Lawrence Lowlands Research Articles

Mercury transfer from watersheds to aquatic environments following the erosion of agrarian soils: A molecular biomarker approach

Lake St. Pierre, an important freshwater location for sports and commercial fisheries in Canada, is composed of a 120 km2 stretch of the St. Lawrence River, located at the center of the St. Lawrence Lowlands. Receiving its waters from the St. François, Yamaska, Ottawa, and St. Lawrence Rivers, it is subjected to important inputs of mercury (Hg) and suspended particles eroded from its watershed. This study aims at tracing back the origin of terrigenous Hg loadings to Lake St. Pierre. The specific phenol signatures yielded by a mild oxidation of the terrestrial organic matter (TOM) carried in the water column was used as a tracer to identify the different sources of terrigenous Hg to the lake. Our results demonstrate that most of the Hg bound to suspended particulate matter (SPM-bound Hg) found in Lake St. Pierre is associated with TOM. We were also able to distinguish the relative influence that forested soils, mainly drained by the Ottawa River, and agrarian soils, located on nearby watersheds, exert on the lake's Hg burden. Our data strongly suggest that the erosion of vast areas of agrarian soils, drained by the Yamaska and St. François rivers to Lake St. Pierre, greatly facilitates the transfer of Hg from the watersheds to the lake. This study stresses the need to improve the management of agrarian soils and protect them from extensive erosion in order to preserve the integrity of the fish resources harvested in Lake St. Pierre. Key words: Mercury, soil erosion, Lake St. Pierre, lignin biomarkers

Impact of disturbance on the Holocene development of a temperate peatland (Southern Québec)

A temperate peatland located in the St. Lawrence lowlands (Southern Quebec) was studied in order to specify the past influence of natural and anthropogenic disturbances on its postglacial development. Seven profiles were analysed for pollen, macroremains, microfossils, macro- and microcharcoal, and loss-on-ignition. Fire incidence was important between 9300 and 8200 cal b.p. and during the last 300 years. The Early Holocene fires were contemporaneous with local waterlogged conditions, which prevented the destruction of peat. In contrast, the effects of the recent fires were enhanced by surface dryness resulting from the creation of at least six successive ditch networks over the entire peatland. The modification of the local hydrology and the regular occurrence of fire induced profound vegetation changes. The bryophytic carpet notably regressed in favour of a shrubby coverage, dominated by Chamaedaphne calyculata, Aronia melanocarpa and Betula populifolia. The results obtained suggest that rehabilitation of such an ecosystem is realistic, although relatively urgent in order to preserve the relict populations of endangered species, such as Woodwardia virginica and Dendroica palmarum.

Influence of arbuscular mycorrhizae on soil P dynamics, corn P-nutrition and growth in a ridge-tilled commercial field

Ridge-tilled corn (Zea mays L.) could benefit from arbuscular mycorrhizal (AM) fungi. Under low soil disturbance, AM hyphal networks are preserved and can contribute to corn nutrition. A 2-yr study was conducted in the St. Lawrence Lowlands (Quebec, Canada) to test the effects of indigenous AM fungi on corn P nutrition, growth, and soil P in field cropped for 8 yr under ridge-tillage. Phosphorus treatments (0, 17, 35 kg P ha-1) were applied to AM-inhibited (AMI) (fungicide treated) and AM non-inhibited (AMNI) plots. Plant tissue and soil were sampled 22, 48 and 72 days after seeding (DAS). P dynamics was monitored in situ with anionic exchange membranes (PAEM) from seeding to the end of July. AMNI plants showed extensive AM colonization at all P rates. At 22 DAS, AMI plants had decreased growth in the absence of P inputs, while AMNI plants had higher dry mass (DM) and P uptake in unfertilized plots. The PAEM was lower in the AMNI unfertilized soils in 1998 and at all P rates in 1999, indicating an inverse relationship between P uptake and PAEM. At harvest, grain P content of AMNI plants was greater than that of AMI plants. In 1998, only AMI plants had decreased yield in the absence of P fertilization. In 1999, AMNI plants produced greater grain yield than AMI plants at all P rates. AM fungi improve the exploitation of soil P by corn thereby maintaining high yields while reducing crop reliance on P inputs in RT. Key words: Arbuscular mycorrhizae, ridge-tillage, soil P dynamics, corn, P nutrition

Ten‐year water table recovery after clearcutting and draining boreal forested wetlands of eastern Canada

