Assessment of a Coastal Aquifer in the Framework of Conjunctive Use of Surface Water and Groundwater—The Case of the River Nestos Western Delta, NE Greece

: This paper presents research regarding the assessment of the hydrogeological system of the River Nestos Western Delta, NE Greece, during the period of 2019. The procedure included the collection and analysis of relevant hydrological and hydrogeological data concerning the aquifer system of the study area. Speciﬁcally, groundwater level measurements and sampling were carried out in a monitoring well network in the shallow unconﬁned and the deep conﬁned aquifers of the study area, respectively; and surface water sampling was conducted from the River Nestos at selected locations in each of the main drainage canals, as well as in lagoons of the study area; followed by analysis and processing of the relevant chemical analyses results. Finally, piezometric, hydrochemical maps and diagrams were constructed to augment the evaluation of results and the assessment of the system. The present study contributes to the development and management of water resources in the River Nestos Delta area by providing insight into the hydrodynamic and hydrochemical status of the system based on comprehensive contemporary data that can support and justify the compilation of realistic measurements. The conjunctive management of the surface and groundwater in the study area can improve the quantitative and qualitative characteristics of the water. The water level in piezometric maps varies from − 4 m up to 16 m for both time periods (May 2019 and October 2019). Moreover, the maximum values of EC are 2700 µ S/cm and 2390 µ S/cm for the conﬁned and unconﬁned aquifer systems, respectively. The maximum values of Cl − concentrations are 573.89 mg/L for the conﬁned aquifer system and 514.73 mg/L for the unconﬁned aquifer system for both time periods (May 2019 and October 2019).


Introduction
Conjunctive use of surface water and groundwater is generally determined as the allotment of quantity and/or quality of surface water and groundwater so as to achieve one or more aims while satisfying certain limitations [1]. Conjunctive use of surface water and groundwater is regarded as the optimal beneficial practice that entails the coordinated and planned utilization of both surface and groundwater resources in order to satisfy water requirements in basins that are nearing their full water resources potential development [2]. Conjunctive use entails the designed and coordinated management of surface water and groundwater in order to maximize the efficiency of using the total water resources. Overall, a correctly managed integrated system will yield more water at a better economic rate than individually managed surface water and groundwater systems [3]. The different and supplementary characteristics, as well as the specificities of both surface water and groundwater, make it feasible to resolve specific requirements of the power of the groundwater model is determined by the conceptual model. Therefore, for assessing and reducing the risk of the model's prediction, the reliability analysis of the conceptual model is meaningful and necessary for groundwater simulation [29,30]. This paper presents research work regarding the conceptual model development of the hydrogeological system of the River Nestos Western Delta, NE Greece, as a prelude to organizing the conjunctive use of surface water and groundwater in the area. The procedure included the collection and analysis of relevant hydrological and hydrogeological data regarding the aquifer system of the study area. Processing and analysis of all the collected data revealed the key hydrodynamic evolution mechanisms of the system. It appears that one of the main advantages gained through the conjunctive use of surface water and groundwater in coastal areas would be to provide more surface water for irrigation using storage reservoirs and collective irrigation networks for controlling the overpumping of groundwater as a remedy for seawater intrusion.

Location of the Study Area
The study area is located at the eastern coastal part of the Prefecture of Kavala, NE Greece, and specifically occupies most of the western part of the River Nestos Delta ( Figure 1). It is bounded by the River Nestos on the east and by the Aegean Sea on the south and west.
Hydrology 2022, 9, x FOR PEER REVIEW 3 of 20 zone study, etc.) and hydro-chemical measurements (in situ groundwater sampling and chemical analyses) [26]. According to [27], selecting the appropriate conceptual model for a given problem is one of the most important steps in the modelling process. The authors of [28] state that the power of the groundwater model is determined by the conceptual model. Therefore, for assessing and reducing the risk of the model's prediction, the reliability analysis of the conceptual model is meaningful and necessary for groundwater simulation [29,30]. This paper presents research work regarding the conceptual model development of the hydrogeological system of the River Nestos Western Delta, NE Greece, as a prelude to organizing the conjunctive use of surface water and groundwater in the area. The procedure included the collection and analysis of relevant hydrological and hydrogeological data regarding the aquifer system of the study area. Processing and analysis of all the collected data revealed the key hydrodynamic evolution mechanisms of the system. It appears that one of the main advantages gained through the conjunctive use of surface water and groundwater in coastal areas would be to provide more surface water for irrigation using storage reservoirs and collective irrigation networks for controlling the overpumping of groundwater as a remedy for seawater intrusion.

