In the south of Córdoba (Argentina), groundwater from the unconfined aquifer, presents scarce suitability due to high values of salts, sulfates, arsenic and fluorides. This situation makes it necessary to explore other possibilities, for example, conjunctive use of groundwater and surface water. The objective of this work was to describe the dynamic and geochemical relationships between surface water and groundwater in the medium-low basin of the Ají stream. The conceptual model was supported by the use of stable 18O/16O and 2H/1H isotopes and it was validated through numerical modeling and 222Rn activity results. The stream water is sodium bicarbonate fresh type (460–1,400 μS/cm). The aquifer presents fresh to salt water (720–14,000 μS/cm), from sodium bicarbonate to sodium chloride type. The isotopic results show that there is a straight mixing line between surface water and groundwater samples. The geochemical numerical modeling revealed that in the mixing models between stream and aquifer, for hydraulically linked samples, the aquifer only provides 2% of the total stream flow. This was validated through 222Rn results, which led us to the estimation of about 2% of groundwater supply in summer.

INTRODUCTION

Although water law and water policy often consider groundwater and surface water as separate resources, both water bodies are functionally interdependent. Surface-water bodies such as rivers, lakes and wetlands can receive groundwater inflow, recharge the groundwater or do both actions in different sectors or seasons. This interaction between water systems leads to the mixing of their water qualities (Winter 1999; Sophocleous 2002; Hancock et al. 2005; Goldscheider et al. 2006). High quantities of nutrients or other dissolved chemicals derived from natural processes and human activities in surface water can be transferred to the connected groundwater system and vice versa. Thus, the interaction of ground water and surface water has been shown to be a significant concern in issues related to water supply, water quality and degradation of aquatic environments (Winter 1999). The interactions of streams, lakes, and wetlands with groundwater are governed by the positions of the water bodies with respect to groundwater flow systems, geologic characteristics of their beds, and their climatic settings (Tóth 1999; Winter 1999).

Despite the fact that groundwater and surface water interactions are controlled by their hydraulic connection and may be considered essentially one resource, the interactions are difficult to observe and measure (Winter 1999; Sophocleous 2002; Custodio 2005). All methods have shown significant uncertainty, as usually happens when humans try to quantify nature.

In the south of Córdoba Province (Argentina), the most used hydrological resource is groundwater from the unconfined aquifer, which quite often presents scarce suitability for several uses due to high values of salts, sulfates, arsenic and fluorides. This situation and the high salinity of confined aquifers in some areas make it necessary to explore other possibilities, for example, conjunctive use of groundwater and surface water. In this framework, a sector of a basin located in this region was selected for the investigation. The objective of this work is to show the dynamic and geochemical relationships between surface water and groundwater in the low basin of the Ají stream situated in the sandy plain in the south of Córdoba Province (Argentina, Figure 1). The results were interpreted to obtain a hydrogeochemical conceptual model which could be used as a tool for hydrological and environmental management. The conceptual model was supported by the use of stable 18O/16O and 2H/1H isotopes and was validated through numerical geochemical modeling and the preliminary results of 222Rn activity in water.

Figure 1

Study area.

Figure 1

Study area.

METHODS

The regional geological and hydrological features were identified in the field and hydrogeological data were obtained from 25 wells, most of them penetrating the upper 20 m of the unconfined sedimentary aquifer. Climatic and land use features were analyzed for the study area. The historical precipitation data were collected from ‘La Perdiz’ station (33 °54′52.54″S – 64 °24′16.39″W). In this first stage of the study, the hydrological survey was carried out in summer and the stream flow was measured in this season in different gaging sections. Twenty-five groundwater and surface water samples were obtained and field parameters were measured in situ: pH (Electrode Orion 9104), electrical conductivity (EC) (Hanna Instruments, HI 9033) and temperature (T) (YSI Model 95 Handheld Temperature System).

The samples were collected in 1 L plastic bottles and were analyzed within 24 h of collection, in the laboratory of the National University of Rio Cuarto. The following parameters were analyzed: Na+, K+, HCO3, Cl, Mg2+, Ca2+, SO42− (American Public Health Association (APHA) et al. 2005,).

Stables isotopes (2H and 18O) analyses were done in the Institute of Geochronology and Isotope Geology (INGEIS-CONICET-UBA) by means of Off-Axis Integrated Cavity Output Spectroscopy (OICOS) (Lis et al. 2008), DLT-100 Liquid-Water Isotope Analyzer from LGR Inc. Results were expressed in the usual form, i.e. δ(‰) defined as: 
formula
1
where R = isotope ratio 2H/1H or 18O/16O; δ2H or δ18O, isotopic deviation in ‰, respectively. S denotes sample and ST reference standard. Uncertainties are ±1‰ for δ2H and ±0.3‰ for δ18O. Reference standard is V-SMOW (Gonfiantini 1978).

Although this work was carried out in the medium-low basin as a result of a requirement of a water company, some samples were collected in the upper and middle basin to improve hydrological interpretations. Moreover, and even though this research was carried out in summer, an expeditious survey was carried out in winter to measure EC and isotope values. The numerical geochemical models were developed using the PHREEQC software (Parkhurst and Appelo 1999) taking into consideration the major ions (HCO3, SO42−, Cl, Na+, K+, Ca2+, Mg2+) and the stable isotopes (δ18O/δ2H). The activity of 222Rn in surface water and groundwater was measured in situ in five samples using the RAD7 radon detector (Durridge Co.) belonging to INGEIS.