AbstractIn boreal forested wetlands, the observed increase in the water table level after clearcutting (watering‐up) is often a threat to sustained ecosystem productivity. Hydrologic recovery refers to the processes by which a water table progressively drops back to its initial level after the cut. In eastern Canada, drainage is used operationally after clearcutting wet sites in order to lower the water table level and accelerate hydrologic recovery. The objective of this study was to evaluate the duration of the watering‐up caused by timber harvesting and the extent to which drainage affected the water table recovery on five peatlands and three hydromorphic mineral sites located in the St. Lawrence Lowlands of Québec (Canada). The mixed wood stands studied are dominated by balsam fir (Abies balsamea (L.) Mill.), eastern white cedar (Thuja occidentalis L.), and red maple (Acer rubrum L). Results indicate that, 10 years after clearcutting, water table levels in undrained plots are still 5 to 7 cm higher than the pre‐cut levels. The slight recovery in water table level plateaued after the third year. Rainfall interception by vegetation was also monitored, and after 10 years had reached nearly 50% of the pre‐cut rate. The immediate water table drawdown following drainage mitigated watering‐up within 40 m of a ditch. The persistent watering‐up observed in this study should encourage using sylvicultural systems adapted to boreal forested wetlands in order to prevent productivity loss and stand conversion. Copyright © 2008 John Wiley & Sons, Ltd.

Thermoluminescence Dating of Late Pleistocene Sediments, St. Lawrence Lowland, Québec

This paper presents the results of a thermoluminescence (TL) dating program applied to waterlaid sediments of Late Pleistocene age, collected in the central St. Lawrence Lowland, in eastern Canada. The apparent TL ages were obtained using a partial bleach method (R-Gamma) in which the TL from light-sensitive traps is separated from the total TL. Fluvial (modern), marine (late-glacial) lacustrine (interstadial) and glacio-lacustrine (stadial) sediments yielded apparent ages ranging from 4 to 135 ka, and in general, these ages agreed with available stratigraphic evidence. Whilst testing the zeroing assumption, apparent TL ages obtained from modern fluvial and late glacial sediments suggest that laboratory overbleaching or insufficient natural bleaching may result in over-estimation of the apparent TL ages by 4-5 ka. All samples display anomalous fading, a problem that can be overcome by introducing a three month delay between artificial irradiation and glowing. Future research should focus on sediments for which the age as well as the sedimentology are well documented, such as the late glacial Champlain Sea sediments.

A New Framework for the Pleistocene Stratigraphy of the Central St. Lawrence Lowland, Southern Québec

For three decades, a stratigraphic framework involving one glaciation with two major ice advances (represented by the Bécancour Till and Gentilly Till), separated by one brief interstade (represented by the St. Pierre Sediments), has been invoked to explain the lithostratigraphic succession of Pleistocene sediments exposed in the St. Lawrence Lowland of southern Québec. New exposures found along the bluffs of the St. Lawrence River and recent borehole data provide evidence that the Pleistocene depositional sequence is the result of three glacial advances and two nonglacial events, each represented by organic-bearing units. Two lithostratigraphic units (Lotbinière Sand and Lévrard Till) and three climatostratigraphic units (St. Lawrence Stade, Grondines Interstade and Les Becquets Interstade) are introduced in the stratigraphie nomenclature. No definite age can be assigned to the lowermost till (Bécancour?) but it is now believed to be pre-Sangamonian. Field observations and geochronological data suggest the lower and upper interstadial sediments, and an intervening glacial unit represent brief but severe environmental changes that occurred at the beginning of the Wisconsin Glaciation, ca. 90-70 ka BP. This sequence may correlate with marine isotope stage 5a, stage 4, and the earliest part of stage 3. The age of the onset of the last glacial advance (Gentilly Till) is problematic, possibly ranging from 60 to 30 ka BP.

