Water Level Drawdown Research Articles

Effects of an Experimental Water-level Drawdown on Methane Emissions from a Eutrophic Reservoir

Reservoirs are a globally significant source of methane (CH4) to the atmosphere. However, emission rate estimates may be biased low due to inadequate monitoring during brief periods of elevated emission rates (that is, hot moments). Here we investigate CH4 bubbling (that is, ebullition) during periods of falling water levels in a eutrophic reservoir in the Midwestern USA. We hypothesized that periods of water-level decline trigger the release of CH4-rich bubbles from the sediments and that these emissions constitute a substantial fraction of the annual CH4 flux. We explored this hypothesis by monitoring CH4 ebullition in a eutrophic reservoir over a 7-month period, which included an experimental water-level drawdown. We found that the ebullitive CH4 flux rate was among the highest ever reported for a reservoir (mean = 32.3 mg CH4 m−2 h−1). The already high ebullitive flux rates increased by factors of 1.4–77 across the nine monitoring sites during the 24-h experimental water-level drawdown, but these emissions constituted only 3% of the CH4 flux during the 7-month monitoring period due to the naturally high ebullitive CH4 flux rates that persist throughout the warm weather season. Although drawdown emissions were found to be a minor component of annual CH4 emissions in this reservoir, our findings demonstrate a link between water-level change and CH4 ebullition, suggesting that CH4 emissions may be mitigated through water-level management in some reservoirs.

Local climate change induced by groundwater overexploitation in a high Andean arid watershed, Laguna Lagunillas basin, northern Chile

The Laguna Lagunillas basin in the arid Andes of northern Chile exhibits a shallow aquifer and is exposed to extreme air temperature variations from 20 to −25 °C. Between 1991 and 2012, groundwater levels in the Pampa Lagunillas aquifer fell from near-surface to ~15 m below ground level (bgl) due to severe overexploitation. In the same period, local mean monthly minimum temperatures started a declining trend, dropping by 3–8 °C relative to a nearby reference station. Meanwhile, mean monthly maximum summer temperatures shifted abruptly upwards by 2.7 °C on average in around 1996. The observed air temperature downturns and upturns are in accordance with detected anomalies in land-surface temperature imagery. Two major factors may be causing the local climate change. One is related to a water-table decline below the evaporative energy potential extinction depth of ~2 m bgl, which causes an up-heating of the bare soil surface and, in turn, influences the lower atmosphere. At the same time, the removal of near-surface groundwater reduces the thermal conductivity of the upper sedimentary layer, which consequently diminishes the heat exchange between the aquifer (constant heat source of ~10 °C) and the lower atmosphere during nights, leading to a severe dropping of minimum air temperatures. The observed critical water-level drawdown was 2–3 m bgl. Future and existing water-production projects in arid high Andean basins with shallow groundwater should avoid a decline of near-surface groundwater below 2 m bgl and take groundwater-climate interactions into account when identifying and monitoring potential environmental impacts.

Annual variation of landslide stability under the effect of water level fluctuation and rainfall in the Three Gorges Reservoir, China

Landslides in the Three Gorges Reservoir (TGR) are widely distributed and are a serious threat to the environment and the local people. Since the impoundment of the reservoir in 2003, many of the landslides have been reactivated which were triggered by water level fluctuation and rainfall. Taking the Maliulin landslide in the TGR as a case study, field investigation and displacement monitoring are conduced to study the characteristics of the landslide. According to the annual variation, the fluctuation of reservoir water is divided into four periods. The cumulative rainfall corresponding to different rainfall return periods is computed by Gumbel model. The variation of landslide stability and failure probability under the effect of water level fluctuation and rainfall in a complete annual cycle is calculated in terms of the Morgenstern–Price and Monte Carlo model. Based on the monitoring by inclinometer, a secondary shallow sliding surface is detected which controls its current activities. The annual variation of landslide stability tends to coincide with the change of reservoir water level. The minimum factor of safety occurs during the period of water level drawdown. Combining with the effect of extreme rainfall 50-year return period and water level dropdown, the calculated minimum factor of safety is below unit and the landslide is unstable. The scenario of annual failure probability of landslide is completed in the paper that is the basis for further risk evaluations.

