PWN is responsible for supplying sufficient quantities of high-quality water to its customers. The growing population and the effects of climate change, such as heat waves and droughts, are straining the capacity of the water supply network, especially during dry and warm periods when water demand increases. A significant portion of the drinking water is produced in Andijk. From there, the drinking water is transported to Hoorn, where it is further distributed to customers. The connection between Andijk and Hoorn is therefore crucial, and in the event of a pipeline failure due to a calamity or maintenance, capacity issues can arise. To maintain the redundancy of this connection and to smooth out the daily water demand and supply fluctuations, PWN is exploring the implementation of an Aquifer Storage and Recovery (ASR) system in Hoorn.

The ASR system in Hoorn faces strict requirements, which address the challenge of maintaining water quality standards and optimising recovery efficiency. These requirements must ensure that the extracted water remains suitable for consumption, with no more than 1% dilution with ambient groundwater. The objective of this study is to identify a method to improve the recovery efficiency of the ASR system in Hoorn. The ASR system operates by injecting drinking water into an aquifer during periods of water availability and recovering it when needed. Compared to installing a new pipeline, the ASR system offers a more cost-effective solution with additional benefits such as space efficiency and temperature stability. However, the ASR system in Hoorn faces challenges related to maintaining water quality standards and optimising recovery efficiency. Processes such as lateral flow, dispersion, and buoyancy affect the system’s performance, and a thorough understanding of these processes is crucial for accurately predicting recovery efficiency. A comprehensive analysis of a pilot ASR system in Hoorn was conducted by PWN to address these challenges. The pilot system consists of a single well where two pumps operate at two different filter depths in an aquifer. In the final layout of the ASR, additional wells are necessary to achieve the desired capacity.

A radially symmetric model was used to simulate groundwater flow, conservative solute transport, and heat transport. Due to the stringent water quality requirements, the radially symmetric model must accurately capture essential processes in an Aquifer Storage and Recovery (ASR) system, such as flow, dispersion, retardation, and buoyancy. The performance of a radial symmetric model in SEAWAT and MODFLOW 6 was assessed based on analytical methods and 3D models. Through this analysis, it was decided to utilise a radial symmetric model in SEAWAT due to the presence of numerical dispersion in a model using MODFLOW 6.

After this analysis, the model’s performance was tested against various measurements, including hydraulic head, temperature, and electrical conductivity. It is evident that the model effectively captures both solute transport and heat transport. Discrepancies between measurements and the model can be attributed to assumptions made during the study and uncertainties in the measured values. However, the presence of clay layers between the deep and shallow filters in the pumping well significantly contributes to local differences between the model and the measurements. The main reason for this difference is that these layers are not homogeneous throughout the depths, allowing water to flow between them. This heterogeneity cannot be simulated with a radially symmetric model. However, despite this heterogeneity, these clay layers consistently result in low recovery efficiency in the current system.

The objective of this study was to identify a method to improve the recovery efficiency of the ASR system in Hoorn. The current system has a recovery efficiency of about 30%. This can improved by implementing a check valve in the shallow filter of the pump well, with 60% to 65% of the filter dedicated to recovery to achieve a recovery efficiency of 80%. During the testing of this system, an injection period was followed by a recovery period with specific pumping rates. It took three cycles to achieve the desired recovery efficiency. It is important to note that these cycles did not include storage and rest phases. The system’s recovery efficiency may change when these phases are incorporated. However, an important assumption is that homogeneous layers are present. Heterogeneity of layers can lead to deviations from the modelled recovery efficiency. This research contributes to a better understanding of the pilot ASR system in Hoorn and provides insights into improving its recovery efficiency. With the lessons learned from this study,
PWN can assist in developing the final design for the ASR system. This design will involve multiple wells to meet the required capacity.

