Environmental Management (2012) 50:490–503 DOI 10.1007/s00267-012-9891-9

A Gis-Based Assessment of Groundwater Suitability for Irrigation Purposes in Flat Areas of the Wet Pampa Plain, Argentina Asuncio´n Romanelli • Marı´a Lourdes Lima • Orlando Mauricio Quiroz London˜o • Daniel Emilio Martı´nez • He´ctor Enrique Massone

Received: 21 December 2010 / Accepted: 24 May 2012 / Published online: 30 June 2012 Ó Springer Science+Business Media, LLC 2012

Abstract The Pampa in Argentina is a large plain with a quite obvious dependence on agriculture, water availability and its quality. It is a sensitive environment due to weather changes and slope variations. Supplementary irrigation is a useful practice for compensating the production in the zone. However, potential negative impacts of this type of irrigation in salinization and sodification of soils are evident. Most conventional methodologies for assessing water irrigation quality have difficulties in their application in the region because they do not adjust to the defined assumptions for them. Consequently, a new GIS-based methodology integrating multiparametric data was proposed for evaluating and delineating groundwater suitability zones A. Romanelli  M. L. Lima  D. E. Martı´nez Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Instituto de Investigaciones Marinas y Costeras (IIMyC), Mar del Plata, Argentina e-mail: [email protected] D. E. Martı´nez e-mail: [email protected] A. Romanelli (&)  M. L. Lima  O. M. Quiroz London˜o  D. E. Martı´nez  H. E. Massone Instituto de Geologı´a de Costas y del Cuaternario, Universidad Nacional de Mar del Plata, FCEyN, Funes 3350, Nivel 1, 7600 Mar del Plata, Argentina e-mail: [email protected]; [email protected] O. M. Quiroz London˜o e-mail: [email protected] H. E. Massone e-mail: [email protected] O. M. Quiroz London˜o Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Instituto de Investigaciones Marinas y Costeras (IIMyC), Mar del Plata, Argentina

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for irrigation purposes in flat areas. Hydrogeological surveys including water level measurements, groundwater samples for chemical analysis and electrical conductivity (EC) measurements were performed. The combination of EC, sodium adsorption ratio, residual sodium carbonate, slopes and hydraulic gradient parameters generated an irrigation water index (IWI). With the integration of the IWI 1 to 3 classes (categories of suitable waters for irrigation) and the aquifer thickness the restricted irrigation water index (RIWI) was obtained. The IWI0 s index application showed that 61.3 % of the area has ‘‘Very high’’ to ‘‘Moderate’’ potential for irrigation, while the 31.4 % of it has unsuitable waters. Approximately, 46 % of the tested area has high suitability for irrigation and moderate groundwater availability. This proposed methodology has advantages over traditional methods because it allows for better discrimination in homogeneous areas. Keywords GIS  Groundwater  Hydrogeological information  Irrigation water suitability  Pampa plain

Introduction Water quality plays an important role in the management of irrigation and leaching fractions, as well as in the treatment of water itself, so as to achieve an optimal level of production in situations where irrigation systems are used (Castellanos and others 2002). A supplementary approach to new irrigation water development will be to increase water use efficiency with the improvement of irrigation technology, crops, and land productivity adversely impacted by high water tables and salinity (Ayars and others 2006). The longtime effects of irrigation water on soil’s physical properties and crop productivity depend on the total salt,

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sodium, bicarbonate and carbonate concentrations of the irrigation water, and also the soil’s initial physical properties (Richards 1954; FAO 1988). Therefore, the principal variables to be evaluated in the classification of water quality are: (a) electrical conductivity (EC), (b) sodium adsorption ratio (SAR), (c) residual sodium carbonate (RSC), and (d) the concentration of other specific ions, such as chlorine (Castellanos and others 2002; Yidana and others 2008). The consequences of irrigating soils with poor quality water have been broadly studied, and it is known that hydraulic conductivity (K) is negatively affected by soil sodication (Shainberg and Letey 1984; Sumner 1993; Levy 1999). Sodication is a process of exchange between sodium ions (Na?) in the solution and divalent cations (mainly Ca?2), absorbed in the exchange process. Sodium is one of the main cations in most natural waters. Although it is not an essential plant nutrient, it is one of the most injurious cations to plant physiological development and soil’s physical properties. Clay particles that adsorb sodium have a tendency to disperse and form hard clods that melt when wet and tend to fill and seal pore spaces, thus inhibiting water movement into and throughout the soil. In this case, the water infiltration capability of the soil is drastically reduced. In addition to reducing the soil infiltration capacity, high sodium may be toxic to plants (Peinemann and others 1998; Yidana and others 2008). The Pampa in Argentina is a large plain of about 1,500,000 km2, with dependence between agriculture, water availability and its quality. This region is a very sensitive environment due to weather changes and slope variations. The drought-flood period alternation in this area is a wellknown problem, quoted initially by Ameghino (1896). The periodic occurrence of droughts of different intensities is one of the main causes of interannual variability of crop yields in the area. Therefore, supplementary irrigation is a useful practice to compensate the production of the zone. However, potential negative impacts of this type of irrigation in salinization and sodification of soils need to be evaluated in order to prevent soil and water degradation (Lossino and others 2002). This fact is enhanced because of the extreme low topographic gradients existing in the region, being usually lower than 0.1 %. Moreover, groundwater constitutes the main water supply for agricultural, residential and industrial activities, although in these extremely flat areas it usually becomes salty, causing limitations for some agricultural practices and for human use. The risks and consequences of applying irrigating waters of poor quality onto soils of the Pampa plain have been acknowledged for a long time. Arens (1969) indicated that few years are necessary to make a good soil turn into a sodic soil. In this region, the irrigated surface with waters that have RSC is increasing, and experimental evidence of soil deterioration has already been shown by Andriulo and

