Theo Olsthoorn

Hydrogeochemical transport modeling of 24 years of Rhine water infiltration in the dunes of the Amsterdam Water Supply.

Abstract Water quality changes were modelled along a flowpath in a plume of artificially recharged, pretreated Rhine water in the dunes of the Amsterdam Water Supply, after 24 years of infiltration. The hydrogeochemical transport model PHREEQC was extended with dispersion/diffusion and kinetics for selected chemical reactions. In the model the following reactions were included: cation-exchange, calcite dissolution and precipitation, and kinetic oxygen consumption and denitrification by oxidation of organic matter. Monthly-averaged values were used for the infiltration water quality. Traveltimes from infiltration area to sampling points were determined with chloride and tritium, and used to place the 3D field-observations in the 1D column-model. Values for CEC were variable for seven layers in the model. Infiltration of pretreated Rhine water in the dune aquifer can be considered an intrusion of more saline water. It caused desorption of Ca2+, in exchange for Na+, K+ and Mg2+ from Rhine water. Because of variations in total solute concentrations in infiltration water, local small scale freshening fronts (Ca2+ sorption, Na+ desorption) were created by seasonally decreasing salt concentrations. The undersaturation with respect to calcite in the infiltration water, and the CO2 produced during consumption of oxygen, resulted in dissolution of calcite. Precipitation of calcite occurred in response to desorption of calcium from the exchanger in the downstream parts. Overall, a net dissolution of calcite was simulated. Good results were generally achieved for all components: sulfate, nitrate, chloride, alkalinity, calcium, magnesium, potassium, sodium, 3H and O2. The contributions of the different geochemical reactions to the water quality are illustrated with computer simulations for the individual processes.

Do a Bit More with Convolution

Convolution is a form of superposition that efficiently deals with input varying arbitrarily in time or space. It works whenever superposition is applicable, that is, for linear systems. Even though convolution is well-known since the 19th century, this valuable method is still missing in most textbooks on ground water hydrology. This limits widespread application in this field. Perhaps most papers are too complex mathematically as they tend to focus on the derivation of analytical expressions rather than solving practical problems. However, convolution is straightforward with standard mathematical software or even a spreadsheet, as is demonstrated in the paper. The necessary system responses are not limited to analytic solutions; they may also be obtained by running an already existing ground water model for a single stress period until equilibrium is reached. With these responses, high-resolution time series of head or discharge may then be computed by convolution for arbitrary points and arbitrarily varying input, without further use of the model. There are probably thousands of applications in the field of ground water hydrology that may benefit from convolution. Therefore, its inclusion in ground water textbooks and courses is strongly needed.

Software for hydrogeologic time series analysis, interfacing data with physical insight

The program Menyanthes combines a variety of functions for managing, editing, visualizing, analyzing and modeling hydrogeologic time series. Menyanthes was initially developed within the scope of the PhD research of the first author, whose primary aim was the integration of data and physically-based methods for modeling time series of groundwater heads. As such, time series analysis forms the heart of Menyanthes. Within Menyanthes, time series can be modeled using both the ARMA and PIRFICT methods. The PIRFICT method is a new method of time series analysis that has practical advantages and facilitates physical interpretation and implementation of knowledge on physical behavior. Analytic solutions to specific hydrogeologic problems may be used as response function, along with their physically-based parameters. A more general approach is possible using Skew-Gaussian distribution functions, which prove to fit the behavior of hydrogeologic (and other) systems well. Use of such functions within the PIRFICT method substantially simplifies the model identification procedure, as compared to the traditional Box-Jenkins procedure. PIRFICT models may be fitted to a large number of time series in batch. Spatial patterns that emerge in the results provide useful, additional, and independent information, which adds another dimension to time series analysis. Their interpretation is supported by the spatial visualization and analysis tools of Menyanthes. The PIRFICT method also facilitates the integration of time series and spatially-distributed models via, e.g., moment-generating differential equations. The PIRFICT method may prove to be of use for other types of time series as well, both within and outside the realm of environmental sciences.