Extensive usage of vital freshwater is highly undesirable in the current era of climate change and population growth. However, the 6 millionm3/y of brackish seepage that occurs in the deep Horstermeer Polder, located in between the cities of Amsterdam and Hilversumin the Netherlands, is mitigated by using up to 207 millionm3/y of freshwater. This is very unsustainable, especially since the
pressure on the freshwater reserves in the Netherlands is rising. Therefore, this research focuses on evaluating a new mitigation measure of the brackish
upconing below the polder: the extraction of brackish water. In March 2021 AD, a pilot well has been installed in the polder, and its effects were assessed in this research. To enable the evaluation, an extensive analysis of all available measurements of the geology, hydraulic head, and chloride distribution in the area was performed, and a 3D regional variable-density groundwater model using MODFLOW 6 was built. The model result has a reasonable good agreement with these measurements, since at 70% of the measurement locations, the simulated hydraulic head deviates less than 30 cm, and at 80% of them, the simulated chloride concentration deviates less than 200 mg/l. Moreover, compared to previous research, the representation of the brackish upconing was significantly improved. The effects of the pilot well were calculated using five different operational settings. The results indicated that a shallow well, starting below -50 m NAP, reduces the chloride load in the polder more than a deep well (-100 m NAP). However, even in the best scenario, i.e. an active shallow well with a constant pumping rate of 100 m3/h, the reduction of the chloride load is limited to 5.5%. In perspective, these results suggested that at least 24 of these wells are needed to lower the chloride concentration in the discharge water out of the polder to 250 mg/l. On the contrary, the shallow wells cause a considerable average drawdown of 4 cm at surface level in a radius of 50 meters around the well, which might result in damage to wooden pile foundations in the northeast of the polder.

In the winter of 2014 upon 2015 the Hondsbossche dike, between Petten and Camperduin in the Netherlands, has been transformed into a dune area through sand replenishment at the seaside of the dike. The area inland has been appointed as a Natura-2000 area because of the highly appreciated ecological value. A reduction of saltwater seepage would cause potential harm to the flora and fauna which is undesired. Two sectional models of the density dependent groundwater flow have been developed to map out the growth of a freshwater bubble forming beneath the Hondsbossche dunes and the corresponding decline of saltwater seepage to the polders inland. The two sections chosen are a section perpendicular to the coastline through the middle of a small lake De Putten and a section perpendicular to the coastline just south of a small lake Abtskolk. Both sectional models have been tested through a calibration process for the measurement data considering the freshwater head and geophysical properties of the groundwater. From the models the development of the groundwater regime for the coming 95 years has been calculated (100 years in total and 5 years until the present situation). Besides this the development of the groundwater regime in several sectional variations to the developed sectional models have been investigated. This has been done to investigate the impact of certain assumptions in the layout of the sectional models. These variations to the sectional models also are an indication of the range of possible changes to the groundwater regime for the whole area because elsewhere in the area these resulting sections could be present. Looking at the results of both sectional models and all model variations the conclusion can be drawn that the biggest reduction of saltwater seepage takes place in the years 2026 till 2056. Therefore it is advised to take action against the reduction of saltwater seepage before 2031. A sensible way of doing this is to firstly identify the most critical sections perpendicular to the sea in the area of interest. A good first step would be to identify the most permeable spots in the clay layer beneath the dunes. The results show that in these spots a faster spread of the freshwater bubble is expected. Besides this it has become apparent from the model results that the small lake De Putten attracts a bigger fraction of the seepage currents in comparison to the surrounding area. It would be a good idea to investigate where other hotspots for seepage are present in the area. In these areas the reduction of saltwater seepage would most likely be more intense. The identified critical sections would be the best places to start implementing measures against the decline of saltwater seepage. When actions are taken timely and precisely it is expected that the decline of saltwater seepage will remain small and the ecological value of the polders will remain.

