Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe
Groundwater Quality 2001 Conference held al Sheffield. UK., .lime 2001 ). IAHS Publ. no. 275. 2002.
85
Groundwater quality considerations related to
artificial recharge to the aquifer of the Korinthos
Prefecture, Greece
M. HIONIDI, A. P A N A G O P O U L O S \ I. K O U M A N T A K I S ,
Department of Mining and Metallurgical
Athens, Greece
Engineering, National Technical University of Athens,
e-mail: panagopoulos.Iri@nagref.gr
K. V O U D O U R I S
Department of Geology, University oj'Patras,
Greece
Abstract Artificial recharge was employed in a preliminary study as a means
to restore the aquifer system of the Korinthos Prefecture and it was concluded
that this method can significantly contribute to groundwater quality improvement. Extreme care, however, must be taken as temporary quality
deterioration may occur due to washout of the unsaturated zone. Boreholes
and dug wells appear most vulnerable to pollution and damage compared to
other methods of artificial recharge, since injected water directly reaches the
saturated zone. Diluted air and suspended solids content are key parameters
for the recharge rate sustainability. A direct relationship between aquifer
matrix lithology and the aforementioned factors has been observed. Injection
water should be of identical chemical characteristics to resident groundwater.
Despite limitations in its application, artificial recharge with water of different
hydrochemical characteristics to resident groundwater appeared to be a
comparatively low cost, effective remedial measure for aquifer restoration.
Key words groundwater artificial recharge; groundwater quality improvement;
high turbidity recharge water; Korinthos coastal plain; limited water resources
INTRODUCTION
A number of artificial recharge experiments were carried out in the northern coastal
part of the Korinthos Prefecture, Greece, to investigate the applicability of the method
and the associated problems, and to assess its efficiency for replenishing the aquifer
system and improving the groundwater quality. The current study aims at presenting
groundwater quality issues that need to be taken into consideration during the
application of an artificial recharge programme.
Water needs in the area are principally covered by groundwater abstraction and
additionally by surface water, mainly from the River Asopos. These are the only fresh
water resources in the area. However water resources development has caused
considerable groundwater quality deterioration (Daskalaki et al, 1998). Water demands
have considerably increased over the last 15 years (Koumantakis et al, 1999b).
Consequent overexploitation by the ever-increasing number of deep wells led to a
''Currently al: The National Agricultural Research Foundation, Land Reclamation Institute, Greece
M. Hionidi et al.
86
hydraulic head decline that triggered saline water intrusion. Salinization has become
evident along some parts of the coastal sedimentary aquifer system. Groundwater
quality deterioration is also being caused by the over-fertilization of crops and the use
of abandoned shallow wells as septic tanks, as indicated by the reported increased
nitrogen compound concentrations (Voudouris et al, 2000b). Surface water is also
subject to quality deterioration due to illegal waste disposal from olive oil mills.
Groundwater artificial recharge has been considered as a promising remedial
measure for the aquifer system, which is of paramount importance to the socioeconomic development of the region. Its successful application requires good
understanding of the geometry, the prevailing flow mechanisms and the evolution of
the aquifer system. Despite problems in its application, artificial recharge appeared to
offer a short- to long-term, comparatively low cost, effective remedial measure for
aquifer restoration.
G E O L O G I C A L A N D H Y D R O G E O L O G I C A L SETTING
The study area is situated in the northern coastal part of the Korinthos Prefecture and
its geological structure is presented in Fig. 1. An aquifer system occurs in the recent
basin deposits, which consist of unconsolidated materials: sands, pebbles, breccias and
fine clay to silty sand sediments, characterized by a high degree of heterogeneity. The
thickness of the basin deposits varies from 30 m to 70 m. Recent and older fluviotorrential deposits originating from the streams and rivers that flow across the study
area disrupt the lateral continuity of these sediments. Their thickness exceeds 100 m
along the River Asopos (Koumantakis et al. 1999a).
In the south, Tyrrhenian deposits of coastal origin outcrop and their continuity is
disrupted as a result of erosion. They consist of highly consolidated breccioconglomerates, sand and gravel, with intermittent marl intercalations. It is believed that
these deposits locally thicken to the north, underneath the recent unconsolidated
sediments of the plain. Pliocene marl series occupy most of the hilly region further
south of the study area and form the bedrock of the aquifer system (Panagopoulos et
al, 1999).