RESULTS AND DISCUSSION

Climatic and geological characterization

Climatic changes which occurred in South America during the Quaternary provoked deflation and deposition of large masses of silt and fine sand which formed loess and loessoid units of regional extension in several areas of the continent. The winds derived from the Patagonian ice field during the Last Glacial Maximum transported to the Pampean plain silts and fine sands formed by frost action in the Andean Mountains Range. In humid periods, these loessical and sand plains were crossed by different fluvial systems while, in the interfluvial areas, soils were developed (Iriondo 1997).

At present, the studied area (2,500 km2) has a subhumid climate, characterized by an annual rainfall of 750 mm (Figure 2(a)), of which 85% is concentrated in spring-summer (September–March), (Figure 2(b)). The regional rainfall salt concentration is in the order of 30 mg/L and, according to its chemical composition it supplies bicarbonate, sulfate, chloride, sodium, calcium and magnesium ions to the groundwater (Cabrera et al. 2013).

Figure 2

(a) Annual rainfall chronological curve. (b) Distribution of precipitations.

Figure 2

(a) Annual rainfall chronological curve. (b) Distribution of precipitations.

The area has a flat to gently undulated relief, with longitudinal and parabolic dune systems of different origin and age which are crossed by the Ají stream fluvial strip (Bécher Quinodóz 2014). The upper basin of this stream is located in the Comechingones Mountains (Córdoba Province), outside the studied area, where the Precambrian-Paleozoic bedrock outcrops. In the fluvial system, erosion processes prevail. Both lateral and vertical fluvial incisions were identified while floods occasionally occur in the low basin. The most relevant geomorphological units can be observed in Figure 3. The unconfined aquifer is formed by sandy-silt brown sediments with dispersed calcareous nodules or layers (calcretes).

Figure 3

Geomorphological map.

Figure 3

Geomorphological map.

Hydrodynamic and hydrochemical characteristics

The Ají stream starts at the confluence of the Cortaderas and Zelegüa streams with some supply from the Suco lagoon. From the middle basin, it is a permanent stream as the result of groundwater contribution. The flow varies between summer (0.8 m3/s) and winter (in the order of 0.40 m3/s) with an average water velocity of 0.33 m/s (Table 1).

Table 1

(a) Stream flow (Q) in Ají stream (summer) and (b) (winter)

Gaging stations Latitude Longitude Area [m2Mean velocity [m/s] Flow (Q) [m3/s] 
Summer 
 A1 33 °55′47.36″S 64 °32′25.45″W 2.18 0.37 0.81 
 A2 33 °55′47.98″S 64 °24′52.81″W 2.67 0.31 0.83 
 A3 33 °57′09.04″S 64 °17′17.98″W 2.29 0.34 0.78 
 A5 34 °08′39.70″S 64 °05′16.89″W 2.49 0.29 0.74 
Winter 
 A1 33 °55′47.36″S 64 °32′25.45″W 1.44 0.34 0.50 
 A2 33 °55′47.98″S 64 °24′52.81″W 1.83 0.29 0.54 
 A3 33 °57′09.04″S 64 °17′17.98″W 1.08 0.42 0.46 
 A5 34 °08′39.70″S 64 °05′16.89″W 1.28 0.28 0.36 
Gaging stations Latitude Longitude Area [m2Mean velocity [m/s] Flow (Q) [m3/s] 
Summer 
 A1 33 °55′47.36″S 64 °32′25.45″W 2.18 0.37 0.81 
 A2 33 °55′47.98″S 64 °24′52.81″W 2.67 0.31 0.83 
 A3 33 °57′09.04″S 64 °17′17.98″W 2.29 0.34 0.78 
 A5 34 °08′39.70″S 64 °05′16.89″W 2.49 0.29 0.74 
Winter 
 A1 33 °55′47.36″S 64 °32′25.45″W 1.44 0.34 0.50 
 A2 33 °55′47.98″S 64 °24′52.81″W 1.83 0.29 0.54 
 A3 33 °57′09.04″S 64 °17′17.98″W 1.08 0.42 0.46 
 A5 34 °08′39.70″S 64 °05′16.89″W 1.28 0.28 0.36 

As can be seen in the unconfined aquifer equipotential map and the hydrogeological sections (Figure 4(a) and 4(b)) the hydraulic relationships are variable but, in most reaches, the watercourse is a gaining stream as a result of groundwater contribution. In the upper basin, groundwater does not influence the stream water (Bécher Quinodóz 2014). Hydrogeological data are shown in Table 2.