Ecological impacts of winter water level drawdowns on lake littoral zones: a review

Freshwater littoral zones harbor diverse ecological communities and serve numerous ecosystem functions that are controlled, in part, by natural water level fluctuations. However, human alteration of lake hydrologic regimes beyond natural fluctuations threaten littoral zone ecological integrity. One type of hydrologic alteration in lakes is winter water level drawdowns, which are frequently employed for hydropower, flood control, and macrophyte control, among other purposes. Here, we synthesize the abiotic and biotic responses to annual and novel winter water level drawdowns in littoral zones of lakes and reservoirs. The dewatering, freezing, and increased erosion of exposed lakebeds drive changes in the littoral zone. Shoreline-specific physicochemical conditions such as littoral slope and shoreline exposure further induce modifications. Loss of fine sediment decreases nutrient availability over time, but desiccation may promote a temporary nutrient pulse upon re-inundation. Annual winter drawdowns can decrease taxonomic richness of macrophytes and benthic invertebrates and shift assemblage composition to favor taxa with r-selected life history strategies and with functional traits resistant to direct and indirect drawdown effects. Fish assemblages, though less directly affected by winter drawdowns (except where there is critically low dissolved oxygen), experience negative effects via indirect pathways like decreased food resources and spawning habitat. We identify eight general research gaps to guide future research that could improve our understanding about the complex effects of winter drawdowns on littoral zone ecology.

Perturbation with a constant drawdown of the underground water level in the perturbing well

On the basis of the method of integral relationships, an approximate solution of the problem of groundwater filtration in pressure aquifers has been obtained when they are tested with pumpings. The solution obtained is applicable for moments of time less than two orders than the approximate representation of the exact solution on which the existing technique for interpreting the results of the experimental filtration testing of horizons is based.

An experimental investigation on slope stability under drawdown conditions using transparent soils

This paper presents an experimental investigation into slope failure during water-level drawdown using transparent soil. The internal characteristics of slope failure are not well-known due to the limitations of the techniques used in the experiments conducted to date. In this study, transparent soil is used to visualize the process of slope failure. We developed a water-level control system to implement simulation of the drawdown of the water level at various speeds and used a charge-coupled device camera to capture images during the entire slope failure process. Particle image velocimetry was used to measure the displacement of the sand particles and identify the sliding zones. The flow paths of the fluid inside the slope were illuminated using an organic dye. The results show that the slope failure process can be divided into two stages: surface and overall sliding. The overall sliding of the slope is caused by the gradual development of partial instability, and the failure mode is a cyclic failure. The slope angle is different above and below the water level during the process of sliding. In our experiments, the slope angle is about 20° above the water level, which is the same as the final stable slope angle, and about 35° below the water level, which is the same as the initial slope angle. This means that the drawdown influences the angle above the water level but has little influence on the angle below the water level. The results of this paper provide a better understanding of the physical behavior and failure mode of soil slopes caused by drawdowns near the coast.

Stability Analysis of Hydrodynamic Pressure Landslides with Different Permeability Coefficients Affected by Reservoir Water Level Fluctuations and Rainstorms