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others (1998). Pilatti and others (2004) found that supplementary irrigation with waters containing RSC greater than 1.25 milliequivalent per liter (meq/l) and a low saline concentration have caused sodication in soils. More precise recommendations for irrigation than the traditional classification, based on salinity and sodicity criteria (Richards 1954), are needed in the Pampa region (Baccaro and others 2006) since many authors criticize its application in humid regions with supplementary irrigation (Costa 1995; Abrego and others 1998; Irurtia and Mon 1998; Ge´nova 2006). Geographical information systems (GIS) can be powerful tools for developing solutions for water resources problems: assessing water quality, determining water availability, preventing flooding, understanding the natural environment and managing water resources on a local or regional scale (Ashraf et al. 2011). Some studies in different countries have used GIS as a database system in order to prepare maps of water quality according to concentration values of different chemical constituents (Yammani 2007, Rangzan and others 2008; El-Hames and others 2010). In such studies, GIS is utilized to locate groundwater quality zones suitable for different uses such as irrigation and domestic. Other researchers Simsek and Gunduz (2007) and Ashraf and others (2011) developed GIS-based irrigation water indexes just by combining well-known hydrochemical indicators (SAR, RSC or soluble sodium percentage, among others). The aim of the present study is to assess groundwater potential for irrigation in flat areas by using a multiparametric data set which is obtained by conventional hydrogeological field surveys. In this sense, a GIS-based irrigation water index is proposed not only for evaluating but also for delineating groundwater suitability zones for irrigation purposes. In comparison with other available indexes, this new index spatially integrates hydrochemical, hydrogeological and topographic parameters (EC, SAR, RSC, slopes, hydraulic gradient, aquifer thickness) which have a significant influence on the irrigation water suitability of groundwater. A portion of the wet Pampa plain was chosen to test the usefulness and applicability of this suggested method. The pilot area is located in the southeast of the Buenos Aires province, and it is representative of the ‘‘Pampas’’ plain (the main grain-producing region in Argentina). It was chosen according to criteria that included the high level of agricultural activities and the extensive available data regarding aquifer features. From a hydrological point of view this area involves significant local extraction of groundwater resources for drinking water and irrigation. However, until now, in the Pampa plain there are no previous studies with these characteristics. It is expected that this new GIS-based proposed methodology could be applied in flat areas with similar characteristics, providing useful information in order to improve water resource and land use planning. In spite of the strategic importance of a

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sustainable plan for water resources and land use, there is no record of local precedents.

Study Area The Pampa plain in Argentina is characterized by a geomorphological environment which corresponds mostly to that of gently sloped plains (slope values\0.5 %) crossed by two block mountain systems (‘‘Tandilia and Ventania ranges’’). Over the past 15 years, annual precipitation values have ranged from 703 to 1,400 mm/year, with an average of 943 mm/year. Climatic conditions are highly variables, but a more restricted area, called Wet Pampa is characterized by a humid climate, very good soils and a resulting high agricultural productivity (soya beans, wheat, sunflowers, corn, potatoes) which is the main sustenance of the country’s economy. Flat-land landscapes in humid to sub-humid climates, like the Wet Pampa Plain, have rather negligible surface relief, the basin boundaries are diffuse or undetermined, with shallow water courses which do not integrate a well-defined surface drainage system, with groundwater levels close to the surface, and soils made up of fine-grained sediments. Such characteristics make the hydrological cycle components different from those of typical hydrological scenarios. The infiltration proceeds at a very slow rate and the water may remain a long time ponded on the surface putting agricultural lands at a greater risk of flooding and/or salinization (Usunoff and others 1999). The area under study is 24,754.39 km2 and it is located in the southeast of the Buenos Aires province (Fig. 1). It constitutes a portion of territory of the Wet Pampa Plain, representative of this type of geomorphological environment, comprising of the northern and southern slopes of the Tandilia range. Important cities such as Mar del Plata, Balcarce, Necochea and Loberı´a are located within it. In the area, the Tandilia system has a maximum altitude of about 510 m above sea level (m asl). It consists of two big geological units: a Precambrian crystalline bedrock called Complejo Buenos Aires (Marchese and Di Paola 1975), and a set of sedimentary rocks of Precambrian-Lower Paleozoic origin, grouped under the name of Balcarce Formation (Dalla Salda and In˜iguez 1979). They are both considered as a hydrogeological impermeable sequence. An inter-range fringe that is a few hundred meters wide surrounds the blocks; it is formed by hills which quickly give way to the plains areas that reach the sea. Hills and plains are formed by Cenozoic loess-like sediments (especially of Pleistocene-Holocene age). The upper Pleistocene-Holocene cover of the area is a sequence of silt, silt-clayed and fine sand sediments of aeolian and fluvial origin that constitute an aquifer system known as Pampeano Aquifer (Sala 1975). The Pampeano Aquifer in the Wet Pampa Plain is an unconfined aquifer, with a thin