Table 2

Topographic and hydrogeological data

Name Latitude Longitude Well depth (m) Topographic elevation (m) Water table depth (m) Water table level (m) 
M81 33 °55′45.84″S 64 °27′18.66″O 15 249.0 3.0 246.0 
M82 33 °53′54.31″S 64 °30′13.25″O 15 265.5 3.5 262.0 
M83 33 °54′19.72″S 64 °33′45.75″O 20 276.5 3.5 273.0 
M84 33 °52′25.14″S 64 °41′20.34″O 15 309.0 3.0 306.0 
M85 33 °50′52.93″S 64 °46′25.32″O 15 335.0 7.0 328.0 
M86 33 °58′11.84″S 64 °40′18.85″O 20 291.5 3.5 288.0 
M94 34 °01′42.31″S 64 °36′11.01″O 20 268.0 3.0 265.0 
M95 34 °01′15.40″S 64 °27′26.95″O 20 242.5 2.5 240.0 
M96 34 °01′45.36″S 64 °27′18.98″O 20 241.5 2.5 239.0 
M97 34 °05′39.58″S 64 °32′14.47″O 20 253.0 2.5 250.5 
M99 34 °08′19.88″S 64 °23′00.64″O 18 223.4 3.0 220.4 
M101 34 °10′33.64″S 64 °14′19.21″O 15 202.5 3.5 199.0 
M102 34 °03′49.22″S 64 °16′16.05″O 20 206.5 2.5 204.0 
M103a 33 °56′58.51″S 64 °25′02.27″O 14 241.0 3.0 238.0 
M105a 34 °11′19.83″S 64 °04′05.74″O 15 178.0 2.5 175.5 
M106 34 °09′53.66″S 64 °10′53.11″O 15 198.5 3.5 195.0 
M107 33 °59′41.66″S 64 °06′31.29″O 20 193.5 2.5 191.0 
M110 33 °56′57.11″S 64 °14′28.33″O 15 207.5 2.5 205.0 
M118 33 °57′00.73″S 64 °25′00.52″O 15 241.5 3.0 238.5 
M131 33 °55′38.57″S 64 °31′55.40″O 15 264.0 3.0 261.0 
M132 33 °55′47.98″S 64 °24′52.81″O 10 242.0 3.0 239.0 
M133 34 °08′39.70″S 64 °05′16.89″O 10 183.5 2.5 181.0 
M134 34 °00′23.59″S 64 °12′30.78″O 15 201.5 2.5 199.0 
M138 33 °31′39.95″S 64 °56′26.36″O 20 505.0 7.0 498.0 
M139 33 °54′59.60″S 64 °22′17.63″O 15 236.5 2.5 234.0 
M140 33 °19′32.93″S 64 °51′38.46″O 20 631.0 10.0 621.0 
Name Latitude Longitude Well depth (m) Topographic elevation (m) Water table depth (m) Water table level (m) 
M81 33 °55′45.84″S 64 °27′18.66″O 15 249.0 3.0 246.0 
M82 33 °53′54.31″S 64 °30′13.25″O 15 265.5 3.5 262.0 
M83 33 °54′19.72″S 64 °33′45.75″O 20 276.5 3.5 273.0 
M84 33 °52′25.14″S 64 °41′20.34″O 15 309.0 3.0 306.0 
M85 33 °50′52.93″S 64 °46′25.32″O 15 335.0 7.0 328.0 
M86 33 °58′11.84″S 64 °40′18.85″O 20 291.5 3.5 288.0 
M94 34 °01′42.31″S 64 °36′11.01″O 20 268.0 3.0 265.0 
M95 34 °01′15.40″S 64 °27′26.95″O 20 242.5 2.5 240.0 
M96 34 °01′45.36″S 64 °27′18.98″O 20 241.5 2.5 239.0 
M97 34 °05′39.58″S 64 °32′14.47″O 20 253.0 2.5 250.5 
M99 34 °08′19.88″S 64 °23′00.64″O 18 223.4 3.0 220.4 
M101 34 °10′33.64″S 64 °14′19.21″O 15 202.5 3.5 199.0 
M102 34 °03′49.22″S 64 °16′16.05″O 20 206.5 2.5 204.0 
M103a 33 °56′58.51″S 64 °25′02.27″O 14 241.0 3.0 238.0 
M105a 34 °11′19.83″S 64 °04′05.74″O 15 178.0 2.5 175.5 
M106 34 °09′53.66″S 64 °10′53.11″O 15 198.5 3.5 195.0 
M107 33 °59′41.66″S 64 °06′31.29″O 20 193.5 2.5 191.0 
M110 33 °56′57.11″S 64 °14′28.33″O 15 207.5 2.5 205.0 
M118 33 °57′00.73″S 64 °25′00.52″O 15 241.5 3.0 238.5 
M131 33 °55′38.57″S 64 °31′55.40″O 15 264.0 3.0 261.0 
M132 33 °55′47.98″S 64 °24′52.81″O 10 242.0 3.0 239.0 
M133 34 °08′39.70″S 64 °05′16.89″O 10 183.5 2.5 181.0 
M134 34 °00′23.59″S 64 °12′30.78″O 15 201.5 2.5 199.0 
M138 33 °31′39.95″S 64 °56′26.36″O 20 505.0 7.0 498.0 
M139 33 °54′59.60″S 64 °22′17.63″O 15 236.5 2.5 234.0 
M140 33 °19′32.93″S 64 °51′38.46″O 20 631.0 10.0 621.0 
Figure 4

(a) Equipotential map for the unconfined aquifer. (b) Hydraulic relation between stream and aquifer in sections A2 and A4.

Figure 4

(a) Equipotential map for the unconfined aquifer. (b) Hydraulic relation between stream and aquifer in sections A2 and A4.

Surface water salinity shows a notable homogeneity along the studied stream reach. Thus the EC was from 458 μS/cm to 1,410 μS/cm in the flow direction and all the samples were classified as sodium bicarbonate geochemical type (Table 3, Figure 5). Measurement of winter EC in surface water (467–1,414 μS/cm) showed that there is almost no salinity change in relation to summer, indicating high influence of the fresh water that flows from the mountains.

