Assesment of shallow groundwater vulnerability in the Las Conchas basin of Entre Ríos using the DRAstiC model

106 ARTICLES RIA / Vol. 37 / N.º 2 Assesment of shallow groundwater vulnerability in the Las Conchas basin of Entre Ríos using the DRAstiC model SASAL MC. 1 ; WILSON MG. 1,2 ; SANTI M. 3 ; OSZUST JD. 2 ; SCHULZ GA. 1 ; PAUSICH G. 1,4 ; BEDENDO D. 1 ABSTRACT The increasing demand of society for health and environmental care requires assessing the potential negative impacts of agricultural practices, including the contamination of natural resources by agrochemicals. The aim of this study was to assess the vulnerability to contamination of an unconfined aquifer located in the highlypopulated area of Arroyo Las Conchas basin, Entre Ríos, (2156.6 km 2 ). To this end, the DRASTIC index with seven hydrogeological parameters was applied and maps were developed for each parameter after analyzing 82 water sources in the basin and selecting 39 water sources in the unconfined aquifer. The unconfined aquifer is superficial, between 0.39 m and 12 m depth and soils are well drained. Vulnerability of the basin to contamination was moderate and homogeneous. The most vulnerable area is located in the extreme NE (near María Grande) and coincides with a flat area of water accumulation. The central-south area is less vulnerable and coincides with a deeper area of the aquifer with low recharge. The information developed provides a basis for environmental management and the development of good agricultural practices. Keywords: DRASTIC, vulnerability, unconfined aquifer, pollution. INTRODUCCIÓN Society is increasingly concerned about preserving natural resources and in recent years agriculture has been identified as responsible for their degradation. There is a need to generate and publish information about the impact of agriculture on water quality in order to respond to the social demand for health and environmental care. Agriculture in Entre Ríos province is becoming increasingly more specialized and homogeneous. However, the study of pollution by agrochemicals is just beginning. Two problems posing a risk to water quality due to agricultural use are expected to occur: on one hand, a diffuse source of agrochemicals as a result of lixiviation or drainage from agricultural lands and, on the other, a point source caused by the inadequate agricultural practices such as filling and washing of pesticides application equipments in surface water, accumulation of empty containers on the farms and aerial spraying of large surfaces with no safeguard of surface water bodies. Assessing potential negative impacts of agricultural practices on the environment is a complex task and requires taking into account climatic, edaphic, geomorphologic, phy siographic, hydrographic and agro-economic factors. An analysis of variations in water quality resulting from the impact of agro-systems and the definition of comprehensive and viable solutions or mitigation measures require understanding the role of water at the local and regional level. Identification and control of the environmental impact is only feasible if functional time and space characteristics of underground and surface flows are defined. The risk of groundwater pollution results from the interaction between the vulnerability to pollution of an aquifer and the load of contaminants applied (Foster et al., 1987). The source of pollutants may be either controlled or modified but not the vulnerability of the environment. The first step towards protecting groundwater is to develop awareness of the scale and severity of the problem. Maximum priority should be given to preventive rather than to corrective measures (Reynoso et al., 2005). Therefore, quantifying the vulnerability 1 Natural Resources and Abiotic Factors Group - INTA EEA Paraná 2 FCA-UNER 3 Hydraulics Office of Entre Ríos 4 FHUC-UNL Received December 29 th 2010 // Accepted April 25 th 2011 // Published on line June 15 th 2011 Assesment of shallow groundwater vulnerability in the Las Conchas basin of Entre Ríos using the DRASTIC model

