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.