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unsaturated zone ranging form 0.50 to 25 m. Most typical values of unsaturated zone thickness are in the range from 2 to 10 m. Recharge is due to infiltration of precipitation excess, and discharge occurs towards surface streams, river and water bodies, and directly to the Atlantic Ocean, but other forms of indirect discharge are evapotranspiration when the water table is near the surface (Luo and Sophocleous 2010) and evaporation of water bodies that are fed by groundwater discharge otherwise widespread in lowland areas (Varni and others 1999). Recharge in the Pampeano Aquifer has been calculated from different approaches, giving results as different as 50 or 200 mm/y. On the other hand, discharge takes place into streams and shallow lakes (Massone 2003; Quiroz London˜o and others 2008; Romanelli and others 2011).

Methods Hydrogeological field studies were performed in the study area during the period of 2005–2010 (Fig. 1). These surveys involved water level measurements (n = 218), collection of groundwater samples (n = 368) for chemical analysis and electrical conductivity (EC) measures. Measurements were mainly taken from existing exploitation wells (mills or domiciliary/irrigation wells). The collection, preservation and chemical analysis for major ions of water samples were made following the standard methods given by the American Public Health Association (APHA 1998). Chemical analyses were performed applying standard methods: calcium and magnesium by complexometric titrations with Ethylenediaminetetraacetic acid (EDTA), sodium by flame spectrometry and carbonatebicarbonate by potentiometric titrations. Hydrochemical analysis allowed the assessment of groundwater quality and its suitability for irrigational purposes. The obtained information was included in a GIS implemented for this study in order to integrate data for their spatial analysis. A GIS-managed hydrogeological database has been developed to support geographical data used in the integrated management of groundwater and surface water. A data model relating each hydrological variable with four dimensions (latitude, longitude, height and time) was designed (Fig. 2). Storing and manipulating data while maintaining their spatial relationships can be achieved with GIS software by using a Geodatabase model. This model links a relational database to geometrical features. The combination of the parameters of EC, sodium adsorption ratio (SAR), residual sodium carbonate (RSC), slopes and hydraulic gradient generates an irrigation water index (IWI) (Fig. 3). It uses variables that have an influence on irrigation water suitability of groundwater. With the integration of the IWI suitable classes and the aquifer thickness a restricted irrigation water index (RIWI) is

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Fig. 1 Location map and piezometry of the southeast of the Buenos Aires province

obtained. All this parameters have been included in the hydrogeological database. Salt concentration can be measured by the electrical conductivity of irrigation water (Custodio and Llamas 1976; Appelo and Postma 1993). In order to evaluate the salinity hazard, electrical conductivity measures were categorized into several classes. In the categorization of this parameter the regional typical values plus the threshold limit values of established classifications (Scofield 1935, Wilcox and Magistad 1943, Richards 1954), for assessing water suitability for irrigation, were considered. The sodium adsorption ratio (SAR) is used because of its direct relation to the adsorption of sodium by soil. The sodium content of water relative to the sum of calcium and magnesium is assessed using an index calculated by the following equation (Todd 1980): Naþ SAR ¼  ð1Þ ðCaþ2 þ Mgþ2 Þ=2g1=2 Where the quantities for the ions are in meq/l. Ravi Shankar and Mohan (2006) considered that the ranges for

classifying waters according to SAR are: 10: excellent water; 10–18: good water; 18–26: doubtful water and [26: unsuitable water. The residual sodium carbonate (RSC) was also calculated to determine the hazardous effect of sodium carbonate and bicarbonate on the quality of water for irrigation purposes. RSC is calculated from the equation below: 1 þ2 RSC ¼ ðCO2 þ Mgþ2 Þ 3 þ HCO3 Þ  ðCa