August 2011, Argentina 107 of aquifers and, based on this information, adopting precautionary measures to avoid their contamination is necessary (Foster et al., 1992; Foster and Hirata, 1991). Numerous methods have been developed to evaluate the vulnerability of aquifers: statistics, simulation models and maps and indexes overlaying, among others. One of the methods that is currently the most widely used to estimate the overall vulnerability to contamination of unconfined aquifers is the DRASTIC Index (ID) developed by the Environmental Protection Agency of the United States (USEPA) (Aller et al., 1987). This method was first published in the US and adopted in Europe and Asia (Martínez et al., 1998; Secunda et al., 2001; Sharadghah, 2001; Thirummalaivasan et al. 2003). The method has also been used in Argentina (Auge, 2003). Reynoso et al. (2005) estimated the vulnerability to contamination of the Pampean aquifer in the north of the Province of Buenos Aires. Massone et al. (2007) also evaluated the vulnerability of unconfined aquifers in piedmont basins of Balcarce, Province of Buenos Aires. The aim of this study was to assess the vulnerability to contamination of an unconfined aquifer in an agricultural area. The basin of the Arroyo Las Conchas in the Province of Entre Ríos was selected. Although, water table availability is insufficient to allow for a significant extraction, it is used in rural areas for domestic purposes and to provide drinking water to animals even though the continuity of its flow is unknown. MATERIALS AND METHODS Site characteristics The selected site was the basin of the Arroyo Las Conchas in the County of Paraná (Entre Ríos). The basin has an area of 2,156.6 Km 2 and a perimeter of 207.6 Km. Its geometry is moderately regular, almost circular. Maximum and minimum levels are 100 m and 13.5 m, respectively. Average basin slope is 14.52 m/km (Hydraulics Office, 2003). The County of Paraná has a humid temperate climate typical of plains which is very favorable for growing grains, oilseeds and dry-land forage crops. Mean annual rainfall is 1000 mm. Temperatures are moderate. Mean annual temperature is 18.5ºC and varies between 25ºC in January and 12ºC in July with a thermal amplitude of 13ºC. One of the most extensive and characteristic physiographic landscapes is the peneplain, that is, a gently-undulating to flat plain modelled by erosion cycles. The existing peneplain is mainly gently undulated and covered by moderately-thick wind-blown materials (loess) interrupted by the presence of colluvium and alluvium sediments filling the streams. The influence of the wind-blown material decreases towards the East where it becomes significantly thinner allowing the outcropping of older, clayey materials from which Vertisols soils have developed (Soil Map, 1998). The flattened or slightly-flattened low hills or cuchillas are the highest area in the County and are stable sectors that developed over clayey sediments of lacustrine origin (Hernandarias Formation). Drainage in the area is poor due to insufficient slopes and slow soil permeability. Predominant soils in the County of Paraná are Mollisols (41%) and Vertisols (36%) with a lower proportion of Alfisols and Entisols (Soil Map, 1998). The hydrogeological profile of the Arroyo Las Conchas basin is shown in Figure 1. The thickness of the quaternary cover (where the water table is found) is observed. Beneath this quaternary cover the aquifers which are mainly used for human, irrigation and industry use are found in the geological formations Ituzaingó (Plio Pleistoce) and Paraná (Miocene) (Santi and Sanguinetti, 2000). Q= Quaternary cover N2 1-2 Ituzaingó formation (semi-confined aquifer) N1 2 Paraná formation (semi-confined aquifer). Figure 1. Geological Formation Paraná-El Pingo (Santi and Sanguinetti, 2000) SASAL MC. 1 ; WILSON MG. 1,2 ; SANTI M. 3 ; OSZUST JD. 2 ; SCHULZ GA. 1 ; PAUSICH G. 1,4 ; BEDENDO D. 1

108 ARTICLES Analysis of the vulnerability of local groundwater flows (water table) using databases of drillings at a greater depth in the Paraná and Ituzaingó formations was ruled out as these are semi-confined aquifers. The continuity of the groundwater flow in the quaternary cover in Entre Ríos (Tezanos Pinto and Hernandarias formations, among others) is still unknown. Measurement of the water table depth was performed in pre-existing wells that were sampled for this study. Of the 82 existing water sources in the Arroyo Las Conchas basin that were surveyed, 39 wells drawing water from the quaternary cover (Figure 2) were selected and the remaining 41 were discarded because they are in the semiconfined Ituzaingó/Paraná aquifer. RIA / Vol. 37 / N.