Location of the Study Area
The study area is located at the eastern coastal part of the Prefecture of Kavala, NE Greece, and specifically occupies most of the western part of the River Nestos Delta (Figure 1). It is bounded by the River Nestos on the east and by the Aegean Sea on the south and west. The study area is part of the National Park of Eastern Macedonia and Thrace, which was established in 2008, and included the already protected areas of the Nestos Delta, Vistonida Lagoon, Ismarida Lagoon, and their wider area. The study area is part of the National Park of Eastern Macedonia and Thrace, which was established in 2008, and included the already protected areas of the Nestos Delta, Vistonida Lagoon, Ismarida Lagoon, and their wider area.
The lowland area of the Nestos River, which is essentially identical to the delta of the river, is bounded by the southern boundary of the Rhodope Mountain mass at the position of the Toxotes Dam and continues south down to the Thracian Sea coastline, including the entire length of the river from the dam to the sea. East and west, the delta spreads in about two equal parts from the villages of Nea Karvali to the west, up to Toxotes to the north and Avdira to the east. The study area is a lowland area with mild slopes ranging from +40 m to 0 ± 1 m at the southern end of the coastline [31]. In this section, there are generally small morphological inclinations (<2%), from north to south, resulting in the formation of shallow sections that create seasonally stagnating surfaces (lakes or swamps). This feature, the marshes in particular, was the most prominent in earlier evolution phases of the delta and mainly owe their presence to the terminals of smaller secondary Nestos hills or mountain-zone currents [32].

Geological Setting
The deltaic area is formed of Holocene sediments with a thickness of some tens of meters deposited by the River Nestos and its sub-streams ( Figure 2). These sediments consist alternately of clay, sand, and silt layering, reflecting a broad range of formational and depositional environments, producing a very diverse geological domain. In addition, due to delta marshes, the existence of organic clay is identified at some spots [26].
The lowland area of the Nestos River, which is essentially identical to the delta of the river, is bounded by the southern boundary of the Rhodope Mountain mass at the position of the Toxotes Dam and continues south down to the Thracian Sea coastline, including the entire length of the river from the dam to the sea. East and west, the delta spreads in about two equal parts from the villages of Nea Karvali to the west, up to Toxotes to the north and Avdira to the east.
The study area is a lowland area with mild slopes ranging from +40 m to 0 ± 1 m at the southern end of the coastline [31]. In this section, there are generally small morphological inclinations (<2%), from north to south, resulting in the formation of shallow sections that create seasonally stagnating surfaces (lakes or swamps). This feature, the marshes in particular, was the most prominent in earlier evolution phases of the delta and mainly owe their presence to the terminals of smaller secondary Nestos hills or mountainzone currents [32].