Table 3

Major ions and stable isotope concentrations in surface water in 2013

Name Date EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
S. Lagoon Summer (15/03/2013) 1,266 8.7 46 480.0 103.6 82.9 242.7 15.4 20.0 13.2 −2.0 −11.6 
 AZ1 458 8.2 185.0 43.3 16.6 30.9 5.3 38.4 4.9 −5.2 −26.6 
 AZ2 1,050 8.5 21.82 502.5 88.0 42.9 273.0 7.0 12.0 21.0 −5.1 −25.9 
 AC1 1,025 8.4 9.70 490.0 107.8 42.9 257.8 5.6 16.0 8.8 −5.1 −25.5 
 A1 1,212 8.9 26.7 535.0 123.4 51.4 282.1 9.2 20.0 12.7 −4.9 −24.7 
 A2 1,272 8.8 26.7 532.5 113.0 60.0 291.2 9.8 19.2 17.1 −4.4 −23.5 
 A3 1,333 8.2 625.0 88.9 71.4 264.9 10.8 24.0 12.2 −4.5 −23.1 
 A4 1,368 8.8 33.9 517.5 102.5 71.4 315.5 11.0 21.6 14.6 −4.5 −23.3 
 A5 1,410 8.6 12.1 565.0 113.0 82.9 291.2 17.6 20.0 17.1 −4.2 −23.0 
S. Lagoon Winter (12/08/2013) 1,266 8.7 46 480.0 103.6 82.9 242.7 15.4 20.0 13.2 −2.0 −11.6 
 AZ1 467 8.2 187.5 47.2 17.1 36.4 5.4 40.0 4.9 −5.3 −26.7 
 AZ2 1,080 8.7 21.82 527.5 92.5 45.7 273.0 7.2 13.6 21.0 −4.7 −23.6 
 AC1 1,076 8.6 12.12 512.5 123.4 45.7 257.8 5.7 17.6 8.8 −4.7 −24.4 
 A1 1,270 8.2 585.0 144.2 57.1 297.3 7.3 17.6 8.8 −4.4 −22.9 
 A2 1,329 8.3 585.0 144.2 62.9 308.4 7.6 17.6 16.1 −4.3 −22.1 
 A3 1,406 8.4 4.85 590.0 144.2 91.4 328.6 7.8 17.6 20.5 −4.3 −22.7 
 A4 1,410 8.5 12.12 575.0 144.2 97.1 328.6 7.8 17.6 16.6 −4.2 −22.9 
 A5 1,414 8.6 12.12 575.0 113.0 102.9 328.6 7.9 17.6 29.3 −4.1 −21.8 
Name Date EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
S. Lagoon Summer (15/03/2013) 1,266 8.7 46 480.0 103.6 82.9 242.7 15.4 20.0 13.2 −2.0 −11.6 
 AZ1 458 8.2 185.0 43.3 16.6 30.9 5.3 38.4 4.9 −5.2 −26.6 
 AZ2 1,050 8.5 21.82 502.5 88.0 42.9 273.0 7.0 12.0 21.0 −5.1 −25.9 
 AC1 1,025 8.4 9.70 490.0 107.8 42.9 257.8 5.6 16.0 8.8 −5.1 −25.5 
 A1 1,212 8.9 26.7 535.0 123.4 51.4 282.1 9.2 20.0 12.7 −4.9 −24.7 
 A2 1,272 8.8 26.7 532.5 113.0 60.0 291.2 9.8 19.2 17.1 −4.4 −23.5 
 A3 1,333 8.2 625.0 88.9 71.4 264.9 10.8 24.0 12.2 −4.5 −23.1 
 A4 1,368 8.8 33.9 517.5 102.5 71.4 315.5 11.0 21.6 14.6 −4.5 −23.3 
 A5 1,410 8.6 12.1 565.0 113.0 82.9 291.2 17.6 20.0 17.1 −4.2 −23.0 
S. Lagoon Winter (12/08/2013) 1,266 8.7 46 480.0 103.6 82.9 242.7 15.4 20.0 13.2 −2.0 −11.6 
 AZ1 467 8.2 187.5 47.2 17.1 36.4 5.4 40.0 4.9 −5.3 −26.7 
 AZ2 1,080 8.7 21.82 527.5 92.5 45.7 273.0 7.2 13.6 21.0 −4.7 −23.6 
 AC1 1,076 8.6 12.12 512.5 123.4 45.7 257.8 5.7 17.6 8.8 −4.7 −24.4 
 A1 1,270 8.2 585.0 144.2 57.1 297.3 7.3 17.6 8.8 −4.4 −22.9 
 A2 1,329 8.3 585.0 144.2 62.9 308.4 7.6 17.6 16.1 −4.3 −22.1 
 A3 1,406 8.4 4.85 590.0 144.2 91.4 328.6 7.8 17.6 20.5 −4.3 −22.7 
 A4 1,410 8.5 12.12 575.0 144.2 97.1 328.6 7.8 17.6 16.6 −4.2 −22.9 
 A5 1,414 8.6 12.12 575.0 113.0 102.9 328.6 7.9 17.6 29.3 −4.1 −21.8 
Figure 5

Water geochemical type in the stream and the unconfined aquifer.

Figure 5

Water geochemical type in the stream and the unconfined aquifer.

The groundwater from the unconfined aquifer exhibits variable EC from 650 μS/cm (fresh water) linked to recharge in dunes (Bécher Quinodóz 2014), up to 14,000 μS/cm (salty water) in the hydrohalomorphic areas (Figure 3). Groundwater shows a geochemical evolution in the flow direction from fresh to salty water, changing from sodium bicarbonate to sodium sulfate and, finally, to sodium chloride geochemical type (Table 4, Figure 5).