º 2 Parameters of the DRASTIC model Vulnerability of the unconfined aquifer was determined with the DRASTIC method (Aller et al. 1987) by constructing an index (ID) based on 7 hydrogeological parameters that were mapped separately and overlaid to identify areas with different level of vulnerability. The parameters were: depth to the groundwater (D), net recharge (R), aquifer material (A), soil (S), topography (T), impact of non-saturated area (I) and hydraulic conductivity (C). Each of the 7 parameters was valued against the others to determine their relative importance which was expressed as a relative value (weight) varying between 1 and 6 (Table 1), as suggested in the original methodology (Aller et al., 1987). Figure 2. Selected wells and depth (m) of the water table in the Arroyo Las Conchas basin. Assesment of shallow groundwater vulnerability in the Las Conchas basin of Entre Ríos using the DRASTIC model

August 2011, Argentina 109 Table 1. Relative weight of DRASTIC parameters. Table 2. Categories of overall vulnerability to contamination. The most important parameter for vulnerability was assigned a weight of 6 whereas the parameter of least importance was assigned a weight of 1. Overall vulnerability values are shown in Table 2. Parameter D is the depth to the groundwater level. Values used to construct this parameter were obtained by field measurements in existing wells. The water table was measured at each site using a manual probe equipped with a sound and light alarm and water samples were collected with a Van Dorn sampler. Samples were conditioned for testing with a multiparameter probe. The following parameters were analyzed: concentration of nitrate, ammonium, chloride and sodium, ph and electrical conductivity. Each well was geo-referenced to know their geographic distribution. Parameter R is the recharge capacity of the aquifer. The scale varied between 1 and 9 for a recharge variation between 0 and over 25 cm year -1. Recharge values were determined by calculating the hydrological balance for each point. Precipitation values provided by the nearest weather stations were used and infiltration was estimated by the number curve (method developed by USDA to estimate the runoff of small agricultural basins) taking into account the type of soil at each site, soil cover and antecedent soil moisture. Soil cover was determined taking into account if the site was an area with agricultural activity, livestock farming with grassland or pasture, or rangeland. Mean antecedent moisture (Type II) was used as the criterion for antecedent soil moisture. Evapotranspiration values were taken from a 74-year series of the weather station at the Paraná Experimental Station. All infiltrated water was considered to reach the aquifer (since it is an unconfined aquifer). Parameter A is related to the lithology of the aquifer; higher granulometry was considered to provide higher permeability and therefore higher vulnerability. This parameter was constructed using the constituents of the material that formed the soils at each sampling point and applying the extreme values used by Aller et al. (1987). Parameter S is the buffer capacity of the soil to a contaminant due to, basically, two factors: biological activity and soil texture. That is the capacity of soil organisms to metabolize a contaminant load and the soil texture that regulates the speed of lixiviation of a contaminant. In this study, the score assigned to each site was based on the texture of the soil horizon with the greatest limitation to water flow (B-textural horizon). The depth of this horizon and both the content and the type of existing clay were considered. Although soil texture varied between sandy and clayey, most of the sites were in the latter textural class. The range of the score variation assigned to each texture class by USEPA (1985) was extended based on the type and quantity of clay and the depth of the limiting horizon. The assigned scores varied between 3 and 9 for clay contents between 30% and 53%, respectively. The following equation was used: S=0.1739 x clay content (%) 2.2174. Important to note is that the predominant clay in the soil series is an expanding-type clay. A cross section of the soil map was developed at a scale of 1:100,000 to develop the S layer of the ID. Additionally, the type of soil was checked in the field with a soil sampling auger and a spade. Parameter T is the slope of the landscape at the sampling site assuming that less slope implies greater vulnerability. Slope values were taken from topographical sheets (scale: 