Lsotopic and Palynological Records of the Late Pleistocene in Eastern Canada and Adjacent Ocean Basins

Correlations of isotopic and palynological records from deep sea cores in Baffin Bay and Labrador Sea with terrestrial palynological sequences, supported by a few Th/U chronological controls, allow the establishment of a regional climatostratigraphic scheme for the Late Pleistocene climatic fluctuations in eastern Canada. During the climatic optimum of isotopic substage 5e, warmer conditions than present prevailed both on land and in oceanic surface water masses. The 5e/5d transition is marked by an abrupt shift in 818O values in Baffin Bay and Labrador Sea as a consequence of ice growth over circumpolar areas of northeastern Canada. From substage 5d to substage 5a, the Baffin Bay border lands experienced glacial conditions while subarctic to cool-temperate and humid climates persisted over Labrador Sea and Atlantic Canada. A short (<104yrs) stage 4 is recorded in deep sea cores with high 8'8O values. It corresponds to the Early Wisconsinan southward extension of the Laurentide Ice Sheet dated at ca. 80,000 yrs in the central St. Lawrence Lowland. There is no clear evidence of full glacial conditions in the Atlantic Provinces during this episode. Stage 3 (Middle Wisconsinan) corresponds in the isotopic records to large oscillations in 818O values suggesting meltwater transits in both Baffin Bay and Labrador Sea. The ice cover remained relatively extensive over eastern Canada, although some areas experienced ice-marginal conditions : in the Atlantic Provinces, notably on Cape Breton Island, hemiarctic to subarctic climate is inferred from palynological records; in the Appalachian foothills of Québec glacial Lake Gayhurst developed some 46,000 yrs ago. During isotopic stage 2 (Late Wisconsinan), the Laurentide Ice Sheet reached its maximum extent while satellitic ice-caps developed over the Atlantic Provinces. In deep sea cores, high 818O values mark the full glacial conditions of isotopic stage 2, although slightly lower values in western Labrador Sea indicate discrete but continuous meltwater influxes. An early melting phase of the northeastern margin of the Laurentide Ice Sheet is recorded shortly after 16,700BP. The full ice-retreat is observed after ca. 11,000BP. At that time, southeastern Canada was already largely ice-free. Finally, the optimum climatic conditions of isotopic stage 1 settled diachronously in the adjacent basins of eastern Canada.

The Sangamonian Pointe-Fortune Site, Ontario-Québec Border

Inter-till sediments (Units 2, 3, 4) in a sand pit located 115 km east of Ottawa, Ontario, contain a predominance of deciduous tree pollen including oak. elm, beech and hickory {Quercus, Ulmus, Fagus, and Carya) and minor amounts of basswood, ash and sweetgum (TiHa, Fraxinus and Liquidambar). Unit 4 also contains macrofossils of several plant taxa which presently do not grow much north of the site. The fossils portray an inter-glacial environment (the Sangamonian) with conditions as warm as or warmer than the present in the area. By contrast, overlying Unit 4a reveals a dominance of boreal indicators such as pollen of spruce, pine, willow and alder (Picea, Pinus, Salix and Alnus) and the beetles, Bembidion transparens, Eucnecosum, and Olophrum boréale. Unit 4a fossils indicate a climate that was colder than at present but no colder than the climate of central Québec - suggesting a correlation with the waning phase of the warm interval, or alternatively, with the St. Pierre Interstade of the St. Lawrence Lowlands. Thus the lower till is interpreted as lllinoian in age; the upper till may be Middle to Late Wisconsinan or Early to Late Wisconsinan.

Evidence from the Lake Champlain Valley for a later onset of the Champlain Sea and implications for late glacial meltwater routing to the North Atlantic

Ocean circulation models indicate that freshwater runoff from the North American continent during the last deglaciation may have had an effect on North Atlantic Ocean circulation, and thereby have altered regional climate. One such example is a flood from Lake Agassiz, which has been proposed by previous workers to have caused the onset of the Younger Dryas at around 12,850 calibrated years B.P. by entering the North Atlantic through the Gulf of St. Lawrence. We present two radiocarbon ages from terrestrial organic sources in the Champlain Valley that we associate with the pre-Champlain Sea proglacial lake phases; a musk-ox bone with an AMS age of 11,362 ± 115 14C years B.P. (13,438–13,020 calibrated years B.P.), and a wood fragment with an AMS age of 10,901 ± 76 14C years B.P. (12,995–12,793 calibrated years B.P.). These ages together with a glacial lacustrine varve estimate suggest that the initiation of the Champlain Sea may not have occurred until after the onset of the Younger Dryas. If the Lake Agassiz flood event occurred before the opening of the Champlain Sea then the floodwaters would have been diverted down the Champlain and Hudson valleys to the North Atlantic. Another possibility is that the Agassiz flood may have been contemporaneous with the opening of the Gulf of St. Lawrence. It is also possible that there may have been a large enough flood produced by meltwater originating in the Champlain Valley and St. Lawrence Lowlands at the inception of the Champlain Sea to have affected ocean circulation without the influence of Lake Agassiz.

