Groundwater contamination: risk management and assessment for landfills in Mar del Plata, Argentina

 This study presents a program for risk management in the contamination of groundwater resulting from leachate in landfills at Mar del Plata (Argentina). The program includes prediction, prevention, monitory observation and mitigation as actions that must be taken prior to, during, and after contamination occurs. This routine aids in the identification of weak points and failures of the program for those who are in charge of making decisions, and will allow better use of limited financial and technical resources based on planning, thus minimizing disconnected determinations. Results of the prediction stage were confirmed through monitoring. The presence of a plume of contamination more than 100 m from the landfill was shown, thus proving that implementation of a monitoring plan to follow the development of the contaminated plume must be implemented, particularly in populated areas which depend on residential wells for their water supply. Received: 8 May 2000 · Accepted: 14 August 2000     

Hydrogeochemistry and cation-exchange processes in the coastal aquifer of Mar Del Plata,Argentina

The aquifer of Mar del Plata is unconfined and composed of silt and fine sand. The sand fraction is mainly quartz, potassium feldspars, chalcedony, and gypsum. Volcanic-glass shards (40–60%) dominate the silt fraction, and the clays are of the smectite and illite groups. Calcium carbonate, in caliche form, constitutes about 10–20% of the sediment. Groundwater flow is from west to east, and discharge is in the Atlantic Ocean. Because of overexploitation, the flow direction was reversed in a coastal belt about 3.5 km wide, and this has resulted in seawater intrusion. The groundwater is the CaHCO₃ type in the recharge zone, and becomes NaHCO₃ type towards the discharge area. Salinization by marine intrusion and seawater/fresh-water mixing produces an increase in the major-ion concentrations of the groundwater. The calcium content of the groundwater is higher and the sodium content is lower than those that would be expected if the mixing is considered as just the addition of seawater and fresh water in determined proportions without reactive processes taking place. Hydrogeochemical modeling was applied to the study of hydrogeochemical processes, mainly cation exchange, using the codes NETPATH and PHREEQM. Calcite and gypsum equilibrium, together with cation exchange, are the main hydrogeochemical processes. Cation-exchange capacity of the solid phase was determined by empirical calculations and experimental methods. The affinity order for the groundwater in contact with the aquifer matrix is Ca>Mg>Na in the regional-flow system, but the order is reversed in the salinization process. Reactive transport modeling using the code PHREEQM is useful for analyzing cation exchange in a marine-intrusion process. Electronic Publication     

Distribution and origin of nitrate in groundwater in an urban and suburban aquifer in Mar del Plata,Argentina

The impact of urbanization on groundwater quality is of special concern for water managers dealing with the provision of drinking water to large urban centers. Nitrate is one of the most common contaminants found in urban aquifers. This paper presents a case study aiming at evaluating the distribution and sources of nitrate in an urban aquifer in the city of Mar del Plata, Argentina. Four study zones under different land uses, including a pristine, a semi-rural, an intermediate, and an urban area, were evaluated as a part of this study. The three latter zones are linked by the groundwater flow system. The average nitrate concentration in the pristine area is 6.7 mg/L as nitrate and is over the permissible level of 50 mg/L for drinking water in the other areas. In the semi-rural area it ranges from 39.2 to 107.1 mg/L with an average value of 38.2 mg/L and the nitrate concentration tends to decrease in the intermediate zone to an average value of 38.2 mg/L; however, values above 60 mg/L are also observed there. Then the nitrate concentration in the urban area water is higher than that in the intermediate zonewater ranging from 48.2 to 100.3 mg/L with an average value of 67.3 mg/L. Data on the stable isotopes 15N and 18O in nitrate show that the main sources of nitrate in the study area are manure associated to agriculture uses and cesspools in the semi-rural area, and leakage of the sewage distribution network in the urban area, respectively. This is supported by a previous study which found that 20 % of the water flooding many underground structures in the city came from leakage of the sewage network. No evidence of nitrate attenuation by denitrification was found in the groundwater. This study has shown that aquifers in urban areas can be affected by agricultural activity in the upstream areas and leakage of the sewage network in the urban area.     