ð2Þ

Where the quantities for the ions are in meq/l. Castellanos and others (2002) indicated that the ranges for classifying waters according to the variables RSC are : \1.25 meq/l: good water; 1.25–2.50 meq/l: marginal water and [2.50 meq/l: water with a greater risk of causing higher concentrations of sodium in soil. Slope is a critical parameter with a direct control on runoff and therefore on infiltration (Ravi Shankar and Mohan 2006). This data was generated from a digital elevation model (DEM) of the shutter radar topographic mission (SRTM) of NASA, with 90 m of spatial resolution. Then it was classified into four slope groups according to

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Fig. 2 GIS 0 s structure

Marsh (1991) as optimal for different land uses. Moreover, this slope classification was also applied by Massone (2003) in a nearby basin. The best conditions for groundwater quality usually occur in high topographic areas, facilitating water movement and surface run off while avoiding evaporation processes in the non saturated zone. On the other side, extreme flat zones may remain inundated yielding to evaporation and salt concentration in the soil. The hydraulic gradient determines the direction and rate of groundwater movement in a specific area (Davis and De Wiest 1966; Johnson 1975). A higher hydraulic gradient probably implies both a faster groundwater displacement resulting in a lower salt concentration, and, a higher permeability allowing

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better conditions for water exploitation. Piezometric data was used to perform the hydraulic gradient map. This map was classified into five classes based on the natural gradient changes in the region. Aquifer thickness is an indirect estimation of groundwater quantity assuming homogenous hydraulic conditions. This data was obtained by using lithological information (rock basement) and water table level measurements of the area. Then it was classified into three main categories relating to the potential groundwater offer. The assignment of zero values to the range system and the measured piezometric level of each well in the plains areas were done. Moreover, a depth rock basement of 80 m was assigned to the sites further away from the ranges.

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Fig. 3 Methodological framework for the proposed GIS-based methodology

Several thematic maps necessary for the IWI and RIWI were prepared on the basis of information obtained from each of the six parameters, by using the GIS software ArcGis 9.2 (ESRI 2007). After the preparation of the base thematic maps (by a geostatistical analysis of spatial data distribution and kriging interpolation) for each variable under consideration, a transformation of each map into raster format (using the spatial analysis module of ArcGIS) took place. A spatial cell resolution of 100 9 100 m was used. All GIS information was projected in the Gauss Kru¨ger system, zone 5, (Campo Inchauspe). Correlation among variables was assessed by the Pearson analysis. Those variables with significant correlation were omitted. As a consequence, chloride content was not included in the index. Thematic maps were prepared for EC, SAR, RSC, slope, hydraulic gradient (HG) and aquifer thickness (ATH). The

classification of each map involved subdividing the information into several categories to enable the zoning of groundwater quality for irrigation. Each map was reclassified into rating values according to the range of each category: 1. excellent; 2. good; 3. moderate; 4. doubtful and 5. unsuitable (Table 1). The rating values ranged from 1 to 5. Here, a value of 5 would indicate an area with the lowest potential for irrigation, while a value of 1 would indicate the highest potential for irrigation. The ranking of the IWI identifies sites where the groundwater quality for irrigation may be relatively better than that of other sites. Although ranges have been given in this version of the index, it is imperative that each user establishes their range of values for each parameter according to the features of a specific study area. To obtain the IWI a multiplication of the reclassified maps (EC, SAR, RSC, slope and hydraulic gradient) is

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Table 1 Range of each category of parameter and rating value after the reclassification Parameters Rating

Categories

EC lS/cm

SAR adimensional

RSC meq/l

Slope°

Hydraulic gradient adimensional

Aquifer thickness m

1

Excellent

\500

\10

–

[6

[0.005

– [30

2

Good

500–750

10–18

\1.25

2–6

0.003–0.005

3

Moderate

750–1000

18–26

1.25–2.25

1–2

0.002–0.003

10–30

4

Doubtful

1000–1500

[26

[2.25

\1

0.0007–0.002

\10

5

Unsuitable

[1500

–

–

–

\0.0007

–

performed, by the application of map algebra. Reclassification of these IWI map according to the Natural Breaks (Jenks) method (provided by ArcGIS 9.2), to obtain the ‘‘IWI priorities’’, which recognize five classes from priority 1 (lower values in the series) to priority 5 (higher values), was done. In this reclassification, classes are based on natural groupings in the data distribution. This method identifies breakpoints between classes using a statistical formula called Jenks’ optimization (Jenks and Caspall 1971; Jenks 1977). It is rather complex, but basically it minimizes the sum of the variance within each class (Slocum 1999; Murray and Shyy 2000). The RIWI results are obtained after multiplying the IWI 1 to 3 classes (categories of suitable waters for irrigation) and the aquifer thickness map. The reclassification of this index was similar to the IWI, obtaining the ‘‘RIWI priorities’’, which recognize three classes from priority 1 (lower values) to priority 3 (higher values). The final irrigation indexes can be computed using the following formulas: IWI ¼ ECr  SARr  RSCr  Sloper  HGr

ð3Þ

Where IWI is the irrigation water index for a mapping unit and r is the rating for each parameter. RIWI ¼ IWI13  ATHr

ð4Þ

Where RIWI is the restricted irrigation water index for a mapping unit and r is the rating for each parameter.