Paleostress Analysis of Atlantic Crustal Extension in the Quebec Appalachians

Abstract A paleostress analysis using normal fault slip data and Jurassic dike trends has been conducted in the Quebec and New England Appalachians. Stress tensor analysis of fault slip by a numerical inversion method has distinguished homogeneous and directionally consistent E‐W‐ and NW‐SE‐oriented extensional stress regimes in the Quebec Appalachians and St. Lawrence Lowlands. The directions of extension compare well with the mean direction of extensional stress inferred from the Jurassic dolerite dike trend in New England. Both extensions are interpreted as the result of rift‐related stress associated with the separation of North America and Africa, early in the Late Triassic–Early Jurassic fragmentation of Pangea. The paleostress field configuration is assumed to be related to the direction of divergent plate motion. The N‐S and NE‐SW Late Proterozoic faults that bordered the Quebec Reentrant and Taconian‐Acadian structures of the Quebec Appalachians appear to have played a significant role in the dis...

The late Quaternary stratigraphic record northwest of Montréal: regional ice-sheet dynamics, ice-stream activity, and early deglacial events

The Quaternary sediments of previously unstudied buried valleys and sections near Montréal are analyzed and other sites are revisited to further develop the stratigraphic framework of the St. Lawrence Lowland and to establish regional glacial and deglacial models. The southwest-trending buried valleys were investigated by stratigraphic drilling and high-resolution seismic profiling. The Quaternary succession consists, from base to top, of proximal glaciolacustrine sediments, two superposed till sheets (Argenteuil and Oka tills) of inferred Late Wisconsinan age, and Champlain Sea sediments. The glacial sediments of this sequence record an ice advance toward south (Argenteuil Till) followed by an abrupt ice-flow shift toward the southwest (Oka Till). Compositional and geomorphic data indicate that Oka Till is ubiquitous and is associated with a regional set of glacial landforms. The analysis of a regional digital elevation model in combination with published ice-flow indicators shows convergent flow patterns from the Ottawa–Montréal–Adirondack regions toward the Lake Ontario basin. Landforms produced by the inferred ice stream are locally crosscut by southward-trending ice-flow features. Hence southward flow in the upper St. Lawrence Valley seemingly took place in two distinct contexts: (1) during full glacial conditions, as ice margins stood at or near the late glacial maximum limits, and (2) during late deglaciation, as a post-ice stream reequilibration mechanism. Early deglacial events in the study area were also characterized by subglacial meltwater channelling and erosion along the valleys, subaquatic outwash deposition in glacial Lake Candona, and rapid infill of the valleys during the early stages of the ensuing Champlain Sea.

Hydrogeochemistry and groundwater origin of the Basses-Laurentides sedimentary rock aquifer system, St. Lawrence Lowlands, Québec, Canada

A comprehensive hydrogeochemical study was carried out in the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Quebec over a 1500 km2 study area. Groundwater samples were collected at 153 sites, characterizing all geological and hydrogeological units to a maximum depth of 140 m. Groundwater was analyzed for major, minor and trace inorganic constituents, stable isotopes δ 2H, δ 18O, and δ 13C of dissolved inorganic carbon (DIC), and some samples were analyzed for 3H, and 14C of DIC. The regional distribution of groundwater types shows that the hydrogeological conditions exert a dominant control on the major ions chemistry of groundwater. Preferential recharge areas are characterized by tritiated Ca-Mg-HCO3 groundwater, and confined conditions by submodern Na-HCO3 and Na-Cl groundwater types. Two groundwater end-members are identified in the aquifer system, modern meteoric water and Pleistocene Champlain Sea water. The region displays significant variations of groundwater geochemistry and quality controlled by glaciation, Champlain Sea invasion, lithological rock diversity, and flow system scales. This situation leads to varied groundwater types and origins within a restricted area.