Table 4

Major ions and stable isotope concentrations in groundwater in Summer 2013

Name EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
M82 1,700 7.8 460.0 259.5 222.9 242.7 19.3 80.8 54.6 −4.5 −24 
M83 1,650 8.2 800.0 88.0 65.7 385.2 11.4 24.8 26.3 −4.5 −25 
M84 1,670 8.6 24.2 825.0 92.5 28.6 448.9 8.2 4.8 9.3 −4.6 −24 
M85 1,950 8.2 900.0 146.3 85.7 520.7 9.2 7.2 11.2 −4.1 −22 
M86 1,850 8.3 715.0 332.7 51.4 348.8 9.1 17.6 14.6 −4.6 −24 
M94 4,250 8.2 1,000.0 499.9 388.6 1,061.7 19.9 21.6 33.7 −4.6 −24 
M95 8,990 7.9 545.0 1,543.4 2,057.1 2,042.5 65.6 144.0 165.9 −4.1 −24 
M96 5,020 8.2 835.0 714.3 857.1 1,294.2 29.9 44.0 69.3 −4.2 −23 
M97 9,540 7.9 700.0 1,867.0 1,828.6 2,548.0 41.8 70.4 77.6 −4.7 −29 
M99 7,600 7.6 435.0 1,602.9 1,657.1 1,820.0 37.8 222.4 185.4 −4.6 −28 
M101 1,470 8.5 19.4 705.0 102.5 34.3 321.5 7.0 6.4 13.2 −4.6 −24 
M102 14,040 8.0 835.0 2,565.9 3,171.4 3,731.0 75.0 168.0 204.9 −4.4 −23 
M103 650 7.7 347.5 32.3 14.3 72.8 19.0 32.0 30.7 −4.4 −24 
M105b 5,740 8.4 12.1 907.5 1,168.2 885.7 1,415.6 28.1 33.6 64.9 −4.8 −25 
M106 5,230 8.1 705.0 1,016.8 828.6 1,162.8 32.8 51.2 58.5 −4.4 −24 
M107 3,580 7.6 360.0 758.8 562.9 697.7 27.5 100.8 11.2 −4.4 −23 
M110 9,190 7.5 837.5 1,995.9 1,631.4 2,184.0 74.4 65.6 123.4 −4.4 −23 
M118 2,970 8.6 38.8 935.0 206.8 205.7 551.1 39.6 34.4 37.6 −4.4 −23 
M131 1,198 8.2 620.0 102.5 62.9 273.0 8.8 14.4 13.2 −4.7 −24 
M132 1,270 8.2 575.0 102.5 57.1 291.2 12.9 18.4 11.2 −4.4 −24 
M133 1,410 8.8 26.7 542.5 102.5 85.7 291.2 10.8 21.6 15.1 −4.4 −24 
M134 6,820 7.5 1,012.5 997.5 1,200.0 1,698.7 49.8 56.8 32.7 −4.4 −23 
M138 1,450 7.9 725.0 113.0 42.9 306.4 6.3 23.2 49.3 −5.1 −26 
M139 7,830 7.9 1,250.0 1,348.5 1,214.3 1,739.1 40.4 56.0 112.2 −4.7 −27 
M140 1,393 7.7 677.5 98.4 68.6 291.2 15.7 24.8 10.2 −2.2 −10 
Name EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
M82 1,700 7.8 460.0 259.5 222.9 242.7 19.3 80.8 54.6 −4.5 −24 
M83 1,650 8.2 800.0 88.0 65.7 385.2 11.4 24.8 26.3 −4.5 −25 
M84 1,670 8.6 24.2 825.0 92.5 28.6 448.9 8.2 4.8 9.3 −4.6 −24 
M85 1,950 8.2 900.0 146.3 85.7 520.7 9.2 7.2 11.2 −4.1 −22 
M86 1,850 8.3 715.0 332.7 51.4 348.8 9.1 17.6 14.6 −4.6 −24 
M94 4,250 8.2 1,000.0 499.9 388.6 1,061.7 19.9 21.6 33.7 −4.6 −24 
M95 8,990 7.9 545.0 1,543.4 2,057.1 2,042.5 65.6 144.0 165.9 −4.1 −24 
M96 5,020 8.2 835.0 714.3 857.1 1,294.2 29.9 44.0 69.3 −4.2 −23 
M97 9,540 7.9 700.0 1,867.0 1,828.6 2,548.0 41.8 70.4 77.6 −4.7 −29 
M99 7,600 7.6 435.0 1,602.9 1,657.1 1,820.0 37.8 222.4 185.4 −4.6 −28 
M101 1,470 8.5 19.4 705.0 102.5 34.3 321.5 7.0 6.4 13.2 −4.6 −24 
M102 14,040 8.0 835.0 2,565.9 3,171.4 3,731.0 75.0 168.0 204.9 −4.4 −23 
M103 650 7.7 347.5 32.3 14.3 72.8 19.0 32.0 30.7 −4.4 −24 
M105b 5,740 8.4 12.1 907.5 1,168.2 885.7 1,415.6 28.1 33.6 64.9 −4.8 −25 
M106 5,230 8.1 705.0 1,016.8 828.6 1,162.8 32.8 51.2 58.5 −4.4 −24 
M107 3,580 7.6 360.0 758.8 562.9 697.7 27.5 100.8 11.2 −4.4 −23 
M110 9,190 7.5 837.5 1,995.9 1,631.4 2,184.0 74.4 65.6 123.4 −4.4 −23 
M118 2,970 8.6 38.8 935.0 206.8 205.7 551.1 39.6 34.4 37.6 −4.4 −23 
M131 1,198 8.2 620.0 102.5 62.9 273.0 8.8 14.4 13.2 −4.7 −24 
M132 1,270 8.2 575.0 102.5 57.1 291.2 12.9 18.4 11.2 −4.4 −24 
M133 1,410 8.8 26.7 542.5 102.5 85.7 291.2 10.8 21.6 15.1 −4.4 −24 
M134 6,820 7.5 1,012.5 997.5 1,200.0 1,698.7 49.8 56.8 32.7 −4.4 −23 
M138 1,450 7.9 725.0 113.0 42.9 306.4 6.3 23.2 49.3 −5.1 −26 
M139 7,830 7.9 1,250.0 1,348.5 1,214.3 1,739.1 40.4 56.0 112.2 −4.7 −27 
M140 1,393 7.7 677.5 98.4 68.6 291.2 15.7 24.8 10.2 −2.2 −10 

The analyses of relationships show that surface water exhibits marked homogeneity, with freshwater from the beginning to the end of the studied area (Figure 5), despite having traveled tens of kilometres and maintaining, in most sections, hydraulic connection with an aquifer that provides brackish to salty groundwater. This situation allows the interpretation that the aquifer has little influence on the mixture.