SW–NE (northern slope) increase, in agreement with flow direction. Just 1.05 % of the zone has EC values \500 lS/cm (‘‘Excellent’’ category). About 16.57 % of the study area (4,103.44 km2) falls within the ‘‘Good’’ category (500–750 lS/cm). Moderate salinity waters (750–1000 lS/ cm) are the predominant ones, corresponding to 47.38 % of the total area (11,730.17 km2), followed by high salinity waters (1,000–1,500 lS/cm) occupying 28.97 % of the zone (7,172.58 km2). The area with the most probability of a salinity hazard ([1,500 lS/cm) is the northeastern one (6.03 % and 1,476.72 km2 of the total area), which coincides with the stream discharge zone to the Atlantic Ocean on the southern side of the Tandilia system. SAR The SAR varies from 0.38 to 31.24, with a mean value of 7.03. Approximately, 51 % of the area (12,695.55 km2) has very low sodicity water (SAR \ 10), coinciding with the range sector. Moreover, 45.45 % of this part of the Buenos Aires Province has SAR values ranging from 10–18, representing 11,251.72 km2 and restricted to the boundaries between plain and range areas. Just 3.27 % of the total area (797.10 km2) has moderate SAR values. High sodium in water (SAR [ 26) was not detected in the zone, but the most affected area is the northeastern one with moderate sodicity.

Results

RSC

Thematic Maps

Residual sodium carbonate in groundwater ranges from 36.51 to 34.55 meq/l with a mean value of 6.04 meq/l. About 92.70 % of the area falls within the ‘‘Doubtful’’ category of RSC ([2.25 meq/l). Just 3.71 % (918.82 km2) and 3.59 % (875.80 km2) correspond to ‘‘Moderate’’ (1.25–2.25 meq/l) and ‘‘Good’’ categories (\1.25 meq/l), respectively. Groundwater samples that had RSC indices of positive value imply that the cumulative concentration of CO-2 3 and ?2 ?2 HCO3 is higher than the combined Ca and Mg concentrations. This would indicate that there is a residual

A brief description of the different thematic maps (Fig. 4) needed for evaluating groundwater suitability for irrigation, is described below: Electrical Conductivity Electrical conductivity (EC) ranges between 328 and 4,710 lS/cm, with a mean EC value of 1,054.19 lS/cm. There is a clear regional NW–SW (southern slope) and

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Fig. 4 Thematic maps prepared for the IWI and RIWI: a EC, b SAR, c RSC, d slope, e hydraulic gradient and f aquifer thickness

carbonate to react with sodium, presenting sodium hazard to the soil when irrigated with such water. A negative value indicates no residual carbonate (Al-Ariqi and Ghaleb 2010). Slope The slope in the area ranges from 0 to 61.16° with a mean value of 1.10° with a standard deviation of 1.74. The slope map shows that the dominant slope (70.71 % of the total area) is ‘‘Doubtful’’ (\1°), occupying 17,517.85 km2 corresponding to the plain area. Besides, the ‘‘Moderate’’ category is also significant (17.00 %) correlating with the hills area. The steepest slope areas (‘‘Excellent’’ and ‘‘Good’’ categories), higher than 2°, coincide with the flat summit and perirange-piedmont of the ranges (12.29 %). Hydraulic Gradient This parameter is defined as the rate of change of piezometric height between two points of the aquifer per unit of distance and in a given direction (Freeze and Cherry 1979). In the study

area it does not show significant changes. In general, the variation of hydraulic gradient in the aquifer is smooth and constant, similar to the topography. The values range from 0 to 0.023, with an overall average of 0.0017 and a standard deviation of 0.0011. The map shows that the most common values (52.60 % of the total area) vary between 0.002 and 0.007, falling within the ‘‘Doubtful’’ category. Besides, the 28.68 % is associated to the ‘‘Moderate’’ category. The lower values (\0.0007) correspond to the ‘‘Unsuitable’’ category (10.70 %), while the higher ones ([0.005) are mainly located near the area of hills corresponding to the ‘‘Good’’ and ‘‘Excellent’’ categories (8.02 %).

Aquifer Thickness The lowest aquifer thickness values (\30 m), with moderate to doubtful potential for irrigation purposes, correspond to the range and hill areas (40.14 %). The 59.86 % of the territory (‘‘Good’’ category) has the higher values of this parameter ([30 m) coinciding with the plain area.

