Conduit Flow Process Research Articles

CFPy-A Python Package for Pre- and Postprocessing of the Conduit Flow Process of MODFLOW.

The conduit flow process (CFP) for MODFLOW's groundwater flow model is an advanced approach for investigating complex groundwater systems, such as karst, with coupled discrete-continuum models. CFP represents laminar and turbulent flow in a discrete pipe network coupled to a matrix continuum. However, the preprocessing demand is comparatively high to generate the conduit network and is usually performed with graphical user interfaces. To overcome this limitation and allow a scalable, reproducible, and comprehensive workflow, existing and new routines were aggregated to a Python package named CFPy, to allow script-based modeling that harmonizes well with the available and widely used FloPy package. CFPy allows information about the location and geometry of the conduit network to be considered by user-specific approaches or by sophisticated methods such as stochastic conduit network generators. The latter allows the automatic generation of many model variants with differing conduit networks for advanced investigations like multi-model approaches in combination with automatic parameter estimation. Additional postprocessing routines provide powerful control and valuable insights for CFP applications. In this methods note, a general technical description of the approach is complemented with two examples that guide users and demonstrate the main capabilities of CFPy.

Toward the estimation of the transfer coefficient in karst systems: Using baseflow recession coefficient under matrix-restrained flow regime

Using the reservoir model theory, this study developed an equation for the relationship between the transfer and recession coefficients under a matrix-restrained flow regime. Assuming Darcy's law, the proposed equation was developed based on conventional parameters of karst systems, such as recession coefficient, specific yield, and catchment area. Firstly, the equation was evaluated through forward and inverse modeling of an idealized karst system with MODFLOW-2005 Conduit Flow Process (CFP) and KarstMod, respectively. Then, calibration of a KarstMod and a CFP model of the Baget karstic watershed in France was carried out to check the efficiency of the developed equation.Results of the first step showed that the optimal value of the transfer coefficient based on KarstMod for the linear transfer flow is very close to the baseflow recession coefficient values. Therefore, the transfer coefficient in KarstMod may be considered as the recession coefficient from the matrix to the conduit compartment. Moreover, substituting these two coefficients in the developed equation caused less than 10% errors in estimating the transfer coefficients assigned to the idealized models in CFP. In addition, adding the recession coefficient as prior information in the calibration process affects the recession coefficient of the simulated time series and provides a reliable evaluation of the internal dynamics of the system. Therefore, the results of this research show that the recession coefficients can be used as an initial estimate of the transfer coefficient in the baseflow condition. This achievement can significantly assist the discrete-continuum model calibration and further support the water balance calculations based on the recession data.

A Dynamic Modeling Approach to Predict Water Inflow during Karst Tunnel Excavation

During tunnel construction in strongly developed karst terrain, water inrush hazards often occur due to the complex hydrogeological conditions, which require accurate prediction of water inflow. In this study, a dynamic modeling approach for water inflow prediction of karst tunnels using the conduit flow process (CFP) is developed that considers both karst duality and changing boundary conditions of the tunnel. The CFP model has a good agreement with field-observed hydraulic head after calibration, and the Nash–Sutcliffe model efficiency (NSE) for the CFP model is 97.3%. Numerical calculation of water inflow was conducted in a successive drilling scenario with permeability change of the surrounding rocks. Additionally, a modular three-dimensional finite-difference ground-water flow model (MODFLOW) has been applied to predict the water inflow, for comparison with the CFP model. The prediction results obtained from the CFP model are generally in close agreement with the field-observed results; the percentage errors were 13.3% and 5.4%, respectively. For the MODFLOW model, the percentage errors were 34.2% and 36.8%, respectively. The proposed CFP model is both closer to reality and more reasonable than the MODFLOW model in predictive analysis of water inflow into karst tunnels, reflecting the influence of karst conduits on the water inflow process.