Geological Setting
The deltaic area is formed of Holocene sediments with a thickness of some tens of meters deposited by the River Nestos and its sub-streams ( Figure 2). These sediments consist alternately of clay, sand, and silt layering, reflecting a broad range of formational and depositional environments, producing a very diverse geological domain. In addition, due to delta marshes, the existence of organic clay is identified at some spots [26]. The main study area belongs to the wider Nestos Delta region. The Nestos Delta is one of the most important wetlands in Greece, as well as Europe, due to the extent and diversity of its biotopes. It is included as one entry in the List of Wetlands of International Importance under the Ramsar Convention, belonging to the Natura 2000 network, as well as to the Special Protection Areas of Birds of the European Union. It is part of the National Park of Eastern Macedonia and Thrace, stretching from the Nestos Delta to the lagoons of The main study area belongs to the wider Nestos Delta region. The Nestos Delta is one of the most important wetlands in Greece, as well as Europe, due to the extent and diversity of its biotopes. It is included as one entry in the List of Wetlands of International Importance under the Ramsar Convention, belonging to the Natura 2000 network, as well as to the Special Protection Areas of Birds of the European Union. It is part of the National Park of Eastern Macedonia and Thrace, stretching from the Nestos Delta to the lagoons of Vistonida and Ismarida. In fact, the Nestos Delta consists of a mosaic of individual wetlands of which the largest and most distinct are the following (from D to A): The Bassova Lagoon, Eratinos Lagoon, Agiasmos Lagoon, Kokala Lagoon, Chaidou Lagoon, Keramoti Lagoon, Monastiraki Lagoon, and Magana Lagoon.
In the entire extent of the study area (~206 km 2 ), most of the land use is agriculture (>77%). Flood irrigation is still practiced in large parts of the area, and this contributes Hydrology 2022, 9, 172 5 of 20 to groundwater recharge through deep percolation. Additionally, this process seems to contribute to the prevention of further seawater intrusion into the aquifer system, as groundwater abstractions are also limited. It is worth mentioning that currently, shallow wells are those which are used for irrigation purposes, while pumping from deep wells is very limited and occurs only for a short period of time during the irrigation season.
The entire amount of irrigation water comes from the Toxotes Dam ( Figure 1). From this point, the two main canal routes (east and west) begin. Irrigation is carried out with open-type surface networks through reinforced concrete canals that cover an area of about 152,000 acres. In the rest of the region, at the present time, projects are being carried out to construct new irrigation canals to replace approximately 24,000 acres of existing earthen canals. Overall, collective surface irrigation networks serve the irrigation needs of approximately 176,000 acres. In the irrigated area of 176,000 acres, the water supply from the Toxotes Dam is facilitated by free flow irrigation (maximum quantity: 24 m 3 /s), and flood irrigation is the main method practiced.

Hydrometeorological Conditions
Considering that the average annual rainfall value for the period of 1985-2019 is 496.80 mm, the total volume of precipitation in the study area, the spatial extent of which is 206.13 km 2 , is 102.40 × 10 6 m 3 .
The rainfall data presented in Figure 3 for the period, 2002-2019, comes from the Chrysoupolis meteorological station, which is situated in the town of Chrysoupolis ( Figure 4). Specifically, Figure 3  In the entire extent of the study area (~206 km 2 ), most of the land use is agriculture (>77%). Flood irrigation is still practiced in large parts of the area, and this contributes to groundwater recharge through deep percolation. Additionally, this process seems to contribute to the prevention of further seawater intrusion into the aquifer system, as groundwater abstractions are also limited. It is worth mentioning that currently, shallow wells are those which are used for irrigation purposes, while pumping from deep wells is very limited and occurs only for a short period of time during the irrigation season.
The entire amount of irrigation water comes from the Toxotes Dam ( Figure 1). From this point, the two main canal routes (east and west) begin. Irrigation is carried out with open-type surface networks through reinforced concrete canals that cover an area of about 152,000 acres. In the rest of the region, at the present time, projects are being carried out to construct new irrigation canals to replace approximately 24,000 acres of existing earthen canals. Overall, collective surface irrigation networks serve the irrigation needs of approximately 176,000 acres. In the irrigated area of 176,000 acres, the water supply from the Toxotes Dam is facilitated by free flow irrigation (maximum quantity: 24 m 3 /s), and flood irrigation is the main method practiced.