In this report, the tidal method is used to estimate hydraulic aquifer parameters. The principle of the tidal method is to use head fluctuations in observation wells, caused by tidal motion in a sea or river, to determine regional hydrological characteristics of an aquifer system. As the wave propagates into the aquifer, the amplitude of the signal decreases and the phase increasingly lags behind the tide at sea. The extent of this is determined by hydrological aquifer characteristics, hence aquifer parameters can be estimated, by calibrating a model that reproduces the measured tidal propagation. Knowledge of hydraulic aquifer parameters is important since these are needed in groundwater models. The case study was performed on Schouwen-Duiveland where use was made of hydraulic head fluctuations from three groundwater observation wells and water-level observations in the Oosterschelde. First, these time series were analyzed to estimate the amplitudes and phases of the constituents present in the data. Noise in the data was reduced with the use of Pastas (a model to analyze hydrological time series; Collenteur et al., 2019) and a Butterworth filter, after which both methods were compared. Pastas was used to decompose the fluctuations observed in the groundwater to different contributions of hydrological stresses (e.g. rain and evaporation) and Butterworth was used to flatten the frequency response for frequencies that are not of interest. The use of the Butterworth filter is preferred, partly because it produces the smallest standard deviations for the amplitude and phase estimates and because it is easier to use. Moreover, especially for the wells with a low signal to noise ratio, the Butterworth filter is better at extracting the tidal signal. In addition, a graphical determination of the amplitude and phase was performed to check if this relatively quick and easy analysis gives accurate estimates of the M2 amplitude and phase as well. It was concluded that with a high signal to noise ratio and a dominantly present constituent (M2 in this case), the amplitude can be reliably estimated. Determining the phase with this method did not give satisfactory results. In the inverse modeling part, the hydraulic aquifer parameters are determined, which is done with a least-squares minimization of the observed and modeled amplitudes and phases. For the optimization, both a one-aquifer and a two-aquifer model were used, both based on the one-dimensional, multi-layer solution presented by Bakker (2019). The optimization was performed with a global optimizer. The obtained fit to the observations was somewhat poor, moreover, unrealistic optimal parameter estimates indicate that the model, to some extent, incorrectly represents the real system. This also implies that some of the model simplifications do significantly impact the results. Simplifications presumed to mostly influence the model results include homogeneity, one-dimensional flow and the use of a straight shoreline. The presence of some model error signifies that the resulting parameter estimates should be treated with care. The hydraulic conductivity and the resistance of the one-layer model were consistently estimated at their upper boundary and the storage in the leaky layer was estimated to be negligible. Therefore, only one parameter group (i.e. the diffusivity) could be estimated with the optimization. The fit of the model was not perfect, a compromise must be sought based on what residual the model minimizes. As a result, all diffusivity estimates within and around the range [1.07E6, 1.31E6] m²/day are all considered to be reasonable estimates. For the resistance of the aquitard (CU) and the storage in both the aquifer (S_sU) and the aquitard (σU) it was analyzed which parameter values gave unreasonable results. This resulted in a lower bound for CU (CU=663 days) and an upper bound for σU  (σU =1.22E-4 m^-1). For S_sU reasonable results are obtained between 2.28E-6 m^-1 and 1.87E-5 m^-1. The lower bound for the CU estimate seems to comply with estimates based on two models (REGIS-II and GeoTOP). Finally, the two-aquifer model was considered to be useless for parameter estimation. Presumably, this is predominantly caused by a small amount of amplitude and phase data compared to the number of parameters to be optimized.

Seawater intrusion modeling is challenging either because of a lack of many approaches' ability to cope with complex environments or significant computational expense. Seawater intrusion models also face issues with parameterization and validation due to problems with many parameters and also a lack of observational data. Bakker and Schaars (2013) proposed a method to solve for the steady-state interface using a single-density flow model (MODFLOW). In this thesis, this method was applied to 3 existing coastal flow models and shown to be able to compute the steady interface. The work also contains examples of applying critical pumping solutions to the models to demonstrate its applicability as a management tool. The research also contains an effort to solve for an interface when the saltwater is moving but was ineffective. Modifications may be made to resolve the issues with the latter approach that could lead to a advances in interface modeling methods.