Applying a modified conduit flow process to understand conduit-matrix exchange of a karst aquifer

Due to the high heterogeneity and complexity of water flow movement for multiple karst water-bearing mediums, the evaluation, effective development, and utilization of karst water resources are significantly limited. Matrix flow is usually laminar, whereas conduit flow is usually turbulent. The driving mechanisms of water exchange that occur between the karst conduit and its adjacent matrix are not well understood. This paper investigates the hydrodynamic characteristics and the mechanism of flow exchange in dual water-bearing mediums (conduit and matrix) of karst aquifers through laboratory experimentation and numerical simulation. A karst aquifer consisting of a matrix network and a conduit was proposed, and the relationship between the water exchange flux and hydraulic head differences generated from the laboratory experiments was analyzed. Two modes of experimental tests were performed with different fixed water level boundaries in the laboratory karst aquifer. The results indicate that the water exchange capacity was proportional to the square root of hydraulic head differences. The linear exchange term in the conduit flow process (CFP) source program was modified according to experimental results. The modified CFP and the original CFP model experimental data results were compared, and it was found that the modified CFP model had better fitting effects. These results showed that the water exchange mechanism between conduit and matrix is very important for solid-liquid interface reaction, water resource evaluation, and understanding of karst hydrodynamic behavior.© 2022 China Geology Editorial Office.

Simulation of horizontal injection wells in Managed Aquifer Recharge facilities using the conduit flow process (CFP) code for MODFLOW-2005

Groundwater management of coastal aquifers is a scientific and engineering challenge, as coastal aquifer systems are hydrogeologic features that are hydraulically connected with the sea while at the same time are stressed with extensive abstraction practices for irrigation. To remedy this, Managed Aquifer Recharge -MAR- is proved to be a sound and effective technology that is able to fill the gap between groundwater supply and demand. Essential part of this procedure is reliable groundwater modeling that simulates effectively all involved hydrologic processes in order to be able to predict the response of the system under different natural or artificially induced conditions. This paper describes the application of a modeling tool, for the simulation of the relative hydrologic processes between the open filter pipe of the injection well and the surrounding aquifer material with the finite difference method. More specifically, a MAR pilot setup with a horizontal direction injection well was simulated with the Conduit Flow Process (CFP) of MODFLOW-2005 code. To investigate further the applicability of CFP code, the HDDW was conceptualized with the simpler MODFLOW WEL package and the results of the two models were compared. For the two models, sensitivity analysis was conducted using UCODE_2014. Both models were calibrated successfully as the simulated values showed an acceptable fit to the observed data. The main difference between the two packages was observed in the rise of groundwater level along the horizontal well. To this end, it was indicated that CFP offered a more realistic representation of the horizontal well pipe.

Breakthrough curves of dye tracing tests in karst aquifers: Review of effective parameters based on synthetic modeling and field data

Breakthrough curves (BTCs) are known as adequate evidence to characterize karst aquifers which mainly specified by dual flow systems (matrix and conduit) as well as some degree of connection between dual media. Conduit Flow Process version 2 (CFPv2) is a discrete conduit continuum model applying which it is possible to simulate such characteristics. In this research, first CFPv2 was employed to evaluate the effect of several factors including hydraulic conductivity, exchange flow, tracing distance, hydraulic gradient, and conduit diameter and roughness on BTCs in a synthetic karst aquifer. Then, 253 BTCs from 24 dye tracing tests, conducted in karstic aquifers of Iran, were analyzed to evaluate the effect of hydraulic gradient and tracing distance on the BTCs with taking into account the effect of medium type (matrix or conduit) and karst development. These tests were conducted under either natural or elevated man-made hydraulic gradient conditions. The results revealed that peak concentration decreased with enlarging conduit diameter because of considerable flow rate. Likewise, it decreases over longer distances specifically in the elevated man-made hydraulic conditions. However, with increasing hydraulic gradient, peak concentration and its timing have upward and downward trends, respectively. When BTCs start to demonstrate tailing and become flattened, it could be an indication of low matrix hydraulic conductivity or lower hydraulic gradient in conduit as a result of less exchange flow from the matrix into conduits. Based on this study a new intermixture of flow and solute transport parameters was engineered to classify karst development. High peak concentration, short peak time, high velocity particularly over long distances and under the natural hydraulic gradient conditions and feasible conduit flow regime whether laminar or turbulent suggested as indication of well-developed karstic system.