A New Operational Paradigm for Small-Scale ASR in Saline Aquifers

A new operational paradigm is presented for small-scale aquifer storage and recovery systems (ASR) in saline aquifers. Regular ASR is often not feasible for small-scale storage in saline aquifers because fresh water floats to the top of the aquifer where it is unrecoverable. In the new paradigm, fresh water storage is combined with salt water extraction from below the fresh water cone. The salt water extraction counteracts the buoyancy due to the density difference between fresh water and salt water, thus preventing the fresh water from floating up. The proposed approach is applied to assess the feasibility of ASR for the seasonal storage of fresh water produced by desalination plants in tourist resorts along the Egyptian Red Sea coast. In these situations, the continuous extraction of salt water can be used for desalination purposes. An analytical Dupuit solution is presented for the steady flow of salt water toward a well with a volume of fresh water floating on top of the cone of depression. The required salt water discharge for the storage of a given volume of fresh water can be computed with the analytical solution. Numerical modeling is applied to determine how the stored fresh water can be recovered. Three recovery approaches are examined. Fresh water recovery rates on the order of 70% are achievable when salt water is extracted in high volumes, subsurface impermeable barriers are constructed at a distance from the well, or several fresh water recovery drains are used. The effect of ambient flow and interruptions of salt water pumping on the recovery efficiency are reported.

Combining climatic and geo-hydrological preconditions as a method to determine world potential for aquifer thermal energy storage

A heat pump combined with Aquifer Thermal Energy Storage (ATES) is proven technology to economically and sustainably provide space heating and cooling. The two most important preconditions for the applicability of ATES are favorable climatic conditions and the availability of a suitable aquifer. This paper shows how these two preconditions can be combined to identify where in the world ATES potential is present, or will become present as a consequence of climate change. Countries and regions are identified where regulation and stimulation measures may increase application of ATES technologies and thus help reduce CO2-emissions. Two types of data determine ATES suitability, and their combination with a 3rd identifies potential hot-spots in the world: 1) geo-hydrological conditions, 2) current and projected climate classification and 3) urbanization. Our method combines the data into an ATES-suitability score as explained in this paper. On the one hand the results confirm the suitability for ATES where it is already applied and on the other they identify places where the technology is or will become suitable. About 15% of urban population lived in areas with high potential for ATES at the start of the 21st century, but this figure will decrease to about 5% during the 21st century as a consequence of expected climate change. Around 50% of urban population currently lives in areas of medium ATES suitability, a percentage that will remain constant. Demand for ATES is likely to exceed available subsurface space in a significant part of the urban areas.

A technical investigation on tools and concepts for sustainable management of the subsurface in The Netherlands

In response to increasing use of the subsurface, there is a need to modernise policies on sustainable use of the subsurface. This holds in particular for the densely populated Netherlands. We aimed to analyse current practice of subsurface management and the associated pressure points and to establish a conceptual overview of the technical issues related to sustainable management of the subsurface. Case studies on the exploitation of subsurface resources (including spatial use of the subsurface) were analysed, examining social relevance, environmental impact, pressure points and management solutions. The case studies ranged from constructing underground garages to geothermal exploitation. The following issues were identified for the technological/scientific aspects: site investigation, suitability, risk assessment, monitoring and measures in the event of failure. Additionally, the following general issues were identified for the administrative aspects: spatial planning, option assessment, precaution, transparency, responsibility and liability. These issues were explored on their technological implications within the framework of sustainable management of the subsurface. This resulted into the following key aspects: (1) sustainability assessment, (2) dealing with uncertainty and (3) policy instruments and governance. For all three aspects, different options were identified which might have a legal, economic or ethical background. The technological implications of these backgrounds have been identified. A set of recommendations for sustainable management of the subsurface resources (incl. space) was established: (1) management should be driven by scarcity, (2) always implement closed loop monitoring when the subsurface activities are high-risk, (3) when dealing with unknown features and heterogeneity, apply the precautionary principle, (4) responsibility and liability for damage must be set out in legislation and (5) sustainability should be incorporated in all relevant legislation and not only in environmental legislation. Other aspects to be considered are the reversibility of the impacts from subsurface activities and the abandonment of installations.