Aquifer thermal energy storage (ATES) is an energy efficient technology to temporarily store groundwater of different temperatures in an aquifer. The basic idea behind ATES is to store thermal energy in warm and cold wells so that the energy can be used for heating and cooling of buildings in the next season. Design and planning of ATES systems requires numerical simulation tools such as SEA- WAT, COMSOL, and FEFLOW. COMSOL and FEFLOW are expensive commercial products. SEAWAT is a free computer program based on MODFLOW and MT3DMS. The flow field is modelled with the finite difference method while there are several methods to simulate the heat transport including the finite difference (FD) method and the total variation diminishing (TVD) method. OpenGeoSys (OGS) is an open-source alternative based on the finite element method which is able to simulate groundwater flow and heat transport processes and can potentially be used for design and planning of ATES systems. There are very few applications of OGS to simulate ATES systems. The main goal of this research is to assess the usability of OpenGeoSys in the simulation of ATES sys- tems. The numerical solutions of OGS and SEAWAT (both the TVD scheme and FD scheme) are compared to the analytical solutions for a single well that injects water with a constant flow rate of water and constant temperature. For a doublet with a cold well and a warm well, the OGS solution is compared to the SEAWAT solution for both a sin- gle cycle system and a multiple cycles system with constant flow rates and temperatures. Finally, a field case of a doublet with varying flow rates and injection temperatures is studied to compare the performance of OGS to SEAWAT. The main findings of this study are as follows. Both OGS and SEAWAT can reproduce the heat transport process of the analytical solutions for a single well injecting warm or cold water. There are some deviations between the temperature distributions computed by the numerical models and the analytical solutions which are primarily caused by nu- merical dispersion. The TVD scheme results in the smallest numerical dispersion while OGS shows slightly more numerical dispersion when applying the same number of cell- s/nodes. Numerical dispersion can be reduced by application of finer spatial resolution for the SEAWAT since the time step is adjusted automatically. For OGS, the time step must be reduced manually when the grid is refined. Both OGS and SEAWAT show energy balance errors smaller than 1%. Numerical dis- persion has an effect similar to physical dispersion so that the thermal radius increases when the numerical dispersion is larger. The interaction between the warm well and cold well of a doublet increases when the thermal radius increases, which may result in an increase of the energy balance error. According to the analytical solution for the model where the heat transport between the aquifer and aquitards is included, OGS shows the smallest overestimation of thermal energy that remains in the aquifer part compared to the two solution schemes of SEA- WAT. The vertical temperature distribution in aquitards simulated with OGS is closer to that of the analytical solution compared to SEAWAT. Both numerical dispersion and the overestimation of thermal energy in the aquifer have an influence on thermal recovery efficiency of an ATES system. Larger numerical dis- persion indicates a longer heat transport distance, resulting in more time needed for thermal energy to transport back to the wells during the extraction period. Similarly, the overestimation of the thermal energy in the aquifer provides more thermal energy dur- ing the extraction period. There are some overshoots/undershoots when simulating an ATES system with varying flow rates and injection temperatures by the TVD scheme. This can be eliminated by refining the spatial resolution. It is more complicated to simulate an ATES system with OGS than SEAWAT. The com- putational time is much longer for OGS compared with SEAWAT by a factor of around 10 for the same number of cells/nodes. Despite these drawbacks, OGS is suitable for the simulation of ATES systems as the simulated temperature distribution in both the aquifer and aquitards compares well to the analytical solution, and there are no prob- lems of overshoots/undershoots in the simulation of varying flow rates and injection temperatures.

Transfer function noise (TFN) modelling is a form of time series analysis which regularly uses the recharge as a stress to explain the groundwater table fluctuations. Often the recharge flux is estimated as a linear combination of the precipitation and the (potential) evaporation. However, this is a simplification of the actual hydrological processes in the unsaturated zone. This is tried to be overcome by implementing a nonlinear recharge model in TFN time series models. Additionally, TFN models can use different impulse response functions, where some of them account for dispersion and retardation due to the unsaturated zone.

In this report the performance of a linear and nonlinear recharge model, inside the TFN model, are tested against synthetic time series of the groundwater table. These time series for the groundwater table are created with the unsaturated/saturated zone model HYDRUS-1D. With HYDRUS-1D, thirty-five synthetic time series are created for five different soil types and seven different unsaturated zone thicknesses (up to 5 m). The three most commonly used response functions, exponential, gamma and four-parameters are also tested for these thirty-five time series.

The results show that TFN models using the nonlinear recharge model are almost always better in estimating the groundwater table time series than the linear recharge model. This is confirmed in both the calibration and validation period. The common disadvantage of the linear recharge model, undershooting the groundwater table in (dry) summers, is not observed for the nonlinear recharge model. This can improve the forecasting abilities of TFN models during droughts.

