from Science Direct: www.sciencedirect.com
Lytton, L. (2008). Deep Impact: Why post-tsunami wells need a measured approach. ICE , 42-48.
Groundwater Drilling and Development _Session GWD 1_2
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Characteristics of Groundwater Systems
Panabokke, & Perera. (2005, January). Groundwater Resources of Sri Lanka. Retrieved March 2010,
from http://tsunami.obeysekera.net/documents/Panabokke_Perera_2005_Sri_Lanka.pdf
Piyadasa, R., Weerasinghe, K., Lakmal, H., & Maier, D. (2006). Groundwater quality changes in the
tsunami affected coastal belt - Southern Sri Lanka. 32nd WEDC International Conference (pp. 320326). Colombo: SUSTAINABLE DEVELOPMENT OF WATER RESOURCES, WATER SUPPLY AND
ENVIRONMENTAL SANITATION.
Saltori, R., & Giusti, A. (2006). Challenges of tsunami and conflict affected rural water supply in Sri
Lanka. 32nd WEDC International Conference (pp. 523-529). Colombo: SUSTAINABLE DEVELOPMENT
OF WATER RESOURCES, WATER SUPPLY AND ENVIRONMENTAL SANITATION.
Tamil Information Centre. (2006, March 15). Sri Lankan Tsunami Situation Report: Report Number 6.
Retrieved April 2010, from UNEP: http://www.unep.org/tsunami/reports/Tsunami_Report_No_6.pdf
UNICEF. (2005). Guidelines for the rehabilitation of tsunami effected wells. Galle: UNICEF.
Villholth, K., Manamperi, A., & Buergi, N. (2006). Chemical Characteristics of Tsunami-Affected
Groundwater and Lagoon on the East Coast of Sri Lanka. 32nd WEDC International Conference (pp.
334-340). Colombo: SUSTAINABLE DEVELOPMENT OF WATER RESOURCES, WATER SUPPLY AND
ENVIRONMENTAL SANITATION.
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Characteristics of Groundwater Systems
Climatic influences:
Influences on aquifers
Climatic influences including seasonal variations in amount of water that can infiltrate.
As discussed earlier, rainfall and evaporation are key influences in the amount of water that may
enter the soil. This can also be a seasonal factor.
To illustrate this Dillon et al (2009) plotted the ratio of rainfall in the driest six months /total rainfall
against the ratio of evaporation / rainfall for a range of locations. This shows the huge difference in
the potential for recharge across the world
Source: Dillon et al 2009
The impact on aquifers at a particular location can also change with time, with influence on
groundwater availability. The seasonal recharge that occurs from flood waters is a desired
consequence of the wet season in many arid environments. There is a potential impact of climatic
events (floods particularly) on water quality with the introduction of fresh water in areas of
groundater recharge
The effect of seasonal climate impact on aquifers can be seen from plots of groundwater level over
time (this is a bore hydrograph), especially when compared to climatic events. An example of a
representative bore from the Murray Basin in Australia (after Macumber 2009) shows the rise in the
water level in the bore after a wet period (1973) when extensive recharge occurred, followed by
seasonal fluctuations due to groundwater abstraction pumping. Of particular note is the decline of
groundwater level from 1992- 2006 which coincides with a combination of groundwater pumping
and a prolonged period of below average rainfall particularly from 2000 onwards.
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Characteristics of Groundwater Systems
After:
Macumber, P. G., 1991. Interaction between groundwater and surface systems in northern Victoria. Prepared for Department
of Conservation and Environment – Victoria.
Macumber, P. G., 2007. Groundwater Occurrence and Process in the Mid-Loddon WSPA. Report prepared for GoulburnMurray Water, June 2007.
Group Discussion:
What are the potential implications of climate change on groundwater ?
Discussion points:



Reduced recharge in areas where there is lower rainfall
o lower availability
o also potential for increased demand
o perhaps increase in evaporation
Some wetter areas to have higher rainfall and greater intensity
o perhaps increased recharge
o not necessarily increased groundwater recharge in the immediate area because of
increased surface runoff
o May be more recharge remotely from increased stream flows and leakage
o with higher surface runoff, possible drainage into existing wells (will mention again
later in pollution section),
Changing water demand
o as agriculture changes with climate change
o perhaps as population moves
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Characteristics of Groundwater Systems
Variability of Water quality
Groundwater is a solvent that is in contact with a wide range of earth materials (Fetter, 1992).
The chemistry of groundwater is complex and varies on location. Even in the same aquifer there are
differences depending on the time that water takes to flow through the aquifer, the recharge rate of
fresh water, the rocks that the groundwater flows through and whether there are reactions of the
water with the mineral particles in the rocks. This is discussed later.
Groundwater salinity is a primary indicator of water quality and there can be a wide variation in
groundwater salinity in aquifers. Naturally occurring groundwater can be less than 100mg/L
(milligrams per litre) to more than 50,000 mg/L (Hem, 1985). Salinity greater that 600mg/L becomes
increasingly unpalatable (World Health Organisation), so not all groundwater is drinkable.
There are numerous maps of groundwater salinity, and there is a series by Water Aid and the British
Geological Survey describing salinity in the countries in which Water Aid works.
An example of Aquifer salinity in two sedimentary aquifers in Sudan near Khartoum, which appears
to be related to recharge from the Nile. Lower salinity (<500mg/L) is found near the rivers.
From Elrail et
al 2009
In addition to salinity, there are naturally occurring chemicals eg Arsenic, Fluoride, Iron that are
dissolved in groundwater and that make it unfit for or undesirable for drinking. (We will discuss
these and contamination from anthropogenic pollutants later).
Relevance to Emergencies
The variation in salinity means that even if there is groundwater, the quality may be unsuitable for
drinking.
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Characteristics of Groundwater Systems
There are also naturally occurring chemicals which can make groundwater unfit for drinking
Groundwater Drilling and Development _Session GWD 1_2
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