Applying a modified conduit flow process to understand conduit-matrix exchange of a karst aquifer

Due to the high heterogeneity and complexity of water flow movement for multiple karst water-bearing mediums, the evaluation, effective development, and utilization of karst water resources are significantly limited. Matrix flow is usually laminar, whereas conduit flow is usually turbulent. The driving mechanisms of water exchange that occur between the karst conduit and its adjacent matrix are not well understood. This paper investigates the hydrodynamic characteristics and the mechanism of flow exchange in dual water-bearing mediums (conduit and matrix) of karst aquifers through laboratory experimentation and numerical simulation. A karst aquifer consisting of a matrix network and a conduit was proposed, and the relationship between the water exchange flux and hydraulic head differences generated from the laboratory experiments was analyzed. Two modes of experimental tests were performed with different fixed water level boundaries in the laboratory karst aquifer. The results indicate that the water exchange capacity was proportional to the square root of hydraulic head differences. The linear exchange term in the conduit flow process (CFP) source program was modified according to experimental results. The modified CFP and the original CFP model experimental data results were compared, and it was found that the modified CFP model had better fitting effects. These results showed that the water exchange mechanism between conduit and matrix is very important for solid-liquid interface reaction, water resource evaluation, and understanding of karst hydrodynamic behavior.© 2022 China Geology Editorial Office.

Evaluating the performance of BAM-based blanket filter on nitrate reduction in a karst spring

Understanding the effects of best management practices (BMPs) on nitrate reduction in karst aquifers is critical for spring restoration. In this study, Conduit Flow Process (CFPv2) and Conduit Mass Three-Dimensional (CMT3D) models were applied to evaluate nitrate removal for Silver Springs in Florida using blanket filters composed of biosorption-activated media (BAM). As the inputs of the model, the spatial and temporal variability of nitrate-N concentration in groundwater recharge was estimated as a function of population and land use, reflecting both point and non-point sources of nitrate. Additionally, the spatial heterogeneity of nitrate removal efficiency in the soil was considered in the evaluation. The model estimates that conduit flow accounts for 48% of spring discharge and 47% of nitrate-N mass transport on average, and the contributions of conduit to flow and nitrate mass transport are higher during wet years. The net effect of nitrate reduction in spring discharge was evaluated for two BMP scenarios, i.e., implementing BAM-based blanket filters in 26 stormwater retention basins and 50% of the urban area. The net effect of the BMP on nitrate reduction for Scenario 1 is limited; whereas, for Scenario 2, the nitrate-N concentration will be reduced to 1.08 mg/L in 2026 for a nitrate removal efficiency of 74% (i.e., a 4.9% reduction relative to the baseline condition). This study suggests that the BAM-based blanket filters benefit is likely to scale with penetration (i.e. greater water quality benefits can be expected with greater BMP implementation). Further, this study shows that whether BAM-based blanket filters leads to net water quality improvements or degradation depends on whether BAM removes more or less nutrients from stormwater than the unaltered soil profile. The simulated effects of the BMP on nitrate reduction may serve as an additional guidance for water resources managers to make decisions on investments in nutrient reduction technologies for spring restorations in karst systems.

Assessment of groundwater ingress to a partially pressurized water-conveyance tunnel using a conduit-flow process model: a case study in Iran

Construction of a conveyance tunnel through rock often induces the ingress of groundwater into the tunnel, a flow that changes both the hydrogeological regime of the tunnel and its environment. To explore this key interaction, a novel modeling approach using the conduit flow process (CFP) is developed that considers both the hydraulic head and the ingress of water from the rock matrix during excavation. The resulting flow values are predicted through an adapted MODFLOW numerical model into which the tunnel is introduced with the aid of the new CFP approach. The CFP approach can simulate both laminar and turbulent flow in the tunnel whether the flow is free surface or pressurized. Several simulations, including one for which the permeability of the tunnel perimeter is assumed to be identical to the hydraulic conductivity of the surrounding rock matrix, are then used to explore the sensitivity of the predicted head and flow conditions to the permeability of the tunnel perimeter. Comparisons of the numerical results with field data from the Kerman Water Conveyance Tunnel in Iran show that the proposed approach accurately predicts the spatial variation of both groundwater ingress and hydraulic head.

