Original article
Development of a coupled surface-groundwater-pipe network model for the sustainable management of karstic groundwater R. Adams Æ G. Parkin
Abstract This paper considers the hydrogeological simulation of groundwater movement in karstic regions using a hydrological modelling system (SHETRAN) which has been adapted for modelling flow in karstic aquifers. Flow and transport through karstic aquifers remains poorly understood, yet quantitative hydrogeological models are essential for developing and implementing groundwater protection policies. The new model has been developed and used within the STALAGMITE (Sustainable Management of Groundwater in Karstic Environments) project, funded by the European Commission. The SHETRAN model is physically based insofar as most of the parameters have some physical meaning. The SHETRAN model represents all of the key processes in the hydrological cycle, including subsurface flow in the saturated and unsaturated zones, surface flow over the ground surface and in channels, rainfall interception by vegetation canopies, evapotranspiration, snow-pack development and snowmelt. The modifications made to SHETRAN to simulate karstic aquifers are (1) the coupling of a pipe network model to a variably saturated, three-dimensional groundwater component (the VSS-NET component), to simulate flow under pressure in saturated conduits; (2) the coupling of surface water features (e.g. sinking streams or ‘‘ponors’’, and spring discharges) to the conduit system; (3) the addition of a preferential ‘‘bypass’’ flow mechanism to represent vertical infiltration through a high-conductivity epikarst zone. Lastly, a forward particle tracking routine has been
Received: 17 September 2001 / Accepted: 16 October 2001 Published online: 13 February 2002 ª Springer-Verlag 2002 R. Adams (&) Æ G. Parkin Water Resources Systems Research Laboratory, Department of Civil Engineering, University of Newcastle, Claremont Rd., Newcastle Upon Tyne NE1 7RU, UK E-mail:
[email protected] Tel.: +44-191-2226419 Fax: +44-191-2225950
developed to trace the path of hypothetical particles with matrix and pipe flow to springs or other discharge points. This component allows the definition of groundwater protection zones around a source for areas of the catchment (watershed) which are vulnerable to pollution from non-point sources (agriculture and forestry). Keywords Hydrological modelling Æ Groundwater protection zone Æ Turbulent flow Æ Bulgaria Æ Slovenia Æ Slovakia
Introduction Springs in karstic aquifers are often used for public water supply, but are particularly vulnerable to contamination owing to the rapid transit times through the aquifer. The activities which may affect karstic waters include (Gillieson 1996) changes in vegetation cover, enhanced soil loss, agricultural activity, landfill sites, dumping of rubbish directly into the cave system, and runoff from roads. Guidelines have been produced in many countries to try to alleviate or prevent the types of activities which may be most damaging to karst waters. However, it is usually the case that there is insufficient understanding of karst hydrogeological systems to provide a quantitative basis for groundwater protection. A new approach has been developed which attempts to improve understanding of the internal functioning of karstic aquifers, to help to provide this quantitative basis for implementing protection measures. The approach uses an existing hydrological modelling system (SHETRAN), extended to represent turbulent flows in subsurface conduits as well as interactions between surface and subsurface waters (via sinkholes and springs). The work was carried out within the STALAGMITE (Sustainable Management of Groundwater in Karstic Environments) project. This is concerned with the development of decision support tools and procedures for the holistic management of karstic groundwater in a sustainable manner, focussing on the eastern European countries of Bulgaria, Slovenia and Slovakia where karst waters account for a significant proportion of the water supply. The extended SHETRAN model will be used to
DOI 10.1007/s00254-001-0513-8 Environmental Geology (2002) 42:513–517
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predict the effects of different catchment (watershed) management strategies on water supplies, including the possible effects of climate change on water resources.