Hydrometeorological Conditions
Considering that the average annual rainfall value for the period of 1985-2019 is 496.80 mm, the total volume of precipitation in the study area, the spatial extent of which is 206.13 km 2 , is 102.40 × 10 6 m 3 .
The rainfall data presented in Figure 3 for the period, 2002-2019, comes from the Chrysoupolis meteorological station, which is situated in the town of Chrysoupolis (Figure 4). Specifically, Figure 3        An analysis of the presented data yields the following results: 1. The average annual rainfall is 496.80 mm. The minimum and maximum annual rainfall are 227.30 mm and 968.20 mm, respectively, for the same period. 2. The peak value of rainfall is observed in November and December. The period of July-September contains the months with the minimum rainfall values.

1.
The average annual rainfall is 496.80 mm. The minimum and maximum annual rainfall are 227.30 mm and 968.20 mm, respectively, for the same period.

2.
The peak value of rainfall is observed in November and December. The period of July-September contains the months with the minimum rainfall values.

Hydrogeological Setting
In the area of the Nestos River Delta, two water-reserve zones are developing: (a) the zone of the unconfined aquifer and (b) the zone of the confined aquifer, located in the range of the Miocene series.
The first zone is directly recharged by precipitation, percolation through the Nestos riverbed, and lateral crossflows from the karst system of the wider area along the northern boundary of the studied system. Lateral crossflows mainly contribute to the recharge of the aquifers of the Miocene series. In the Quaternary deposits of the delta plain, the unconfined aquifer is formed, which is currently characterized by limited groundwater level fluctuation [25].
In the western part of the Nestos River Delta, according to earlier reports, the first zone of the aquifer system had begun to weaken. This zone experienced a great decline in the observed groundwater level in shallow wells over the years. There are two main reasons for this phenomenon: overpumping and the construction of lined canals. In addition, the construction of some deep drainage canals created a continuous discharge from the unconfined aquifer.
The second zone of the confined aquifer is made up of the permeable formations of the delta (gravel, sands), which alternate with clays, in the vertical as well as in the horizontal dimension, forming superimposed aquifers. Below the unconfined aquifer, there are other aquifers up to a depth of 150 m. Among these confined aquifers, some are artesian and are detected from a depth of about 15 m. These artesian aquifers have been located a depths of up to 120 m in the area of Keramoti [34].
The irrigation network in the study area, with water supply from the River Nestos, was constructed during the 1950s. Comparison of older hydrological and hydrogeological data to the monitoring data collected in the framework of this study demonstrate that groundwater levels in the study area have recovered considerably, and consequently, quality indicators have improved over the last 20 years and especially after the operation of the dams constructed by the Greek Public Power Corporation (PPC) at the upstream portions of the River Nestos. This is due to the fact that surface water, instead of groundwater, has been used for irrigation purposes in the study area since 2000, thus enabling the aquifer system to recover to its baseline state, before systematic abstractions were initiated.
In the present research in the study area, two networks of groundwater monitoring points were created (Figure 4). One network includes monitoring wells focusing on the unconfined aquifer, with a depth of less than 15 m, and the other is comprised of monitoring wells that focus on the confined aquifer, with a depth of more than 75 m. Figure 4 presents the geological sections of four selected wells in the confined aquifer. Piezometric maps were designed based on the relevant groundwater level measurements for the four time periods of May to October 2019 (Figures 5-8).
In the present research in the study area, two networks of groundwater monitoring points were created (Figure 4). One network includes monitoring wells focusing on the unconfined aquifer, with a depth of less than 15 m, and the other is comprised of monitoring wells that focus on the confined aquifer, with a depth of more than 75 m. Figure 4 presents the geological sections of four selected wells in the confined aquifer. Piezometric maps were designed based on the relevant groundwater level measurements for the four time periods of May to October 2019 (Figures 5-8).     After analyzing these maps, the following conclusions were drawn: 1. The main groundwater recharge areas of the upper unconfined aquifer system occur mainly from the N-NW portion of the study area from the River Nestos and old riverbeds, as well as from the local irrigation network. 2. There were no significant groundwater level fluctuations detected in either the confined or unconfined aquifers during the four periods. After analyzing these maps, the following conclusions were drawn: 1.
The main groundwater recharge areas of the upper unconfined aquifer system occur mainly from the N-NW portion of the study area from the River Nestos and old riverbeds, as well as from the local irrigation network.