Laboratory and numerical simulations of spatio-temporal variability of water exchange between the fissures and conduits in a karstic aquifer

Fissures and conduits are main void of karstic aquifer. Karst aquifers are highly vulnerable to contamination as water exchange between fissuresystem andconduitsystem greatly influences the karstification of limestone terrains, heterogeneity of karstic aquifer, and spring flow regime. A two-dimensional tank model was constructed, and a corresponding numerical model (MODFLOW-2005 Conduit Flow Process Mode 1, CFPM1) was developed to explore the effects of rainfall intensity and intrinsic structure of karst system (hydraulic conductivity of fissures and conduit diameter) on conduit-matrix water exchange. Nine different scenarios were designed to quantitatively simulate the spatial and temporal variability of water exchange between the fissures and conduits. The study reproduced the area of fissure water recharging to conduit (recharge zone), the area of conduit water discharging to fissure water (discharge zone) and the area of fissure water moving laterally (transport zone) in both physical model and numerical model. The results from rainfall scenarios show a relatively stable ratio of exchange zone (recharge zone and discharge zone) with increasing rainfall intensity, indicating that conduit-matrix water exchange is not sensitive to rainfall intensity, and conduit water discharging to fissure water is just a short-term phenomenon. It is also found that an increase in hydraulic conductivity of fissure system enhances extensiveness of water transport, but restrains conduit-matrix water exchange. However, the enlargement of conduit can prompt the extent of conduit-matrix water exchange. A positive feedback between the extent of conduit-matrix water exchanges and karstification can be confirmed. These findings might be essential for better understanding groundwater flow dynamics in karst aquifers.

Nitrate transport in a karst aquifer: Numerical model development and source evaluation

Protecting karst aquifers from nitrate contamination is an issue of global concern due to the inherent vulnerability of karst aquifers to contamination and the negative health effects of elevated nitrate levels. Hydrological and transport models were developed to assess nitrate transport within a karstic aquifer located in an area with both semi-arid and sub-humid climate conditions. Groundwater flow through the aquifer matrix and conduits was simulated with CFPv2, an experimental version of the Conduit Flow Process (CFP) version of the MODular FLOW (MODFLOW) groundwater model. Nitrate transport was simulated with the experimental Conduit Mass Transport Three-Dimensional Model (CMT3D). The hydrologic calibration of the steady-state model was successful based on a scaled Root Mean Squared Error (RMSE) of 4.9% for hydraulic heads and percent errors of 3.0% and 6.6% for the two major springs in the area. Performance of the transient hydrologic model exceeded the performance of the existing Equivalent Porous Media (EPM) model with a scaled RMSE of 2.2% and springflow errors of 1.5% and 2.8% for the major springs. Similarly, nitrate calibration was successful with all Percent Bias (PBIAS) values between ±25%. Additional calibration statistics including a two-tailed t-test indicated successful calibration. The CMT3D model corroborates prior research findings that anthropogenic sources as well as biogenic wastes are important sources of nitrate to the aquifer. The methodology used in this study represents an additional tool for watershed managers to address nitrate contamination within karst aquifers.

Simulation de l’intrusion saline dans un aquifère côtier karstique complexe en utilisant un modèle amélioré d’écoulement dans les conduits et de transport de solutés à densité variable

VDFST-CFP (variable-density flow and solute transport–conduit flow process) is a density-dependent discrete-continuum numerical model for simulating seawater intrusion in a dual-permeability coastal karst aquifer. A previous study (Xu and Hu 2017) simulates variable-density flow only in a single conduit, and studies the parameter sensitivities only in the horizontal case (2D domain as horizontal section) by the VDFST-CFP model. This paper focuses on the density-dependent vertical case (2D domain as vertical section) with two major improvements: 1) when implementing double-conduit networks in the domain, simulated intruded plumes in the porous medium are extended in the double-conduit scenario, compared to the single-conduit system; 2) by quantifying micro-textures on the conduit wall by the Goudar-Sonnad equation and considering macro-structures as local head loss. Sensitivity analysis shows that medium hydraulic conductivity, conduit diameter and effective porosity are important parameters for simulating seawater intrusion in the discrete-continuum system. On the other hand, rougher micro-structures and additional macro-structure components on the conduit wall would reduce the distance of seawater intrusion to the conduit system, but, rarely affect salinity distribution in the matrix. Compared to the equivalent mean roughness height, the new method (with more detailed description of structure) simulates seawater intrusion slightly landward in the conduit system. The macro-structure measured by local head loss is more reasonable but needs further study on conduit flow. Xu and Hu (2017) Development of a discrete-continuum VDFST-CFP numerical model for simulating seawater intrusion to a coastal karst aquifer with a conduit system. Water Resources Research: 53, 688-711.

