Pumping‐Induced Drawdown and Stream Depletion in a Leaky Aquifer System
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Abstract The impact of ground water pumping on nearby streams is often estimated using analytic models of the interconnected stream‐aquifer system. A common assumption of these models is that the pumped aquifer is underlain by an impermeable formation. A new semianalytic solution for drawdown and stream depletion has been developed that does not require this assumption. This solution shows that pumping‐induced flow (leakage) through an underlying aquitard can be an important recharge mechanism in many stream‐aquifer systems. The relative importance of this source of recharge increases with the distance between the pumping well and the stream. The distance at which leakage becomes the primary component of the pumping‐induced recharge depends on the specific properties of the aquifer, aquitard, and streambed. Even when the aquitard is orders of magnitude less transmissive than the aquifer, leakage can be an important recharge mechanism because of the large surface area over which it occurs. Failure to consider aquitard leakage can lead to large overestimations of both the drawdown produced by pumping and the contribution of stream depletion to the pumping‐induced recharge. The ramifications for water resources management and water rights adjudication can be significant. A hypothetical example helps illustrate these points and demonstrates that more attention should be given to estimating the properties of aquitards underlying stream‐aquifer systems. The solution presented here should serve as a relatively simple but versatile tool for practical assessments of pumping‐induced stream‐aquifer interactions. However, this solution should not be used for such assessments without site‐specific data that indicate pumping has induced leakage through the aquitard.Keywords:
Drawdown (hydrology)
Aquifer test
Sustainable groundwater management is founded on the sound understanding of the effects of water extraction on the aquifer water level and the springs and streams receiving groundwater discharge. Pumping test data are commonly used in extraction licence applications to evaluate aquifer properties and assess the magnitude of storage depletion resulting from pumping. However, a short duration (eg 48 hours) pumping test can fail to detect the presence of aquifer boundaries, as the cone of depression is not large enough to reach the boundaries. This may cause an underestimation of long-term drawdown and an overestimation of permissible extraction rate (ie safe yield). In the rural town of Irricana in Alberta, groundwater extraction licences for municipal water supply wells were issued in the early 1980s based on the analysis of 48-hour pumping tests. Actual water extraction rates were substantially below the licensed rates, but the unanticipated and excessive drawdown in the aquifer forced the town to discontinue pumping and switch to surface water supply after 25 years. To examine the cause of overallocation, a new 48-hour pumping test was conducted in the same aquifer, which included an extended drawdown analysis using 26 days of recovery data. Geological formation logs for existing wells in the area surrounding Irricana were used to infer the extent of sandstone aquifer units within the heterogeneous bedrock formation. The new data analysis showed that the aquifer is semi-closed, contrary to the infinite-aquifer assumption used in the original pumping test, which caused additional drawdown due to the aquifer boundary effects. This study suggests an improved procedure for estimation of storage depletion using standard hydrogeological methods and readily available data. The new procedure provides a useful tool as part of adaptive groundwater management, in which water levels and other relevant variables are monitored and licensed extraction rates are adjusted accordingly.
Drawdown (hydrology)
Cone of depression
Aquifer test
Water well
Groundwater discharge
Bedrock
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The buried‐valley aquifers that are common in the glacial deposits of the northern hemisphere are a typical case of the strip aquifers that occur in many parts of the world. Pumping from a narrow strip aquifer leads to much greater drawdown and much more distant drawdown effects then would occur in a sheet aquifer with a similar transmissivity and storage coefficient. Widely used theories for radial flow to wells, such as the Theis equation, are not appropriate for narrow strip aquifers. Previously published theory for flow to wells in semiconfined strip aquifers is reviewed and a practical format of the type curves for pumping‐test analysis is described. The drawdown response of strip aquifers to pumping tests is distinctive, especially for observation wells near the pumped well. A case study is presented, based on extensive pumping test experience for the Estevan Valley Aquifer in southern Saskatchewan, Canada. Evaluation of groundwater resources in such buried‐valley aquifers needs to take into account the unusually large drawdowns in response to pumping.
Drawdown (hydrology)
Aquifer test
Aquifer properties
Water well
Surficial aquifer
Cone of depression
Slug test
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Abstract A quasi, a mixed, and a weighted three‐dimensional model are applied to approximate the three‐dimensional, unsteady drawdown in vertical pumping and observation wells, fully or partially penetrating a single, vertically heterogeneous, anisotropic aquifer. Cases of confined, semiconfined, and unconfined flow conditions are considered. Numerical examples are used to quantify the numerical error of the methods, introduced by the vertical heterogeneity of the aquifer. Results of pumping test data analysis in a three‐layer, anisotropic aquifer with multiple screens in the pumping well are presented.
Drawdown (hydrology)
Aquifer test
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ABSTRACT A numerical simulation and analytical study of a constant‐rate pumping test, for a well situated at the centre of a disc of anomalous transmissivity and storage coefficient, have been used to aid in the interpretation of tests performed in a “patchy” aquifer in India. Equations describing the long‐time behaviour of drawdown show that Jacob's method can be employed to estimate the regional transmissivity from drawdowns measured at any point in the aquifer or in the pumping well. However, these equations also show that an average storage coefficient should be calculated from drawdowns measured outside the aquifer discontinuity. The results of this study support the hypothesis that the average transmissivity of a heterogeneous aquifer can be calculated from rates of drawdown observed after long periods of pumping.
Drawdown (hydrology)
Aquifer test
Discontinuity (linguistics)
Cone of depression
Aquifer properties
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Prediction of aquifer drawdown is necessary for the correct management of an aquifer. The use of mathematical models is one important tool to predict the status of an aquifer drawdown. In this study, we have used the MODFLOW model to simulate the SarzeRezvan Aquifer. After having proven an excellent match between the generated model and the aquifer's natural condition, we used this model to predict the aquifer drawdown. To simulate the change in water head under unsteady state, we used water head data from 2001-2010. The PEST computational code was used for calibration. Water head data from April to September 2011 has been used for verification. The Root Mean Square Error (RMSE) between observed and calculated (by the model) water head is 0.854, which indicates excellent agreement with the aquifer's natural condition. Therefore, this model was used to predict the aquifer drawdown. The results of the model prediction have shown that if pumping is going to be constant for 8 years, the drawdown will be approximately 10.56 m, after 8 years.
Drawdown (hydrology)
MODFLOW
Aquifer test
Piezometer
Hydraulic head
Groundwater model
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