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Irrigation Water (IWI) and Restricted Irrigation Water (RIWI) Indexes Figure 5 shows the resulting maps obtained by applying the IWI and RIWI indexes in the study area. The IWI and RIWI ‘‘priorities’’ and classes are listed in Tables 2 and 3. The description of each ‘‘IWI priority’’ is the following: 1:

2:

3:

This site has a VERY HIGH potential for irrigation. No damaging effects due to the high groundwater quality are evident. Good hydrogeological conditions for water exploitation. This site has a HIGH potential for irrigation. If farming practices are maintained at current level, the probability of an adverse impact to water resources and soil would be low. This site has a MODERATE potential for irrigation. The probability of an adverse impact to water resources and soil is greater than that from a high rated site; moreover, moderate hydrogeological

Fig. 5 a IWI and b RIWI maps

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4:

5:

conditions for water exploitation dominate. Some remedial action should be taken to lessen the probability of salinity problems. This site has a LOW potential for irrigation. There is a high probability of an adverse impact to water resources unless remedial action is taken. Soil and water conservation as well as management practices are necessary to reduce the risk of salinity hazard, probable water quality degradation and soil structure loss. This site has a VERY LOW potential for irrigation. The probability of an adverse impact to water resources is very high. Remedial action is required to reduce the risk of salinity problems, water quality degradation and soil structure loss. All necessary soil and water conservation practices plus a management plan must be adopted to reduce the potential of water quality degradation. Periodic application of non salinesodic irrigation waters, a routine monitoring of soil solution chemistry and irrigation water quality, and a

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Table 2 IWI and RIWI ‘‘priorities’’

Table 4 Number and percentage of irrigation wells with respect to the different IWI and RIWI priorities

Priority

Irrigation potential

IWI Classes

RIWI Classes

1

Very high

4 C IWI B 81

1 C RIWI B 2

2

High

81 [ IWI B 162

2 [ RIWI B 6

3

Moderate

162 [ IWI B 320

6 [ RIWI B 9

1

Very high

29

15.8

44

23.2

4

Low

320 [ IWI B 720

2

High

41

22.4

76

40.0

5

Very low

IWI [ 720

3

Moderate

50

27.3

3

1.6

4

Low

63

34.5

5

Very low

0

Priority

IWI irrigation water index, RIWI restricted irrigation water index

Irrigation potential

IWI Irrigation wells (N8)

RIWI %

Irrigation wells (N8)

%

IWI irrigation water index, RIWI restricted irrigation water index Table 3 Obtained area and percentage of the different IWI and RIWI priorities Priority

Irrigation potential

IWI 2

km

RIWI %

km2

%

1

Very high

2466.89

9.96

3080.23

12.44

2

High

5705.87

23.05

11451.14

46.26

3 4

Moderate Low

7001.98 6902.53

28.28 27.88

1502.49

6.07

5

Very low

867.53

3.50

IWI irrigation water index, RIWI restricted irrigation water index

periodic application of chemical amendments to salt and sodium affected soils is needed (Bauder 2001). From a hydrogeological point of view this site is unsuitable for water exploitation. The description of each ‘‘RIWI priority’’ is shown below: 1: 2: 3:

Discussion

This site has VERY HIGH potential for irrigation and HIGH groundwater availability. This site has HIGH potential for irrigation and MODERATE groundwater availability. This site has MODERATE suitability for irrigation and LOW groundwater availability.

The application of these indexes shows a good approximation of the analysis and delineation of areas with different suitabilities for irrigation. A spatial distribution map of the irrigation wells provided by the National Institute of Agricultural Technology- INTA (2007–2008) of the southeast of the Buenos Aires province was used to validate the obtained IWI and RIWI. Regarding the IWI map, 65.5 % of the irrigation wells are mainly located within the priorities 1–3, being waters with very high to moderate potential for irrigation. On the contrary, 34.5 % of them are situated in priority 4, whilst no pumping wells are detected in priority 5 (Table 4). In the case of the RIWI map, 64.8 % of the irrigation wells correspond to priorities 1–3 while the 35.2 % of them pump from unsuitable waters.

