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Sustainable Water Resources Management

, Volume 5, Issue 1, pp 187–201 | Cite as

Replacing the groundwater supply by a surface source: effects on the aquifer in Santo Tomé city (Argentina)

  • Marcela PerezEmail author
  • Marta Paris
  • Mónica D’Elía
  • Lukas Brandt
Original Article
  • 38 Downloads

Abstract

Santo Tomé city (Santa Fe Province, Argentine Republic), with a population of approximately 60,000 inhabitants, is supplied by pumping wells operated by the Municipal Government. The wells are located in the urban area and they are fed up from a leaky aquifer, which lodges good-quality water. Because of the increasing demand, the aquifer system is more and more exploited. Even though, up to the moment, the drop of the water level in the exploited aquifer is not significant, groundwater shows a progressive deterioration in its quality due to the rising of saline water that underlain the leaky aquifer. To cope with this situation, the local authorities have decided to evaluate the possibility of replacing groundwater by a surface water source. On the other hand, and to guarantee good-quality water to all its inhabitants the Government of the Province began the construction of aqueducts with their intakes in the Paraná River and its tributary. Under this situation is strategy to evaluate the effects that would be produced on groundwater levels the importation of surface water and the subsequent abandonment of groundwater wells in Santo Tomé City. This paper shows the results of the hydrogeological studies carried out to define the conceptual model, the 3D numerical model implemented to simulate the groundwater level evolution and some guidelines to contribute to the sustainable management of the water supply in Santo Tomé city.

Keywords

Sustainable water management Water supply Groundwater modelling Argentina 

Introduction

A source of water supply has always conditioned the location and development of a human settlement. The proximity of watercourses such as rivers or lakes, or the presence of aquifers of good quality, yield and accessible depth are the places chosen by a population. When this is not possible, it is sought some type of driving system to obtain water of better quality located at more distance.

In accordance with this and with the objective of guaranteeing the human right to access to drinking water to all its inhabitants, the Government of Santa Fe Province adopted a solution consisting of the design and construction of 12 aqueducts which capture the raw water from the Paraná River and its tributaries. According to the diagnosis made, the building of these aqueducts would favour the establishment of industries and initiatives of various kinds and would overcome the issues of water quality and the lack of potable water in most of the localities of the province (Binner and Bonfatti 2009). It should be noted that only 15 of the 362 localities in the province are supplied by Aguas Santafesinas S.A.—the state-owned utility. The water supply in the remaining localities is done by local government or cooperatives using groundwater.

The decision of the Provincial Government to implement the Program of Great Aqueduct System emphasises the fact that capturing surface raw water, with all the necessary facilities (intake, treatment plant, pipelines) is the only efficient way to provide a good service of water supply to all the inhabitants of the Province. In addition, Forestieri, et al. (2009) assume that “as the Aqueduct System is put into operation, the use of the aquifers will be reduced allowing their recovery both in quality and quantity”.

In this regard, it is worth mentioning the existing problems, in our country and worldwide, associated with the import of surface water to urban centres that have historically been supplied with groundwater.

At national level one of the most resonance cases is the rise of the water table in the suburbs of Buenos Aires city due to:

  • the change of the supply source because the water removed from the aquifer do not fit the quality standards

  • the cease of pumping, because of the closure of industries due to the decrease in the economic activity or the settle of factories in more distant areas

  • the increase of natural or artificial recharge

with implications very similar to those mentioned above (Auge 2003; Hernández and González 1997; Bianchi et al. 2005). Another example at national level is La Pampa Province, where the rise of the water table in some urban areas was strengthened by importing water from more distant zones (Giai 2003).

In Santa Fe Province, studies carried out by the National Water Institute in agreement with the Provincial Department of Hydraulics (Dirección Provincial de Hidráulica 2002) can be mentioned. Giacosa et al. (2005) analysed the influence of the change of the water supply source, on the groundwater levels, in certain localities of Santa Fe Province, posing different hypothetical possible scenarios.