Determination of Zn partition coefficient and simulation of reactive transport from landfills in Mar Del Plata, Argentina

The distribution coefficient (Kd) expresses the relationship between the concentration of an element, which is adsorbed in the solid surface and its remaining concentration in the solution. The Kd is a very important factor in reactive transport, representing the source/sink term, and explaining the difference between the velocity of transport of non-conservative elements (Kd>0) and water flow velocity. In this paper, the Kd value for Zn element in loess like sediments forming the Pampeano aquifer is determined and this value is used in the modeling of reactive transport from the landfill of the city of Mar del Plata (Argentina). The determination of Kd value was done by means of batch experiments. The results obtained showed good agreement with Freundlich isotherm, with a value of K_F=300.95 ml g⁻¹ and a super index value b of 0.3961. These values were applied to reactive transport modeling using Visual Modflow code. The Zn plume obtained showed the low mobility of the element in the oxidizing conditions of the environment.     

Origin of Dissolved Groundwater Sulphate in Coastal Plain Sediments of the Rio de la Plata, Eastern Argentina

Groundwater of the coastal plain of the Rio de La Plata, Argentina,contains up to 17 g L^-1 SO₄ and 37 gL^-1 TDS. Some of this SO₄ is from paleo-seawater intrusion; however, SO₄ : Cl ratios can be>2 : 1, and most of the SO₄ must, therefore, have another source. Three possible sources were investigated: gypsum, organic matter,and iron sulphides. Dissolved SO₄ showed δ³⁴S valuesfrom -7 to 0‰, typical values for S from iron sulphides or organicmatter, but distinct from that of seawater (+22‰). To test whetherthe SO₄ was derived from oxidation of reduced S, four 4-mcores were taken from marine sediments of the coastal plain. Two were takenfrom higher, drier areas where the highest dissolved SO₄values were encountered, and two were taken from lower, wetter areas thathad much lower SO₄ concentrations. Pore waterSO₄, Cl and alkalinity were determined; solids were analyzedfor SO₄, sulphide-S and organic-S. Sulphide-S was the dominant form of reduced S, averaging about0.5% S in the lower interval (2.5–4 m) of the cores. Sulphidewas absent in the upper 2.5 m in both topographically higher and lowerareas. Sulphate was present in the entire unit in the higher, drier areas,but almost absent in lower areas. Organic-S was insignificant. Our model for the origin of dissolved SO₄ is: fine-grainedpyrite was oxidized during hotter or drier periods. Some resulting dissolvedSO₄ was precipitated as gypsum. Iron from the pyriteprecipitated as FeOOH. Lower, wetter areas formed over time where recharginggroundwater dissolved most of the gypsum. In higher areas with low hydraulicgradients and high net evapotranspiration, SO₄ remained asgypsum and in the dissolved phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sedimentological maps of the typical loessic units in North Pampa,Argentina

Application of grain size data allowed the construction of maps of two typical loessic formations of North Pampa. A Sand Sea covering the central Argentine plain and a Peripheral Loess Belt to the northeast, compose the Pampean Aeolian System. A region of 20,000 km² in the Loess Belt (Santa Fe and Córdoba provinces) was studied. Thick loess outcrops are located in this region. The Tezanos Pinto Loess, Upper Pleistocene-Lower Holocene in age, and the San Guillermo Fm, Late Holocene in age, are analysed in this article. The silt content has been taken as the main parameter for mapping. In general, the pattern of the isoplethic curves is similar in both maps. These maps show that the silt content in both loessic units progressively increases from SSW to NNE. In the Loess Belt, the silt content increases from 55 to 60% in the SSW (at the transition belt to the Sand Sea) up to 75–80% in the NNE (along a distance of 120–200 km). Other sedimentological data of the outcropping loess-paleosols sequences are presented as complementary information.     