In subhumid regions, where irrigation is provided on a standby or supplemental basis, salinity is usually of little concern because rainfall is sufficient to leach out any accumulated salts. But in semiarid or arid regions, salinity is usually an ever-present hazard and must be taken into account at all stages of planning and operation (Richards 1954). In the Pampa plain, the alternation between drought and flood periods is a common episode; for this reason salt leaching or accumulation may occur. Moreover, in such flat-land landscapes, water excesses are hard to evacuate because of the topography promoting a greater risk of flooding and salinization of agricultural lands. In this context, periodic assessment and groundwater irrigation planning is required. Ge´nova (2006) invalidates the classification method for irrigated water quality (Richards 1954) of universal use in arid zones, based on salinity and sodicity criteria, since it is improperly applied in areas with supplementary irrigation. Most of the available water resources in the Wet Pampa Plain are classified as barely suitable to unsuitable for agricultural use by this method. Some studies use the RSC to predict the hazardous effect of sodium associated to calcium carbonate precipitation. This ionic relationship has a limitation since it assumes the precipitation of all the bicarbonate in water; however, this depends on the quantity of water that infiltrates through the soil. Most of the groundwater samples in the area have RSC values higher than 2.5 meq/l and are considered harmful in all conditions (Bohn and others 1993). Traditional assessment methods of irrigation water quality (SAR, RSC or EC) in flat areas yield results whose homogeneity make decision-making altogether difficult. These decisions may be about land use, monitoring plans, water resource/soil protection strategies, or environmental impact assessment. Therefore, the homogeneity of results in the lower priority categories, even though in principle make decision-making easier, may lead to excessive

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confidence, leading to a rejection of protection measures or decisions taken with scarce attention to prevention. In this sense, the proposed GIS-integrated indexes (IWI and RIWI) show a higher discrimination in the groundwater suitability map for irrigation, an easier application in a wide range of study areas and the possibility to analyze the spatial variation of water quality in an integrated way. The IWI0 s priorities are related to the geomorphologic features of the region. Areas with very high to high potential for irrigation correspond to perirange fringe and hills with well drainage soils. On the other hand, sites with moderate to low suitability for irrigation, ‘‘3’’ and ‘‘4’’priorities, are associated to plains areas with poor drainage soils. The ‘‘5’’ priority, with very low suitability for irrigation, coincides with the surface and groundwater discharge area of the Northern slope of the Tandilia Range. Near 33% of the study area has ‘‘1’’ and ‘‘2’’ priorities, with very high to high potential for irrigation purposes. These sites have EC values \750 lS/cm, very low sodicity water (SAR \ 18), mainly [28 slopes and hydraulic gradients [0.003. The sites with ‘‘3’’ priority (28 %) have in general EC values ranging from 750 to 1,000 lS/cm, 18–26 SAR values, slopes \28 and primarily 0.002 to 0.003 hydraulic gradients. The unsuitable sites (31 %), ‘‘4’’ and ‘‘5’’ priorities, are characterized by EC values [1,000 lS/cm reaching 4,710 lS/cm, especially SAR values [26, \18 slopes and \0.0007 hydraulic gradients. The remaining 8 % correspond to ranges and urban fabric. It is relevant to mention that in the slope map, the low values were considered as ‘‘Doubtful’’ and ‘‘Unsuitable’’ categories since low slope areas can lead to productivity problems (poor drainage, flood area, salinity and alkalinity), determining, in many cases, low agricultural suitability (Martı´n and others 2007). Related to the hydraulic gradient map, the lower values were also classified as ‘‘Doubtful’’ and ‘‘Unsuitable’’ categories. The flow velocity of groundwater depends on the hydraulic gradient and permeability (Freeze and Cherry 1979). In this sense, the lower the gradient the less groundwater flow, causing drainage problems, salinity and alkalinity. In areas where water is cheap and large volumes are used, as in the Wet Pampa Plain, irrigation practices are often inefficient. Overuse and the waste of irrigation water contribute to drainage difficulties and salinity problems (Richards 1954). As a consequence, a method for assessing water quality and availability is required in order to propose a sustainable planning of water resources in these particular areas. The RIWI is a viable proposal. It derives from the IWI, and allows making an accurate delimitation of suitable sites for irrigation including the aquifer thickness, as a parameter. This variable gives information related to the groundwater offer in the area. It was proposed for suitable irrigation areas (‘‘1’’ to ‘‘3’’ IWI