At international level, the difficulty suffered in Barcelona due to replacing the groundwater source by an imported surface water source is well known and reported. Custodio (1997), Vázquez-Suñé et al. (1997, 2003, 2005), Ondiviela Monté et al. (2003) are some of the authors who address this issue. The principal damages mentioned by these authors refer to the progressive increase of water leaks, even flooding, at the various public and private underground structures (subways, basements, sewers, parking lots, etc.). The authors also deal with the economic impact derived from this situation (drainage costs, energy, waterproofing, pumping facilities, pumped water evacuation) and the additional problem of structural safety, among other geotechnical problems. Regarding groundwater quality, due to level rise, they affirm that there was remobilization of pollutants mainly due to direct leaching and changes in the chemical conditions of certain compounds. Serious drawbacks were mentioned in areas close to ancient wells and clandestine trenches, as well as in the urban wastewater network.

Therefore, the objective of this paper is to evaluate the effects that would be produced on groundwater levels due to the importation of surface water and the subsequent abandonment of pumping wells in a city supplied for more than 40 years with groundwater. In this case, the local government (Santo Tomé City, Santa Fe Province, Argentina) is the Utilities Authority. The execution of this research allowed analysing, knowing and interpreting the hydrogeological peculiarities of this area. The results of the hydrogeological studies carried out to define the conceptual model, the 3D numerical model implemented to evaluate different scenarios, and some guidelines to manage the replacement of the supply source will contribute to the decision-making process for the sustainable development. These investigations were carried out with the necessary interdisciplinary required to achieve the integration of knowledge.

Characteristics of the study area

Santo Tomé City, Santa Fe Province, Argentina, has approximately 66,000 inhabitants (INDEC 2010) and an area of approximately 80 km2. Geographically, it is located at 31°40′S–60°46′W, on a plain area with elevations between 13.0 and 17.0 m above sea level (m a.s.l.). It is settled on the right margin of the Salado River, 5 km southwest from Santa Fe City, capital of the homonymous province, forming part of its conurbation (Fig. 1). The climate of the region is temperate, with average annual rainfall of about 1000 mm. The average annual temperature has minimum and maximum values of 12 °C and 25 °C, respectively. Most of its inhabitants work in Santa Fe and in its surrounding industrial areas. Locally commerce and services are the most important economic activities.

Fig. 1

Location of the study area and the area to be modelled

Characteristics of the water supply service

Although most of the localities in Santa Fe Province are supplied with groundwater, it is common not to conduct studies to previously know the amount of water the aquifer can provide. Quality is the only factor that matters. That is why once a well stops operating another is built nearby the well closed without considering the hydrodynamic behaviour of the aquifer. This practice began in the ‘70s and is still common.

Until the 1960s, Santo Tomé was supplied exclusively with drinking water through domestic private wells. Since the 1970s, the city has a public water supply service provided by the local government. The unique water source for this service is the semiconfined aquifer layer that underlain the area.

Currently, the drinking water distribution system is composed by a central system and five independent systems: Libertad, Loyola, General Paz, Adelina Este and Monseñor Zazpe. Each of these systems has their own production wells, storage tank and main system. The whole public water supply system counts a total of 42 groundwater abstraction wells sparse spatially as it can be seen in Fig. 2.

Fig. 2

Location of the groundwater abstraction wells and area to be modelled

The wells have an average depth of 32 m and a screened section of 6 or 12 m. Their flow rates range between 500 and 1200 m3/d, depending on the water distribution system to which they belong.

Nowadays, the water supply main covers 78% of the urban area while only 53% has sewerage (Plan Base de la Ciudad de Santo Tomé 2014). This information is important to further consider the anthropic recharge to the groundwater system.

Geomorphological, geological and hydrogeological settings

The study area is located in the geological region called “Llanura Chaco Pampeana” (Chaco Pampeana Plain). According to Russo et al. (1979) and Iriondo (1987), this is one of the greatest plain in the world. The main geological features of this plain are its very uniform relief, scarce tectonic deformation, predominance of fine and medium grain size sediments and a considerable continuity and areal extension.

On the other hand, Kruse and Zimmermann (2002) affirm that the relief of the Chaco Pampeana Plain is a product of different events happened in the past. In the geomorphology of this plain two aspects must be mentioned: one is linked to the spatial distribution of the hydrolithological outcrops and the other is related to the characteristics of the energy that modelled the landscape. The former can indicate several possibilities such as greater infiltration, storage and/or recharge capacity. The importance of the second one is that geomorphology can help to define hydrological behaviour based on the understanding about the erosion and sedimentation processes recognised in the outcropping materials.

