When building tunnels in rock, an inflow of groundwater is likely induced. Depending on the hydraulic properties of the rock and the surrounding soils, the inflow may cause a decline in the groundwater level above the tunnel line with possible consequences for nearby well facilities or groundwater dependent environments. Discharge of the inflowing groundwater represents an operation of water (vattenverksamhet in Swedish) according to the Swedish environmental law, meaning that permission must be applied for at the Environmental Court. In the application, the operator presents an area of influence for groundwater, which defines the area where the groundwater levels could change due to the planned operation. In this work, the area of influence has been defined as the area where the groundwater level is lowered by more than five centimeters. As part of the project Tvärförbindelse Södertörn in southern Stockholm, the Swedish Transport Administration is planning for the construction of a tunnel in rock adjacent to Glömstadalen in Huddinge municipality. The aim of this master thesis was to investigate the extension of the area of influence of the tunnel through two-dimensional groundwater modelling. A site-specific model was created in the program SEEP/W by establishing a cross-section orthogonal to the tunnel and through examination of the hydrogeological conditions in the area. Steady-state modelling of both unsaturated and saturated flow was then carried out including and excluding the tunnel. Through this, the change in groundwater levels due to the tunnel could be reviewed. Since the development of the model required simplifications and assumptions of the site-specific conditions, a simple sensitivity analysis was also performed where a few model parameters were altered to examine how the area of influence changed. The modelling results showed that the tunnel, when sealed suitably, at most changed the groundwater levels 680 meters north and 840 meters south along the studied cross-section. The smallest effect on the groundwater levels was observed 400 meters north and 560 meters south of the tunnel. Differences in the extension of the area of influence were noticed depending on how the outflow of groundwater in Glömstadalen was represented, and which sealing properties around the tunnel were used. Furthermore, the modelling showed that the hydraulic conductivity of the rock is of major importance for the location of the groundwater table, and therefore it is considered relevant to perform hydraulic tests in the rock to increase the reliability of the model.

Grundvatten finns över allt i marken och vid byggnation under markytan är det därför viktigt att tagrundvattnets påverkan i beaktning för att inte skada miljö eller närområde. Hur en lokalgrundvattenstörning sprids i närområdet är normalt svårbedömt, och modelleringsverktyg har blivit ett vanligt verktyg för att undersöka flöden och potentiell påverkan på grundvatten. I detta arbete utvecklasen grundvattenmodell i ModelMuse med Modflow 6 för att undersöka påverkansområdet för en planeradbyggnation som kräver en grundvattensänkning, i Veddesta, Järfälla kommun. Arbetet syftar även tillatt bedöma de olika parametrar som krävs för modelleringen och hur de kan mätas eller uppskattas. Inför modelleringen undersöktes akviferens egenskaper med slugtester och ett kort pumpningstestoch tidigare undersökningar användes för att skapa modellens lagerföljd. Modellen använde tre lager,som generellt från ytan representerade lera, friktionsjord (morän) samt ett berglager, där domänenbestämdes utifrån ett avgränsat avrinningsområde. Modellen kalibrerades manuellt mot observationer i flera grundvattenrör kring byggnationsområdet.För övriga delar av modellen gjordes endast en grov värdering av grundvattennivåerna, där vissa oklaranivåer kvarstod under vidare modellering. Den avsänkning som modelleras uppgår som mest till 3,3 m,medan grundvattenobservationer visar att avsänkningen endast bör uppgå till 1,8 m vid samma punkt.Denna avsänkning resulterar i en omfattande och kraftig påverkan över ett stort område, men denmodellerade påverkan är troligen överdriven då den ursprungliga grundvattensänkningen haröverskattats. Den modellerade grundvattenpåverkan riskera att skapa sättningar och skada byggnader inom ettstort område, men dess effekter kan kraftigt minskas om avsänkningen görs inom en vattentät spont. En enklare känslighetsanalys över parametrarna som använts visar att akvifertjockleken har stor påverkanpå det beräknade utloppet för att upprätthålla avsänkningen, men relationen till påverkansområdetsstorlek är mindre tydligt. Andra betydande osäkerheter finns inom den modelleradegrundvattenbildningen samt det avrinningsområde som användes som rand. Detta arbete visar på hur de mest kritiska parametrarna för grundvatten kan bestämmas eller väljasoch hur de påverkar resultatet. Arbetet belyser också riskerna med felaktiga värden, som trots att de kananses rimliga utifrån teorin inte är lämpliga vid modellering i tätbebyggda områden. Stora avvikelserfrån de initiala värden upptäcktes för grundvattenbildning och problem relaterade tillavrinningsområdet, som upptäcktes under modellens kalibrering. / Groundwater is found everywhere in the ground and when building below ground level, the groundwatermust be considered. Failure to do so may result in damage the environment by reducing wateravailability for vegetation, nearby structures through reduced ground stability or endanger drinkingwater resources. Groundwater is difficult to assess as it can only be directly observed throughobservation wells, and digital modelling tools have become a common tool for investigating flows andpotential impacts on groundwater. In this work, a groundwater model is being developed in ModelMusewith Modflow 6 to investigate the effective influence area for a planned construction in Veddesta,Järfälla municipality in Sweden. During this construction, the groundwater levels will be loweredtemporarily to enable excavation and construction below the natural groundwater level The work alsoaims to assess the various parameters required for the modelling and how they can be retrieved. Prior to the modelling, the properties of the aquifer, which is where the main groundwater transportoccurs, were examined with slug tests and a short pumping test. Data from previous investigations wereused to create the model's stratigraphy. The model used three layers, which generally represented clay,granular soil (till) and a thin rock layer and the model’s boundary was set based on an estimatedcatchment area.    The model was manually calibrated against observations in several groundwater pipes around theconstruction area. For other parts of the model, only a rough assessment was made of the groundwaterlevels, where areas of unclear pressure heads remained throughout the modelling. The modelledlowering of the groundwater amounts to 3.3 m, while groundwater observations show that the expectedlowering should be 1.8 m. The modelled displacement results in a widespread and significant impact,but the modelled impact is probably excessive as the initial lowering of the groundwater is larger thanwhat would be necessary during the construction. The modelled displacement of groundwater risks creating subsidence which will likely damagebuildings within a large area. The impact can be greatly reduced if the lowering of groundwater is donewithin a waterproof sheet pile, which enables dewatering within the excavation without effecting thesurroundings. A simple sensitivity analysis of the parameters used in the modelling shows that theaquifer thickness has a large impact on the calculated outlet across the model boundary, which representsthe required pumping to maintain the reduced groundwater level. The relation to the size of the effectiveinfluence area is less clear. Other significant uncertainties exist within the modelled groundwaterrecharge and the catchment area that was used as the model’s boundary. This work has shown how the most significant parameters for groundwater modelling can bedetermined or selected and how they affect the result. The work also highlights the risks of incorrectvalues, which, although they can be considered reasonable based on theory may not be suitable formodelling groundwater within built up area. Notable deviations from the initial values were rechargerate and issues related to the model domain, which were discovered during the calibration process.