2.
There were no significant groundwater level fluctuations detected in either the confined or unconfined aquifers during the four periods.

Hydrochemical Setting
In the framework of this research work, in situ measurements of groundwater temperature, pH, and electrical conductivity were performed. In addition, groundwater sampling from 24 and 22 wells from the unconfined and the confined aquifers, respectively, in the study area was carried out in two time periods (May and October 2019). Then, relevant chemical analyses were performed at the accredited laboratory of the Soil and Water Resources Institute-Hellenic Agricultural Organization, Sindos, Greece. The laboratory measurements included the determination of several physicochemical and chemical parameters such as: temperature, pH, electrical conductivity, alkalinity, NH 4 + , NO 3 − , NO 2 − , Cl − , Na + , Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ , SAR, total hardness, and SO 4 2− . Hydrochemical maps were also compiled, presenting the spatial distribution of temperature, pH, electrical conductivity, NH 4 + , NO 3 − , NO 2 − , Cl − , Na + , Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ , SAR, and SO 4 2− values. In                       Table 3. Statistical analysis of some of the major chemical constituents from groundwater samples (October 2019) (unconfined aquifer).   Table 4. Statistical analysis of some of the major chemical constituents from groundwater samples (October 2019) (confined aquifer).  Moreover, in situ measurements of surface water temperature, pH, and electrical conductivity were performed, and surface water sampling was performed in three monitoring sites along the River Nestos course (Nestos 1, 2, and 3) and in four monitoring sites on the main drainage canals (T1, T2, T3, and T4) in the study area (Tables 5-8) for the same time periods (May and October 2019) ( Figure 17). Relevant laboratory measurements regarding the surface water sampling included the determination of several physicochemical and chemical parameters such as: temperature, pH, electrical conductivity, alkalinity, NH 4 + , NO 3 − , NO 2 − , Cl − , Na + , Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ , SAR, total hardness, and SO 4 2− .              From the analysis of the Piper and Durov diagrams (Figures 18 and 19), for both the confined and the unconfined aquifers, it is concluded that most of the samples are in the range of magnesium bicarbonate type, and some of the samples show mixed type behaviors [35]. From the analysis of the Piper and Durov diagrams (Figures 18 and 19), for both the confined and the unconfined aquifers, it is concluded that most of the samples are in the range of magnesium bicarbonate type, and some of the samples show mixed type behaviors [35].  The EC values range from 388 µS/cm to 2700 µS/cm for the confined aquifer system and from 320 µS/cm to 2390 µS/cm for the unconfined aquifer system for both time periods.

Ca
The highest values are observed in both the unconfined and the confined aquifer in the western and southern portions of the study area. This fact was expected because these are the areas closest to the sea, and they are more susceptible to the seawater intrusion phenomena.  The EC values range from 388 μS/cm to 2700 μS/cm for the confined aquifer s and from 320 μS/cm to 2390 μS/cm for the unconfined aquifer system for both tim ods. The highest values are observed in both the unconfined and the confined aqu the western and southern portions of the study area. This fact was expected because are the areas closest to the sea, and they are more susceptible to the seawater int phenomena.
The values of Cl − concentrations range from 4.05 mg/L to 573.89 mg/L for the co aquifer system and from 4.78 mg/L to 514.73 mg/L for the unconfined aquifer syste The values of Cl − concentrations range from 4.05 mg/L to 573.89 mg/L for the confined aquifer system and from 4.78 mg/L to 514.73 mg/L for the unconfined aquifer system for both time periods. These values are observed mainly in the western part of the study area and in wells that are adjacent to the sea and away from the River Nestos.
The values of NO 3 − concentrations range from 0.00 mg/L to 140.50 mg/L for the confined aquifer system and from 0.00 mg/L to 61.66 mg/L for the unconfined aquifer system for both time periods, with most values being around 10.00 mg/L for all monitoring wells and for all seasons. In general, most values are below 50.00 mg/L, which is the maximum admissible concentration for human consumption, and it is commonly used as a threshold value for the purpose of assessing the chemical status of groundwater bodies in Greece.
None of the values of all measured parameters at the four sampling points of the drainage canals for all periods show exceedances, according to relevant national hydrochemical regulations for irrigation and potable water. Particularly, the maximum value of EC was 499.00 µS/cm, with a minimum value of 337.00 µS/cm. Moreover, the maximum value of NO 3 − was 5.43 mg/L in the drainage canal T3 for the period October 2019, while the values of Cl − were >5.16 mg/L for all canals and for all periods.
Finally, it must be noted that most isolines are obtained automatically from the GIS interpolation software.