It is significant to study the variations in the stability coefficients of hydrodynamic pressure landslides with different permeability coefficients affected by reservoir water level fluctuations and rainstorms. The Sifangbei landslide in Three Gorges Reservoir area is used as case study. Its stability coefficients are simulated based on saturated-unsaturated seepage theory and finite element analysis. The operating conditions of stability coefficients calculation are reservoir water level variations between 175 m and 145 m, different rates of reservoir water level fluctuations, and a three-day continuous rainstorm. Results show that the stability coefficient of the hydrodynamic pressure landslide decreases with the drawdown of the reservoir water level, and a rapid drawdown rate leads to a small stability coefficient when the permeability coefficient ranges from 1.16 × 10−6 m/s to 4.64 × 10−5 m/s. Additionally, the landslide stability coefficient increases as the reservoir water level increases, and a rapid increase in the water level leads to a high stability coefficient when the permeability coefficient ranges from 1.16 × 10−6 m/s to 4.64 × 10−5 m/s. The landslide stability coefficient initially decreases and then increases as the reservoir water level declines when the permeability coefficient is greater than 4.64 × 10−5 m/s. Moreover, for structures with the same landslide, the landslide stability coefficient is most sensitive to the change in the rate of reservoir water level drawdown when the permeability coefficient increases from 1.16 × 10−6 m/s to 1.16 × 10−4 m/s. Additionally, the rate of decrease in the stability coefficient increases as the permeability coefficient increases. Finally, the three-day rainstorm leads to a significant reduction in landslide stability, and the rate of decrease in the stability coefficient initially increases and then decreases as the permeability coefficient increases.

The impact of long-term water level draw-down on microbial biomass: A comparative study from two peatland sites with different nutrient status

We examined the effects of long-term (51 years) drainage on peat microbial communities using phospholipid fatty acid (PLFA) analysis. We analysed the peat profiles of natural and adjacent drained fen and bog sites. Viable microbes (i.e. microbial PLFA) were present in relatively large amounts even in the deepest peat layers of both peatland sites, a finding that warrants further investigation. Microbial biomass was generally higher in the fen than in the bog. Microbial community structure (indexed from PLFA) differed between the fen and bog sites and among depths. Although we did not exclude other factors, the effect of drainage on the total microbial biomass and community structure was not limited to the surface layers, but extended to the deepest layers of the fen and bog. Long-term drainage increased the total microbial PLFA biomass in the surface, subsurface and bottom layers of the fen, but decreased it in the surface and bottom layers of the bog site. Drainage also increased the characteristic FAs of Gram-positive and Gram-negative bacteria in the surface and subsurface layers of the fen, and decreased them in the bottom layers of the bog site. The characteristic fungal FA was only reduced in the surface layers of the bog site by drainage. Thus, by affecting the microbial community beyond the surface layers, long-term peatland water-level draw-down can alter the microbial contribution to deeper peat organic matter stabilization. This suggests that long-term drainage may have a more significant climate change effect than revealed by the surface layer analyses alone.

Discussion on the Safety Factors of Slopes Recommended for Small Dams

The design and assessment of the slope stability of small embankment dams is usually not carried out using slope stability calculations but rather by the comparison of proposed or existing dam slopes with those recommended by technical standards or guidelines. Practical experience shows that in many cases the slopes of small dams are steeper than those recommended. However, most of such steeper slopes at existing dams do not exhibit any visible signs of instability, defects or sliding. For the dam owner and also for dam stability engineers, the safety of the slope, expressed e.g. via a factor of safety, is crucial. The aim of this study is to evaluate the safety margin provided by recommended slopes. The factor of safety was evaluated for several dam shape and layout variants via the shear strength reduction method using PLAXIS software. The study covers various dam geometries, dam core and shoulder positions and parameter values of utilised soils. Three load cases were considered: one with a steady state seepage condition and two with different reservoir water level drawdown velocities – standard and critical. As numerous older small dams lack a drainage system, variants with and without a toe drain were assessed. Calculated factors of safety were compared with required values specified by national standards and guidelines.