Derivation of karst-fracture conduit flow and laboratory studies using a 3-D printer

It is crucial to karst aquifer groundwater numerical simulation that the karst-fracture conduit flow is accurately calculated. The mostly used conduit flow process (CFP) approach is more precise than the conventional equivalent porous media approach. However, the CFP approach ignores the karst conduit structural feature, considers the shapes of cross sections to be circles, introduces empirical friction coefficient, and uses the same formulas for different flow regimes. To solve the above problems, this study combines formula derivation with physical testing verification, starting with the karst-fracture conduit structural feature using 3-D printing technology. Our results show that classical CFP models can be greatly enhanced by taking into account the geometric properties such as conduit anisotropy and roughness. The new approach has great application prospects in karst-fracture conduit flow aquifer groundwater seepage calculation, and provides a theoretical foundation for the development of relevant numerical simulation software.

Turbulent and Laminar Flow in Karst Conduits Under Unsteady Flow Conditions: Interpretation of Pumping Tests by Discrete Conduit‐Continuum Modeling

AbstractDue to the duality in terms of (1) the groundwater flow field and (2) the discharge conditions, flow patterns of karst aquifer systems are complex. Estimated aquifer parameters may differ by several orders of magnitude from local (borehole) to regional (catchment) scale because of the large contrast in hydraulic parameters between matrix and conduit, their heterogeneity and anisotropy. One approach to deal with the scale effect problem in the estimation of hydraulic parameters of karst aquifers is the application of large‐scale experiments such as long‐term high‐abstraction conduit pumping tests, stimulating measurable groundwater drawdown in both, the karst conduit system as well as the fractured matrix. The numerical discrete conduit‐continuum modeling approach MODFLOW‐2005 Conduit Flow Process Mode 1 (CFPM1) is employed to simulate laminar and nonlaminar conduit flow, induced by large‐scale experiments, in combination with Darcian matrix flow. Effects of large‐scale experiments were simulated for idealized settings. Subsequently, diagnostic plots and analyses of different fluxes are applied to interpret differences in the simulated conduit drawdown and general flow patterns. The main focus is set on the question to which extent different conduit flow regimes will affect the drawdown in conduit and matrix depending on the hydraulic properties of the conduit system, i.e., conduit diameter and relative roughness. In this context, CFPM1 is applied to investigate the importance of considering turbulent conditions for the simulation of karst conduit flow. This work quantifies the relative error that results from assuming laminar conduit flow for the interpretation of a synthetic large‐scale pumping test in karst.

Development of a discrete‐continuum VDFST‐CFP numerical model for simulating seawater intrusion to a coastal karst aquifer with a conduit system

AbstractA hybrid discrete‐continuum numerical model, Variable‐Density Flow and Solute Transport—Conduit Flow Process (VDFST‐CFP), is developed to simulate seawater intrusion to a coastal karst aquifer with a conduit network. The Darcy‐Weisbach equation is applied to simulate the nonlaminar groundwater flow in the conduit system that is conceptualized as pipes, while the Darcy equation is used for laminar groundwater flow in the continuum porous medium. Density‐dependent groundwater flow with appropriate additional density terms in the conduit is analytically derived. The flow and transport equations are coupled, and numerically solved by the finite difference method with an implicit iteration procedure. Two synthetic benchmarks are developed to compare the VDFST‐CFP model results with other numerical models, such as the variable‐density SEAWAT, constant‐density continuum MODFLOW/MT3DMS, and constant‐density discrete‐continuum CFPv2/UMT3D models. The VDFST‐CFP model compares reasonably well with the other model results in both conduit and porous medium domains, and well describes water and salt exchange between the two systems. Under turbulent flow conditions within the conduit, the Darcy‐Weisbach equation calculates the flow rate more accurately without overestimation by the Darcy equation. Sensitivity analysis indicates that conduit diameter, friction factor, matrix hydraulic conductivity, and effective medium porosity are important parameters in the VDFST‐CFP model. The pros and cons of the VDFST‐CFP model are discussed, including the model assumptions and simplifications, limitations of the discrete‐continuum modeling method, and the convergence criteria. In general, the newly developed VDFST‐CFP model provides a new numerical modeling method for simulating seawater intrusion in a coastal karst aquifer with conduits.