According to Marengo et al. (2002), the scarcity of natural outcrops in Santa Fe Province has been a limiting factor for the geological knowledge of the region. In fact, background studies are very scarce, and they are circumscribed to partial stratigraphic and geomorphological studies. Figure 3 shows the surface geology in the neighbouring of Santo Tomé City extracted from the surface geological map Santa Fe 3160-III (Marengo et al. 2002).

Fig. 3

Surface geological map of the study area

(extracted from Marengo et al. 2002)

This plain is formed largely by continental quaternary sediments tens of metres thick. These sediments were deposited on tertiary geological formations of continental and marine origin. The geological sequence of hydrogeological interest is described (from top to bottom) as follows:

  • Sandy clay silts, generically called loess: the dominant colour is brown, with yellowish to reddish tonalities. They are informally referred as “Sedimentos Pampeanos” (Pampean sediments) from Quaternary. The name of “Grupo Pampa” (Pampa Group) was proposed by Tujchneider (Tujchneider and Tineo 2005) to consider this set of sediments. Generally, they do not present a stratification. If they have one, it is local and little perceptible composed frequently by nodules or calcareous intercalations. They have diverse origin, mainly, aeolian and lacustrine. The thickness of these sediments is between 0 and 8 m (bottom elevation approximately at 10 m a.s.l.). Its hydrogeological importance is that they lodge an unconfined aquifer of low yield. Tujchneider also recognises that there is a layer of fine sediments (silt and clay) at the bottom of the Pampa Group. It is up to 3 m thick and behaves as an aquitard layer.

  • Fine and medium quartz sands, brownish yellow to whitish: The grain size increases downwards and, sometimes, coarse sand with some gravel is found in the lower portion of this layer. However, in its upper part there are sandy silts and/or very fine sand, ochre to yellow. The set is known as Ituzaingó Formation (Pliocene) and informally as “Puelches” sands or “Puelche” aquifer. The thickness of this layer ranges from 25 to 35 m and it is considered an aquifer of great importance in the province.

  • Green clays and grey sands of marine origin belonging to the Paraná Formation (Miocene): Aceñolaza (1976) and Iriondo (1973) studied specifically this lithostratigraphic unit. For the last author, Paraná Formation represents a Miocene marine ingression extended over the entire Argentinian Chaco Pampeana region. In the region, it is considered the hydrogeological basement of the productive aquifer.

From the hydrogeological point of view, the local aquifer system underlain Santo Tomé city behaves as a multilayer aquifer. The unconfined aquifer, lodges in the “Sedimentos Pampeanos”, has an average thickness of 8 m; a hydraulic conductivity of 5 m/d and a specific storage of 0.05. It is locally recharged from precipitation (D’Elía et al. 2013). It is exploited in suburban areas, where the drinking water service does not reach.

The leaky (or semiconfined) aquifer is in the “Puelches” sands between an elevation ranges from 8.5 to − 17 m a.s.l. This aquifer has a transmissivity (T) of about 1100 m2/day, a hydraulic conductivity of 45 m/day and a storage coefficient (S) of 2 × 10−3. It is currently the source of water supply in Santo Tomé city. In the subsurface of Santo Tomé City, and at the top of the Ituzaingó Formation there are fine sediments behaving as an aquitard with a thickness of 1.5 m (Tujchneider et al. 2008).

Figure 4 shows the A–B cross-section with the principal geological and hydrogeological characteristics of the study area.

Fig. 4

Cross-section of the study area projected upon Y-axe

According to D’Elía et al. (2013), the recharge of the semiconfined aquifer is both local and regional. The local recharge comes from the vertical downward flow that takes place according the hydraulic head relationships between the unconfined and semiconfined aquifers. These local and regional groundwater flows were distinguished in previous studies and corroborated by isotopic investigations. The regional direction of the groundwater flow is from west to east.

Hydrogeochemical characteristics

The available information corresponds to water samples taken from the pumping wells belonging to the semiconfined aquifer. The collection and analysis of these water samples is overseen by the Sanitation Department of the Secretary of Public Infrastructure and Construction of the Municipality of Santo Tomé. This Department performs chemical and bacteriological analyses of water samples taken from the supplying wells, periodically, since 2001. For this department, the most significant constituents in groundwater samples are: total iron (Fe), manganese (Mn), total dissolved solids (TDS), electrical conductivity (EC), turbidity (T), nitrates (NO3), chlorides (Cl) and sulphates (SO42−).