Stable isotope composition of surface and groundwater

Isotopic results (δ2H and δ18O) show that groundwater and surface water exhibit more depleted values at the western part of the basin. This situation can be explained because the rainfall water that recharges the aquifer and surface water bodies in the piedmont area of the Comechingones Mountains (Figure 5) comes from depleted rains originated in a wet warm front that comes from the Atlantic Ocean (1,000 km away). This front has suffered isotopic impoverishment as a consequence of continental and altitude effects (Cabrera et al. 2010; Bécher Quinodóz 2014). Moreover, Figure 6 shows that there is a straight mixing line between surface water and groundwater samples. The most depleted are those surface water samples related to the rainy season and clearly linked to the flows coming from the mountains.

Figure 6

δ2H vs δ18O for the Ají stream and the unconfined aquifer samples.

Figure 6

δ2H vs δ18O for the Ají stream and the unconfined aquifer samples.

In some samples, the similarity between aquifer and stream isotopic features are closely linked to induced recharge from the stream freshwater when groundwater is extracted in wells located at the stream side (Figure 4(b), Section A2). This singular situation holds an environmental positive connotation, because the induced recharge improves the groundwater condition. More studies are necessary to evaluate the winter behavior, but preliminary δ2H and δ18O isotope results showed more fractionation in winter stream samples as a result of evaporation. The surface water will also be more enriched in winter as a result of the aquifer contribution.

Numerical geochemical model

Numerical modeling was carried out for summer, taking into account the explained hydrological features and linking the different sampled sections (Figure 7), in order to evaluate the adjustment to the conceptual model.

Figure 7

Location of the modeled sites.

Figure 7

Location of the modeled sites.

Taking into account the conceptual model, it was considered that AZ2 (upper basin) is a mixture between AZ1 (surface water from mountain area) and SL (surface water from Suco lagoon). Table 5 shows chemical and isotope data of samples used in the numerical models.

Table 5

(a) Samples used in the numerical modeling and (b) results of numerical modeling carried out with PHREEQC

(a) Sample EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
SL (IS) 1,266 8.7 46 480.0 103.6 82.9 242.7 15.4 20.0 13.2 −2.0 −11.6 
AZ1 (IS) 458 8.2 185.0 43.3 16.6 30.9 5.3 38.4 4.9 −5.2 −26.6 
AZ2 (FS) 1,050 8.5 21.82 502.5 88.0 42.9 273.0 7.0 12.0 21.0 −5.1 −25.9 
(b) 
Sample Mixture fraction NaX (mol/L) CaX2 (mol/L) MgX2 (mol/L) CaSO4:2H2O (mol/L)               
SL (IS) 0.08 0.24 0.17 0.05 0.08               
AZ1 (IS) 0.92 2.46 1.78 0.55 0.82               
(a) Sample EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
SL (IS) 1,266 8.7 46 480.0 103.6 82.9 242.7 15.4 20.0 13.2 −2.0 −11.6 
AZ1 (IS) 458 8.2 185.0 43.3 16.6 30.9 5.3 38.4 4.9 −5.2 −26.6 
AZ2 (FS) 1,050 8.5 21.82 502.5 88.0 42.9 273.0 7.0 12.0 21.0 −5.1 −25.9 
(b) 
Sample Mixture fraction NaX (mol/L) CaX2 (mol/L) MgX2 (mol/L) CaSO4:2H2O (mol/L)               
SL (IS) 0.08 0.24 0.17 0.05 0.08               
AZ1 (IS) 0.92 2.46 1.78 0.55 0.82               

IS: initial solution; FS: final solution

The results (Table 5) showed that to obtain the sodium bicarbonate water type of Zelegüa stream in AZ2 (final solution), the initial solutions must be mixed in a ratio 92/8 (AZ1/SL) and cation exchange and salt dissolution processes (CaSO4:2H2O) must be produced.

The A1 sample was considered a mixture (Table 6) of AZ2 (surface water, Zelegüa stream), AC1 (surface water, Cortaderas stream) and M83 (groundwater from unconfined aquifer). The results (Table 6) showed that to obtain the sodium bicarbonate water type of Ají stream in the A1 section (final solution), the initial solutions must be mixed in a ratio 19/72/9 (AZ2/AC1/M83) and cation exchange and salt dissolution processes (CaCO3;CaSO4:2H2O) must be produced.

Table 6

(a) Samples used in the numerical modeling and (b) results of numerical modeling carried out with PHREEQC

(a) Sample EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4=(mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
AZ2 (IS) 1,050 8.5 21.82 502.5 88.0 42.9 273.0 7.0 12.0 21.0 −5.1 −25.9 
AC1 (IS) 1,025 8.4 9.70 490.0 107.8 42.9 257.8 5.6 16.0 8.8 −5.1 −25.5 
M83 (IS) 1,650 8.2 0.00 800.0 88.0 65.7 385.2 11.4 24.8 26.3 −4.5 −25.1 
A1 (FS) 1,212 8.9 26.7 535.0 123.4 51.4 282.1 9.2 20.0 12.7 −4.9 −24.7 
(b) 
Sample Mixture fraction NaX (mol/L) CaX2 (mol/L) KX (mol/L) CaCO3 (mol/L) CaSO4:2H2O (mol/L)             
AZ2 (IS) 0.190 0.20 −0.10 0.02 0.09 0.03             
AC1 (IS) 0.720 0.78 −0.41 0.05 0.35 0.04             
M83 (IS) 0.090 0.10 −0.05 0.006 0.04 0.02             
(a) Sample EC (μS/cm) pH CO3−2 (mg/L) CO3H (mg/L) SO4=(mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
AZ2 (IS) 1,050 8.5 21.82 502.5 88.0 42.9 273.0 7.0 12.0 21.0 −5.1 −25.9 
AC1 (IS) 1,025 8.4 9.70 490.0 107.8 42.9 257.8 5.6 16.0 8.8 −5.1 −25.5 
M83 (IS) 1,650 8.2 0.00 800.0 88.0 65.7 385.2 11.4 24.8 26.3 −4.5 −25.1 
A1 (FS) 1,212 8.9 26.7 535.0 123.4 51.4 282.1 9.2 20.0 12.7 −4.9 −24.7 
(b) 
Sample Mixture fraction NaX (mol/L) CaX2 (mol/L) KX (mol/L) CaCO3 (mol/L) CaSO4:2H2O (mol/L)             
AZ2 (IS) 0.190 0.20 −0.10 0.02 0.09 0.03             
AC1 (IS) 0.720 0.78 −0.41 0.05 0.35 0.04             
M83 (IS) 0.090 0.10 −0.05 0.006 0.04 0.02             