Conclusions
The area of the western part of the River Nestos Delta plays an important role in the economic development and the environmental balance of the broader area. In the area, two water-reserve zones are developing: (a) the zone of the unconfined aquifer and (b) the zone of the confined aquifer located in the ranges of the Miocene series, the first zone of the aquifer. According to earlier reports, this area had begun to weaken. Over the years, this zone has faced a significant lowering in the observed groundwater level in shallow wells. Two main reasons were identified for this phenomenon: overpumping and the construction of concealed canals. On the contrary, the construction of some deep drainage canals created a continuous discharge to the unconfined aquifer. The second zone of the confined aquifer is hosted in the permeable formations of the delta (gravel, sands) that alternate with clays.
For the purposes of the present research, two networks of groundwater sampling points were created in the study area. One network included wells of the unconfined aquifer, with a depth of less than 15 m, and another well network was made up of the confined aquifer, with a depth of more than 75 m. Piezometric maps were designed based on the relevant groundwater level measurements for the four time periods (May and October of 2019, respectively).
In the framework of this research, groundwater level measurements, including in situ measurements of groundwater temperature, pH, and electrical conductivity, and groundwater sampling from 24 and 22 wells from the unconfined and the confined aquifer, respectively, at the study area were carried out in two time periods (May and October 2019). Moreover, in situ measurements of surface water also took place, which included temperature, pH, and electrical conductivity, as well as surface water sampling from three monitoring sites in the Nestos River and in four monitoring sites on the main drainage canals of the study area for the same time periods. Laboratory measurements included determination of temperature, pH, electrical conductivity, alkalinity, NH 4 + , NO 3 − , NO 2 − , Cl − , Na + , Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ , SAR, total hardness, and SO 4 2− . Hydrochemical maps were also compiled presenting the spatial distribution of several parameters.
The processing and analysis of all the collected data resulted in significant findings with regards to the hydrodynamic evolution of the system, where one of the main advantages of the conjunctive use of surface water and groundwater in coastal areas appears to be the provision of more surface water irrigation, with the help of storage reservoirs and irrigation networks, for controlling the overpumping of groundwater as a remedy for seawater intrusion.
Furthermore, the present study is expected to contribute to the development and management of water resources in the eastern and western delta of the River Nestos, where two different methods of using irrigation water occur. In addition, it is considered that the results of this research can enrich the database emerging from the transfer of surface water from the River Nestos planned for the irrigation of a total area of more than 200 km 2 .
As surface water has been used intensively for irrigation for the last 20 years, the conjunctive management of surface water and groundwater is very interesting for further study. Relevant measurements should be continued in order to assess how the use of surface water can improve the quantitative and qualitative characteristics of groundwater [36].
Finally, the goal of this study was to establish a solid conceptual model for the study area in the framework of hydrogeological investigation regarding the conjunctive use of water, without a need of a detailed approach for each contributing factor. An additional, more detailed approach is planned as a future stage for this research.
Author Contributions: Conceptualization and methodology, G.K. and I.G.; writing-original draft preparation, review, and editing, G.K., I.G. and F.-K.P.; validation and supervision, A.P., F.-K.P. and I.D. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Data Availability Statement:
The study did not report any data.