Due to increasing water demands the withdrawal volumes grow constantly. Although, up to the present, the decreasing of groundwater levels does not seem to be significant, changes in water quality are. In general, the concentrations of the groundwater constituents accomplish the Provincial Drinking Water Quality Standards (Provincial Law N° 11220), but the sustained increase in the contents of Total Dissolved Solids (TDS), chlorides and sulphates in many wells shows a progressive deterioration of the aquifer system quality (Tujchneider et al. 2008):

  • TDS increase to values close to the maximum allowable concentrations according to Provincial Standards (1500 mg/L),

  • Contents of both Cl and SO42− progressively increase. Cl concentrations increase slower than SO42−. While the contents of Cl are around 250 mg/L, SO42− almost reaches the obligatory limit (400 mg/L).

  • Contents of Fe and Mn are quite above the values indicated in the standards (0.2 and 0.1 mg/L, respectively). Fe contents are generally around 0.3 mg/L and 0.18 mg/L of Mn.

Materials and methods

Piezometric map

At the time of the present study, background available information about groundwater level measurements in pumping wells indicates that they were performed sporadically. Only occasional piezometric level records are found in some of the wells of the supply system. The lack of systematic measurements makes difficult to appreciate previous temporal variations. However, it could be possible to outline a piezometric map with the depth information of static level obtained in 20 (twenty) wells in 2014.

Natural recharge

The natural recharge to the phreatic aquifer was estimated using the Thornthwaite–Matter method (Thornthwaite and Matter 1955) and the Curve Number Method (CN) (USDA 1972). The first method allowed to perform a monthly water balance with the meteorological information (temperature and precipitation) from Santa Fe meteorological Station. The second one was used to estimate the water runoff.

Withdrawals and anthropic recharge before water import

Considering Foster et al. (1998) and Lerner (2002), the total withdrawal volume and the anthropic recharge before water imports were estimated following the scheme proposed in Fig. 5.

Fig. 5

Withdrawals and anthropic recharge before water imports

The situation presented in Fig. 5 is the current situation in Santo Tomé (before the beginning of surface water importations). The total withdrawals are the sum of the private well withdrawals (Epriv) and the volume withdrawn from the public supplying wells (Wpub). A part of Wpub gets lost in the distribution before reaching the consumer and recharges the aquifer (Lpub). The sum of Epriv and Wpub without Lpub reaches the consumer. After water is consumed it is disposed either in private waste water systems, which emit the waste water to the ground (Wwas,priv) or in the sewer system (Wsew). Part of Wsew gets lost to the ground and recharges the aquifer and the remaining is emitted to the surface water body (Salado River) after being treated in the municipal waste water treatment plant. Like the sewer system, the urban drainage system is also subject to losses. The total anthropic recharge is the sum of the losses of urban drainage (Ldrain), sewer (Lsew) and water mains (Lpub) and the volume disposed in private waste water systems (Lwas,priv).

Total withdrawals

Based on the pumping schedules available, Wpub was estimated as the sum for each wells of the product of the mean pumping time (ti) per day multiplied by the mean withdrawal rate (Qi) and 365 days.
$${W_{{\text{pub}}}}=\mathop \sum \limits_{{i=1}}^{{34}} {Q_i} \times {t_i} \times 365\;{\text{~days}}$$
According to municipal values, the daily water consumption is 261 l/inhabitant/day. Assuming the same daily consumption both for inhabitants connected to the water supply net and for those who use their own wells for water supply, the annual volume of the private withdrawals can be calculated as follows:
$${E_{{\text{priv}}}}={W_{{\text{con}}}} \times I \times ~\left( {100\% - ~{i_{{\text{pub}}}}} \right) \times ~0.365~\left( {{{\text{m}}^3} \times {\text{day}}} \right)/\left( {L \times {\text{year}}} \right)$$
with \({E_{{\text{priv}}}}\) = water withdrawals for private water supply [m3/year], \({W_{{\text{con}}}}\) = daily per capita water consumption [L/inhabitants/day], \(I\) = number of inhabitants of Santo Tomé city [inhabitants], and \({i_{{\text{pub}}}}\) = percentage of inhabitants connected to public water supply system [%].

