CONTRIBUTION TO HYDROLOGICAL ANALYSIS OF THE COASTAL KARST SPRING ALMYROS (CRETE, GREECE)

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CONTRIBUTION TO HYDROLOGICAL ANALYSIS OF THE
COASTAL KARST SPRING ALMYROS (CRETE, GREECE)

• Ognjen Bonacci Ivana Fistanic
• Faculty of Civil Engineering Architecture
• Split, CROATIA

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INTRODUCTION TO THE PROBLEM OF SEA WATER INTRUSION

• Sea water intrusion into the aquifers exists in
  many coastal areas of the world particularly in
  karst areas
• Fresh water quantities that are in this way lost
  for human consumption are significant (25x109
  m3 annually in Mediterranean coastal karst
  areas)
• Intensive studies of brackish karst springs
  France, Spain, Greece, Italy, Croatia, Malta,
  Lebanon, Libya, etc.
• All these studies have been done with the goal to
  take efficient protective measures against sea
  water intrusion into the karst aquifers
• Results better understanding of brackish springs
  functioning, but still, successfully realized
  solutions of coastal karst springs protection
  from sea water intrusion were not achieved.

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Case study Crete Almyros spring
The island of Crete has serious problem with
shortage of water during summer tourist
season. The Almyros spring on the north of the
island in the vicinity of the town Heraklio 90
of the year the spring water is brackish. Spring
discharge fluctuates between 4 m3/s and 70-80
m3/s. In summer period concentration of
chlorides is about 5000 mg/l Almyros spring
could be greatest fresh water resource on the
island.
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CATCHMENT AREA OF THE ALMYROS SPRING

• Location 1 km from the northern coast 3 meters
  above sea level
• Catchment area is from 300 to 500 km2.
• Sub-aquifer B spring is mainly recharged with
  water from this sub-aquifer which is considered
  as the main sub-aquifer.
• Sub-aquifer A not directly connected to the
  spring by large karst conduit.

Tw16 C
Tw20 C
Tw16 C
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CORRELATION ANALYSIS

• A correlation analyses, as a type of time series
  analyses, gives valuable information about
  aquifer characteristics.
• A correlation analyses relates inputs to outputs
  of the system through the use of statistical
  functions.

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Cross-correlation analysis rainfall-spring
discharge

• Anogia station - west central part of the aquifer
  (740 m a.s.l) Marathos station - close to the
  spring on the west (400 m a.s.l)
• maximum coefficient value for two days lag.
• Iirregularly distributed peaks-complex
  underground features (existence of few
  sub-aquifers)
• The stronger dependence between discharge and
  Anogia rainfall-spring is mainly recharged by the
  water from the inland sub-aquifer B.

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CROSSCORRELATION ANALYSIS FRESH WATER
DISCHARGE-SEA WATER DISCHARGE/CHLORIDES

• Strong inverse relationship
• Highest crosscorrelation coefficient is -0,85 for
  chlorides and -0,77 for sea water discharge
• Fresh water discharge has longer influence on
  chlorides than on sea water discharge
• Chloride increasing is not caused only by higher
  sea water inflow but it is the result of fresh
  water discharge decrease.

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Time series of rainfall, discharges (total, fresh
and seawater discharge) and chlorides for one
rainfall event
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RELATIONSHIP BETEWEEN SEA WATER DISCHARGE AND
CHLORIDES FOR 13 RAINFALL EVENTS

• In phase I. and phase II. relationship between
  sea water discharge and chloride concentration is
  more or less linear
• At the beginning of phase III. relationship is
  linear and with higher values of chlorides and
  sea water discharge it becomes nonlinear
• The sea water discharge in this phase reaches
  maximum value of about 0,8 m3/s
• Further chloride increase is the result of fresh
  water discharge decrease.

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Mechanism of sea water intrusion in karst areas
A2
B
A1
A/ Depending on pressure in branching place two
different situation are happening sea water
intrusion (A1) and freshwater outflow into the
sea (A2) B/ Diffuse inflow into the conduit from
the karst matrix
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CONSLUSIONS ABOUT ALMYROS SPRING

• Recharge system of Almyros spring is composed of
  two sub-aquifers
• Large karst features exist in karst aquifer
• In karst aquifer matrix exists considerable
  volume of small voids with laminar flow
• Spring is mainly recharged by the water from the
  inland aquifer
• Existence of karst conduit coneccted to the sea
  and siphon like shape of main conduit
• Chloride behavior can be interpreted through
  three phases depending on fresh water and sea
  water discharge changing - General shape of this
  function exists for each rainfall event with
  small modification depending on initial
  conditions in aquifer when rainfall event
  started
• Further analyses of the characteristics of these
  functions would contribute to the better
  understanding of the complex process of sea water
  intrusion.

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CONCLUSIONS ABOUT SALINISATION PROCESS

• Generally based on processes of sea water level
  changes in geological history it is assumed that
  there is main channel connected to the sea and
  siphon like shape of the main conduit
• Monitoring data is needed for the purpose of of
  recognizing underground processes
• Each karst spring is unique and has its unique
  geometry of underground conduits and mechanism of
  sea water intrusion
• Knowledge of this geometry and position of this
  channel and main channel can be used for
  implementing necessary measures for spring
  desalinization and management.

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RELATIONSHIP BETWEEN PEAK SPRING DISCHARGE RISING
?Q AND RAINFALL P

• The analytical expression is defined using 12
  hydrographs measured in period between September
  1999. and May 2001.
• Strong relationship high karst matrix
  hydraulical conductivity and existence of large
  karst fractures and conduits in the spring
  catchment area.

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• Discharge of the sea and fresh water was defined
  according to the following expression

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RELATIONSHIP BETEWEEN SEA WATER DISCHARGE AND
CHLORIDES FOR ONE RAINFALL EVENT

• One rainfall event
• Relationship between sea water discharge and
  chloride concentration is linear
• Three phases are clearly present.