Study on formation and expansion condition of hidden soil cavity under condition of groundwater exploitation in karst areas

This paper uses an analytical method to analyze soil cavity forming conditions and the associated mechanics. On this basis, the paper studies the soil cavity expansion rule and reveals the nature of the soil cave collapse from the mechanism angle. The results show the following: (1) the spalling force caused by pumping at different points around a soil cavity varies. The nearer the point to the pumping well, the easier it is for the soil cavity to expand. (2) The nearer the point to the soil cavity roof, the greater the spalling force, indicating that the roof is easier to expand outward than a wall. (3) The expansion condition of a soil cavity is that the spalling force is greater than the cohesive force of the soil mass. With the expansion of the soil cavity, the spalling force increases. When the soil cavity begins to expand outward, the soil cavity shows trends of development due to the increasingly larger spalling force. (4) The spalling force influencing the expansion of the soil cavity is proportional to the pump drawdown and hydraulic gradient. Thus, the water level drawdown and flow must be controlled while pumping to control generation of soil cavities. The research results of soil cavity forming processes can be used as a theoretical reference for controlling karst collapse in the region.

Slope Stability Analysis of an Earth Dam

The study of slope stability is important in the design and construction of the earth dams under influence of earthquake and some surcharge loads. Some factors affect the slope stability for the earth dam such as change the water level in the reservoir or rapid drawdown of the water level. In the present study, limit equilibrium methods and finite element method have been used to calculate the factor of safety of earth dam. The main objective is studying the influence the soil strength parameters including cohesion, angle of internal friction and unit weight of soil on the values of factor of safety of the upstream slope for earth dam. The results show that the values of factor of safety increase when the values of soil strength parameters (cohesion, angle of internal friction)and water level increased, and the value of unit weight of the soil decrease, and the values of factor of safety, decreasing fast in rapid draw down of the water level. The main conclusion is the stability of the earth dam increases when the soil strength parameters (cohesion, angle of internal friction) increase and unit weight of the soil decrease with increasing the water level in the reservoir and the earth dam may be exposed to the collapse in the case of the rapid drawdown of water level.

Macrophyte beds in a subtropical reservoir shifted from a nutrient sink to a source after drying then rewetting

AbstractMacrophytes play a key role in assimilating and storing nutrients in shallow aquatic ecosystems, but their capacity to act as a long‐term nutrient sink can be affected by water level fluctuations. Water level drawdown in reservoirs followed by rewetting may mobilise a significant nutrient pool. These nutrients can be stored in the littoral zone in dead or dormant macrophytes, and in the desiccated sediments within the macrophyte beds. However, the contribution of desiccated macrophyte beds to nutrient release upon rewetting has not been well quantified.Our study examined the effect of rewetting the previously desiccated waterlilyNymphoides indica(Menyanthaceae) in treatments (1) without sediments (N.i.−Sed), and (2) with sediments inN. indicabeds (N.i.+Sed) on water quality.We found that longer drying duration increased dissolved nutrients (nitrate/nitrite, ammonium and total dissolved organic nitrogen/phosphorus) and organic carbon release fromN.i.+Sed andN.i.−Sed treatments after rewetting. In theN.i.+Sed treatment with <4 weeks of desiccation, allN. indicaplants regenerated from roots after subsequent rewetting. In addition, the resulting nutrient/carbon release was not significantly different to the control treatment which did not have desiccation. A significant increase in dissolved nutrient and carbon concentrations in the water column was found in treatments with more than 10 weeks of desiccation followed by rewetting. This coincided with the sediment reaching its minimum moisture content. Furthermore, chlorophylla(Chl‐a) concentrations in the overlying water also increased with more than 10 weeks of desiccation, presumably in response to the increased nutrient availability and the removal of competition for nutrients from macrophytes.On the basis of our laboratory experiments, we calculated the potential effect of desiccated and rewettedN. indicabeds on water quality in a localN. indica‐dominated reservoir after water level drawdown. We also separated the contribution ofN. indicaplants from their macrophyte beds on water quality changes. Fourteen days after rewetting, the total dissolved nutrients released fromN. indicaalone (N.i.−Sed) could contribute 0.5% of the total nitrogen and 29% of total phosphorus to the water column concentrations in the whole reservoir. In contrast,N. indicabeds (N.i.+Sed) contributed more total dissolved nitrogen (4.3%) but less total dissolved phosphorus (0.3%) release into the water column. The higher nitrogen release for theN.i.+Sed treatment was likely due to the organic matter decomposition in the sediment in macrophyte beds. In contrast, the less dissolved phosphorus release, compared with theN. indicaalone, was likely the result of phosphate adsorption by previously desiccated soil particles and/or assimilation by phytoplankton, since the phytoplankton biomass (as measured by Chl‐aconcentrations) was significantly higher in theN.i.+Sed treatment.This study highlights the importance of managing both the duration and rate of water level drawdown in reservoirs to prevent rooted macrophytes, likeN. indica,from becoming a source of nutrients, which may cause deterioration in water quality.