1:100,000). Parameter I is the effect of existing materials in the nonsaturated area (from the soil to the water level). As the aquifer is not very deep this parameter was considered equal to parameter A. Parameter C is the hydraulic conductivity of the aquifer. Information on the conductivity of the aquifer material was not available. A mean score of 5, equivalent to a conductivity range between 1.5 10-2 and 5 10-2 cm s -1 (corresponding to silty-clayey material), was used. RESULTS AND DISCUSSION Maximum, minimum and mean values of the parameters for the physicochemical quality of the groundwater at the 39 wells selected to apply the model are shown in Table 3. Although the DRASTIC index does not include water quality as a parameter, the information on groundwater that was collected may be considered as a reference for the Province SASAL MC. 1 ; WILSON MG. 1,2 ; SANTI M. 3 ; OSZUST JD. 2 ; SCHULZ GA. 1 ; PAUSICH G. 1,4 ; BEDENDO D. 1

110 ARTICLES RIA / Vol. 37 / N.º 2 Table 3. Physicochemical quality of groundwater at the 39 wells included in the model. of Entre Ríos. Mean values obtained are below the accepted thresholds defined by the World Health Organization for drinking water for human consumption. This information may serve as a baseline for future work on the environmental impact of farming systems and feasibility studies on crop and pasture irrigation or use as drinking water for animal use. Vulnerability of the aquifer Depth of the groundwater varied between 0.39 m and 12 m (mean depth: 3.7 m). In general, groundwater in the entire basin is near the surface, with values for the D parameter between 7 and 10 (Figure 3). The DRASTIC index for the Arroyo Las Conchas basin was between 109 and 151 indicating that the intrinsic vulnerability was moderate (Figure 4). Intrinsic vulnerability to contamination determined by the DRASTIC index was similar for the entire basin due to the work scale applied and because there were no significant differences in the parameters for soil type, hydraulic conductivity, recharge and topography. For this reason, moderate vulnerability categories were subdivided and a new map was drawn to differentiate areas within the basin (Figure 5). Vulnerability of the groundwater was lower in the centralsouth area (near Seguí and Crespo) coinciding with the area of greater depth of the aquifer (Figure 3) and low recharge (Figure 6). The most vulnerable area is located in the extreme NE (near María Grande) coinciding with a flat area where there is water stagnation (Figure 6). Relatively highrecharge values for average historical precipitation (15 to 18 cm year -1 ) were associated to the presence of native vegetation and lagoons. Parameter S (Figure 7) varied from E to Figure 3. Parameter D (groundwater depth) of the DRASTIC index for the Arroyo Las Conchas basin. Assesment of shallow groundwater vulnerability in the Las Conchas basin of Entre Ríos using the DRASTIC model

August 2011, Argentina Figure 4. DRASTIC vulnerability index for the Arroyo Las Conchas basin Figure 5. Moderate vulnerability categories of the DRASTIC index for the Arroyo Las Conchas basin. SASAL MC.1; WILSON MG.1,2; SANTI M.3; OSZUST JD.2; SCHULZ GA.1; PAUSICH G.1,4; BEDENDO D.1 111

ARTICLES RIA / Vol. 37 / N.º 2 Figure 6: Parameter R (recharge of the aquifer) of the DRASTIC index for the Arroyo Las Conchas basin. Figure 7: Parameter S (soil) of the DRASTIC index for the Arroyo Las Conchas basin. Assesment of shallow groundwater vulnerability in the Las Conchas basin of Entre Ríos using the DRASTIC model

August 2011, Argentina W due to different type of predominant soils: Alfisols at the East, and Mollisols at the West along the coast of the Paraná River. These variations were not closely associated to the vulnerability levels due to the moderate weight of parameter S in the DRASTIC index. FINAL CONSIDERATIONS On the maps, the results have identified areas with different levels of vulnerability to contamination of the unconfined aquifer in the Arroyo Las Conchas basin. The use of the DRASTIC model to assess the intrinsic vulnerability to contamination required adjusting the parameters to the particular characteristics of the Entre Ríos environment. The methodology is now available and adequate to use in other basins of interest. The basin has a shallow unconfined aquifer and soils with good drainage due to their granulometric composition. Current use is predominantly agriculture and