Small-Scale ASR Between Flow Barriers in a Saline Aquifer

Regular aquifer storage recovery, ASR, is often not feasible for small-scale storage in brackish or saline aquifers because fresh water floats to the top of the aquifer where it is unrecoverable. Flow barriers that partially penetrate a brackish or saline aquifer prevent a stored volume of fresh water from expanding sideways, thus increasing the recovery efficiency. In this paper, the groundwater flow and mixing is studied during injection, storage, and recovery of fresh water in a brackish or saline aquifer in a flow-tank experiment and by numerical modeling to investigate the effect of density difference, hydraulic conductivity, pumping rate, cyclic operation, and flow barrier settings. Two injection and recovery methods are investigated: constant flux and constant head. Fresh water recovery rates on the order of 65% in the first cycle climbing to as much as 90% in the following cycles were achievable for the studied configurations with constant flux whereas the recovery efficiency was somewhat lower for constant head. The spatial variation in flow velocity over the width of the storage zone influences the recovery efficiency, because it induces leakage of fresh water underneath the barriers during injection and upconing of salt water during recovery.

Multidepth Pumping Tests in Deep Aquifers

Multidepth pumping tests (MDPTs), in which different sections of a screen are pumped in sequence, are not being used by hydrogeologists, despite the capability of such tests to resolve uncertainties in the estimation of aquifer characteristics. MDPTs can be used to discern the effects of partial penetration and vertical anisotropy. This article demonstrates the use of MDPTs for a deep and vertically anisotropic aquifer, based on a real and unique series of pumping tests conducted in the Indus Basin. Traditional single-layer methods, which incorporate partial penetration and vertical scaling, were employed to evaluate these tests. However, the drawdowns of the 19 piezometers at different depths for which times series data were available could not be matched, presumably because of the layered structure of the aquifer. Numerical (MODFLOW) and multilayer analytical (Hemker and Maas 1987; Hemker 1999) approaches were used to assess the benefits of using MDPTs in the analysis of deep layered and anisotropic aquifers. The multilayer analytical solution results are consistent with the measured and numerically computed drawdowns. The original step-drawdown data were used to verify the model independently. The results of statistical analyses indicate that the parameters for a three-layer system are uniquely estimated. A sensitivity analysis showed that aquifer depths greater than 900 m do not affect the drawdown. The multilayer analytical solution was implemented in MATLAB and can be found in the online version of this article. This multilayer analytical approach was implemented in MLU by Hemker and Randall (2013) for up to 40 layers. The results of this study will be useful in groundwater management, exploration, and optimal well depth estimation for the Indus Basin aquifer and other vertically heterogeneous aquifers.

Models by decision makers and ecologists, can they be coupled?

Ecologists tend to improve their models by including more and more detail, while decision makers prefer models at a very high aggregation level with a minimum of detail. This seems to lead to a controversy between the two groups of modelers. Ecologists may wonder if their results will ever be used by policy makers. On the basis of experience gained by the development of management models, possible solutions to this problem are indicated. The principle of the Pollution Management Model of the Netherlands National Institute of Public Health and Environmental Protection (RIVM) is described. Existing knowledge was successfully coupled using a framework, containing a source term part which describes a particular action, e.g, a release of a pollutant, a pathway part which describes the behaviour of the pollutant in the environment and a dose effect part which describes the harmful effects at a particular location. Advantages and disadvantages of this approach are discussed.

Climate Proof Fresh Water Supply in Coastal Areas and Deltas in Europe

In the near future, climate change will likely increase pressures on transition zones such as deltas and coastal areas (IPCC, 2013). This special issue focuses on how to “climate proof” these zones with special concerns for the stresses stemming from droughts and salinization. Coastal zones and Deltas are under pressure worldwide. High water demand in these regions put pressure on the availability of freshwater resources and on coastal ecosystems. This leads to problems like water shortage, overexploitation of groundwater resources, saltwater intrusion, and degradation of wetlands. Economic growth, population increase, and climate change may potentially intensify these problems. This, therefore, is of strategic importance for Europe, which has a long coastline where many human activities are concentrated. Coastal aquifer development is often intensive and prone to induce salinization because of seawater intrusion, up-coning of deep saline water, and residual salinity in aquitards (Custodio 2010). Severity and frequency of droughts appear to have increased in the southern European countries. Minimum river flows are projected to decrease significantly not only in southern and southeastern Europe, but also in many other parts of the continent, especially in summer (EEA 2012). The Mediterranean region is particularly stressed (e.g. Giorgi 2006), due to the combined effect of rising sea levels, increased water demand due to global warming, and reduced aquifer recharge.