Simplified stability analysis of strain-softening slopes under drawdown conditions

The progressive failure of slopes is often induced by drawdown and strain-softening of the soil. Thus, the stability level of a strain-softening slope under drawdown conditions should be evaluated while considering progressive failure. A simplified method was developed to analyze the stability of a strain-softening slope based on a new algorithm for determining the subsidiary shear deformation under drawdown conditions. The parameters of the method could be easily determined. The predictions of the failure behavior of the slope that were obtained using the proposed method were close to the observations of centrifuge model tests under drawdown conditions. Thus, the method was confirmed to be effective in analyzing the stability evolution of the strain-softening slopes induced by the drawdown of water level. This proposed method was used to evaluate the stability of a real-world slope under drawdown conditions. The results showed that there is a significant evolution in the safety factor of the slope as the water level decreases. The safety factor of a slope is overestimated if the progressive failure behavior is not considered. The strain-softening behavior and initial water level have a significant effect on the safety factor and critical slip surface of the slope.

LAND SUBSIDENCE DUE TO EXCESSIVE GROUND WATER WITHDRAWAL. A CASE STUDY FROM STAVROS - FARSALA SITE, WEST THESSALY GREECE

Land subsidence manifestation due to ground-water overexploitation in the Stavros – Farsala site (eastern part of Western Thessaly Basin) has been noticed since 1990. Because of this overexploitation, an excessive drawdown of the ground water level (20 to 40m) was noticed in the various successive aquifers the last decades. The subsidence phenomena resulted to the formation of tensile surface ruptures, which occurred since 2002, affecting roads and buildings. The geological environment of the study area consists of terrestrial Pleistocene deposits containing sands and gravels interbedded with clayey silt to silty clay horizons. These alternations of aquifers (permeable coarse-grained deposits) with aquitards (impermeable to low permeability strata) create a number of successive semi-confined to confined aquifer, initially artesian. This study examines the geological and the hydrogeological conditions of the wider study area, aiming to clarify their relationship with subsidence phenomena and to provide solutions for their suspension. Note that those phenomena are expected to be more incensed during the next few years.

Reservoir Water-Level Drawdowns Accelerate and Amplify Methane Emission.

Water-level fluctuations due to reservoir management could substantially affect the timing and magnitude of reservoir methane (CH4) fluxes to the atmosphere. However, effects of such fluctuations on CH4 emissions have received limited attention. Here we examine CH4 emission dynamics in six Pacific Northwest U.S. reservoirs of varying trophic status, morphometry, and management regimes. In these systems, we show that water-level drawdowns can, at least temporarily, greatly increase per-area reservoir CH4 fluxes to the atmosphere, and can account for more than 90% of annual reservoir CH4 flux in a period of just a few weeks. Reservoirs with higher epilimnetic [chlorophyll a] experienced larger increases in CH4 emission in response to drawdown (R2 = 0.84, p < 0.01), suggesting that eutrophication magnifies the effect of drawdown on CH4 emission. We show that drawdowns as small as 0.5 m can stimulate ebullition events. Given that drawdown events of this magnitude are quite common in reservoirs, our results suggest that this process must be considered in sampling strategies designed to characterize total CH4 fluxes from reservoirs. The extent to which (and the mechanisms by which) drawdowns short-circuit connections between methanogenesis and methanotrophy, thereby increasing net CH4 fluxes to the atmosphere, should be a focus of future work.