IS: initial solution; FS: final solution.

The A3 sample was considered a mixture (Table 7) of A1 (surface water of Ají stream, which is identical to A2) and M139 (groundwater from unconfined aquifer).

Table 7

(a) Samples used in the numerical modeling and (b) results of numerical modeling carried out with PHREEQC

(a) Sample EC (μS/cm) pH CO3−2 (mg/L) CO3H(mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
A1 (IS) 1,212 8.9 26.7 535 1,23.4 51.4 282.1 9.2 20.0 12.7 −4.9 −24.7 
M139 (IS) 7,830 7.9 1,250 1,348 1,214 1,739 40.4 56.0 112.2 −4.7 −27 
A3 (FS) 1,333 8.2 625 88.9 71.4 264.9 10.8 24.0 12.2 −4.5 −23.1 
(b) 
Sample Mixture fraction NaX (mol/L) CaX2 (mol/L) CaCO3 (mol/L) CaSO4:2H2O (mol/L)               
A1 (IS) 0.98 −0.12 0.63 −0.48 −0.45               
M139 (IS) 0.02 −0.02 0.01 −0.03 −0.02               
(a) Sample EC (μS/cm) pH CO3−2 (mg/L) CO3H(mg/L) SO4= (mg/L) Cl (mg/L) Na+ (mg/L) K+ (mg/L) Ca+ (mg/L) Mg+ (mg/L) δ18O (‰) δ2H (‰) 
A1 (IS) 1,212 8.9 26.7 535 1,23.4 51.4 282.1 9.2 20.0 12.7 −4.9 −24.7 
M139 (IS) 7,830 7.9 1,250 1,348 1,214 1,739 40.4 56.0 112.2 −4.7 −27 
A3 (FS) 1,333 8.2 625 88.9 71.4 264.9 10.8 24.0 12.2 −4.5 −23.1 
(b) 
Sample Mixture fraction NaX (mol/L) CaX2 (mol/L) CaCO3 (mol/L) CaSO4:2H2O (mol/L)               
A1 (IS) 0.98 −0.12 0.63 −0.48 −0.45               
M139 (IS) 0.02 −0.02 0.01 −0.03 −0.02               

IS: initial solution; FS: final solution.

The results (Table 7) showed that to obtain the sodium bicarbonate water type of Ají stream in the A3 section (final solution), the initial solutions must be mixed in a ratio 98/2 (A1/M139), and cation exchange and salt dissolution processes must be produced (CaCO3;CaSO4:2H2O).

Application of 222Rn for assessment of the relationship between surface water and groundwater

In order to add quantitative evidence to the conceptual hydrogeological model, a preliminary characterization of the stream-aquifer relationships was made using dissolved 222Rn in water. The characterization of 222Rn activity in groundwater and surface water was performed by punctual water measurements at five sites (stream and wells), (Table 8, Figure 8) located along the entire basin.

Table 8

222Rn concentration (pCi/L) in the surface and ground water

Sample 222Rn concentration (pCi/L) 
1. Suco lagoon 2.2 
2. Unconfined aquifer upper basin (M140246 
3. Ají stream (A34.5 
4. Unconfined aquifer study area (M139274 
5. Confined aquifer study area (C100351 
Sample 222Rn concentration (pCi/L) 
1. Suco lagoon 2.2 
2. Unconfined aquifer upper basin (M140246 
3. Ají stream (A34.5 
4. Unconfined aquifer study area (M139274 
5. Confined aquifer study area (C100351 
Figure 8

Cross section of a groundwater-surface water relationship scheme taking into account 222Rn values.

Figure 8

Cross section of a groundwater-surface water relationship scheme taking into account 222Rn values.

Suco Lagoon, located in the upper basin (outside the study area) showed results which were consistent with large surface runoff contribution (2.2 pCi/L). On the other hand, the Ají stream within the study area, with a flow of about 0.8 m3/s at the selected gaging station, showed low 222Rn values (4.5 pCi/L). This value, higher than that of Suco lagoon, was consistent with the hydrological section location, i.e. the low basin where the stream receives groundwater supply. The unconfined aquifer near the gaging station exhibited high 222Rn values, typical of groundwater contained in the regional aeolian sediments (Bécher Quinodóz 2014). The highest value (351 pCi/L) corresponded to a confined aquifer (C100) and it was probably related to the aquifer depth and slow groundwater flow (Figure 8). At the A3 gaging station, an estimation of groundwater contribution was performed, taking into account the 222Rn mass balance method (Ellins et al. 1990) and using the following equation: 
formula
2
where Qgw: groundwater flow; Qsw: surface water flow; Rnsw: 222Rn activity in surface water; Rngw:222Rn activity in groundwater and Rnf: regional background of 222Rn activity in surface water.