Anthropic recharge

The total anthropic recharge volume without considering the changes in natural recharge due to changes of the infiltration rate as a consequence of land use, imply taking into account: losses in public water supply system (Lpub), Losses in the sewer system (Lsew), Discharge of waste water of households with in-situ sanitation (Wwas,priv) and Losses in rainwater drainage systems (Ldrain).

  • Losses in public water supply system (Lpub)

$${L_{{\text{pub}}}}={l_{{\text{pub}}}} \times {W_{{\text{pub}}}}$$
being \({L_{{\text{pub}}}}\) = water volume lost between production well and consumer [m3/year], \({l_{{\text{pub}}}}\) = loss factor in the water supply system [%], and \({W_{{\text{pub}}}}\) = water volume necessary for public water supply [m3/year].
  • Losses in the sewer system (Lsew)

The sewer system in Santo Tomé conducts its waste water to the municipal waste water treatment plant which emits the treated water to the Salado River. But part of the conducted waste water is lost to the ground and recharges the aquifer. Assuming losses of 10% of the sewer system, the loss volume is 219,390 m3/year and it can be calculated with the following equation:
$${L_{{\text{sew}}}}={l_{{\text{sew}}}} \times {W_{{\text{sew}}}}$$
being \({L_{{\text{sew}}}}\) = waste water volume supply with sewer system [m3/year], \({l_{{\text{sew}}}}\) = loss factor in the sewer system [%], \({W_{{\text{sew}}}}\) = waste water volume disposed with the sewer system [m3/year].
  • Discharge of waste water of households with in situ sanitation (Wwas,priv) was calculated with the following expression:

$${W_{{\text{was,priv}}}}={W_{{\text{con}}}} \times \left( {100\% - {u_{{\text{con}}}}} \right) - {W_{{\text{sew}}}}$$
with \({W_{{\text{was,priv}}}}\) = waste water volume disposed with private systems [m3/year], \({W_{{\text{con}}}}\) = daily per capita water consume [L/(inhabitant day)], \({u_{{\text{con}}}}\) = proportion of consumptive uses [%], and \({W_{{\text{sew}}}}\) = waste water volume disposed with the sewer system [m3/year].
  • Losses in rainwater drainage systems (Ldrain):

They were considered negligible since they do not produce a continuous contribution to the aquifer system.

Withdrawals and anthropic recharge after water import

Considering a similar scheme as proposed above, the total withdrawal volume and the anthropic recharge after water imports is shown in Fig. 6.

Fig. 6

Withdrawals and anthropic recharge after water imports

After water imports, the only change is that the wells stop pumping and water imports begin. The volume necessary for public water supply will no longer be withdrawn from the local aquifer. It will come from a surface water body (Coronda River, with the intake in Desvío Arijón) and it will be conducted to Santo Tomé city. In consequence the only aquifer withdrawals are from private wells. The anthropic recharge and discharge rates are calculated before depending on the loss factors and coverage rates of the urban drainage system, the extension of water and sewer main, the daily per capita water consume and the population of Santo Tomé.

Numerical model

Hydrogeological numerical modelling was implemented using Visual MODFLOW Flex 2015.1 (Waterloo and Hydrogeologic 2015). The model domain has an area of about 20 km2. It is located between UTM coordinates 5,425,000 and 5,428,500 on the East–West direction (5000–8500 coordinate model); and 6,493,500 and 6,499,000 on the North–South direction (3500–9000 coordinate model). The domain was discretized in quadrangular and/or rectangular cells of 100, 50 and 20 m side, depending on the location of the pumping wells (Fig. 7). The non-uniform grid has 111 rows x 193 columns, that is, 21,423 cells in each layer. In the vertical direction the modelled domain was discretized in three layers (Fig. 8):

Fig. 7

Discretization of the study area

Fig. 8

Vertical discretization of the study area and hydraulic parameters used in each layer

  1. 1.

    The first layer has an average thickness of about 8 m. It represents the unconfined aquifer. The top of this layer is the ground surface, and its bottom was established at an elevation of 10 m. Due to the lack of available information the layer is considered isotropic and homogeneous. The hydraulic parameters were estimated according the grain size. That is why the hydraulic conductivity K = 4 m/day and the storage coefficient Sy = 0.2.