The Coupling Effect of Rainfall and Reservoir Water Level Decline on the Baijiabao Landslide in the Three Gorges Reservoir Area, China

Rainfall and reservoir level fluctuation are two of the main factors contributing to reservoir landslides. However, in China’s Three Gorges Reservoir Area, when the reservoir water level fluctuates significantly, it comes at a time of abundant rainfall, which makes it difficult to distinguish which factor dominates the deformation of the landslide. This study focuses on how rainfall and reservoir water level decline affect the seepage and displacement field of Baijiabao landslide spatially and temporally during drawdown of reservoir water level in the Three Gorges Reservoir Area, thus exploring its movement mechanism. The monitoring data of the landslide in the past 10 years were analyzed, and the correlation between rainfall, reservoir water level decline, and landslide displacement was clarified. By the numerical simulation method, the deformation evolution mechanism of this landslide during drawdown of reservoir water level was revealed, respectively, under three conditions, namely, rainfall, reservoir water level decline, and coupling of the above two conditions. The results showed that the deformation of the Baijiabao landslide was the coupling effect of rainfall and reservoir water level decline, while the latter effect is more pronounced.

Flowing fluid electrical conductivity logging of a deep borehole during and following drilling: estimation of transmissivity, water salinity and hydraulic head of conductive zones

Flowing fluid electrical conductivity (FFEC) logging is a hydrogeologic testing method that is usually conducted in an existing borehole. However, for the 2,500-m deep COSC-1 borehole, drilled at Are, central Sweden, it was done within the drilling period during a scheduled 1-day break, thus having a negligible impact on the drilling schedule, yet providing important information on depths of hydraulically conductive zones and their transmissivities and salinities. This paper presents a reanalysis of this set of data together with a new FFEC logging data set obtained soon after drilling was completed, also over a period of 1 day, but with a different pumping rate and water-level drawdown. Their joint analysis not only results in better estimates of transmissivity and salinity in the conducting fractures intercepted by the borehole, but also yields the hydraulic head values of these fractures, an important piece of information for the understanding of hydraulic structure of the subsurface. Two additional FFEC logging tests were done about 1 year later, and are used to confirm and refine this analysis. Results show that from 250 to 2,000 m depths, there are seven distinct hydraulically conductive zones with different hydraulic heads and low transmissivity values. For the final test, conducted with a much smaller water-level drawdown, inflow ceased from some of the conductive zones, confirming that their hydraulic heads are below the hydraulic head measured in the wellbore under non-pumped conditions. The challenges accompanying 1-day FFEC logging are summarized, along with lessons learned in addressing them.

Canada goose nest survival at rural wetlands in north-central Iowa

The last comprehensive nest survival study of the breeding giant Canada goose (Branta canadensis maxima) population in Iowa, USA, was conducted >30 years ago during a period of population recovery, during which available nesting habitat consisted primarily of artificial nest structures. Currently, Iowa's resident goose population is stable and nests in a variety of habitats. We analyzed the effects of available habitat on nest survival and how nest survival rates compared with those of the expanding goose population studied previously to better understand how to maintain a sustainable Canada goose population in Iowa. We documented Canada goose nest survival at rural wetland sites in north-central Iowa. We monitored 121 nests in 2013 and 149 nests in 2014 at 5 Wildlife Management Areas (WMAs) with various nesting habitats, including islands, muskrat (Ondatra zibethicus) houses, and elevated nest structures. We estimated daily nest-survival rate using the nest survival model in Program MARK. Survival was influenced by year, site, stage, presence of a camera, nest age, and an interaction between nest age and stage. Nest success rates for the 28-day incubation period by site and year combination ranged from 0.10 to 0.84. Nest survival was greatest at sites with nest structures (β = 17.34). Nest survival was negatively affected by lowered water levels at Rice Lake WMA (2013 β = −0.77, nest age β = −0.07). Timing of water-level drawdowns for shallow lake restorations may influence nest survival rates. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

Mine water supplyassessment and evaluation of the systemresponse to the designed demand in a desert region, central Saudi Arabia.