A karst hydrogeological model for SHETRAN
The combined SHETRAN and VSS-NET models form the basis for an extended model for flow in karstic aquifers. A similar modelling approach has previously been used, by The SHETRAN modelling system adding conduits to an existing, porous-media groundwater SHETRAN is a physically based, distributed hydrological flow model (MODFLOW) to simulate the formation of a cave system through calcite dissolution (Clemens and modelling system. Currently it is capable of simulating water flow, contaminant transport and sediment transport others 1996). As far as possible the new model is physically based insofar as the model parameters have some physical (Ewen and others 2000). A modular system of interconmeaning. Where direct parameter measurements are not nected components representing the hydrological cycle available (as is usually the case for karst conduits), various has been adopted for the structure of SHETRAN. This sources of data to parameterise the model (for example, structure allows existing components to be upgraded or new components to be added to the model without serious spring hydrograph analysis) are being explored (e.g. Felton 1994). The following additional flow processes to those alterations to the basic structure of the system. Each represented in the basic SHETRAN model have been component is based around the finite-difference equations identified as important in karst aquifers and are shown for flow and transport which are solved by robust and schematically in Fig. 2. accurate numerical methods. A variably saturated subsurface (VSS) component has been recently added to Flow in a subhorizontal cave network represent three-dimensional, variably saturated flow in in the saturated zone heterogeneous porous media (Parkin 1996). VSS allows the simulation of multiple confined and unconfined aquifers, The existing VSS-NET component can directly simulate perched aquifers and stream-aquifer interactions. Figure 1 flow through a network of interconnected conduits in the saturated zone. The exchange flows between the conduits shows a schematic diagram of the flow processes repreand the matrix cells are calculated using Darcy’s law, and sented in the SHETRAN model. therefore a coefficient of proportionality is required for To allow the simulation of groundwater flow in abandoned deep mine systems, the VSS-NET component has each pipe. The flows in the conduits are calculated using the Darcy-Weisbach or Hazen-Williams formulae for turbeen developed (Adams and Younger 1997, 2001). VSS-NET has been used to model mine water rebound – a bulent flow. The model requires values of the conduit diameter (D) and effective roughness (k) to define the term used to describe the flooding-up of abandoned mines to the ground surface. The rebound process usually headloss-flow relationship in addition to the length of each gives rise to long-term discharges of highly polluted wa- pipe. Gale (1984) tabulated some values for the ratio k/D for karstic conduits, and Jeannin and Mare´chal (1995) ter. VSS-NET comprises a turbulent flow component used a value of 0.25 for a cave system in Switzerland when (NET) which is based on a water-supply distribution modelling the system using a discrete conduit network network model, coupled to the existing laminar-flow (VSS) component, to enable flows in the shafts and access model. A boundary condition with a fixed head (e.g. a spring discharge where the water surface elevation is tunnels (roadways) found in abandoned mines to be simulated. These shafts and roadways are of a similar size measured) is required for each network of conduits. Each to karst shafts and conduits and were usually engineered cave section can be represented by a straight length of pipe (a conduit), and a network consisting of looped and/or for permanence.
Fig. 1 Schematic diagram of flow processes represented in the VSS component
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Fig. 2 Karstic flow processes modelled in SHETRAN. 1 Flow in a subhorizontal cave network in the saturated zone; 2 shafts; 3 epikarst bypass flow generation; 4 interactions with surface water
Original article
To identify vulnerable areas of karst aquifers and the catchment areas supplying sources, a forward particle tracking model has been developed for SHETRAN. This method allows the particles to be released at the start of the simulation from a number of locations in the catchment (usually an aquifer grid cell) which the user can specify. Particles can be tracked in both the VSS component (in the saturated and unsaturated zones, through Shafts These represent sub-vertical features connecting the cave both the rock matrix and epikarst) and the NET component (through conduits) to an outlet source, usually a networks in the saturated and unsaturated zones and spring or borehole, and the travel time to the source can be sometimes extending to the ground surface. In the new calculated. The user can then draw isochrones of travel model, shafts may receive rainfall or flow from sinking streams in addition to inflow from the conduit network and time around the source, or path lines showing the origin of connected aquifer. The water level in shafts can also define each particle (thus delineating the catchment area of the fixed head boundaries in the saturated cave network. The source). new model simulates only major, clearly mapped shafts which connect the ground surface to the cave network. branched caves can be simulated by the model. Cave systems are rarely mapped with the same resolution or certainty as mine tunnels (White 1999), so this component of the model is suitable for systems where the inflows and outflows to the cave network are well known, for example, through tracer tests.