Groundwater arsenic in the Chaco-Pampean Plain,Argentina: case study from Robles county,Santiago del Estero Province

In large parts of rural Argentina people depend on groundwater whose As content exceeds the Argentine drinking water standards (0.05 mg l⁻¹). The most affected areas are located in the Chaco-Pampean Plain, where aquifers comprise Tertiary loess deposits (in the Pampean Plain) and Tertiary and Quaternary fluvial and aeolian sediments (in the Chaco Plain). Robles county is located in the alluvial cone of the Dulce River consisting of loess (aeolian), and gravel, silt, sand and clay (alluvial) deposits. In the shallow aquifers, more than 48% of the 63 studied wells show As at toxic levels (maximum 4.8 mg l⁻¹), while in the deep groundwater the concentration is below 0.05 mg l⁻¹. The pH of the shallow groundwaters range between 6.5 and 9 and generally have high electrical conductivity with mean values of 2072 and 1693 μS/cm⁻¹ in the years 1998 and 1999, respectively. Arsenic concentrations are high in the alkaline Na–HCO₃ type groundwaters, where As correlates positively with Na⁺ and HCO₃⁻. Moreover, As correlates positively with Mo, U, and V, while a negative correlation was observed with Ca²⁺ and Mg²⁺. The potential sources of groundwater As are: (i) layers of volcanic ash with 90% of rhyolitic glass; (ii) volcanic glass dispersed in the sediments; and (iii) clastic sediments of metamorphic and igneous origin. Great lateral variability in the concentration of groundwater As is caused by several hydrogeological and hydrogeochemical factors.     

Estimation of transport hydraulic parameters in loessic sediment, Argentina: Application of column tests

Dispersivity and porosity parameters needed to simulate flow and non-reactive transport were calculated and validated for loess-like sediments of the Pampean aquifer located in southeastern Buenos Aires province, Argentina, using infiltration experiments in laboratory columns filled with selected undisturbed sediment. To obtain values for these parameters, the experimental curves of arrival for chloride were visually adjusted to the curves obtained from numerical simulation using PHREEQC 2.0. Visual adjustment consisted of trying different parameter values until the most representative one was obtained. The best adjustment was obtained for dispersivity and porosity values of 0.01 m and 0.35, respectively. Since analytical solutions for the flow and transport equations are applicable to simplified cases such as this, an attempt was made to adjust the parameters automatically, using a model based on this type of solution. This automated analytical solution was developed using the Fortran programming language, and the automatic parameter calibration was achieved by minimizing the distance between the observed and calculated values of two parameters. The automated technique gave values of 0.0084 m and 0.39 for dispersivity and porosity, respectively. The sediments studied here constitute the uppermost unit of the Pampean aquifer. Given the relevance of this layer and the growing anthropic influence on it, parameter quantification is important for both laboratory tests and for hydrogeochemical simulations that investigate contaminant transport.

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Resumen Se han obtenido los parámetros hidráulicos dispersividad y porosidad efectiva para sedimentos loessoides del acuífero denominado Pampeano cuya extensión abarca casi todo el sector central de la República Argentina. Los sedimentos estudiados en este trabajo se localizan en el sudeste de la provincia de Buenos Aires, Argentina. Los parámetros fueron calculados y validados mediante la obtención de curvas de arribo de cloruro en ensayos en columnas de laboratorio con el sedimento no perturbado, y su ajuste visual a curvas obtenidas en modelos numéricos con el programa PHREEQC 2.0. Los mejores ajustes se alcanzaron con valores de porosidad y dispersividad de 0.01 y 0.35 respectivamente. Asimismo, se implementó una solución analítica del problema en código FORTRAM para lograr el ajuste automático de estos dos parámetros mediante la minimización de las distancias en función de los dos parámetros. Los valores obtenidos fueron: dispersividad 0.0084 m y porosidad efectiva 0.39. Los resultados fueron validados mediante la realización de nuevos ensayos y ajustes numéricos. Los sedimentos aquí estudiados son de importancia por constituir la unidad mas superficial del sistema acuífero Pampeano, la cual es usualmente explotada por perforaciones domiciliarias. La cuantificación de sus parámetros hidráulicos es necesaria para ensayos de laboratorio y simulaciones hidroquímicas orientadas al estudio de la contaminación.