Use of the Conduit Flow Process for the simulation of passive mitigation measures against the piezometric damming effect at the new underground High Speed railway station of Florence

The new High Speed railway station of the city of Florence will be located below ground level inside a huge excavation, 25 m b.g.s deep, 450 m long and 50 m wide; the pit, already built, is surrounded by concrete cut-off walls causing a piezometric damming effect against the main aquifer of the Arno River. As permanent mitigation measure against the piezometric mounding, the design of the station considers a series of drain pipes drilled horizontally on up and down gradient sides of the pit and connected by blind wall tubes. A numerical finite differences model has been implemented with the Modflow code, in order to quantify the expected damming effect and to properly design the drain-based mitigation measures (i.e. frequency and length of the drains). One of the major challenges of the numeric approach was the coupling between the groundwater flow in the porous medium and the water flow towards and through the drains and the blind wall tubes connecting the sides of the excavation. Thus, the drains were simulated using a recently published Modflow Process originally developed for karst conduits (Conduit Flow Process – CFP), which allows combining the groundwater flow in the porous medium with laminar or turbulent flow into pipes. In order to guarantee an acceptable head differential between both sides of the excavation, the results showed that the system should permit a discharge flow rate of about 0.026 m3/s, considering a hydraulic gradient equal to 0.5% (i.e. hydraulic gradient from the averaged piezometric distribution before pit excavation). Conductance turned out to be the most influent parameter towards the effectiveness of the drainage system.

A New Equation Solver for Modeling Turbulent Flow in Coupled Matrix-Conduit Flow Models.

Karst aquifers represent dual flow systems consisting of a highly conductive conduit system embedded in a less permeable rock matrix. Hybrid models iteratively coupling both flow systems generally consume much time, especially because of the nonlinearity of turbulent conduit flow. To reduce calculation times compared to those of existing approaches, a new iterative equation solver for the conduit system is developed based on an approximated Newton-Raphson expression and a Gauß-Seidel or successive over-relaxation scheme with a single iteration step at the innermost level. It is implemented and tested in the research code CAVE but should be easily adaptable to similar models such as the Conduit Flow Process for MODFLOW-2005. It substantially reduces the computational effort as demonstrated by steady-state benchmark scenarios as well as by transient karst genesis simulations. Water balance errors are found to be acceptable in most of the test cases. However, the performance and accuracy may deteriorate under unfavorable conditions such as sudden, strong changes of the flow field at some stages of the karst genesis simulations.

Numerical study of groundwater flow cycling controlled by seawater/freshwater interaction in a coastal karst aquifer through conduit network using CFPv2

In this study, a groundwater flow cycling in a karst springshed and an interaction between two springs, Spring Creek Springs and Wakulla Springs, through a subground conduit network are numerically simulated using CFPv2, the latest research version of MODFLOW-CFP (Conduit Flow Process). The Spring Creek Springs and Wakulla Springs, located in a marine estuary and 11 miles inland, respectively, are two major groundwater discharge spots in the Woodville Karst Plain (WKP), North Florida, USA. A three-phase conceptual model of groundwater flow cycling between the two springs and surface water recharge from a major surface creek (Lost Creek) was proposed in various rainfall conditions. A high permeable subground karst conduit network connecting the two springs was found by tracer tests and cave diving. Flow rate of discharge, salinity, sea level and tide height at Spring Creek Springs could significantly affect groundwater discharge and water stage at Wakulla Springs simultaneously.Based on the conceptual model, a numerical hybrid discrete-continuum groundwater flow model is developed using CFPv2 and calibrated by field measurements. Non-laminar flows in conduits and flow exchange between conduits and porous medium are implemented in the hybrid coupling numerical model. Time-variable salinity and equivalent freshwater head boundary conditions at the submarine spring as well as changing recharges have significant impacts on seawater/freshwater interaction and springs’ discharges. The developed numerical model is used to simulate the dynamic hydrological process and quantitatively represent the three-phase conceptual model from June 2007 to June 2010. Simulated results of two springs’ discharges match reasonably well to measurements with correlation coefficients 0.891 and 0.866 at Spring Creeks Springs and Wakulla Springs, respectively. The impacts of sea level rise on regional groundwater flow field and relationship between the inland springs and submarine springs are evaluated as well in this study.