Quantitative Assessment of Regional Rock Aquifers, South-Western Quebec, Canada

The population of the St. Lawrence Lowlands platform in south-western Quebec, Canada, is heavily dependent on groundwater. The present study summarizes the quantity, quality and sustainability estimations of the groundwater resources found mainly in sedimentary rock aquifers. Results show that the regional groundwater flow in the considered domain of 1,500 km2 is 97.7 Mm3/y with: 86.6% infiltration from precipitation, 9.6% inflow from neighboring aquifers, and 3.8% induced recharge. The regional sustainability is defined with simulated drawdowns from uniform withdrawal scenarios. The current withdrawal rate of 18 Mm3/y results in median drawdown of 0.6 m, compared to pre-development conditions. This drawdown is situated well in the sustainable range, an indication that regional aquifers are not overexploited. Hypothetical pumping rate of 24 Mm3/y resulting in an average drawdown of 2.2 m is estimated as sustainable limit. Increasing exploitation from 24 to 51 Mm3/y would need tight control and planning. Pumping rates beyond 51 Mm3/y are judged not sustainable as regional drawdowns become high, ∼8 m. The water levels in recharge areas are the most sensitive to groundwater extraction. Combining drawdown maps, groundwater quality zones, and aquifer vulnerability to surface contamination delineated the areas most suitable for future groundwater developments.

"Nindoodemag": The Significance of Algonquian Kinship Networks in the Eastern Great Lakes Region, 1600-1701

N the summer of 1701, the twelve hundred French residents of Montreal played host to some thirteen hundred Native American visitors from communities throughout the Saint Lawrence lowlands and Great Lakes region. They had gathered to ratify a peace agreement, carefully constructed during a decade of difficult and complex negotiations, which was intended to end conflicts among the Haudenosaunee (the Iroquois Confederacy) and the French and their native allies. As Gilles Havard has illustrated in The Great Peace ofMontreal, the ratification ceremony on August 4 concluded a two-week-long trade fair at a spectacular grand council. There the amalgam of European and Native American diplomatic protocols created a hybridized feast for the senses: the scent of tobacco burning in peace pipes mingled with powder and perfume as the members of the assembly, wearing their finest in dress and adornment, listened to the French and Native Americans give elaborate performances drawn from their respective oratorical traditions. Exchanges of gifts, from wampum and beaver pelts to bread and wine, punctuated the speeches.1 The relationships forged and strengthened as

Numerical simulation of groundwater flow in regional rock aquifers, southwestern Quebec, Canada

The St. Lawrence Lowlands platform, Quebec, Canada, is a densely-populated area, heavily dependent on groundwater resources. In 1999, the Geological Survey of Canada initiated a large-scale hydrogeological assessment study over a 1,500 km2 region northwest of Montreal. The objectives were to define the regional groundwater flow, and to give quantitative estimates of the groundwater dynamic parameters and of the available groundwater resources. The applied approach consisted of defining the hydrogeologic framework, hydraulic properties of the aquifer units, and groundwater dynamic components. Lower Paleozoic sedimentary rocks represent regional aquifer units. Coarse Quaternary fluvio-glacial sediments locally overlay the rock sequence and constitute an interface aquifer unit. Fine marine sediments confine most of the regional aquifers. Collected GIS based information was synthesized in a finite element numerical model. The regional saturated steady-state flow was calibrated under current stress conditions assuming an equivalent porous medium approach. Water budget calculations show that the total groundwater flow in regional aquifers amounts to 97.7 Mm3/y. Infiltration from precipitation provides 86.6% of the groundwater supply, while 9.6% comes from subsurface inflow and the remaining 3.8% is induced recharge from surface waters. Discharge from regional aquifers occurs through flow to streams (76.9%), groundwater withdrawal (18.4%), and underground outflow (4.7%).

Contribution aux recherches sociales du Québec par une étude des variations régionales du nombre des vocations sacerdotales

This study, one of the first ever published in that field of research, is devoted to the geography of sacerdotal vocations and is divided in two parts. The first part deals with the differential rates of sacerdotal vocations which characterize some regions of the Province of Québec. The author bas calculated « index numbers of sacerdotal fecundity », and he studies the variability of these indexes within the limits of the diocese of Trois-Rivières. The data show that, before 1950, the St. Lawrence Lowlands region bas given a greater number of priests than the Valley of the St. Maurice River and a much greater number than the Laurentian Uplands. The second part of the article is more elaborated and treats of some « exterior » conditions which usually explain the number of vocations and justify the different contributions of various parts of the diocese. The various factors examined are demographic, familial, religious, economic, social, academic and historical. On this basis, the author formulates a series of observations which, taking into account the limited value of conclusions reached in social research, seem valuable for the understanding of the specific subject of this paper but also of many other problems concerning the social geography of the Province of Québec.