     
  2. 2.

    The second layer, with an average thickness of 1.5 m, represents the aquitard with elevations between 10 and 8.5 m a.s.l. for the top and the bottom of this layer, respectively. Due to its lithological composition, the hydraulic conductivity was established in K = 0.1 m/day and the total porosity m = 0.5.

     
  3. 3.

    The third layer is the semiconfined aquifer exploited for water supply. Its top and bottom elevation are 8.5 and − 17 m, respectively. It is also isotropic and homogeneous due the lack of hydraulic test. The values adopted for the hydraulic conductivity and for the storage coefficient are K = 40 m/d and S = 5 × 10− 3, respectively.

     

To represent the behaviour of the aquifer system the following hypotheses were considered:

  1. a.

    The hydrogeological properties (hydraulic conductivity, storage coefficient, porosity) were estimated according to the lithology of the sediments composing each layer and were considered constant throughout the modelled area as it is shown in Fig. 8.

     
  2. b.

    The well discharge and its schedule were set based on existing records. The well discharge ranges between 500 and 1200 m3/day and the screened section is 6 or 12 m, depending on the water distribution system to which the well belongs, as it is mentioned in characteristics of the water supply service.

     
  3. c.

    The boundary conditions were defined considering the local and regional behaviour of the aquifer system. A general head boundary conditions were set at the west and east border of the domain area. The General-Head Boundary (GHB) is used to simulate head-dependent flux boundaries. In this type of boundary condition, the flux is always proportional to the difference in head. In the west border, boundary heads of 14 and 15 m and a conductance of 1000 m2/day were assigned. While in the east border a boundary head of 11.2 m was used, and the same conductance was considered. At the north and south borders no flow boundary conditions were established.

     
  4. d.

    The initial condition was the static groundwater levels previously obtained.

     
  5. e.

    There were considered also natural recharge to the phreatic layer, and the relationships between aquifer withdrawals and anthropic recharge before and after water imports (as shown in Figs. 5, 6, respectively).

     

Once the model was calibrated in steady-state condition, the stage of simulation begun. To evaluate the effects that would be produced on groundwater levels due to surface water import and the subsequent abandonment of pumping wells, the following scenario was considered:

  1. a.

    From now and for 5 years more (time estimated for replacing the source), the wells would continue pumping with the same flow rate and schedule as they have nowadays.

     
  2. b.

    From the fifth year, the exploitation of groundwater would gradually begin to decrease (the pumping rates are reduced 50% for 3 years and 25% after 2 years more).

     
  3. c.

    From the tenth year on, the pumping ceases completely.

     

It was also considered that the flow boundary conditions remain the same during the whole simulated period (10 years).

Results

Although the lack of systematic measurements to appreciate previous temporal variations in the groundwater levels, it could be possible to outline the piezometric map shown in Fig. 9. This map was elaborated with the depth information of static level obtained from 20 wells. The wells have an average depth of 32 m and their screened sections are in the semiconfined aquifer. The contour lines in the area range from elevations of 13 to 11.5 m. According to this map, the general direction of the groundwater flow is from west to east and the fluvial valley of the Salado river is its discharge area. This fact corroborates the characteristics of the regional groundwater flow.

Fig. 9

Piezometric map of the semiconfined aquifer in 2014 (values in m a.s.l.)

Applying a monthly water balance and the CN method, it was possible to assess the vertical recharge to the phreatic aquifer. The amount of this recharge is 60 mm/year.

As described previously, 22% of Santo Tomé inhabitants are supplied using private wells and the remaining 78% use water from the public water supply system. Losses between private pumping wells and the consumers contribute directly to recharge the aquifer and can hardly be estimated or measured. Therefore, they are not considered in a budget.

The withdrawals of the public pumping wells (Wpub) account for 8,578,279 m3/year (Table 1).