Additionally, the nonlinear recharge model gives a more realistic representation of the fluxes in the root zone. This is confirmed goodness-of-fit parameters when comparing of the recharge flux and evaporation reduction calculated by HYDRUS-1D and the nonlinear recharge model. Especially when using the exponential response function, the recharge flux can be estimated quite well by the nonlinear recharge model. However, the nonlinear recharge model is currently not able to estimate groundwater uptake (upwards recharge) while it is observed in the HYDRUS-1D simulations.

The linear model does perform decently for shallow groundwater tables down to a depth of 150 cm since that is where large groundwater fluctuations and more days with groundwater uptake (upward recharge) are observed. The use of the gamma and four-parameter response functions significantly improves the performance of the linear recharge model. This can be explained by the compensation of these response functions for dispersion and retardation in the root zone. Nevertheless, when performing groundwater table time series analysis on synthetic time series created with HYDRUS-1D, the nonlinear recharge model is preferred to simulate the groundwater table.

Hydraulic properties of aquifer systems are usually estimated by conducting field experiments, which are called aquifer tests. Different approaches have been applied to simulate the drawdown data, such as graphical type curves, analytical solutions, and grid-based models. Since the type of groundwater system varies greatly from location to location, only a few general solutions have been developed. Computation of hydraulic properties are limited by the size and time step of grid-based model. Determining the boundary conditions is also difficult in actual groundwater system. Thus, in order to better simulate various aquifer systems, semi-analytic approaches have been promoted and used in aquifer test analysis. In this report, three different softwares aiming at aquifer tests simulation are presented. Both MLU and TTim are based on semi-analytical solutions and need users to build their own aquifer models, while AQTESOLV provides a base model for adding a variety of data and a stack of choices containing both analytical and semi-analytic solutions. Some benchmark analyses have been performed to assess the performance of TTim with limited types of aquifer systems. This research focuses on the application of TTim to different aquifer systems and the investigation of TTim's performance compared to the other two softwares. In this study, fifteen aquifer tests have been simulated with TTim. Test 0 uses hypothetical data to verify TTim's capabilities of retrieving specified parameters and reporting accurate confidence interval. Test 1 through Test 10 are pumping tests which are taken from reported field experiments and are grouped as confined systems, leaky systems, and unconfined systems. Test 11 through Test 14 present four slug tests with different top boundaries and well construction. The values and confidence intervals of the calibrated parameters are compared to the results of AQTESOLV and MLU. Improvement of conceptual models is carried out by model structure adjustments and parameters set adjustments. Different models' performance are assessed by root-mean-squared-error and AKAIKE Information Criterion (AIC). Most of the pumping tests and slug tests can be conceptualized using either ModelMaq and Model3D within TTim. Model3D is recommended when conceptualizing unconfined systems. The top boundary needs to be specified as `confined', and an additional thin aquifer needs to be added to simulate the specific yield, which is calibrated separately. The performance of TTim is similar, in general, to AQTESOLV and MLU. Well construction parameters cannot be calibrated with AQTESOLV, and only one aquifer system is available. TTim is more flexible and accurate than AQTESOLV when the groundwater system has information of multi aquifers and well construction. Modifications of parameters to be calibrated and model structure have been carried out to improve TTim's performance. It is concluded that aquifers with multi subdivided layers perform better when the well is partially penetrating or the observation wells have different depths. Calibration of well construction parameters may also contributes to a better simulation, but they are usually sensitive to the initial values. It is important to note that adding parameters may give better results, but whether this is significant needs to be tested by the AIC criterion.

A suggested approach for determining groundwater flow in unconsolidated aquifers is tested. Performing a heat pulse response test by using a heat- and fibre-optic cable should result in a vertical profile of groundwater velocities. These cables are installed by using direct push ensuring the direct contact between cable and aquifer. The suggested approach is tested with two case studies. The first case study near the Máximakanaal was meant to determine if the canal is leaking. This experiment failed during the installation which was discovered after analyzing the results. The mistakes that were made during this case study have been analyzed and are discussed. The second case study was not performed by the author of this thesis. Nonetheless, the measurements of this case study enabled the completion of retrieving groundwater velocities from distributed temperature sensing and reviewing the approach.