Simulating long term nitrate-N contamination processes in the Woodville Karst Plain using CFPv2 with UMT3D

• This is an initial attempt of transport simulation by CFPv2 with UMT3D over regional scales. • Nitrate-N from two sprayfields is transported rapidly through karst conduit networks in the WKP. • CFPv2 and UMT3D simulated non-laminar flow and transport in explicit conduit networks. A research version of CFP (Conduit Flow Process) code, CFPv2, is applied with UMT3D to simulate long term (1966–2018) nitrate-N contamination transport processes in the Woodville Karst Plain (WKP), northern Florida, where karst conduit networks are well developed. Groundwater flow in the WKP limestone porous matrix is simulated using Darcy’s law, and non-laminar flow within conduits is described by Darcy-Weisbach equation. Nitrate-N conduit transport and advective exchanges of groundwater and nitrate-N between conduits and limestone matrix are calculated by CFPv2 and UMT3D, instead of MODFLOW and MT3DMS since Reynolds numbers for flows in conduits are over the criteria of laminar flow. The developed numerical model is calibrated by field observations and then applied to simulate nitrate-N transport in the WKP. The numerical simulations verify the theories that two sprayfields near the City of Tallahassee and septic tanks in the rural area are major nitrate-N point sources within the WKP. High nitrate-N concentrations occur near Lost Creek Sink, and conduits of Wakulla Spring and Spring Creek Springs where aquifer discharge groundwater. Conduit networks control nitrate-N transport and regional contaminant distributions in the WKP, as nitrate-N is transported through conduits rapidly and spread over large areas.

Representation of water abstraction from a karst conduit with numerical discrete-continuum models

Abstract. Karst aquifers are characterized by highly conductive conduit flow paths embedded in a less conductive fissured and fractured matrix, resulting in strong permeability contrasts with structured heterogeneity and anisotropy. Groundwater storage occurs predominantly in the fissured matrix. Hence, most mathematical karst models assume quasi-steady-state flow in conduits neglecting conduit-associated drainable storage (CADS). The concept of CADS considers storage volumes, where karst water is not part of the active flow system but hydraulically connected to conduits (for example karstic voids and large fractures). The disregard of conduit storage can be inappropriate when direct water abstraction from karst conduits occurs, e.g., large-scale pumping. In such cases, CADS may be relevant. Furthermore, the typical fixed-head boundary condition at the karst outlet can be inadequate for water abstraction scenarios because unhampered water inflow is possible. The objective of this work is to analyze the significance of CADS and flow-limited boundary conditions on the hydraulic behavior of karst aquifers in water abstraction scenarios. To this end, the numerical discrete-continuum model MODFLOW-2005 Conduit Flow Process Mode 1 (CFPM1) is enhanced to account for CADS. Additionally, a fixed-head limited-flow (FHLQ) boundary condition is added that limits inflow from constant head boundaries to a user-defined threshold. The effects and the proper functioning of these modifications are demonstrated by simplified model studies. Both enhancements, CADS and FHLQ boundary, are shown to be useful for water abstraction scenarios within karst aquifers. An idealized representation of a large-scale pumping test in a karst conduit is used to demonstrate that the enhanced CFPM1 is able to adequately represent water abstraction processes in both the conduits and the matrix of real karst systems, as illustrated by its application to the Cent Fonts karst system.