Epikarst bypass flow generation The epikarst region near the ground surface absorbs and transmits infiltration rapidly into the subsurface region. Infiltrating water flows laterally towards vertical shafts which form conduits to the saturated zone (Jeannin and Grasso 1995). Kiraly and others (1995) assessed the role of epikarst in a three-dimensional, finite-element model and concluded that usually the representation of an epikarst layer was necessary to reproduce the dynamics of a karst aquifer. Therefore, a semi-empirical model for SHETRAN has been developed which allows the instantaneous transfer of water between an upper epikarst layer and the lower aquifer layer. The transfer flux is calculated as a function of the depth of water stored in the epikarst layer above a critical threshold. The model therefore requires two parameters: the threshold water depth, and the constant of proportionality.
Model verification and application The new model described above has recently been applied to steady-state simulations on a simple, hill-slope catchment drained by a single conduit spring connected to a single channel representing a small stream. These simulations have verified the components of the flow model described above. Figure 3 shows a plan view of the catchment with a series of particle path lines depicted by solid lines. The catchment comprises 64 100-m2 elements, and a single particle has been released from the centre of each element at the start of the simulation. The spring is
Interactions with surface flows The new model allows interactions between the VSS-NET and OC (Overland/Channel flow) components of SHETRAN in both directions. Stream inflows into either shafts or conduits can be simulated to represent the sinking streams (ponors) found in karst regions. Spring discharges from conduits into streams (which are modelled by the OC component) can also be simulated.
Implementation of a forward particle tracking model in SHETRAN Defining the area of ground over which the groundwater catchment is vulnerable to surface pollution is a major objective of the STALAGMITE project. In karst regions, actions at the ground surface can rapidly create impacts at large distances away. COST ACTION 65 (1995) produced recommendations for groundwater protection in 16 European countries with karst aquifers. In non-karstic Fig. 3 aquifers, particle tracking is often used to define protec- Plan view of test catchment showing particle path lines (model co-ordinates are in metres) tion zones around groundwater extraction boreholes.
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located near the centre on the southern boundary of the catchment. It is connected to three 100-m-long pipes orientated in the north-to-south direction and which drain the catchment. The upstream (northern) boundary condition is a fixed head. The path lines show the movement of particles towards the pipes and then directly to the spring. Figure 4 shows the discharge from the conduit spring for a time period of three months. Steady-state conditions have been achieved after around 1 month (744 h) when the discharge falls to approximately 5 l/s. Following initial testing, the SHETRAN karst hydrogeological model is being applied to karst aquifers in Slovenia, Slovakia and Bulgaria, and has been transferred to the relevant institutions in the project countries for further modelling studies. Simulations of the Nanos karst massif in Slovenia are currently underway. This massif comprises two sinking streams (Belscica and Lokva ponors) (mean discharge of around 0.1–0.2 m3/s) which drain an area of impermeable flysch (clay), connected by a karst conduit over 12 km long to a group of major springs (Vipava), from which the total daily mean flow is around 6 m3/s (KRI 1997). Tracer tests have proven the connection between the ponors and Vipava springs, and the total recharge area of the springs has been estimated at 149 km2. The study area is shown in Fig. 5, the area of flysch being located in the south-east corner. Simulations will compare several versions of the conceptual model with and without the pipe network included, and calibration will take place using the observed spring discharge. The model showing the best fit will then be used to compare different scenarios of land-use change in the Nanos massif, including deforestation and reforestation.
Fig. 5 Map of Nanos karst massive (Slovenia), showing elevation, locations of ponors and Vipava Springs (elevations are in metres above sea level)
areas around vulnerable karst water supply sources and to calculate the travel times of contaminants to the sources. The model is being used to assess the efficiency of land management policies for the sustainable use of karstic groundwater in Eastern Europe, and also the possible impact of climate change on water quantity. Acknowledgements The SHETRAN model development and testing described in this paper was carried out under EC Project IC15-CT98-0113.
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