The 222Rn regional background value in surface water was tentatively established at 1 pCi/L, on the basis that generally there is no 222Rn in surface water (Ellins et al. 1990). This result indicates that, at the sampling moment, surface flow predominated and groundwater contribution was approximately only in the order of 2% (Table 9, Figure 8).

Table 9

Results of calculation of the %Qgw in the A3 section

Station Qsw (m3/s) 222Rnsw (pCi/L) 222Rngw (pCi/L) 222Rnf (pCi/L) Qgw (m3/s) %Qsw 
A3 (Ají stream) 0.8 4.5 274 0.013 1.60 
Station Qsw (m3/s) 222Rnsw (pCi/L) 222Rngw (pCi/L) 222Rnf (pCi/L) Qgw (m3/s) %Qsw 
A3 (Ají stream) 0.8 4.5 274 0.013 1.60 

CONCLUSIONS

The explained surface water-groundwater relationships showed the control exerted by the relief, the lithology and the climate. The chemical composition of both hydrological systems is essentially the result of different water velocity in each body. This situation leads to mixing processes between surface water and local, intermediate or regional groundwater giving rise to the observed geochemical pattern. The aquifer-stream relationships in the studied stream reach, as was observed from the hydraulic data, showed effluent/influent varying spatially as a result of the fluvial geomorphology and human actions, although a natural gaining stream type prevails.

The isotopic behavior (δ2H – δ18O) of both hydrological systems also indicates water mixtures. This point is validated by 222Rn and the numerical geochemical modeling which indicates also a small contribution from the aquifer (∼2%) in summer.

Taking into account the isotopic and modeling results and although fresh surface water flows in areas with high groundwater salinity (up to 14 mS/cm in the low basin), it may be established that there is strong predominance of surface water in the summer mixing process. As a result of surface water high velocity and the incoming of such small groundwater quantities, salts that come from the aquifer are diluted.

Furthermore, the stream water samples and those extracted from adjacent wells exhibit the same low salinity (in the order of 0.4 g/L) and geochemical type (sodium bicarbonate) due to induced recharge from stream to aquifer during pumping. This is a promising situation for conjunctive use of surface water and groundwater for different purposes. In this regard, the first measurements in the winter season showed that the stream water has low electrical conductivity and it is still influenced by the fresh water coming from the mountains. Thus, it was considered a vital water supply offer and some wells located beside the stream have already been built to generate induced recharge of surface fresh water towards the aquifer. More studies are required, especially extensive and thorough sampling in the two contrasting seasons, if a more comprehensive description of the entire basin is to be attempted.

ACKNOWLEDGEMENTS

The research was supported by FONCYT (Fondo para la Investigación Científica y Tecnológica) and MINCYT (Ministerio de Ciencia, Tecnología e Innovación Productiva) (PID 35/08 Prestamo BID) National University of Río Cuarto and the National Council of Scientific Technical Research (CONICET) and IAEA/RC/17385.

REFERENCES

REFERENCES
APHA (American Public Health Association), AWWA (American Water Works Association) & WPCF (Water Pollution Control Federation)
2005
Standard Methods for the Examination of Water and Wastewater
,
21th edn.
Washington
.
Bécher Quinodóz
F. N.
2014
Implicancias ambientales de las relaciones hidrodinámicas e hidroquímicas entre aguas superficiales y subterráneas en la planicie arenosa del Sur de Córdoba. Argentina (Environmental implications of hydrodynamic and hydrochemical relationships between surface water and groundwater in the sandy plain south of Córdoba. Argentina. 2014)
.
PhD Thesis
,
National University of Río Cuarto (unpublished)
,
Río Cuarto, Córdoba
, p.
412
.
Cabrera
A.
Blarasin
M.
Matteoda
E.
2010
Análisis hidrodinámico, geoquímico e isotópico para la evaluación de sistemas hidrotermales de baja temperatura en la llanura cordobesa (Argentina) (Hydrodynamic geochemical and isotopic analysis for the evaluation of low-temperature hydrothermal systems in Córdoba plain [Argentina])
.
Journal of the Geological and Mining Institute of Spain
121
(
4
),
387
400
.
Cabrera
A.
Blarasin
M.
Dapeña
C.
Maldonado
L.
2013
Composición físico-química e isotópica de precipitaciones del Sur de Córdoba. Estación Río Cuarto-Red Nacional de Colectores. Argentina (Chemical and isotopical rainfall composition in the south of Cordoba Province. Rio Cuarto station. National collector network)
. In:
Groundwater Strategic Resource
(
González
N.
, ed.).
La Plata
,
Argentina
, pp.
35
42
.
Custodio
E.
2005
Técnicas hidroquímicas e isotópicas para el estudio de la relación aguas subterránea agua superficial (Hydrochemical and isotopic techniques for the study of the relationship between surface water and groundwater)
. In:
Hispanic Latin American Seminar II. Current Issues Hydrogeology
(
Blarasin
M.
, ed.).
Río Cuarto
,
Argentina
, pp.
239
248
.
Hancock
P.
Boulton
A.
Humphreys
W. F.
2005
Aquifers and hyporheic zones: towards and ecological understanding of groundwater
.
Hydrogeology Journal
13
,
98
111
.
Iriondo
M. H.
1997
Models of deposition of loess and loessoids in the Upper Quaternary of South America
.
Journal of South American Earth Sciences
10
(
1
),
71
79
.
Parkhurst
D. L.
Appelo
C. A. J.
1999
User's Guide to PHREEQC (version 2). A Computer Program for Speciation, Batch-reaction, one-dimensional Transport, and Inverse Geochemical Calculations
.
US Geological Survey Water-Resources Investigations Report: 99-4259
, p.
312
.