Table 1

Withdrawals of the public pumping wells

Well ID

Flow rate (m3/h)

Mean pumping time per day (h/day)

Annual withdrawal (m3/year)

Nº 1 bis

61.4

21.4

478,342

Nº 2 bis

53.5

11.0

215,559

Nº 3 bis

45.0

18.1

297,433

Nº 4 bis

55.0

14.9

298,168

Nº 5 bis

36.0

21.1

277,162

Nº 6

57.0

21.6

448,517

Nº 7

42.0

20.8

318,484

Nº 8

47.2

14.0

240,791

Nº 9 bis

43.4

15.8

250,316

Nº 10

47.2

15.4

265,365

Nº 11 bis

47.5

21.3

370,261

Nº 12

53.0

16.8

324,966

Nº 14

37.8

17.9

247,163

Nº 15

48.0

21.2

370,636

Nº 16

29.3

20.8

223,180

Nº 17

42.7

13.8

214,767

Nº 18

37.8

8.6

119,358

Nº 19

55.7

16.6

336,749

Nº 20

70.0

7.1

182,415

Nº 1 GP

45.0

19.4

318,493

Nº 2 GP

65.0

20.9

495,397

Nº 3 GP

48.0

19.4

339,726

Nº 4 GP

62.5

20.9

476,343

Nº 5 GP

38.2

19.0

264,585

Nº 1 VA

No data

No data

No data

Nº 2 VA

42.5

14.2

220,462

Nº 3 VA

72.0

7.7

203,626

Nº 1 VL

40.0

13.1

191,017

Nº 2 VL

51.0

13.1

243,546

Nº 3 VL

70.0

4.2

107,397

Nº 1 MZ

65.0

8.2

238,054

Nº 2 MZ

Out of order

Out of order

Out of order

Loyola 1

No data

No data

No data

Loyola 2

No data

No data

No data

  

Σ =  8,578,279 m3/year

The numbers obtained for the private withdrawals and for the anthropic recharge are shown in Table 2:

Table 2

Quantity of withdrawals and anthropic recharge before the water imports

Withdrawal/recharge

Volume per year

Private withdrawals (Epriv)

1,507,915 m3/year

Water losses in the water supply system (Lpub)

4,520,753 m3/year

Losses from the sewer main (Lsew)

2,193,897 m3/year

In-situ waste water systems (Wwas, priv)

2,815,000 m3/year

Urban drainage system (Ldrain)

Negligible

Thus, adding the water volume withdrawn for public water supply and the volume withdrawn in private wells the total withdrawn volume of the aquifer is 10,086,194 m3/year.

For the inflow from the sewer system, neither the infiltration of groundwater nor erroneous induction of surface runoff into the sewerage system was considered. So, the anthropic recharge without considering losses in the urban drainage system is 7,555,143 m3/year. This value means a recharge rate of 180 mm/year in certain areas of the unconfined aquifer (they were considered the differences in the land use).

It should be taken into account that around 75% of the water extracted from the aquifer system returns to the system and the anthropic recharge is more important than the natural one.

To quantify the changes within the balance of anthropic withdrawals and recharges after the water imports, all the factors mentioned above are considered constant.

Under these conditions the total withdrawal volume is 1,507,915 m3/year (only from private wells) while the anthropic recharge volume does not change. The withdrawal volume is only 15.0% of the total withdrawals before the water importation. Anthropic recharges exceed withdrawals by 6,047,228 m3/year.

If the change of the source of water supply is the only change, it will represent an important intervention in the anthropic hydrological balance and consequently in the global balance of the aquifer. This demonstrates that in Santo Tomé City the anthropic discharges and recharges play a very important role in the urban subsurface water cycle.

The numerical model implemented to evaluate the effects that would be produced on groundwater levels due to the importation of surface water and the subsequent abandonment of pumping wells reproduced the behaviour of the hydrogeological system quite well. Figure 10 shows the piezometric map simulated for the steady-state condition in the semiconfined layer (the beginning of the simulation period).

Fig. 10

Simulated groundwater levels for the beginning of the simulation period

Figure 11 shows the calibration obtained. The normalised root mean squared (RMS) error was 4.6%, a value that indicates that the numerical model faithfully represents the conceptual model.

Fig. 11

Calibration of the model for the steady-state condition—Layer 3

The result of the modelled scenario was the gradual rise of groundwater levels till groundwater floods the ground surface, which begins in the fifth year after the start of the simulation. The simulated groundwater levels after 10 years are shown in Fig. 12. This means that all pumping wells stop working.