Waterschap Brabantse Delta (WBD) has the intention to implement measures that enhance the baseflow. Baseflow consists of the groundwater flow and a small part of the interflow. During dry periods, streams are dependent on the baseflow. Enhancing the baseflow has a proper effect on the ecologically relevant quality of waters, and is therefore wanted for WBD according to the regulations of the Water Framework Directive. It is needed to quantitatively examine these measures that have been implemented in subcatchments of WBD. Therefore, sufficient data of good quality is needed. Especially, the stream discharge itself is important to collect. In this research, the stream discharge is obtained by a physically based model (the GR4J rainfall-runoff model). Moreover, a new method is applied: relating groundwater heads to stream discharge by applying machine learning algorithms. These two different methods are used for subcatchment Chaamse Beken, for which it is wanted to simulate stream discharge between 2003-2019 (flow measuring weir has been removed in 2003).
Four different machine learning algorithms are used: decision tree regression (DTR), random forest regression (RFR), gradient boosting regression (GBR) and support vector regression (SVR). The training set of these models is set from 1985-1999, whereas the test set is from 1999-2003. Moreover, different input variables and combinations of these variables are chosen for the models: shallow wells (screen-1 wells), deeper wells (screen-2 wells), precipitation and potential evaporation. The model performance is evaluated with the metrics Nash-Sutcliffe Efficiency (NSE), mean absolute error (MAE), fourth root mean quadrupled error (R4MS4E) and mean squared logarithmic error (MSLE). The first two are considered for overall model performance, whereas the latter two are for high flow and low flow model performance.
The best overall and low model performance is obtained by using the algorithm SVR and using inputs groundwater heads of shallow wells, precipitation and potential evaporation (a NSE of 0.75). In order to examine if this machine learning model can be used in the future for stream discharge simulation, the SVR model is compared with an existing conceptual hydrological model GR4J. The GR4J model has a NSE value of 0.80 and can be rated as good. It has a larger NSE value than the SVR model and performs better than the SVR machine learning model.
It is important to stress that for the GR4J model the memory (or state) of the system is included. This inclusion of the memory of the system is not the case for the machine learning algorithms. Furthermore, an important difference is the fact that groundwater heads play a significant role in the simulation of the stream discharge by using machine learning algorithms. These groundwater heads are not directly used in the GR4J model. Lastly, it is stressed that for building the GR4J model physical understanding of the hydrological system is needed, whereas for machine learning this is not the case.
Overall, it can be concluded that GR4J is still favoured above SVR, but SVR shows promising results for further research in simulating stream discharge.

The northern Ghana climatological conditions are favourable for agricultural production. The annual average precipitation of 800-1250 mm/y is theoretically sufficient for farmers to be year-round self-sufficient. However, the majority of precipitation falls in a 4-month wet season spanning from late May to October. As a consequence, the region is subjected to both seasonal flooding and long periods of drought. The dry season agriculture is of moderate intensity, takes place at small-scale and is groundwater dependent. In the near future it is not unthinkable that extraction exceeds natural recharge and groundwater withdrawal is no longer sustainable in the northern Ghana regions. The small-holder farmers' use of Aquifer Storage and Recovery (ASR) systems can potentially contribute to the continued sustainable use of groundwater in northern Ghana. An ASR system acts as a seasonal bridge. The system recharges flood water and extracts groundwater in periods of drought. A feasibility study on the sustainable use of an ASR system in northern Ghana has been performed by taking present conditions and multiple system improvements into account.

Aquifer tests are carried out at five study sites in northern Ghana to determine local geohydrological conditions. The TTim analytic element modelling environment is used to analyze the obtained groundwater drawdown data and derive parameters for subsurface characteristics. TTim allows for the inclusion of additional model parameters (e.g. borehole storage, well skin resistance and multiple model layers) and outperforms the analytic Theis method in this research. Although some uncertainties are present in the derived subsurface parameters, plausible values for transmissivity (T) and storativity (S) are suggested to be present in the ranges of respectively 1 to 100 (m2/d) and 1e-3 to 1e-2 (-).