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Résumé Les paramètres de dispersivité et de porosité nécessaires à la simulation des écoulements et des transports non-réactifs, ont été calculés et validés pour des sédiments loessiques de laquifère Pampéien dans le Sud-Est de la province de Buenos Aires, Argentine, au moyen dexpériences dinfiltration sur colonnes expérimentales remplies du sédiment non remanié. Pour obtenir les valeurs des paramètres, les courbes expérimentales de larrivée du chlore (élément conservatif) ont été visuellement calibrées sur les courbes obtenues par la simulation numérique sur Phreeqc 2.0.3 Lajustement visuel consistait à tester différents paramètres jusquà trouver le plus représentatif. Le meilleur ajustement était atteint avec des dispersivités et des porosités respectivement de lordre de 0.01 m et 0.35 m. Dés lors que la solution analytique de lécoulement et du transport est applicable dans des cas simplifiés comme celui-ci, il a également été opté dajuster automatiquement les paramètres, en utilisant un programme Fortran ; le calibrage automatique a été obtenu en minimisant la distance fonction des deux paramètres. Les valeurs obtenues ont été de 0.0084 m et 0.39 pour la dispersivité et la porosité. Le sédiment étudié ici constitue lunité supérieure de laquifère. Cette couche est primordiale notamment du point de vue de lactivité anthropique qui est menée dessus ; la quantification des paramètres de transport est importante pour les tests en laboratoire et les simulations hydrogéochimiques orientées vers les études de transport de soluté.

     

Holocene evolution of the Mar Chiquita coastal lagoon area (Argentina) indicated by pollen analysis

Palynologycal information was integrated with the aim to study vegetation history and environmental changes in Mar Chiquita coastal lagoon area (in the southeast of Buenos Aires province, Argentina) related to sea‐level fluctuations during the Holocene. Prior to 5000 yr BP a mosaic of psammophytic and halophytic communities developed on extensive sandy flats. Between 4000 and 2500 yr BP, during a period of sea‐level stabilisation, similar halophytic communities developed in the south and in the middle part of the present lagoon. Between 4500 and 3500 yr BP hydrophytic and halophytic communities developed in Laguna Hinojales. Beginning 2500 yr BP, when sea level was nearing its present position, modern plant communities became established. Pollen records suggest that littoral evolution of the area was much more complex than previous models have described, introducing new evidence of the geomorphological evolution of Mar Chiquita coastal lagoon. The estuarine environment was delimited by a system of barrier islands instead of a linear prograding barrier, as has been proposed. Copyright © 2005 John Wiley & Sons, Ltd.     

Life cycle and population structure of the polychaeteFicopomatus enigmaticus (Serpulidae) in Mar Chiquita coastal lagoon,Argentina

Ficopomatus enigmaticus, a polychaete inhabiting the subtidal zone in the Mar Chiquita coastal lagoon, builds reef-like aggregates that facilitate silt accumulation. To describe the life cycle, samples were take monthly from September 1984 to September 1985. In contrast with other polychaetes, it has two oocyte generations per year; maturation of the oocytes takes approximately 4 mo. Temperature level and day length regime do not appear as environmental signals to the onset of oogenesis, although they possibly affect oogenesis growth. Two periods of spawning and recruitment were observed in November–December and April–May, when water temperature is above 18°C. Hermaphroditic individuals were not found. Sex ratio was male-biased in all months, significantly in November, February, and August (p<0.001). Monthly size-frequency distributions show distinct year-classes with early (November–December) and late (April–May) recruitment. Early recruited cohorts have two spawning periods during a 24-mo lifespan. Late cohorts have one spawning period during their 20-mo lifespan. The life cycle of this species can be summarized as follows: annual iteroparous—1 to 2 batches of small eggs per female during each lifespan, free spawning, planktotrophic larvae, sedentary suspension-feeder worms.     

Feeding ecology of flatfish juveniles (pleuronectiformes) in Mar Chiquita coastal lagoon (Buenos Aires, Argentina)

During a two-year period in Mar Chiquita coastal lagoon (Buenos Aires, Argentina), the diet of juvenileParalichthys orbignyanus andOncopterus darwinii sorted according to size, area, and season was analyzed. Sampling was carried out using a beach seine net in three zones of different salinity. The small size of the individuals collected and the high percentage of full stomachs, corresponding with high food availability in the lagoon, indicated that Mar Chiquita is used as a nursery ground. The diet ofP. orbignyanus consisted of crustaceans and polychaetes, whereasO. darwinii fed predominantly on small crustaceans (gammarid amphipods, harpacticoid copepods, mysids, and ostracods); polychaetes were less important. Diet of both flatfish species varied with length. Smaller individuals were microphagous, while bigger specimens included larger prey in their diet. Prey taxa number increased for both species, seawards to the mouth of the lagoon, probably related to the salinity gradient. Prey diversity forP. orbignyanus was highest in autumn and spring, and the lowest percentage of stomachs with food occurred in winter. Prey diversity was lowest forO. darwinii in spring and the highest percentage of empty stomachs was found in autumn.     