The efficient use of water resources in arid region has become highly relevant in the evaluation and mining planning, since the exploration phase to closure. The objective of the numerical groundwater model was to assess the potential for groundwater extraction to meet mine water demand from one of the driest area in the world. Numerical groundwater models were used to assess groundwater resource. Modelling was undertaken using MODFLOW-SURFACT code, an advanced MODFLOW based code, within the framework of Visual MODFLOW version 4.6. A steady state model was developed to assess the regional groundwater flow pattern and to calibrate the recharge and hydraulic conductivity parameters in the model. The model was calibrated with a correlation of coefficient of 0.997, and root-mean-squared error is 0.3m. A transient simulation model was used to predict the impact of 1.5 million cubic metre/year extraction for 10years on the main aquifer hydrogeological regime, including after cession of pumping. Modelling simulated four hydrogeological scenarios. Model results for the 'worst case' scenario suggested that the Saq Sandstone aquifer should be capable of supplying the mine water demand (1.5 million cubic metre (MCM)/year) for 10years. However, the long-term water-level drawdown shows a continuous decrease without achieving steady state conditions; thus, the majority of water is being taken from aquifer storage, and in the long term, there will be a mutual interference from a borefield located to the north of the model area. In this area, the hydraulic gradient is relatively steep and over-pumped for more than 28years. Other scenario shows that there will be a recovery of around 8m out of the 11.6-m drawdown, after 18years of cession of pumping, implying that the aquifer will be stressed and a large percentage of water taken from aquifer storage. To minimise hydrogeological impacts, it is recommended to laterally spread out production bores, bores should be located where the Saq Sandstone is deepest, and operate the bores at low extraction rates. Overall, this study presents a useful numerical model output for mine water supply assessment and will contribute towards improving water resources management under different conditions in one of the world driest area.

Caracterização hidrofaciológica e monitoramento dos níveis d´água no Sistema Aquífero Guarani na cidade de Araraquara (SP)

O Sistema Aquífero Guarani (SAG) constitui uma das maiores unidades hidroestratigráficas do continente sul-americano e está entre os aquíferos mais explorados no Estado de São Paulo. Nesse contexto o presente trabalho teve como objetivo a caracterização do arcabouço hidroestratigráfico do SAG na cidade de Araraquara, bem como a avaliação do rebaixamento dos níveis d’água no aquífero, decorrente da extração atual, por meio do monitoramento contínuo dos níveis d’água em poço tubular abandonado. Na área de estudo foram reconhecidas três formações geológicas: Formação Pirambóia; Formação Botucatu e a Formação Serra Geral. No SAG foram caracterizadas quatro hidrofácies: A, B e C, que representam a Formação Pirambóia e D constituída pela Formação Botucatu. As unidades estão separadas por descontinuidades geológicas, reconhecidas nos perfis, e foram diferenciadas em função do conteúdo argiloso presente nos arenitos, que condicionam as propriedades hidráulicas do reservatório. Ao longo de 710 dias de monitoramento foi observado um rebaixamento total de 4,98 m no poço monitorado, decorrentes do bombeamento realizado na cidade. A contribuição de cada um dos poços, para o rebaixamento total, foi estimada utilizando-se a equação de Cooper-Jacob. Utilizando as vazões médias históricas para cada poço de bombeamento o rebaixamento calculado foi de 4,96 metros. Observou-se que poços em um raio de 4200 metros do poço de observação apresentaram contribuições significativas no rebaixamento observado.