Uncertainty estimation of end-member mixing using generalized likelihood uncertainty estimation (GLUE), applied in a lowland catchment

End-member mixing models have been widely used to separate the different components of a hydrograph, but their effectiveness suffers from uncertainty in both the identification of end-members and spatiotemporal variation in end-member concentrations. In this paper, we outline a procedure, based on the generalized likelihood uncertainty estimation (GLUE) framework, to more inclusively evaluate uncertainty in mixing models than existing approaches. We apply this procedure, referred to as G-EMMA, to a yearlong chemical data set from the heavily impacted agricultural Lissertocht catchment, Netherlands, and compare its results to the traditional end-member mixing analysis (EMMA). While the traditional approach appears unable to adequately deal with the large spatial variation in one of the end-members, the G-EMMA procedure successfully identified, with varying uncertainty, contributions of five different end-members to the stream. Our results suggest that the concentration distribution of effective end-members, that is, the flux-weighted input of an end-member to the stream, can differ markedly from that inferred from sampling of water stored in the catchment. Results also show that the uncertainty arising from identifying the correct end-members may alter calculated end-member contributions by up to 30%, stressing the importance of including the identification of end-members in the uncertainty assessment

Aquifer thickness along the global coastline: link to shape files

Information on aquifer thickness is one of the crucial inputs to numerical groundwater flow models, especially in the coastal areas that are threatened by increasing salt water intrusion leading to declining fresh groundwater resources. Since aquifer thickness data is missing in many parts of the world we designed a methodology to estimate it on global scale using state of the art datasets. We estimated the aquifer thickness by combining global topographical (Weatherhall et al., 2015, doi:10.1002/2015EA000107), lithological (Hartmann et. al, 2012, doi:10.1029/2012GC004370), soil thickness (Pelletier et al., 2016, doi:10.1002/2015MS000526) and sediment thickness from the global PCR-GLOBWB model (de Graaf et. al, 2015, doi:10.5194/hess-19-823-2015) to find the position and slope of the bedrock formation that are overlaid by unconsolidated sediments forming the aquifer system. The dataset includes the estimates of aquifer thickness at the coastline for each cross-section together with corresponding "anchor points" (last point with known thickness from Pelletier et. al, 2016). With this information it is possible to create 2D cross-sectional groundwater flow coupled with salt transport models to estimate the fresh groundwater resources in coastal areas. More attention should be paid to the composition of unconsolidated sediment aquifers in terms of low permeable layers that can play a large role in the distribution of fresh and saline groundwater

Integrated assessment of variable density-viscosity groundwater flow for a high temperature mono-well aquifer thermal energy storage (HT-ATES) system in a geothermal reservoir

The use of groundwater systems for heat storage increasingly gains interest among water managers, policy makers and researchers as a way to increase the efficiency of energy production and to allow the re-use of waste heat. Typically, mono-well storage systems are thought to require the use of separate aquifers. This study assessed the suitability of using heat and cold storage in a single deep geothermal aquifer for district heating and cooling. An integrated modelling approach was used for evaluating the controls on the energy efficiency of high temperature aquifer thermal energy storage (HT-ATES). The temperature difference (δ. T) of 40 °C between the injection temperatures for the cold and warm storages 20 °C and 60 °C was significant, which required accounting for transient variation of density and viscosity due to temperature and pressure within the modelling code SEAWAT. The developed model was applied for a geothermal reservoir from the Moesian platform, in the Bucharest area, Romania. The sensitivity of the system efficiency was analyzed with respect to the main physical (density, viscosity, longitudinal dispersivity) and operational design parameters (distance between warm and cold storage volumes, flow rates). Uncertainties in geological heterogeneity and the associated range in longitudinal dispersivity values (5-50. m) resulted in significant efficiency differences (80-55%). While reducing the lateral distance between multiple mono-well systems increased their overall efficiency due to positive thermal interference, a minimum vertical distance of (160. m) was required between the injection/extraction filters to prevent interaction between the cold and warm storage volumes. Overall, this study highlights the potential of using a cost-effective mono-well system for HT-ATES in single deep geothermal groundwater systems

Measuring Groundwater Flow Velocities near Drinking Water Extraction Wells in Unconsolidated Sediments

Groundwater is an important source of drinking water in coastal regions with predominantly unconsolidated sediments. To protect and manage drinking water extraction wells in these regions, reliable estimates of groundwater flow velocities around well fields are of paramount importance. Such measurements help to identify the dynamics of the groundwater flow and its response to stresses, to optimize water resources management, and to calibrate groundwater flow models. In this article, we review approaches for measuring the relatively high groundwater flow velocity measurements near these wells. We discuss and review their potential and limitations for use in this environment. Environmental tracer measurements are found to be useful for regional scale estimates of groundwater flow velocities and directions, but their use is limited near drinking water extraction wells. Surface-based hydrogeophysical measurements can potentially provide insight into groundwater flow velocity patterns, although the depth is limited in large-scale measurement setups. Active-heating distributed temperature sensing (AH-DTS) provides direct measurements of in situ groundwater flow velocities and can monitor fluctuations in the high groundwater flow velocities near drinking water extraction wells. Combining geoelectrical measurements with AH-DTS shows the potential to estimate a 3D groundwater flow velocity distribution to fully identify groundwater flow towards drinking water extraction wells

Paleo‐Hydrogeological Modeling to Understand Present‐Day Groundwater Salinities in a Low‐Lying Coastal Groundwater System (Northwestern Germany)

Groundwater abstraction and drainage are considered to be the main drivers for the salinization of low‐lying coastal groundwater systems, while the role of past boundary conditions is less clear. In this study, 3‐D paleo‐hydrogeological variable‐density groundwater flow and salt transport modeling (“paleo‐modeling”) is applied to reconstruct the evolution of groundwater salinities during the Holocene, that is, the last 9,000 years, in Northwestern Germany. Novel aspects of this study include the consideration of highly resolved time‐variant boundary conditions in a 3‐D paleo‐modeling framework, for example, sea‐level rise, surface elevation and coastline changes, development of drainage networks and groundwater abstraction, as well as the quantification of isolated processes impacting salinization. Results show that salinization was a function of sea‐level rise from 9000 BP until 1300 CE. The creation of the dike line ∼1300 CE set the starting point for increasing anthropogenic control of the hydro(geo)logical system: changes in surface elevation and drainage of low‐lying marshes have become main drivers for salinization after 1600 CE when peat was artificially degenerated. Moreover, changes in the dike line caused by storm floods impacted the salinities. Model results for 2020 CE match well with present‐day salinity observations. Yet, salinization will continue in the future, as the hydro(geo)logical system has not reached an equilibrium. The presented paleo‐modeling framework can be viewed as a blueprint for similar low‐lying coastal groundwater systems, influenced by marine transgression and human development. Thereby, it enables the reconstruction of meaningful present‐day salinity distributions, serving as a vital basis for modeling future groundwater systems in a changing climate.Key Points: Salinization of a low‐lying coastal groundwater system during the Holocene, that is, 9000 BP until present‐day, was investigated. Sea‐level rise and the evolution of paleogeography were major controls on groundwater salinization during most of the time. Surface elevation changes due to land cultivation, as well as the development of drainage networks, control salinization since ∼1600 CE.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347https://doi.org/10.5281/zenodo.763338