The year-round performance of a northern Ghana single ASR system is studied with a MODFLOW model. The potential types of ASR system improvements that are examined are (a) the extension of daily pumping time, (b) the enlargement of the borehole diameter, and (c) the reduction of the well skin resistance. The ASR systems sensitivities to changing environmental conditions are explored by (a) the degradation of well depth by clogging, (b) the shortening of the wet season inundation time, and (c) the reduction of the wet season inundation levels. Research results show that well maintenance is key for the performance of existing (and new) ASR systems. The recharge and discharge volumes can be improved by cleaning of the borehole depth and well screen. In the case of a new ASR system, the performance can positively be influenced by an enlargement of the borehole diameter. Furthermore, the construction of a proper permeable well skin (screen and gravel-pack around the well) can also result in increased system capacities. Despite the imposed options of system modifications, the geographic position of an ASR system remains of utmost importance for system performance. The construction of an ASR system at a location sensitive to flooding (riverbank overtopping or rainfall based) can be beneficial from a sustainable perspective. Recharge volumes are normative for the sustainable use of an ASR system. The recharges are (approximately linear) dependent on the time-span and levels of inundation. Moreover, the research contains soil scenarios, and demonstrates that the ASR system performs significantly better in regions with higher transmissivity (T) values.

To give insight on some financial aspects of an operational ASR system, the obtained (improved) ASR system discharge capacities are transformed to agricultural and financial yields. A subdivision of the dry season into a tomato and a groundnut cropping season demonstrates that financial yields are crop type dependent. The ASR system revenues are dominantly affected by the choice in crop type(s) and crop-specific market prices. The yields are compared to the ASR system pumping costs. The importance of pump selection is demonstrated by the implementation of the Pedrollo 4" submersible pump efficiencies. The use of a pump that is tuned to local conditions can be beneficial for the operational costs of an ASR system. Although no distinctive conclusion on the financial feasibility can be drawn, the examined system improvements are substantially beneficial for the revenues of a northern Ghana ASR system.

In a Borehole Thermal Energy Storage (BTES) system, heat is extracted from or injected to the subsurface, taking advantage of the relatively constant temperatures of the underground. Both thermal conduction and advection can influence the performance of BTES systems, but thermal advection is often neglected in the design process. However, in areas with groundwater flow, the heat exchange between the BTES system and the soil can be significantly affected, as the stored heat is transported from the borehole with the flow. This project studies the influence of groundwater flow on the performance of a BTES system. A detailed 3D numerical model is created using MODFLOW and MT3D to simulate groundwater flow and heat transfer under various conditions. The model is verified using an analytical solution as well as experimental data, and is subsequently used to perform a sensitivity analysis and simulate a case study, in order to gain insight on the influence of groundwater flow on the heat exchange process and the conditions that favor this effect. The results reveal that groundwater flow is enhancing the heat exchange rates both in heating and in cooling mode. The magnitude of the effect depends on the total groundwater discharge, and the porosity and background temperature of the soil. In cases of combined heating and cooling, the effect also depends on the magnitude and ratio of the injected and extracted energy loads, as well as the frequency of switching between storage and extraction. Finally, it is revealed that groundwater flow is beneficial for systems with unbalanced energy loads, as it counterbalances the net heat extraction or injection that could decrease the system’s heat exchange capacity in the long term.

This study introduces a new concept of ATES system, which is called ATEST system, to fulfill the transport function while crossing the barriers in water transmission and pipe replacements. Firstly, analytical models were used to help understand the physical process and range the working conditions of the ATEST system. Then numerical models were used to prove the feasibility and the value of this new conceptual system. Acceptable system performance that can meet the heat demand was iterated in case simulation; and the economical advantages were identified by comparing it with traditional ATES systems. The ATEST system showed a greater practical value than the ATES system in: 1) solving the discrepancy between heat service and heat demand in space, and 2) crossing barriers where pipes cannot be buried. However, the system should be further modified in operation to acquire better performance and avoid practical problems.