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priorities) in order to evaluate the availability of groundwater for agricultural use. The dominant priority of this index is ‘‘2’’ (46.26 % area) with high potential for irrigation and moderate groundwater availability. Sites with this priority value are located in the perirange fringe and hill areas, with an aquifer thickness ranging from 10 to 30 m. One particularity of this area is that the structural controls that define the Tandilia range system establish strong changes in the aquifer thickness of the area. The maximum value of this parameter is 80 m, according to the conventional depth of irrigation wells. This depth was considered appropriate for the analysis in this work, knowing that the aquifer near the costal line is higher than 100 m of thickness (Sala and others 1980; Kruse 1987; Quiroz London˜o and others 2008; Martı´nez and others 2010). Although it is well known that the soil texture is an important parameter for assessing salinization and sodification problems in soils, the IWI and RIWI indexes do not include it due to the existing relationship between slope and soil texture in the Pampa Region. Therefore, well drained soils are mainly located in zones with higher slope values, whereas, poor drained soils coincide with plains areas. Development and use of suitability indices without proper consideration of groundwater dynamics may adversely affect the groundwater quality. Pumping of water at any location will cause drawdown and thus flow of groundwater from ‘‘other not suitable areas’’. With suitable areas being so few and sparse in the study area, proximity of the ‘‘not suitable’’ water should be accounted for. Despite this fact, in this work the anthropic impact (pumping wells) on the dynamics of the groundwater system has not been considered since the proposed indexes have a steady state condition. In order to evaluate flows or pumping rates, hydrogeological models and proper simulation scenarios are required. However, at this stage of the work, this exceeds the goal of the paper. Nevertheless, the inclusion of two maps related to hydrogeological features (hydraulic gradient and aquifer thickness maps) provide an irrigation water index integrated by several important issues than just those used for assessing water quality. Generally, groundwater is suitable for irrigation purpose in the analyzed portion of the Wet Pampa Plain. The majority (65.5 %) of the irrigation wells are located on sites with waters with very high to moderate potential for irrigation, whereas the remaining 34.5 % belong to sites with low quality in the southern slope of the Tandilia range. As a consequence, this sector is defined as the highest priority area for the development and management of groundwater irrigation since it combines agricultural soils and hazardous waters. Therefore, (i) to achieve a full yield potential; (ii) to sustain it over a long period of time;

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(iii) to avoid the possibility of increase in salinity, and (iv) to avoid the possibility of occurrence of sodicity and toxicity hazardous in the future, a proper irrigation scheme is required in the form of crop selection, fertilizer usage and suitable alternative management. To maximize the benefits from irrigation activities in this area it is suggested: (i) groundwater potential of the watershed should be investigated in doubtful sites; (ii) farmers should be advised on the site selection and drilling of the wells; (iii) groundwater utilization and management policies should be formulated and implemented to overcome future conflict in the utilization of the groundwater resource in the area and also to maintain the sustainability of the irrigation scheme; (iv) groundwater recharging measures need attention by the community to maintain and maximize the availability of groundwater in the watershed; and irrigation wells should be declared in the Pampean Irrigation Association in order to control their location and the quality and quantity of water extraction.

Conclusions The proposed methodology involves two useful indexes for regional water planning and management in agroecosystems. It includes easily measurable and common used parameters in hydrogeological surveys that can be spatially and temporarily analyzed. They allow comparing and monitoring changes in groundwater suitability for irrigation, rendering decision-making easier for soil protection and agricultural production. A well structured and designed geodatabase helps to integrate, analyze and manipulate information in an easy way, in order to construct the different thematic maps which integrate the proposed indexes. Moreover, these structures allow the updating of information to improve spatial analysis. A GIS-based IWI was proposed for field staffs, watershed planners, and land users, not only for assessing and mapping the groundwater suitability for irrigation purposes in this area, but also to provide the ability to accurately capture spatial variations and quickly reflect changes in irrigation water in response to changes in the environment. Moreover, RIWI was also suggested. This index is a variation of the IWI, considering the aquifer thickness. Thus, RIWI shows the areas where groundwater is suitable for irrigation plus the potential aquifer’s water offer, which is important for groundwater extraction. The IWI index has advantages over the traditional methodology since it allows for a better discrimination in homogeneous areas, which not only involves statistics but also the physical characteristics of the environment. Finally, it also allows the use of a classification based on common information requirements which may be easily

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adapted to each study area. Plains areas with similar features could have the same requirements. The main limitations of applying this index is its incapability to perform an integrated evaluation and planning of groundwater quality suitability for irrigation purposes in areas without (a) an extensive available data regarding aquifer features; (b) representative samples of groundwater; and (c) wellknown geomorphologic units. The IWI0 s index application showed that 65.5 % in the analyzed area has ‘‘Very high’’ to ‘‘Moderate’’ potential for irrigation (‘‘1’’ to ‘‘3’’ priorities). The last one coincides with plains areas, whereas the other ones are located within the perirange fringe and hill areas. The quantity of water available for irrigation is not controlled in the Pampa plain, so irrigation practices are often inefficient. As the RIWI 0 s dominant priority in the tested area is the ‘‘2’’, with high suitability for irrigation and moderate aquifer potential, an irrigation planning in the region is required for an efficient practice. These GIS-based indexes could be used to prevent soil and water deterioration, and therefore, contribute to the sustainable management of these natural resources. Acknowledgments The authors would like to thank the financial support of the Agencia Nacional de Promocio´n Cientı´fica yTecnolo´gica (National Agency for Scientific and Technological Promotion) through PICT 2007 00390, CONICET (PIP 5668), the International Atomic Energy Agency (IAEA) as well as by the Mar del Plata National University through ARQ 168/07 and EXA 388/08. We are also indebted to Mr. A. Ferrante for technical assistance and Mr. G. Bernava for chemical analysis. Two of the authors (AR and MLL) are indebted to the National Scientific and Technical Research Council (CONICET) for fellowship supports.

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