Upper Oligocene–Miocene clinoforms of the foreland Austral Basin of Tierra del Fuego, Argentina: Stratigraphy, depositional sequences and architecture of the foredeep deposits

The Upper Oligocene–Miocene deposits of the foreland Austral Basin of Tierra del Fuego represent the youngest foredeep fill, developed in front of the adjacent fold and thrust belt. They consist of superbly exposed, sub-horizontal clastic successions of more than 600 m of sedimentary thickness. The study of 11 sections by means of facies analysis and sequence stratigraphic criteria enabled the identification of five depositional sequences (SI–SV), bounded by unconformities (dI-dV) involving hiatuses of different magnitudes. The basal sequence (SI) includes two members: A, mudstone dominated, deposited by cohesive flows; and B, glauconite-rich, sandstone dominated, deposited by episodic turbidity currents. The remaining sequences (SII–SV) are composed of complex arrangements of fine conglomerates, coarse- to fine-grained sandstones, and mudstones that were deposited mainly by hyperpycnal flows. The basal unconformities of the SI to SIV involve minor hiatuses, while that of the SV is a major order unconformity. Two types of clinofom geometries are recognized in the foredeep sequences. Type a clinoforms present a wedge shaped geometry and characterize the foredeep infill during the compressional tectonic regime. Regarding this clinoform type, SI is situated closer to the orogen and shows variations in the bedding dip with development of internal unconformities. SII to SIV are situated towards the foreland and are characterized by subhorizontal conformable beds of large lateral extension. Type b clinoforms, with sigmoidal geometry, show a clear northeast progradation related to a progressive foredeep fill under tectonic quiescence. This clinoform type characterizes the deposits in SV. The recognition of hyperpycnites and different types of clinoform geometries in these sequences incorporates new concepts in reservoir prospects, which are critical for the evaluation of the petroleum system in the Austral Basin.     

Sedimentological characteristics of small rivers with loessic headwaters in the Chaco,South America

Some sedimentological characteristics of fluvial environments located in the northeastern region of the Chaco–Pampa plain were analysed in order to determine the possible influence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 μS cm⁻¹). The mean concentration of suspended sediments was 63.1 and 206.3 mg l⁻¹ in flood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy–silty bottoms predominate in channels, followed by silty–sandy and clayey–silty bottom types. Sediment transport mainly occurs in accelerating flow suspension, overcoming the critical erosion velocity. During flood periods, some parameters exceeded by 70% the values recorded during the low water phase.     

Recent changes in flood risk in the Gran La Plata,Buenos Aires province,Argentina: causes and management strategy

The paper analyses the causes of flood occurrence in the Gran La Plata, Buenos Aires Province, Argentina, and the changes that have occurred since the 1970s. The area is characterized by serious deficiencies in the use and management of water resources. The main factors are: (1) flood risk information is not updated; (2) urban building organization does not consider the flood risk; (3) the topography and other physical features are not taken into account in urban development. This research considers some specific problems, especially in data availability, and suggests ways to solve them, including improved analytical methodology. One of the main objectives is to analyse flood risk in social terms, producing a map of flood risk from the “human social vulnerability” point of view. The results suggest that flood risk has increased since the 1980s and that this is associated with changes in precipitation patterns that have also been aggravated by lack of controls on urban development. The poorer areas with the lowest level of infrastructure and public services, many of them developed over the last quarter-century, are especially vulnerable.     

