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

1. CONTRIBUTION TO HYDROLOGICAL ANALYSIS OF THE COASTAL KARST SPRING ALMYROS (CRETE, GREECE) Ognjen Bonacci & Ivana Fistanic Faculty of Civil Engineering & Architecture Split, CROATIA

2. 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 annuallyin 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.

3. 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 between4 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.

4. 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-aquiferwhich is considered as the main sub-aquifer. • Sub-aquifer A – not directly connected to the spring by large karst conduit. Tw=16 C° Tw=20 C° Tw=16 C°

5. 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.

6. 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.

7. 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.

8. Time series of rainfall, discharges (total, fresh and seawater discharge) and chlorides for one rainfall event

9. 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.

10. 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

11. 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.

12. 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.

13. RELATIONSHIP BETWEEN PEAK SPRING DISCHARGE RISING ΔQ 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 hydraulicalconductivityand existence of large karst fractures and conduits in the spring catchment area.

14. Discharge of the sea and fresh water was defined according to the following expression:

15. 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.