therefore the risk of contamination by agrochemicals may be high. This risk reaches relevance taking into account that there are highlypopulated towns such as Viale, Crespo, Villa Urquiza, Seguí and María Grande all over the basin. No high vulnerability areas were identified in the assessed basin. Intrinsic susceptibility of groundwater to contamination is moderate although the vulnerability value that was determined is independent of the current presence of contaminants. The risk of contamination or specific vulnerability to nitrogen of this basin is currently being assessed by measuring time-space nitrate variations to identify areas that are relatively more vulnerable and have higher nitrate concentration in groundwater. This is the first vulnerability assessment of an unconfined aquifer reported in Entre Ríos. These results will contribute to the regional planning for sustainable use of soil and water resources and may serve as a basis for environmental management as well as an important tool for territorial management. ACKNOwLEDgEMENTS This study was financed by the Science, Technology and Innovation Agency of Entre Ríos (ACTIER) and by the INTA PR ERIOS- 630021, PE AEGA-221631 y PE PNECO-093012 projects. The authors thank Dr. Hugo Tasi for his collaboration and suggestions. REFERENCES ALLER, L.; BENNET, T.; LHER, J. H.; PETTY, R.J. 1987. DRAS- TIC A standardized system for evaluating groundwater pollution potential using hydrogeologic settings. U.S. EPA Report 600/2-87-035 Ada Oklahoma AUGE, M. 2003. Vulnerabilidad de Acuíferos. E-book. (http://www.tierra.rediris.es/ hidrored/ebooks/vulnerabilidad.htm) DIRECCIÓN DE HIDRÁULICA, PROVINCIA DE ENTRE RÍOS. 2003. Sistema de Información geográfica de los recursos hídricos de Entre Ríos (en CD). FOSTER, S.; HIRATA, R. 1991. Determinación del Riesgo Ambiental de aguas subterráneas, una metodología basada en los datos existentes. Anales de las Terceras Jornadas de Actualización en Hidrología Subterránea. Huerta Grande, Córdoba, marzo de 1994. Centro Panamericano de Ingeniería Sanitaria y Ciencias del Ambiente (CEPIS). FOSTER, S.; ADAMS, B.; MORALES, M. TENJO, S. 1992. Estrategias para la Protección de Aguas Subterráneas, guía para su implementación. CEPIS/PAHO. Lima, Peru. pp.1-91. FOSTER, S.; VENTURA, M.; HIRATA, R. 1987. Contaminación de las Aguas subterráneas, un enfoque ejecutivo de la situación en América Latina y el Caribe en relación con el suministro de agua potable. Centro Panamericano de Ingeniería Sanitaria y Ciencias del Ambiente (CEPIS), Lima, Peru. MARTÍNEZ, M.; DELGADO, P.; FABREGAT, V. 1998. Aplicación del Método DRASTIC para la evaluación del riesgo de afección a las aguas subterráneas por una obra lineal. Jornadas sobre la contaminación de aguas subterráneas: un problema pendiente. Valencia IH-GE. pp. 413-420. MASSONE, H.; QUIROZ LONDOÑO, M.; TOMAS, M.; FER- RANTE, A. 2007. Evaluación de vulnerabilidad de acuíferos libres en cuencas de llanura Periserranas. Case study: Balcarce, provincia de Buenos Aires. V Arentine Hydrogeology Congress (CON- GHIDRO 2007), 16-19 October, Paraná. PLAN MAPA DE SUELOS, CONVENIO INTA-GOBIERNO DE ENTRE RIOS. (1998). Carta de Suelos de la República Argentina, Departamento Paraná, Provincia de Entre Ríos. Acuerdo Complementario del Convenio INTA-Gobierno de Entre Ríos, EEA Paraná, Serie Relevamiento de Recursos Naturales N.º 17, 114 pp. REYNOSO, L.; SASAL, M.C.; PORTELA, S.; ANDRIULO, A. 2005. Vulnerabilidad del acuífero pampeano a la contaminación en el norte de la provincia de Buenos Aires. Aplicación de la metodología DRASTIC. RIA 34, 1, 85-99. SANTI, M.; SANGUINETTI, J. 2000. Estudio de aguas subterráneas Etapa III. Dirección de Hidráulica. Entre Ríos. Consejo Federal de Inversiones. SECUNDA, S.; COLLIN, M.L.; MOLLOUL, A.J. 1998. Groundwater vulnerability assessment using a composite model combining DRASTIC with extensive agricultural land use in Israel s Sharon region. J. Environ. Manag. 54, 39-57. SHARADQHAH, S. 2001. Evaluación del riesgo de contaminación de las aguas subterráneas en Jordania. Aplicación del modelo DRASTIC. Trabajo de Investigación, Universitat Politécnica de Valencia. pp THIRUMALAIVASAN, D.; KARMEGAM, M.; VENUGOPAL, K. 2003. AHP-DRASTIC: software for specific aquifer vulnerability assessment using DRASTIC model and GIS. Environmental Modeling & Software 18: 645-656. SASAL MC. 1 ; WILSON MG. 1,2 ; SANTI M. 3 ; OSZUST JD. 2 ; SCHULZ GA. 1 ; PAUSICH G. 1,4 ; BEDENDO D. 1