THE DEVELOPMENT OF A GROUNDWATER FLOW MATHEMATICAL MODEL IN THE ANO VIANNOS BASIN, CENTRAL CRETE. SOURCE PROTECTION ZONE DELINEATION AND UNCERTAINTY ANALYSIS

Giorgos Zacharioudakis
MSc Hydrogeology
www.groundwater.gr

Introduction
The scope of the study was to investigate the risks to groundwater in the basin of Ano Viannos in Central Crete delineating the protection zones of the abstractions in the basin using mathematical methodology.
The methodology included running the code Modpath on an already built mathematical model using the code Modflow. The delineation of the source protection zones was based on releasing particles in the area tracing them backwards in time (Reverse particle tracking analysis). The actual groundwater protection zones were created by drawing the isochrones time-travel contours of the particles, using the Modpath model results. Uncertainty analysis was also conducted to investigate the change of the shape of the resulted groundwater protection zones by altering the uncertain parameters in the system and the resulted model.

The area of study
The Ano Viannos basin is a small alluvial basin (about 2.5 km2) located to the SE of the Herakleion prefecture, in central-east Crete. It is surrounded by steep hillsides in the north and east whilst in the west and south, the terrain is not steep. Three streams are flowing in the basin formed from springs at hills in the NE. The basin was filled up with river related sediments comprising coarse gravel in the NE of the basin which grades out to finer sediments in the SW of the basin. This zonation is probably related to the to the role of the surface water network in filling the basin with sediments. The gravels in the basin comprise a very good quality aquifer of an average thickness of 100 m. During the 80s the aquifer was exploited mainly by shallow wells and there was a series of springs discharging the aquifer at the approximate boundary of the coarse with the finer member of the sediments in the basin. In the 90s seven wells were drilled in the basin abstracting larger quantities of groundwater from the aquifer.

Designing a mathematical model
A Modflow steady state model was built as part of the process of delineating the groundwater protection zones. The code Modpath runs using results of the Modflow code, and therefore a Modflow model needed to be constructed. A conceptual model was developed which comprised the main base for the design of the mathematical model. Rainfall recharge, lateral flows from the formations in the North and East of the basin and percolation of the surface water of the streams comprise the main inflows in the basin. Groundwater abstractions and potential lateral outflows to the SW comprise the main water outflows from the basin. The Region of Crete has installed one piezometer that monitors the aquifer since 1981. The data from this borehole were used as a calibration point to calibrate the Modlfow model. The model was also calibrated against the low abstraction period of the eighties when the springs were discharging water in the area.

Reverse particle analysis
Particles were introduced in the Modflow model in small circles around the abstraction points. Using the code Modpath each particle was traced back to its starting location for a long time leaving a trace in the model. The traces were plotted in 50-day period, 400 day period and a very long time period (ie 109 days).

Groundwater Protection zones
By joining the endpoints of the traces the respective time of travel zone is delineated. Repeating this process for the relevant time of travel zone, the definitive set of groundwater protection zones is produced.
The Environment Agency of England and Wales is defining the following groundwater protection zones:
Zone I or Inner Zone. This zone corresponds to 50 days time of travel. This time has been chosen to represent time relevant to decay of bactretiological agents in the water.
Zone II or Outer Zone. This zone corresponds to 400 days time of travel, aiming to represent the living time of the slower decaying contaminants.
Zone III or Total Catchment zone. This zone corresponds to a very high time of travel aiming to represent the total catchment of the borehole. In other words any contaminant that is going to released in this zone, if it is inactive it will arrive in the abstraction after a long period.