Transmissivity and storage coefficient vary between 2 x 10 and 9 x 10 m day"
and 0.1 x 10" and 5 x 10" respectively, as deduced from extensive pumping test
analyses (Panagopoulos et al, 1999).
Recharge to the aquifer system originates from direct rainfall infiltration and
riverbed infiltration. Lateral crossflow from the fluvio-torrential and also from the
Tyrrhenian deposits across the southern edge of the basin, is essential to the system's
replenishment. Returns from flood irrigation, which is traditionally practiced in the
region during springtime, also play a key role in the aquifer's recharge. Mean annual
temperature is 18.3°C, whilst average precipitation is 473 mm year" and occurs mainly
between late October and May, with sparse storm events during the summer months.
Since the late 1980s, however, a change in the rainfall distribution pattern has
occurred. Rainfall now occurs in fewer discrete events having a high intensity and
short duration, resulting in higher runoff, lower recharge to the aquifer system and
increased irrigation demands.
1
5
2
2
1
2
1
87
Groundwater quality considerations related to artificial aquifer recharge
Map compilation is based upon geological maps issued by the Greek Institute of Geology and Mineral Exploration.
Fig. 1 Geological map of the study area, also showing chloride concentrations.
GROUNDWATER QUALITY
Study of the analysed groundwater samples showed that the average p H is 7.2, thus
indicating a neutral environment. The average value of total dissolved solids (TDS) is
high at 954 m g l " and exhibits significant local variation (standard deviation of the
order of 550 m g l" ). Total hardness (TH) in most of the samples exceeds 300 m g 1"' as
C a C 0 . Thus the groundwater is classified as very hard. All major ions exhibit wide
variation in their concentrations across the study region but the dominant ions are C a
and H C 0 " .
Chloride ( C I ) concentration shows a general increase downgradient to the north
towards the coastline (Fig. 1), whereas it is lower along the main river courses and especially along the River Asopos. In addition to this trend, statistical analyses demonsttate
a general increase of the chloride concentrations to the east (Voudouris et al, 2000a).
Nitrate is noticeable throughout the entire region, rendering most of the analysed
waters unsuitable for human consumption, as concentrations far exceed 50 mg l" (EU
Council, 1998). No specific pattern can be identified with regard to the nitrate
concentration distribution. However high concentrations are observed in specific areas.
Electrical conductivity varies between 550 and 4120 U.S cm" , and this is probably
indicative of saline intrusion along the coastal areas of the studied system. It generally
increases eastwards, as also indicated by statistical analysis of the hydrochemical data
(Voudouris et al, 2000a), and reaches its peak at the Lecheo area (EC > 4000 jiS cm" ).
The lowest values occur in the northwestern part ofthe region, where the River Asopos
flows through the plain.
1
1
3
+ +
3
1
1
1
88
M. Hionidi et al.
ARTIFICIAL R E C H A R G E E X P E R I M E N T S
A preliminary study was carried out in order to locate zones where artificial recharge
would be most beneficial to the aquifer system, as well as to decide on the most
suitable method of artificial recharge for each part of the region, taking into account
the local conditions of land use, water use and the geometry of the lithological facies
(Panagopoulos et ai, 1999). The duration o f t h e experiments ranged between 10 h and
23 days. Recharge water physiochemical parameters were continuously recorded throughout the entire duration of the experiments, as was groundwater in a network of monitoring points at various distances from the recharge well.
Eight experimental sites located in the western and central part of the study area
were selected, although ion concentrations in that region are lower. In six of the
experiments, large diameters dug wells (2-3 m), formerly used as production wells,
were employed as recharge wells, whilst the remaining two were flooding field and
flooding trench experiments.