L’évolution récente de l’utilisation du sol sur la rive nord du Saint-Laurent entre Québec et Montréal

In recent years, the face of much of rural Québec bas undergone a series of important transformations : vast areas of marginal farmland have either been abandoned or reforested ; large tracts of cleared land in the Saint Lawrence lowlands have been converted from general farming to specialty crops ; several of these areas of specialty crops are now being swallowed up by urban expansion ; the Laurentide hills and large sections of the Eastern Townships area are rapidly being transformed into sprawling tourist playgrounds. With the aid of 1964 air photos and 1965 land use data the authors record and briefly discuss some of the more striking trends in the evolution of rural land use patterns in the area between Montréal and Québec City, on the north shore of the Saint Lawrence River.

14C chronology for ice retreat and inception of Champlain Sea in the St. Lawrence Lowlands, Canada

AMS radiocarbon cross-dating of plant debris and marine shells trapped in a lake basin on Mount St. Hilaire (Québec, Canada) provides a direct assessment of a reservoir effect totaling ca. 1800 14C years during the early stage of Champlain Sea. Pollen-based extrapolation of bottommost ages on terrestrial plant macrofossils in sediments of this lake, and of another lake nearby support an estimate of 11,100 ± 100 14C yr B.P. for marine invasion in the Central St. Lawrence River Lowlands. Results indicate a 400–1000 years younger regional chronology of ice retreat, now congruent with the one inferred from the New England varve chronology. This is a summary of a longer paper to be published in French.

δ15N in riverine food webs: effects of N inputs from agricultural watersheds

We examined the use of the natural abundance of nitrogen stable isotopes (δ15N) as a tracer of anthropo genic perturbations of the nitrogen (N) cycle at the watershed scale in 82 river sites draining 13 watersheds in the St. Lawrence Lowlands in Quebec. Mean δ15N values of aquatic primary consumers varied greatly among sites (+2‰ to +15‰), most of this variation (88%) being attributable to site effects. Variation in δ15N values among functional feeding groups of primary consumers within sites was comparatively lower (<1‰). Within watersheds, δ15N values of primary consumers (and organisms of higher trophic levels) tracked longitudinal changes in the percentage of agricultural area. Overall, the percentage of total watershed area under agriculture explained up to 69% of the variation in mean primary consumer δ15N values. Similar positive correlations were observed for predatory invertebrates and non-piscivorous fish. In general, our results show that δ15N in riverine food webs reacts strongly to spatial patterns in the intensity of N inputs related to agricultural land use.

Assessing rock aquifer vulnerability using downward advective times from a 3D model of surficial geology: A case study from the St. Lawrence Lowlands, Canada

Se verifica en St. Lawrence Lowlands en SW Quebec un método de evaluación de vulnerabilidad acuífera, el cual relaciona directamente Ia vulnerabilidad con el tiempo advectivo descendiente (DAT) a partir de un modelo geológico 3D. El objetivo fue evaluar Ia vulnerabilidad del acuífero regional, el cual es sobreyacido por unidades Cuaternarias discontinuas y no consolidadas. Parámetros hidrogeológicos como Ia recarga y la porosidad de cada unidad sobreyaciente fueron integrados a una malla estratigráfica gOcad. Esta malla fue generada a partir de un modelo geológico hecho con superficies entrelazadas representando las fronteras de las unidades Cuaternarias y Ia topografía del basamento. Se hicieron cálculos usando 1m enfoque determinístico y valores de DAT fueron obtenidos para el 74% del área en estudio. Los resultados son agrupados en 6 clases DAT, los cuales son interpretados en términos de un índice de vulnerabilidad relativa. La distribución espacial de este índice concuerda con el escenario hidrogeológico y los datos hidrogeoquímicos disponibles. Los resultados indican que 40% del área evaluada cae en clases de moderadamente alta a muy alta vulnerabilidad. Una comparación entre este mapa y no DRASTIC de Ia misma área revelan algunas discrepancias significantes, particularmente en áreas de profundidad somera a! basamento, las cuales son caracterizadas por rápidos DAT a través de Ia delgada zona vadosa. En estas áreas DRASTIC produce vulnerabilidades bajas mientras esta evaluación da valores altos de vulnerabilidad. Esta valoración provee bases legítimas para una planificación regional y !a toma de decisiones.doi: sin doi