Cenozoic phosphatic deposits in North Patagonia,Argentina: Phosphogenesis,sequence-stratigraphy and paleoceanography

Paleoenvironmental analysis of the Cenozoic marine section cropping out near Gaiman (Chubut Province, Argentina) shows that most of the local succession was deposited in a shallow, storm-dominated marine environment, bearing well preserved Ophiomorpha fossil traces. Sequence-stratigraphic interpretation records the effect of three sea-level oscillations. Phosphatic strata are related to mostly in situ concretions developed within transgressive-early highstand system tracts (Type 1) and to reworked and winnowed lags associated with transgressive surfaces (Type 2) which display a concentration of phosphatic concretions, ooids, vertebrate bones, teeth and shells. Close association of Callianasid bioturbation (Ophiomorpha) and phosphatic levels suggest a genetic link for both, via improved early-diagenetic water circulation and Fe and P early-diagenetic cycling. Phosphogenesis would have taken place after cold and corrosive water, probably similar to the present Antarctic Intermediate Water (AAIW), flooded the continental shelf and mixed with warmer surficial waters. This mixing could explain the contrasting indications from the marine vertebrate fauna together with the general corrosion and replacement processes that are common in the Gaiman strata. The development of the phosphorites would have occurred at times of global climatic transition and increased oceanic circulation, probably during the Late Oligocene–Early Miocene.     

Recent Tectonics in the Origin and Evolution of Gran Bajo del Gualicho,Argentina

The results of an interdisciplinary research program on the Gran Bajo del Gualicho (Rio Negro, Argentina) are reported. Geological, morphological, gravimetric, and geochemical studies provide the basis for a reconstruction of the possible origin and evolution of this endorheic depression. On the basis of the available data, the hypothesis is advanced that the depression had a tectonic origin (according to NNW-SSE and ENE-WSW tectonic lines) and subsequently was shaped by exogenous forces that obscured the original fault scarps and caused the depression to assume its present characteristics.     

Determining hydraulic connectivity of the coastal aquifer system of La Plata river estuary (Argentina) to the ocean by analysis of aquifer response to low-frequency tidal components

Hydrogeology Journal - Seawater intrusion occurs in almost all coastal aquifers that are subject to human pressure. Its effects could be reduced by avoiding pumping in those wells better...     

The generation at hot springs of sedimentary ore deposits, microbialites and life

Notwithstanding the current fashion which favours an epigenetic origin for what used to be termed SEDEX deposits, there are several lines of evidence to indicate that Phanerozoic base-metal orebodies of this type have at least some exhalative aspects. The fossil polychaete worms, which occur in Lower Carboniferous pyrite mounds at Tynagh and Silvermines in Ireland, have affinities to Paralvinella, an organism that lives attached to hydrothermal chimneys at the Juan de Fuca hot spring site in the Northeast Pacific. In addition, fossil tube worms, and their moulds, occur both in silica masses underlying the Carboniferous giant Red Dog sulphide orebody in Alaska and in Devonian barite and base-metal deposits in North America and in Russia, respectively. The development of sulphide and carbonate fossil microbialites over exhalative centres further supports generation of some mineral deposits on sea or lake floors. Carbonate microbialite mounds are also developing today over warm springs and seepages.The existence of an environment in which sulphide mineralisation developed at the sea floor has implications also in a different sphere. Life itself may have emerged in a similar milieu at 4.2 Ga from iron monosulphide bubbles. A primitive metabolism could have been driven by the high, long-lived and constant, redox potential of 300 mV made available across an iron monosulphide membrane which would have been spontaneously generated where sulphide-bearing, submarine, alkaline springs issued into the acidic, iron-bearing, Hadean ocean. The alkaline spring provided bisulphide to the iron-rich (carbonic) acid ocean for the precipitation of iron-monosulphide bubbles (probotryoids), as well as acetate (Shock, 1992) — the feeder to the biochemical Krebs cycle, driven in reverse by the high partial pressure of CO₂. In addition to its scientific significance, an understanding of these beginnings may well benefit research into many aspects of economic geology.Even more extreme redox contrasts are revealed by the presence of sedimentary jasper or iron formation in three of the major Carboniferous sulphide orebodies in Ireland. Pyritic sulphide microbialites also grew over some of the associated fossil hot-spring sites and may be recognised by their bacteriogenic δ³⁴S values (−20 to −40‰). Recognition of such fossil hot-spring sites could lead to further discoveries of SEDEX deposits.