In order to apply any artificial recharge method, water of adequate quantity and
quality is required (Huisman & Olsthoorn, 1983). The River Asopos' winter runoff
was the main source of recharge water used in the experiments, and it was transferred
to each site via the existing concrete lined irrigation canal network. The issues
associated with using the selected source are the sparsity of surplus water between
April and October (due to irrigation and climatic conditions), the chemical
incompatibility between November and February (due to contaminant burden), and the
unsuitability of the physical properties of the water. With respect to the latter,
problems were associated with the high turbidity values and the high content of air
bubbles. In particular, due to the morphology and Iithology of the Asopos catchment,
river water is rich in fine suspended particles. Recorded turbidity values were in the
range 150-9000 N T U (nephelometric turbidity units). According to the American
Society for Artificial Recharge of Groundwater (1998), turbidity of recharge water
should not exceed 2-5 N T U and ideally should be less than 1 NTU. The river water
used is also rich in dissolved air due to the flow velocity in the river and in the
irrigation canals that were used to transfer water to the experimental sites.
Figure 2 demonstrates that water level changes in the vicinity of a recharge well
are related to the supplied water quality. As can be seen, the increase on 4 April 1998
of suspended particle concentration is followed by an immediate groundwater level
increase in the adjacent observation well. Water level rises as a result of reduction of
the effective porosity and transmissivity in the aquifer zone surrounding the recharge
well. This is due to well clogging caused by the suspended solids and dissolved air. An
abrupt rise of water level was mainly observed during the afternoon and early evening
of sunny and hot days. This was attributed to the snow thawing in the high ground of
the catchment and the consequent increase of flow rates and the erosive capacity of the
flowing water (Panagopoulos et ai, 1999).
RESULTS AND CONCLUSIONS
Groundwater quality in the monitoring points adjacent to the recharge wells followed
closely, as anticipated, the water chemistry of the recharge water. The experiments
Groundwater quality considerations related to artificial aquifer recharge
Groundwater level (in)
• Recharge (low rate x 10 (m"3/t
Turbidity (NTU)
f
89
End of artificial re>charge^_^
6000
1000
v
30/3/98
1/4/98
3/4/98
5/4/98
Fig. 2 Water level fluctuation in the vicinity of the recharge well (1 m) in comparison
with the turbidity and the flow rate of the recharge water.
Groundwater EC fluctuation (pS/cm)
Recharge water EC fluctuation (u.S/cm)
V
NV
\
\
^-^^
940
8/4/98
11/4/98
End of recharge period
12/4/98
Date
Fig. 3 EC fluctuation at a monitoring point 15 m from one of the artificial recharge
sites compared with the EC fluctuation of the recharge water.
demonstrated that artificial recharge with water of different hydrochemical
composition resulted in quality improvement (up to 2 5 - 3 0 % ) of the resident groundwater. For example, Fig. 3 shows the EC graph plotted from a monitoring point 1 5 m
distant from the recharge well.
In the early stages of the recharge period (Fig. 3), groundwater quality deteriorates
as a result of the unsaturated zone washout induced by the water level increase due to
the artificial recharge. The observed depression of the curve corresponds to the arrival
90
M. Hionidi et al.
of the fresh recharge water at the monitoring point and denotes a considerable
improvement in its quality, which is apparently maintained even after the end of the
recharge period. This magnitude of improvement was short lived (1.5 months after the
cessation of the experiments).
Wells appear most vulnerable to pollution and damage compared with other
groundwater artificial recharge methods, since recharge water moves directly to the
saturated zone, without passing through the unsaturated zone. In contrast, in flooding
methods the recharge water is self-cleaned during its course from the unsaturated
towards the saturated zone. However, extremely high land costs in the region restrict
application of this method. In addition, the success of flooding methods relies on the
existence of continuously permeable sediments, from the soil surface to the saturated
zone, which is not the case in the study area.
The experiments demonstrated that in order to maintain the transmissive capacity
of the recharge wells, cycles of artificial recharge, pumping, and reapplication of
artificial recharge should be implemented in order to remove the fine particles and
trapped air bubbles that clog the well. The whole approach provides an inexpensive
method of groundwater quality improvement that requires no prior treatment of the
recharge water and is suitable for areas with limited resources. No spectacular results
are anticipated, but immediate application can be programmed since no specialized
engineering works are required, hence making the method suitable for crisis situations.
A c k n o w l e d g e m e n t s The authors wish to acknowledge the cooperation of the staff of
the Ministry of Agriculture, and the members of the research team who worked
together in order to successfully complete the project.
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