Fig. 12

Simulated groundwater levels for the 10th year from the beginning of the simulation

The modelled scenario predicts that the area of the central system will have groundwater levels above ground surface 5 years after the beginning of the importations, and in the other areas, groundwater levels will rise 3 m average. Even more, rising groundwater levels would not only cause flooding but also quality problems. If groundwater levels rise above the bottom level of cesspits, cesspools or septic chambers these facilities emit their waste waters directly to the aquifer and the unsaturated zone cannot exert its purifying function on the waste water. This potential hazard of contamination of the upper aquifer layer could affect the water quality of some private wells, which extract water from this layer. According to Giacosa et al. (2005), the areas in Santa Fe Province without sewerage service but with public groundwater supply are most likely affected by rising groundwater levels due to the change by a surface source. Moreover, if groundwater levels continue rising the functioning of the sewer system could be affected. Groundwater that enters the sewerage system may double the waste water volume as occurred in Hannover (Germany), Buenos Aires (Argentina), and Barcelona (Spain), among other cities (Hernández and González 1997; Vázquez-Suñé et al. 2005).

Conclusions and recommendations

Groundwater and surface water are closely linked and within an integrated water resource management approach they should be managed as one resource. Thus, groundwater and surface water management has to be based on an appropriate understanding of their characteristics, relationships and conditions. This knowledge requires good data on the resources from investigations, monitoring and interpretation.

Nowadays, public water supply in Santo Tome city is carried out from a multilayer aquifer system. The upper layer is exploited by private pumping wells and is threatened by pollution from anthropic activity. The semiconfined layer is exploited for the municipal water supply and is threatened by salinization from the underlying aquifer.

In the next years, the provincial water pipeline system will supply the municipality with surface water. This will help to achieve a good water quality for water supply. The results obtained in this study area by numerical modelling show that there will be major impacts on the groundwater levels. The risk of rising ground water levels is of different magnitude in different zones, but it requires attention and so does the protection of the city groundwater resources.

The different problems cannot be resolved separately as they are interconnected. Therefore, an integrated management of the groundwater and surface water resources is necessary. The use of both water resources respecting the quality standards for water supply and the hydrodynamic conditions of the exploited aquifer is one important part of the proposed management strategy. A controlled and maintained scheme of withdrawal volumes of the municipal pumping wells as a part of a programme of joint water use of surface and groundwater probably could help to guarantee an appropriate water quality supplied and the stability of groundwater levels.

In addition, a monitoring programme of the evolution of the hydrogeological system should be designed and implemented to control the hydrodynamic and hydrochemical conditions on the unconfined and semiconfined levels. This will make possible avoiding undesired situations such as flooding, mobilisation and/or entry of pollutants into the groundwater system, structural risk, among others, which would cause damage or loses to the population and important expenses in the future.

It is important to consider that groundwater resources of Santo Tomé city will guarantee the water supply of the population in a short term (until the new water supply system is fully working) and, in a long term they could be jointed with surface water or included if the demands exceed the capacity of the provincial water pipeline system or in case of failure. So, its protection is a priority since now.

Complementarily, other important measurements should be implemented in the water and sanitation sector of Santo Tomé city: the installation of individual water metres and the immediate and necessary expansion of the sewerage network. These measures not only will allow to define a price scheme based on the consumption but also, they will be an important strategy to controlling the demand. The second measure will contribute to decrease the impact of the water importation and improve the groundwater quality. Both measures will require investment and economical, legal and administrative tools. Also, the development of a communicational and participative strategy to involve the different stakeholders and mainly the users should be considered.

Notes

Acknowledgements

The authors would like to thank the Municipality of Santo Tomé who allowed using its information and the Facultad de Ingeniería y Ciencias Hídricas, Universidad Nacional del Litoral (Faculty of Engineering and Water Sciences, National University of El Litoral) for the funding received for this research.

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Copyright information

© Springer Nature Switzerland AG 2018

Authors and Affiliations

  • Marcela Perez
    • 1
    Email author
  • Marta Paris
    • 1
  • Mónica D’Elía
    • 1
  • Lukas Brandt
    • 1
    • 2
  1. 1.Facultad de Ingeniería y Ciencias HídricasUniversidad Nacional del Litoral. Ciudad UniversitariaSanta FeArgentina
  2. 2.Technische Universität DarmstadtDarmstadtGermany

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